SSDP Framework Report Final

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Development of

Simplified Seismic Design

Procedures

Framework Report

September2010


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Table of Contents

Executive Summary. 1

Chapter 1, Introduction... 3

Chapter 2, Background 7Chapter 3, Options For Further Development of Simplified Provisions. 27

Chapter 4, Recommendations.. 35

References 51

Acknowledgments... 55

Appendix A, Work Plans for Further Development of Simplified Provisions 57

Appendix B, Detailed Backup of chapter 2. 67


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Development of Simplified Seismic Design Procedures: Framework Report, September 2010 1

EXECUTIVE SUMMARY

This report covers the efforts of Building Seismic Safety Council of the National Institute

of Building Sciences for Development of Simplified Seismic Design Procedures fundedby Federal Emergency Management Agency (FEMA) under Contract HSFEHQ-09-D-0147, Task Order HSFEHQ-09-J-0002. The initial objective of this project, documentedin this report, was to study past efforts intended to simplify seismic design provisions,obtain input from engineers around the United States, and recommend one or moreinitiatives for further development. The project is coordinated by a Project ManagementCommittee (PMC) consisting of five engineers experienced in building design and codewriting. This report was also reviewed by a Project Review Panel (PRP) and othersconsisting of code writers and users.

The project has been initiated in response to the concerns expressed by engineers and

building officials that the complexity of current code provisions is reducing the efficiencyand reliability of seismic design, particularly for simple and small buildings. Many codedevelopment groups and organizations have previously sought various ways to alleviatethese concerns through the development of design guides, flow charts, and simplifiedprovisions for certain small buildings. However, it is widely recognized that efficientseismic design is often a complex process, particularly for non-standard buildings, andthat the basic seismic design provisions need to be complete and possibly complex tocover all buildings.

Further, some previous attempts at simplification of seismic design for specific buildingtypes have not been pursued because of the need to prove the equivalency of theproposed provisions to the full code. Now, however, a methodology to show equivalencyto code-mandated seismic performance is available (FEMA P 695) and can be used todevelop more innovative provisions for narrow ranges of building types or levels ofseismicity.

The results of this study include recommendations that three initiatives be pursued todevelop simplified seismic provisions:

1. Refinement and further simplification of ASCE/SEI 7-10 Section 12.14,

2. Develop stand-alone seismic design provisions for Seismic Design Category BBuildings, and

3. Develop stand-alone seismic design provisions for buildings with rigid walls andflexible diaphragms.

The three options do not overlap and, in addition to having great promise to provide anincrement of code simplification on its own, each will test the viability of futuredevelopment of similar sets of provisions.


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In addition, it is recommended that each of the three options be pursued by a separateProject Working Group (PWG) consisting of practitioners and academicians withrelevant experience and background.


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CHAPTER 1

INTRODUCTION

BACKGROUND

Over the past 25 years, the seismic design procedures and provisions in the nationalmodel codes and standards have continuously been refined and been made morecomprehensive to cover more building types and sizes located in regions with a widerange of seismicity, to accommodate an ever-expanding state-of-the-art, and toincorporate lessons learned from damaging earthquakes. As a result, the designprovisions have grown in number and complexity. As the design community continues toadopt new systems, the information in and complexity of the standards and codes arelikely to keep increasing. In addition, national standards and codes such as ASCE/SEI 7

(ASCE/SEI, 2005 and 2010) and the International Building Code (IBC, 2006 and 2009)are referencing the material standards for various material details, which also maycontribute to the complexity of a design process.

Some engineers and building officials have articulated their concern that the current codeprovisions are difficult for them to understand and to correctly implement, particularlyfor simple and small buildings that are expected to be designed quickly and efficiently.These concerns are particularly troublesome if lack of understanding or short-cuts lead tothe design and construction of buildings that will perform poorly in strong groundshaking or of buildings that are unreasonably over-designed as a compromise to avoidcomplex code requirements. Many code development groups and organizations have

sought various ways to alleviate the concerns, and theseextensive efforts are summarizedin Chapter 2 of this document.

In recognition of this dilemma, simplified design procedures were developed for the 2003NEHRP Recommended Provisions for Seismic Regulations for New Buildings and OtherStructures (FEMA, 2003, Alternative Simplified Chapter 4, page 73). Subsequently, thenational load standard,Minimum Design Loads for Buildings and Other Structures,ASCE/SEI 7-05 (ASCE/SEI, 2005), was modified to incorporate similar simplifieddesign provisions (Section 12.14, page 135). The design method covers a wide variety ofbuilding types but is limited to low-rise, regular, and inherently stiff structures. Thestreamlining and simplification is focused on the determination of seismic forces andstructural analysis and does not affect material detailing requirements contained in designstandards for the various structural materials and systems.

The effort for the 2003NEHRP Recommended Provisions (FEMA, 2003) was the resultof a more comprehensive planning study completed during the development of the 2000Provisions (FEMA, 2000). The 2000 study concluded that simplification and improvedunderstanding of the code procedures could be accomplished in several ways, includingediting and clarifying the current provisions, reducing prescription and emphasizingperformance-based design, reducing complexity by increasing conservatism, or


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developing structure-specific provisions parallel to the main design standard that mightnot even follow the current general procedures but would yield equivalent performance.

In the past few years, FEMA has noted many indications that the simplified procedures inASCE/SEI 7-05 (ASCE/SEI, 2005) still have not been widely accepted or welcomed bythe design community and the concerns regarding design complexity have not beenreduced. The barriers to more widespread use are believed to be:

Applicability is limited to few building types and sizes and the procedures are notself contained.

Designers who only occasionally perform seismic design still find the processcomplex, particularly the interplay between ASCE/SEI 7-05s (ASCE/SEI, 2005)analysis rules and the materials standards organizations detailing rules.

The simplification comes at a cost of conservatism that may increase projectcosts.

The issues, interests, and viewpoints related to simplification of seismic design aremultiple. They include the public need for provision of adequate seismic safety, highlyvariable designer interest and motivation (particularly in regions of infrequent seismicactivity), the economic interests of various material industries, the inherent complexity ofthe process due to variation of design ground motion by geographic area and site soilconditions, the nonlinear dynamic response of structures to shaking, and the rich varietyof building systems and styles used in the United States. In addition, some are concernedthat seismic code changes are occurring faster than they can be absorbed, that addedcomplexity may not assure safer buildings, and that the costs to design professionals andowners are excessive. Although many different approaches have already been proposedor explored in various attempts to meet the largely dispersed interests, it is widely

recognized that there is no one-size-fits-all solution. The current simplified designprocedures were developed as a sub-set of the existing standard design requirements,primarily because there was concern about proving equivalency for design provisions thattook a different approach.

However, the recently developed procedures for qualifying new structural systems forincorporation in the building code as described in Quantification of Building SeismicPerformance Factors, FEMA P-695 (FEMA, 2009), also can be used to demonstrateequivalency with code objectives for a parallel design methodology that is not necessarilybased on traditional force-based design concepts. This capability was not availableduring the 2000 or 2003NEHRP Recommended Provisions development cycles when

current simplified design provisions were prepared.

It also must be noted that a considerable portion of perceived complexity in overallseismic code provisions stems from the material-specific detailing requirements neededto provide the ductility and toughness assumed in the main body of the code. Thesedetailing requirements are typically provided by expert committees organized andsponsored by various groups that promulgate material-specific design standards.


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In general, this BSSC project will focus on simplification of provisions in the main bodyof the code; however, opportunities to simplify, or eliminate, material-specific detailingrequirements also will be considered.

OBJECTIVES

This project is intended to develop simplified seismic design procedures that will result instructures with seismic performance of increased reliability and consistency with little-to-no additional construction cost. The procedures will be focused on targeted buildingtypes or groups of buildings that are common to the United States and whose design maysuffer from the complexity of the current full code provisions. The seismicperformance resulting from use of the simplified procedures will be at least equivalent tothe standard code objectives.

SCOPE

This project includes review of input received by FEMA in the past 10 years regardingthe complexity of current design methodology, articles and papers discussing simplifieddesign, new input from various regions in the country solicited by this project, and reviewof various guidelines available to assist designers with seismic design, particularly withrespect to material detailing requirements for various seismic regimes. The project willdevelop optional target building groups or building types for simplified design and selectthe most promising for further development by small groups of experts. Finally, theproject will develop design provisions suitable for adoption by building code groups.

The project is coordinated by a Project Management Committee (PMC), a group of five

engineers experienced in building design and code writing. Like this report, the productsof the project also will be reviewed by a Project Review Panel (PRP) consisting of expertcode writers and users.

THE FRAMEWORK REPORT

This report documents the first phase of the BSSC project during which promising targetbuilding groups or building types were identified for further development. In addition,the scope of development work needed is described in some detail.

Chapter 2 describes the history of the development of simplified seismic provisions as

well as other sources of concern about code complexity and recommendations forsimplification. This information provides the background for the development ofoptional simplification efforts to be pursued by the project.

The options for simplification that have been identified by the PMC are described anddiscussed in Chapter 3. Chapter 4 describes the options recommended for furtherdevelopment by this project and the analytical procedures needed to show that the variousmethodologies will provide seismic performance equivalent to the full code.


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Appendix A contains detailed work plans that are to be used to select and direct the teamsthat will be charged with further development of the recommended options. Appendix Bincludes extensive documentation of the historical and background information reviewedfor this project and summarized in Chapter 2.


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CHAPTER 2

BACKGROUND

The growing interest in simplifying seismic design requirements for buildings comesreflects the views of a variety of concerned users. As the principles of seismic designregulations have been spelled out in technical terms in buildings codes, the requirementshave become increasingly complex and building officials, engineers, and others are hardpressed to keep pace with the changing rules. A natural concern has arisen as to whetherthese requirements are being adequately interpreted and applied to new building designsand if the language of seismic design could be clarified and simplified for certainbuilding types.

This present Building Seismic Safety task order explores the continuing need to make

seismic design more accessible to the design professions, building officials, andconstruction personnel involved in building new structures. The end goal of this effort isto minimize errors and assure that the code-prescribed performance objectives are met forthe greatest number of buildings.

The following is a summary of the various efforts to simplify some of the seismicrequirements for the design and construction of new buildings. These undertakings werestudied by the Project Management Committee and used as a basis for developing thisframework report.

PREVIOUS EFFORTS

Structural Engineers Association of California Activities

The Structural Engineers Association of California (SEAOC) has been actively involvedin the development of seismic design provisions for building codes since the late 1950s.The SEAOC Blue Book: Seismic Design Recommendations of the SEAOC SeismologyCommittee (SEAOC, 2009) is considered a visionary publication of earthquakeengineering design and is renowned worldwide.

Early on, concepts like regularity, reliability, and capacity design helped providedirection for the development of the recommendations. With time, these concepts began

to manifest themselves in specific requirements for detailing building components.However, simple observations of post-earthquake building performance have been theguiding lights for these efforts.

In exploring means to simplify seismic design provisions, these early concepts can helpguide and shape the underlying directions for this effort. While technical parameters arethe essence of seismic design, the conveyance of these basic concepts is a desirable goal


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since they inform and educate those responsible for the construction of seismically safestructures.

Conventional Construction

Certain building types have been recognized in building codes as having historicallyperformed well in earthquakes. For a number of years, the Uniform Building CodesConventional Light-Framed Construction Provisions provided prescriptive designmethods for small wood-frame buildings with various height restrictions for a number ofyears. These methods include both gravity and lateral design requirements that aredeemed to comply with the engineering provisions of the building code.

Substantial modifications to the wall bracing requirements occurred after the 1971 SanFernando earthquake. The SEAOC Conventional Construction Task Force undertook thenext significant effort in 1990. This groups product, adopted into the 1994 UniformBuilding Code (ICBO, 1994),added considerations for regularly shaped buildings andrequirements for separating seismic zones 0, 1, 2 and 3 from zone 4.

The prescriptive requirements of these conventional construction provisions held to someof the basic principles of seismic design such as minimum shear wall bracing lengths andconfigurations, continuous wall chords, and continuity ties. A table of nailingrequirements provided minimum permissible connections, helping to assure thatcomponents remain attached during seismic shaking.

Indeed this effort was among the first to simplify seismic design by concentrating onbasic sound principles for good seismic performance and limiting its use to buildingsmeeting certain criteria. This concept could be a model for other building constructionmaterials and its focus on simple building forms with expected, predictable behavior is a

proven example of how regulations can be specified to achieve acceptable performance.

1997 Uniform Building Code Simplified Design Base Shear

The 1997 Uniform Building Code (UBC) was to be the last edition published by theInternational Conference of Building Officials (ICBO) before the three major codewriting agencies underwent a consolidation and became the International Code Council(ICC). As such, the SEAOC Seismology Committee anticipated the need to coordinateand continue efforts to help shape the 2000 International Building Code (ICC, 2000a).

One significant change from the 1994 to 1997 edition of the UBC(ICBO, 1994 and1997) was the introduction of the Simplified Design Base Shearin Section 1630.2.3.This was done in response to practicing engineers concerns about the growingcomplexity of the seismic design regulations. The single code formula, V = 3.0CaW/R,could now be applied to all buildings two stories or less in height (excluding basements)and light-frame construction up to three stories in height (excluding basements).

This static force procedure also allowed for vertical distribution of forces based on thetributary mass at each level. Other simplifications included the removal of checks for


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modeling requirements, P-delta effects, structure period, story drift, and dynamicanalysis.

International Residential Building Code (IRC)

With the merging of the nations building codes into a single resource in the late 1990scame an effort to expand the regulations for light-framed construction for residences intoa separate building code. The International Residential Building Code (IRC) for One-and Two-family Dwellings (ICC, 2000b) was developed to encourage the safe andeconomical construction of the nations most prevalent building type.

Much like the conventional construction provisions of the earlier Uniform Building Codeand the International Building Code, the IRC establishes many prescriptive requirementsfor the seismic design of residential structures and townhouses up to three stories inheight. Among these are regulations regarding the arrangement and lengths of bracingelements, building configuration limitations, and other controls on the applicability of theIRC as it is to be used in the construction of new residences.

While a history of the performance of buildings constructed under the IRC does not yetexist, its basis in the earlier conventional construction requirements of the UBC helpsassure that satisfactory performance can be anticipated. This effort is considered a well-worn example of prescriptive provisions that, for the most part, do not require directengineering design calculations.

R = 3 Simplification for Seismic Design Category B and C Steel Structures

The ultimate in simplification for material-specific detailing requirements was introducedin the 1997NEHRP Recommended Provisions (FEMA, 1997). Any seismic-force-

resisting structural system of structural steel for a structure in Seismic Design Category Bor C could be designed without following any of the provisions of AISC 341, SeismicProvisions for Structural Steel Buildings (AISC, 2005a),if theR factor (responsemodification factor) was set at 3 and the structure did follow the requirements of AISC360, Specification for Structural Steel Buildings (AISC, 2010). This provision has beenin all subsequent editions of theNEHRP Recommended Provisions as well as inASCE/SEI 7 and the IBC. One of the arguments in favor of the provision at the time wasthat conventional practice in many areas of low to moderate seismicity was to:

1. Select anR factor (or the predecessorKfactor),2. Compute a seismic base shear,3. Compare that value to the wind force, which was usually larger,4. Design for the wind force, and5. Ignore the details required for the assumed seismicR factor.

It does appear that the R = 3 provision has reduced the prevalence of that particularerroneous practice (Carter, 2009). However, there has been concern in recent years thatthe R = 3 provision does not meet the objective set forth earlier in this report thatbuildings designed according to a simplified alternative should meet the seismicperformance objectives inherent in theNEHRP Recommended Provisions. One system


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that has been studied and found lacking in some instances is the concentrically bracedframe of structural steel (Hines et al., 2009). The R = 3 provision also has beencriticized on a philosophical basis in that it discourages detailing for ductility, which maywell be the most effective way to provide reliable seismic performance.

Any simplification that is as sweeping and simple as the R = 3 provision is likely to befound wanting in some circumstances. It covers all conceivable seismic-force-resistingsystems for steel construction and it may be relatively unsafe for some while it may resultin more costly construction for others. Validation of the R = 3 provision by the FEMAP-695 (FEMA, 2009) methodology would be an extremely large task that is not proposedin this project. This does limit any other simplification proposal that would be assweeping in scope as the R = 3 provision.

Simplified Procedures Task Group of the 2000 Provisions Update Committee

The BSSCs Provisions Update Committee (PUC) that developed the 2000 edition of theNEHRP Recommended Provisions established a special task force to develop a simplified

set of the provisions for low-rise, stiff structures. In response to practitioners concernsthat seismic design procedures had become too cumbersome and complex for practical,efficient, and reliable seismic design, the task groups objective was to develop one ormore simplified design procedures to:

Reduce engineering errors, thereby making more reliable buildings,

Gain broader acceptance in the design community across the country for seismicdesign procedures,

Facilitate easier learning of seismic design procedures and changes thereto, and

Improve design efficiency.

Some of the principles discussed included the acknowledgement that good seismicperformance is achieved when reliable inelastic behavior can be attained. Ductility,redundancy, and regularity play an important part in shaping these results.

The PUC also acknowledged that the existing seismic design requirements provided thebroadest solutions for building design and therefore encompassed a set of complexregulations. Finding a solution set for buildings with relatively simple characteristics wasan aim that guided the task groups work. Some of the groups general findings andrecommendations (SPTG, 1999) related to:

A road map or flow charts can be provided to reduce the complexity of theseismic design procedures. This would require reorganization and reformatting ofrequirements in Chapter 16 and the materials sectionsof IBC provisions.

Detailing requirements represent the largest change in provisions. One way toreduce or avoid complex detailing could be the Japanese approach of increasingbase shear to elastic levels.

Simplified seismic design procedures should not be limited to small structures


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Simplified seismic design procedures should not be limited to calculation ofseismic design loads. Other aspects of seismic design such as detailingconnections should be addressed.

Simplified seismic design procedures should be able to handle irregularities thatare common to small buildings.

One size fits all approach of the current procedures is the root of complexrequirements.

To be able to use simplified seismic design procedures, the designer may need togive up something to get something

From these considerations, five basic approaches were studied:

Reformat the current code for easier comprehension,

Reduce instructions/prescription,

Develop a simplified base shear equation,

Develop simplified detailing provisions, and

Develop structure-specific simplified procedure for one-story flexible roofbuildings.

Three basic efforts and recommendations resulted from these deliberations:

Building- Specific Simplified Procedures for one-story flexible roof buildings,

Global Simplified Design Procedure for low-rise structures not sensitive to drift,and

Improved presentation of the existingNEHRP Recommended Provisions.

The PUC effort was refined and tested over the 2000 and 2003 cycles. A formal proposalwas developed after a calibration study was undertaken to compare the design forcesusing the simplified procedure with the forces from the fullProvisions considering thevariables of numbers of stories, soil class, and the value ofSs. This resulted in thedevelopment of a stand-alone alternative procedure to Chapter 4, Structural DesignProcedures. This was formally included in the 2003NEHRP Recommended Provisions(FEMA, 2003)as the Alternative Simplified Chapter 4, Alternative Structural DesignCriteria for Simple Bearing Wall and Building Frame Systems. Appendix B provides amore detailed description of this earlier PUC effort to develop simplified seismic designprocedure.

2000 International Building Code Simplified Design Base Shear

The three model code agencies merged into the International Code Council (ICC) andpublished the first national code, the International Building Code, in 2000 (ICC, 2000).The earthquake requirements in Chapter 16 were based on the 1997 edition of the


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NEHRP (National Earthquake Hazards Reduction Program) Recommended Provisions

for the Development of Seismic Regulations for New Buildings (FEMA, 1997).

The Simplified Analytical Procedure for Design of Buildings, IBC Section 1617.5,updated the requirements by converting them to strength design. In essence, there was a20 percent penalty on the equivalent lateral force method and the limitations remainedessentially the same as the older 1997 UBC (ICBO, 1997).

Alternative Simplified Design Procedure of 2003NEHRP Recommended

ProvisionsAs noted above, the 2003Provisions, FEMA 450, (FEMA, 2003)containedfor the first time an Alternative Simplified Chapter 4, Alternative Structural DesignCriteria for Simple Bearing Wall or Building Frame Systems. This was in recognition ofthe need to address the increasing complexity of seismic requirements in building codes.The commentary to the 2003NEHRP Recommended Provisions (FEMA, 2003)states:

It is feared that the typical designers of small, simpler structures, which possiblyrepresent more that 90 percent of construction in the United States, may have difficulty

understanding what the Provisions require in their present complex form.

This alternative simplified design procedure (ASDP) is subject to the followinglimitations:

Structure shall qualify for Seismic Use Group (SUG) I.

The Site Class shall not be Class E or F.

Structure shall not exceed three stories in height above grade.

Seismic force resisting system shall be either a bearing wall system or buildingframe system.

The structure shall have at least two lines of lateral resistance in each of twomajor axis directions.

At least one line of resistance shall be provided on each side of the center of massin each direction.

The sum of strengths of the lines of resistance on each side of the center of massshall equal at least 40 percent of the story shear.

For buildings with non-flexible diaphragms, the eccentricity between the center ofrigidity and center of mass parallel to each major axis shall not exceed 15 percentof the greatest width of the diaphragm parallel to that axis.

Lines of resistance of the lateral-force-resisting system shall be oriented at anglesof no more than 15 degrees from alignment with the major orthogonal horizontalaxes of the building.

The alternative simplified design procedure shall be used for each majororthogonal horizontal axis direction of the building.

System irregularities caused by in-plane or out-of-plane offsets of lateral force-resisting elements shall not be permitted.


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The lateral-load-resistance of any story shall not be less than 80% of the storyabove.

The task group limited this new procedure to structural systems that did not have thecomplicating drift check criteria in the general procedure. Limiting it to bearing wall and

frame systems also required that several prescriptive rules be established to assure thatadditional complications from highly irregular and torsionally challenged buildingswould not be allowed.

In an effort to simplify the design and detailing needs established in the full provisions,they were consolidated in the ASDP into a single set of requirements that do not varywith Seismic Design Category. This could only be accomplished for buildings that didnot contain some of the known problems which, when allowed in construction, requirestrict attention to special design forces generated and the associated special detailing toavoid failures.

Other simplifications such as the removal of the redundancy coefficient, assumption of

default Site ClassFa values, vertical shear distribution based on tributary weights, and a 1percent drift assumption when needed by requirements not covered in the simplifiedprovisions were implemented to help streamline the structural design process.

ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures

ASCE/SEI 7-05(ASCE/SEI, 2005) was adopted by the IBC (ICC, 2006, 2009) as theseismic provisions. ASCE 7, Section 12.14, Simplified Alternative Structural DesignCriteria for Simple Bearing Wall Buildings or Building Frame Systems, contains aseparate Table 12-14-1, Design Coefficients and Factors for Seismic Force-ResistingSystems for Simplified Design, with 42 bearing wall and building frame systems.

Much like the earlier requirements in the 2000 IBC (ICC, 2000) and the 2003NEHRPRecommended Provisions (FEMA, 2003), the ASCE/SEI 7-05 (ASCE/SEI, 2005)provisions limited the use of this method to certain building types and configurations.The twelve limitations listed in Section 12.14.1 mimicked the ASDP method of the 2003NEHRP Recommended Provisions (FEMA, 2003) and updated them to current codelanguage.

The ASCE/SEI 7-05 Simplified Alternative Structural Design Criteria (ASCE/SEI, 2005)was the first stand-alone version of a simplified earthquake design procedure for simplebearing wall and building frame systems. For structures that qualify to use ASCE/SEI 7-

05 Simplified Alternative Structural Design Criteria(ASCE/SEI, 2005), the seismicdesign category is defined based solely on the short-period design spectral coefficient,SDS.


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OTHER INITIAL INPUT

FEMA Solicitation of Opinions

FEMA undertook an effort to solicit opinions from a variety of design professionals and

building officials on the use of the existing simplified seismic design procedures (SSDP)and the need for further improvements. The request asked for comments and suggestionsto help assess the critical issues, practical needs, focus areas, and potential benefits forpursuing such improvements. The following are highlights of some of the responsesreceived (more information about the FEMA solicitation of opinions can be found inAppendix B):

Current code is complex because:

The current language and structure of the code can be confusing.

Detailing requirements are difficult, especially for those who are not familiar withseismic design concepts.

ASCE/SEI 7 Table 12.2-1 is too large and extensive.

Seismic design parameters such as overstrength or redundancy factors are notwell understood.

Construction practices that are traditionally used in many parts of the country butdo not really work very well in earthquakes have been incorporated in the currentcode (which covers more than 80 structural systems).

It is difficult to predict the seismic behavior of an inelastic structural systemwithout complex approaches.

An effort to simplify the code is encouraged because:

The simplified design procedure would need less design time, would simplify (oreliminate) the plan check and peer review process, and would reduce errors.

The simplified design procedure would improve the efficiency of the seismicdesign and construction which would result in cost reduction.

Engineers have embraced the other simplified design procedures.

It is suggested that the current code can be simplified by:

Providing commentary to substantiate the design basis assumptions.

Advocating use of the simplified code to industry.

Consider regional/national practice variations.

Consider regional/national enforcement variations.


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Use concepts that can be retained and applied easily by design professionals andbuilding officials and can be understood by public and clients.

Incorporate flowcharts and checklists.

Use the FEMA P-695 methodology (FEMA, 2009) to show the equivalency of the

simplified methods to the prevailing design procedure. Consider the interaction of various design demands such as seismic, wind, and

security.

Address the necessity of seismic detailing in low- or mid-seismicity regionswhere other forces (wind) may control.

Allow for non-seismic detailing coupled with elastic seismic response.

Avoid comparison of simplified versus complex provisions.

Develop design tools such as CodeMasters (2006a, b, and c) and flowcharts forICC publications (Fanella, 2009) that lead to a series of screens to develop steps

for progression through the provisions.

Eliminate vertical component of earthquakes, rigid versus flexible diaphragms,and redundancy factors.

Address light-frame construction (shear wall buildings) in a stand-alone code.

Address both complexity of seismic demand and material-specific capacity anddetailing.

Pursue other feasible ideas before trying to develop building-specific simplifiedprocedures.

Simplified design methods might be more acceptable if they:

Demonstrate that the simplified method produces safe design and is time and costeffective over current design method.

Provide for monitoring of progress to assure that development of the simplifiedmethod is useful to engineers.

Introduce a General or Uniform Design Method instead of SimplifiedDesign Method.

Designers have not used the existing simplified design procedures frequently because:

The goals of being safe, economical, and simple cannot all be achieved.

The twelve preconditions of the ASCE/SEI 7-05 Simplified Alternative StructuralDesign Criteria (ASCE/SEI, 2005) are not easy to check.

There is a perception that simplified procedures are as not as good or thorough asthe current provisions.


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Among the design professionals who responded to the FEMA solicitation of opinionswere some who did not encourage the effort because they thought:

The complexity of the seismic design procedures has remained essentiallyconstant for the past decade. Therefore, the code is no longer becoming

increasingly complex and code complexity has largely stabilized. If engineers cannot follow the requirements of present seismic provisions, they

should not be practicing in places that require seismic design.

A simplified design guideline may not be appropriate for code revision butpossibly as derivative documents for a designers library.

Efforts such as FEMA-sponsored seismic design seminars that focus on helpingpeople use the current provisions rather than changing them might give designersa better perspective.

Concerns regarding some important provisions may be inadvertently overlookedwhen using simplified procedures.

Research by Project Management Committee

The Project Management Committee was asked to gather information about the differentregional needs for a simplified seismic design procedure. The focus of the studies was topotentially target specific building types and generally exclude considerations for SeismicDesign Categories (SDC) D, E and F but possibly include low SDC D (moreinformation about this research by the Project Management Committee can be found inAppendix B).

Northeast and Southeast (Kelly, 2010)

Speaking with a few individuals about seismic design simplification the followinggeneral comments were made in favor of the effort:

Producing the ASCE/SEI 7 provisions with provisions applicable only to SDC Ato C buildings seems to be a good idea. Such a document would eliminate non-applicable irregularities and would only present the equivalent lateral forceprocedure.

Attempts have been made to do this with Chapter 12 but for SDC B buildingsonly (by Peter Griem).

In terms of model building types/occupancies, the following could be candidates forsimplified seismic design procedures:

Retail/warehouse box buildings, precast bearing wall/shear wall panels, steel joistand joist-girders, and metal deck diaphragms

Residential block and plank structures

Office buildings with steel braced frames and composite steel frames


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Hospitals and institutional science buildings with steel moment frames

Hospitals and institutional buildings with concrete shear walls

Hospitals with beam-and-column concrete moment frames

Office buildings with flat plate moment frames

Metal buildings

Retail block and bar joist buildings for retail or light manufacturing.

Precast parking garages

Residential light-frame construction on a podium structure

Intermountain West: Utah, Idaho, and Nevada (Welliver, 2010)

An audience consisting primarily of structural engineers and building officials wasqueried and the following general comments were received:

Reduce engineering errors making more reliable building designs

Gain broader acceptance in the design community, nationally, for seismic designprocedures

Facilitate easier learning of seismic design procedures and changes thereto

Improve design efficiency

Targeted structural systems with predictable seismic performance

The following were identified as potential areas where simplified seismic design may be

appropriate:

Two seismic codes (Progressive Seismic Code getting more detailed withcomplexity).

SSDP for light-frame construction

SSDP for reinforced concrete and masonry low rise buildings

Candidate Building Type: Warehouses, low-rise steel frame (moment and bracedframes)

Simplifying seismic detailing requirements

Northwest and Alaska (Hooper, 2010)

The following are comments and suggestions by various individuals regarding buildingtypes that may be candidates for a simplified seismic design procedure for SeismicDesign Category B buildings:

Ordinary reinforced concrete shear walls


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Ordinary precast shear walls

Ordinary reinforced masonry shear walls

Intermediate reinforced masonry shear walls

Ordinary plain masonry shear walls

Light-frame construction walls sheathed with wood structural panels rated forshear resistance or steel sheets

Light-frame construction walls sheathed with shear panels of all other materials

Light-frame construction walls systems using flat strap bracing

Ordinary concrete moment frames

Cantilevered column systems

Steel systems not specifically detailed for seismic.

Regarding the general concept of simplified seismic design procedures, the followingcomments and suggestions were noted:

The design and detailing requirements for SDC B structures are not overlycomplex or onerous, but the user/engineer still has to wade through and be able tointerpret applicability of provisions for other SDCs and that creates the perceptionof complexity.

A document that just has the design requirements applicable for SDC B buildingscould be an inclusive model code with reprinted portions of the material standardsapplicable to SDC B buildings as well.

A web-based database driven application where the user enters a SDC, applicableconstruction types/materials, and maybe some other project data and a customcode is created for that project.

Modifications to Chapter 12 of ASCE/SEI 7-10 and removing items that are notapplicable to SDC B. Simply having a chapter that was not clouded byrequirements for higher SDCs would make it easier to navigate and apply.

The simplified method that is already included in ASCE/SEI 7 (ASCE/SEI, 2005and 2010) is truly simpler except for the heinous appearance of the torsionaleffects equation

The AISCR = 3 system does not appear to be allowed by the simplified procedure

and the pros and cons of including it should be considered.

Certain procedures could be skipped in areas with smaller ground motions (thecurrent simplified procedure of in ASCE/SEI 7 seems to imply there are) and thatcould be integrated in a "low seismic" chapter

The BSSC project to develop simplified design procedures was presented to theStructural Engineers Association of Washingtons (SEAW) Earthquake


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Engineering Committee (EEC) on January 28, 2010. A range of options wasdiscussed including developing stand-alone design requirements for specificSeismic Design Categories (SDC). In the end, the group of approximately 25engineers (it should be noted that their experience is for SDC D structures) did notsee the merit in developing specific requirements for each SDC. Such a

separation was actually used in early versions of theNEHRP RecommendedProvisions but was discarded due to the repetitive language necessary. Thisposition did not directly address the concept of development of provisionsapplying only to a lower category such as SDC B.

The rigid walls with flexible diaphragms system, which includes both concreteand masonry wall systems, are thought to be very common and the current designprocedures do not adequately predict their true dynamic characteristics.

Single- and multi-family structures are very common and range in design fromsingle-story, single-family residences up to five-story, multi-family residencessupported by one or more levels of concrete. The hope is that the distribution of

forces to the walls could be simplified (based potentially on linear feet of wall ineach direction) and that the diaphragm/collector design could be based on asimilar simple distribution of forces.

Published Articles

Performance-Based Retrofit of School Buildings in British Columbia, Canada (Taylor

et al., 2009)

Abstract as published:

In 2004, the Province of British Columbia, on the West Coast of Canada, announced a

10-15 year, $1.5 billion seismic retrofit province for the provinces 750 at-risk publicschools. The purpose of this earthquake preparedness initiative is to accelerate theupgrading of school public safety in the moderate and high seismicity regions of theprovince. Given the magnitude of the mitigation program, the provinces Ministry ofEducation made a commitment to support the development of state-of-the-artperformance-based seismic engineering technology for achieving optimum safety withina cost-effective mitigation framework, which could not be achieved based on currentpractice. This paper describes a non-linear performance based methodology that has beendeveloped to provide the framework for the drafting of these technical guidelines. Thismethodology is the culmination of more than five years of applied research and extensiveand intensive peer review.

Nonlinear Performance Based Seismic Assessment for Low-Rise Buildings (Hanson et

al., 2009)

This paper describes the basic concepts and approach for the seismic rehabilitationmethodology developed for use in the British Columbia program described in the paperPerformance-Based Retrofit of School Buildings in British Columbia (Taylor et al.,2009) cited above.


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The methodology was released to the design community in November 2006 and is knownas the Bridging Guidelines; however, the official copyrighted title is Performance-Based Seismic Assessments and Retrofit Design Concepts - Second Edition. Theprocedure is continuing in development and is anticipated to be released in a final form.

Building configuration limitations for the methodology include:

Three-story or shorter school buildings

Primary lateral force deformation resisting system (LDRS) provides acceptablelife-safety risk

Heavy partition walls have adequate restraint

Nonseismic elements have adequate drift compatibility

Retrofitted building must have an adequate load path from foundation to roof.

The procedure implements a nonlinear hysteretic single-degree-of-freedom analytical

model subject to ground motions selected to be representative of those expected at eachsite. These analyses also provide the necessary information as to the desired level ofseismic upgrade necessary to achieve noncollapse for each specific building beingevaluated and provide a process for combining the LDRS properties of the originalbuilding and the retrofit LDRS properties.

Simplified Seismic Design Utilizing Nonlinear Dynamic Analysis (Hanson et al., 2010)

Abstract as published:

One possible way to alleviate the problem of the apparent complexity to performearthquake resistant designs of simple buildings in the low to moderate seismic hazardareas of the USA is to develop spreadsheet data that can be used interactively to achieveappropriate lateral force resisting designs of the structural systems. It is proposed that theFEMA P-695 Quantification of Building Seismic Performance Factors methodology(FEMA, 2009) developed by ATC under contract for FEMA be used to verify theadequacy of this design approach spreadsheet data. This paper attempts to identifypossible steps required to develop and verify such an approach, and discusses severalpotential challenges to the development and implementation of such an approach. It mustbe emphasized that this proposed simplified seismic design procedure is not intended toreplace the current code, but to offer a possible alternative to enhance the application ofthe code provisions.

Does the Building Code Need Simplification? (Hess, 2009)

The author makes several observations about the state of the present building codes:

Codes taking up substantial shelf space.

Plans examiners need more outside, expert help to review designs.

Is every new edition of the code an improvement over the last one or does it reactto a problem?


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The difference between prior editions and new requirements lays not so much in thecontents of the building codes that were used but rather in the knowledge of the engineer.A case for support of this is noted in the failure of Northridge stucco walls havingresulted in reduction in allowable forces (reaction) although construction practices mayhave been more to blame.

Justification for new provisions is often stated as advancements but may in fact bereactions or corrections to observed or predicted failures.

With greater reliance on reference standards, it is much more difficult to introducecorrections in a timely manner. Concrete anchor requirements in the 2006 InternationalBuilding Code (ICC, 2006) being a case in point.

Response to complex building codes suggests the need for better education andunderstanding how to replace them with provisions that facilitate better construction.The result should be not to necessarily replace them with simpler codes but rather withsmarter codes.

On Building Codes and Complexity (Hamburger, 2010)

The author believes that building codes and design standards should not constrainconstruction practice and therefore have produced the broadest range of design tools.

Prior codes have been simple, but buildings were not economical. There is recognitionthat early codes did not always produce reliable results (earthquake tests)

In todays environment there is increasing emphasis on producing designs efficiently andinexpensively; however, the potential solutions are much more varied. This has

necessarily created a response that building owners can have only two of the followingattributes: good, quick, or cheap.

It was noted that the wind provisions have similarly evolving complexities.

Additional comments and suggestions by the author include:

Would it really add that much cost to a building if we went back to 20 psf andgot rid of the different wind pressure zones over the building exterior and forgotabout torsion? I wonder.

Where one or a few load conditions dominate the design, SSDP should be

allowed. Lateral design of most buildings controlled by either seismic, wind, or general

stability.

Future building codes/design standards:

o Complex requirements producing economical and reliable structures (butnot simple)


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o Simplified requirements producing simple and reliable structures (but noteconomical)

o Alternative Performance-based design

Prescriptive procedure restrictive with respect to materials, configurations,

detailing.

MISCELLANEOUS MATERIAL REVIEWED

Information was also provided to the Project Management Committee for review whichincluded guides, flowcharts, manuals, and handbooks to help interpret, organize, andsimplify the seismic requirements of the provisions and building codes. Additionally,building code requirements in other countries were considered and studied.

Guide to the Use of the Provisions

Chapter 2 of theNEHRP Recommended Provisions: Design Examples, FEMA 451(FEMA, 2006) provides flowcharts and tables intended to guide the user of theNEHRPRecommended Provisions (FEMA 450). An overall summary chart breaks down theprocess of navigating theProvisions including the content of the technical chapters.

CodeMaster Wood-frame Dwellings and Townhouses (SDC A, B, and C) (SCI,

2006b)

This 6-page laminated reference guide provides an easy-to-follow 7-step procedure forthe major structural requirements as they relate to wood light-frame construction inSeismic Design Categories A, B, and C in accordance with the 2006 InternationalResidential Code (ICC, 2006b). Illustrations are provided for many of the difficult tounderstand requirements.

Subjects addressed include determination of the IRC's applicability and requirements for:site and soil; foundation, roof, and ceiling framing; floor framing; wall framing; and wallbracing. Three examples are provided illustrating the wall bracing requirements. ThisCodeMaster takes a complex set of code requirements and breaks them down into aneasy-to-follow list of items with corresponding IRC section numbers and sketches anddetails showing exactly what the code provisions are requiring.

CodeMaster Wood-frame Dwellings and Townhouses (SDC D0, D1, and D2)(SCI,

2006c)

This 6-page laminated reference guide provides an easy-to-follow 7-step procedure forthe structural requirements as they relate to wood-frame construction in higher seismicdesign categories in accordance with the 2006 International Residential Code (ICC,2006c). Illustrations are provided for many of the difficult to understand requirements.

Subjects addressed include determination of the IRC's applicability and requirements for:site and soil; foundation, roof, and ceiling framing; floor framing; wall framing; and wall


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bracing. Two examples are provided, one illustrating the continuous wood structuralpanel sheathing method and the other illustrating the alternate braced wall panel method.

This CodeMaster takes a complex set of code requirements and breaks them down into aneasy-to-follow list of items with corresponding IRC section numbers and sketches anddetails showing exactly what the code provisions are requiring.

CodeMaster Seismic Design (SCI, 2006a)

This 6-page laminated reference guide provides an easy-to-follow 11-step procedure forseismic design in accordance with the 2006 IBC (ICC, 2006), 2003NEHRPRecommended Provisions (FEMA, 2003), and ASCE/SEI 7-05 (ASCE/SEI, 2005) withemphasis on the seismic design of a typical one-to-three story building.

Illustrations are provided for many of the difficult to understand requirements. Subjectsaddressed include determination of mapped spectral response accelerations; considerationof exceptions to the seismic code requirements; Seismic Design Category determination;

consideration of plan and vertical structural irregularities; determination of seismic baseshear, redundancy coefficient and seismic load effects; and compliance with drift controlrequirements.

Standard for Cold-Formed Steel Framing - Prescriptive Method for One and Two

Family Dwellings, AISI S230-07 (AISI, 2007)

The American Iron and Steel Institute (AISI) Committee on Framing Standards hasdeveloped AISI S230-07, the 2007 edition of the Standard for Cold-Formed SteelFraming - Prescriptive Method for One and Two Family Dwellings. The standardprovides prescriptive requirements for cold-formed steel framed detached one- and two-

family dwellings, townhouses, attached multifamily dwellings, and other attached single-family dwellings. The standard, which has been adopted into both the 2009 IRC and2009 IBC, uses easy-to-read charts, tables, and details to make code-approved designquick and simple.

Seismic Detailing of Concrete Buildings (Fanella, 2007)

This material-specific publication is intended to illustrate the Chapter 21 detailingrequirements of ACI 318-05 (ACI, 2005). Various tables and figures are used tosummarize the code requirements and show where they apply in concrete members.

This aid graphically illustrates written code requirements and provides a visual guide for

the design engineer, thereby removing some potential confusion for interpreting theprovisions.

Seismic Flowcharts (Fanella, 2009)

This flow chart shows the pathway through the ASCE/SEI 7 Alternate Simplified DesignProcedure (Section 12.14). Each check routes the user to specific requirements for theselection made. Additional flowcharts for seismic demands and nonstructural


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components and the seismic design requirements for nonbuilding structures likewiseguide the engineer in the correct interpretation of the applicable requirements of theprovisions.

Wood-frame Construction Manual (WFCM) (AF&PA, 2005)

This publication provides a design example, typical checklist, and backgroundinformation related to design of wood frame structures in accordance to AF&PAs 2001WFCM for one- and two-family dwellings. The design example uses plans from a two-story residence designed to resist wind, seismic, and snow loads. Contents include thebuilding description, loads on the building, WFCM applicability limitations, prescriptivedesign limitations, load paths, and design checklists.

Direct Design Handbook for Masonry Structures, TMS-0403-10 (Masonry Society,

2009)

TheDirect Design Handbook for Masonry Structures was developed by The Masonry

Societys Design Practices Committee. The handbook provides a direct procedure for thestructural design of single-story concrete masonry structures.

The handbook provides designs for both reinforced and unreinforced masonry. Theprocedure is based on the strength design provisions ofBuilding Code Requirements andSpecifications for Masonry Structures, 2008 MSJC (TMS 402-08/ACI 530-08/ASCE 5-08) and ASCE/SEI 7-05.

The document is applicable to both residential and commercial structures. So that usersare required to do only minimal calculation, parameters are limited and design options aredictated. The handbook applies to common structures over the vast majority of the

United States including mapped ground snow loads up to 60 lb/ft

2

(2.9 kPa), mappedbasic wind speeds up to 150 mph (241 kph), mapped seismic 0.2 second spectral responseaccelerations up to 1.5g, and mapped seismic 1.0 second spectral response accelerationsup to 3.0g.

The handbook was developed as a consensus standard and written in mandatory languageso that it could form a part of a legally adopted building code as an alternative tostandards that address a much broader range of masonry construction.

The direct design procedure is provided for the most common structures and thenmodifications to the direct design procedure are provided for some other conditions. Thehandbook was written so that architects, engineers, contractors, building officials,researchers, educators, suppliers, manufacturers, and others could use the standard intheir practice for various purposes.

In a Structure Magazine article, Benchmark H. Harris (2009) suggests that this will helpreduce the amount of time it takes structural engineers to design relatively simpleconstruction so as to concentrate on more complex issues and building officials will needto review design documents. While useful in its approach to provide tools to simplify


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the design of single-story masonry buildings, the 95-page document is lengthy and lessconcise than desired for a simplified method.

This effort was deemed perhaps useful for the seismic design requirements for masonrybut, in general, not a particularly useful example for the purposes of this study.

Seismic Design Guide for Metal Building Systems (ICC, 2008)

The Seismic Design Guide for Metal Building Systems provides a practical andcomprehensive resource that will help engineers, building officials, and plan checkersensure that designs are compliant with the requirements of the 2006 InternationalBuilding Code (ICC, 2006). Four practical design examples are provided in narrativeform to illustrate acceptable approaches for dealing with the most common seismicdesign issues encountered with metal building systems. This publication also providesbackground on the technical basis of seismic design and insight into the impact of morerecent code developments.

Mexican Building Code

The building code for Mexico contains provisions for three kinds of structural analysis ofbuildings: simplified, static, and dynamic. This code, which has had provisions forsimplified analysis since 1977, was recently updated with improvements.

The simplified method for seismic analysis (SMSA) is a commonly used tool formasonry shear wall structures. The requirements include the following:

Masonry walls must carry more than 75 percent of gravity loads for building.

All walls must be connected to a rigid strong (floor) diaphragm.

Distribution of walls must be symmetric as possible (e < 0.1b).

The plan aspect ratio should not exceed 2.

The ratio between height of structure and least plan dimension should not exceed1.5.

The maximum structure height is five stories or 13 meters.

Under this method, a building can be designed by computing shear forces each wallcarries based on relative shear stiffness (Alcocer and Tena-Colunga, 2007).


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CHAPTER 3

OPTIONS FOR FURTHER DEVELOPMENTOF SIMPLIFIED PROVISIONS

INTRODUCTION

Many options for code simplification were identified based on the background reviewsummarized in Chapter 2. These options all targeted individual building types orsubgroups of the U.S. building stock because:

A complete overhaul of the entire code clearly is not within the scope of thisproject, and

It is not clear if such an overhaul is justified or feasible considering thecomplexities inherent in seismic design provisions intended to cover the widerange of ground motion intensities and building types in this country.

However, simplification of the calculation of the design seismic lateral forces hasgenerally proven to be ineffective in meeting the objectives of simplification so moreencompassing options need to be explored. For example, completely separate and uniquedesign provisions for a specific building type may present opportunities for truesimplification and improved performance, and this technique was recommended by theNERHP Recommended Provisions Simplified Procedures Task Group in 2000 (SPTG,1999). At that time, however, the Provisions Update Committee discouraged such

development based on concerns about demonstrating equivalency with the model codes.Currently, the availability of performance-based design methods in general and thetechniques presented in FEMA P-695 (FEMA, 2009) specifically, present a newopportunity for investigation of such options.

Many suggestions have been made to improve interpretation and implementation of thecurrent seismic code provisions including flow charts, graphical aids, additional designexamples, and educational seminars. Although some or all of these activities mayimprove implementation of the code provisions and are important, the ProjectManagement Committee judges that they are not a part of the core purpose to exploresimplification and therefore are not recommended as part of this project.

Most options are unprecedented and will require special consideration forimplementation. For example, improvement of ASCE/SEI 7-10 Section 12.14 isstraightforward and will require only a change to that standard. But stand-alonedocuments covering single building types, such as buildings with rigid walls and flexiblediaphragms or of light-frame construction, may require special implementation formatssuch as a new chapter in ASCE/SEI 7-10 or development of a new standard to be adopteddirectly by IBC. If a stand-alone set of provisions for Seismic Design Category Bbuildings is developed, will it form a new Chapter in ASCE/SEI 7-10 or be an alternate


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procedure? The practicality of implementation should be considered when judging theoptions.

The level of effort to develop the various procedures and the probability of reasonableand simplifying outcomes also has a wide range and should be considered going forward.Finally. an important characteristic that should be considered is the number of buildingsthat will be covered by the simplified procedure and the importance of such a procedurein improving the seismic performance of the overall U.S. building stock.

All the options that received serious consideration are briefly described below. However,as presented in Chapter 4, only three of the following options are recommended forfurther considerations.

REFINEMENT AND FURTHER SIMPLIFICATION OF ASCE/SEI 7-10

SECTION 12.14

ASCE/SEI 7-10 includes Section 12.14, Simplified Alternative Structural Design Criteriafor Simple Bearing Wall or Building Frame Systems, which is the result of previousdevelopmental work by the BSSC. This section allows a simplified version of theequivalent static force analysis for seismic load effects along with a somewhat simplerstatement of required strengths for selected components. It is applicable to buildings upto three stories tall of regular configuration so long as the seismic-force-resisting system(SFRS) is composed of bearing walls or braced frames. One of the premises is that suchbuildings do not require the computation of lateral drift, which allows substantialsimplification.

The section is useful for qualifying buildings with flexible diaphragms, but it has been

widely criticized as not being particularly useful for qualifying buildings with rigiddiaphragms. The reason is that a check for sensitivity to horizontal torsion is required forsuch buildings. The check itself does require computation of the stiffness of each verticalelement of the SFRS, and it is algebraically complex. Suggestions have been made forsimplifying this portion of Section 12.14 and, given the investment of effort to date indevelopment of Section 12.14 plus the general good acceptance of the remainder of thesection, an attempt to overcome this shortcoming appears to be justified.

A second criticism of the section is that it does nothing to simplify the detailing ofsystems (e.g., the connection requirements for braced frames or checks for boundaryelements of shear walls). The reality is that simplifications in detailing are both desirableand necessary, but they do not necessarily flow from the key characteristic of structureseligible for Section 12.14 and they lack sensitivity to lateral drift. Simplifications indetailing requirements for specific systems do need to be pursued individually, but suchsimplifications are beyond the scope of the present effort.

DEVELOP STAND-ALONE SEISMIC DESIGN REQUIREMENT FOR SEISMIC

DESIGN CATEGORY C AND B BUILDINGS


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Developing stand-alone seismic design requirements for Seismic Design Category (SDC)B and C buildings is an option for consideration. SDCs B and C represent low-to-moderate seismic hazard areas that cover a large portion of the United States and, assuch, would be available for use by many structural engineers.

Currently, however, the seismic design requirements for SDC B and C bifurcate on manykey seismic design issues, which would result in an increase in the design requirementsfor SDC B relative to those currently contained in ASCE/SEI 7-10. Some of these keyseismic design issues are:

Structural System Selection. The systems specified in ASCE/SEI 7-10 Table12.2-1 limit the number of systems available for use in SDC C relative to SDCB.

Limitations and Additional Requirements for Systems with StructuralIrregularities. For both horizontal and vertical structural irregularities, SDC Cstructures are affected by more irregularities than those in SDC B.

Collector Elements Requiring Load Combinations with Overstrength Factor.SDC B structures are exempt from these requirements.

Given the increase in the design requirements for SDC B when combining SDCs B and Cinto a single stand-alone set of seismic design requirements, this option is notrecommended for further investigation.

DEVELOP STAND-ALONE SEISMIC DESIGN REQUIREMENTS FOR

SEISMIC DESIGN CATEGORY B (SDC B) BUILDINGS

This option consists of development of specific seismic design requirements for SDC Bbuildings. These requirements could be developed in a stand-alone document or as aspecial section of the current seismic design standards. For either approach, only theseismic design requirements for SDC B would be included in the stand-alone documentor special section. This option can be pursued by editorial and/or technical changes to theseismic design requirements of ASCE/SEI 7-10.

A relatively straightforward approach to developing seismic design requirements for SDCB is to go through the current chapters of ASCE/SEI 7-10 and eliminate all the provisionsthat pertain to SDC C and higher. The majority of the focus will be on the provisions

located in Chapter 12, Seismic Design Requirements for Building Structures. If thisapproach is taken, developing SDC B seismic design requirements would be relativelystraightforward. However, this effort could take on a more technically-based approachthat would specifically review the design requirements and determine which onesshouldpertain to SDC B.

Another parallel effort would be to prepare a specific seismic design parameter table forSDC B similar to Table 12.2-1 of ASCE/SEI 7-10. Table 12.2-1 lists over 80 vertical


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seismic-force-resisting systems, only a fraction of which are typically used in SDC B. Adetailed investigation of the systems that are actually used in SDC B, perhaps region-by-region, should be performed and the systems that are seldom or never used eliminated.Their elimination, along with any associated seismic design requirements within thestandard, would help to simplify the requirements for SDC B. This process is not

intended to require the implementation of a FEMA P-695 (FEMA, 2009) analysis tovalidate the proposed eliminations. The decision to eliminate systems would be basedsolely on whether they are typically used and not on their technical merits. All systemswould still be included in the general seismic design standard and, as such, would still beavailable to the engineer for use on a project.

Of the numerous seismic design requirements specified in Chapter 12 of ASCE/SEI 7-10,many are exempted for SDC B. However, in many provisions, SDC B is stipulated foronly a select portion of the requirements. The best examples of this are the horizontaland vertical structural irregularity tables (Tables 12.3-1 and 12.3-2). The requirementslisted in these tables for SDC B should be reviewed and technical justification providedfor their removal, if that is deemed appropriate. Other potential reductions in scope infavor of simplification include the sections on analysis procedures, P-delta effects anddiaphragms, chords and collectors to highlight a few.

The process of investigating specific sections for removal or simplification may result inlimitation of the applicability of the stand-alone document or section. Buildings designedusing these simplified provisions may be subject to height limits and whether they areassigned a Risk Category of II or less depending on the extent of the technically basedchanges.

Another study that could be performed using the FEMA P-695 methodology (FEMA,2009) would explore the boundary between SDCs B and C. The current boundaries were

set based on the previous ground motion response accelerations,Aa andAv, and a detailedstudy was not performed to determine whether changes were warranted. It is anticipatedthat SDC B can be expanded based on such a study.

The technical justification for the elimination of seismic design requirements wouldnecessitate the use of FEMA P-695 (FEMA, 2009) to verify that the collapse risk remainsat the intended 1 percent in 50 years level.

DEVELOP STAND-ALONE DESIGN PROVISIONS FOR BUILDINGS WITH

RIGID WALLS AND FLEXIBLE DIAPHRAGMS

Single-story buildings with rigid walls and flexible diaphragms (RWFD buildings) arecommon throughout the United States. These buildings have stiff walls constructed ofreinforced concrete or masonry and relatively flexible diaphragms of bare metal deck orwood structural panels. For large footprint RWFD buildings, movement and strengthdegradation of the flexible diaphragm dominates the response to earthquake groundmotions. Seismic provisions of building codes used in the United States, including thoseof ASCE/SEI 7-10, were developed based on seismic response dominated by deformation


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of the vertical elements of the seismic-force-resisting system. Thus, engineers involvedin building code development have long suspected that the standard code model forseismic provisions does not fit well for RWFD buildings.

The 2000 NEHRP Provisions Update Committee considered developing seismicprovisions for RWFD building that focused on the diaphragms response. Ultimately,separate design provisions were not fully developed, in part because a procedure forestablishing equivalent performance to that intended by building codes was not available.This hurdle has now been overcome with the completion of FEMA P-695 (FEMA, 2009),which establishes a methodology for establishing equivalent performance to that intendedby current building codes.

Developing stand-alone seismic design provisions for RWFD buildings is attractive as itmay result not only in provisions that are simpler to use than current building code designprovisions but also the new provisions may lead to building designs that perform better inearthquakes. Such provisions would build on the concept that elastic and inelasticmovement and strength degradation of the flexible diaphragm dominates the response to

earthquake ground motion. The methodology outlined in FEMA P-695 (FEMA, 2009)would be used to demonstrate that the new provisions result in designs providingequivalent or better seismic performance than that intended by current building codeprovisions.

DEVELOP STAND-ALONE DESIGN PROVISIONS FOR WOOD-FRAME

BUILDINGS

A large fraction of low-cost building construction utilizes light framing of wood bearing, closure, and partition walls of repetitive studs supporting floors and roofs framed

with repetitive joists or trusses. The studs, joists, and trusses can be dimension lumber,engineered wood products, or, in the case of trusses, assemblies of such products withsteel connectors. The walls, floors, and roofs act as shear panels with the woodstructural panels (plywood or oriented strand board) and gypsum wallboard providing thestrength and stiffness for the shear panel action. The actual strength of the shear panels isusually governed by the fasteners (nails or screws) that attach the panels to the framing,but the strength also can be governed by global panel overturning.

The design of this type of structure for gravity loads has been optimized over the years,such that much of the work after defining the loads and configuration is confined toselecting members from design tabulations. Many engineers specialize in design of such

structures, and they would like to optimize the design effort for seismic resistancebecause the design for seismic loads is certainly not simple.

The shear panel systems develop substantial ductility, mostly through inelastic action inand around the panel connectors. The fact that there are many components in the systemmeans that there are many boundaries across which forces must be transferred (this is astrue for wind resistance as it is for seismic resistance). Providing details for transfer offorce across the many boundaries within the typical light-frame structure is a painstaking


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and time-consuming task. Many of the detailed provisions in ASCE/SEI 7-10 can be tieddirectly to experience with wood structures in real earthquakes. On the other hand, thereare provisions in ASCE/SEI 7-10 that are, or may, not be particularly pertinent to woodconstruction. (It deserves to be noted that the loss of life in strong earthquakes in thiscountry has been very low in comparison to other countries and that the performance of

wood light-framed construction is a significant factor in this pleasant truth.)

The development of a set of seismic provisions limited in its application to wood light-framed structures will not solve the apparent problem with complexity in the seismicresistant design of wood structures. All of the requirements for transfer of forcesbetween components will still exist. Furthermore, the proof of such a method followingthe FEMA P-695 methodology (FEMA, 2009) will require a very extensive set ofarchetypical designs because there are so many actual configurations of structuralsystems of wood. Furthermore, it may not be appropriate in the context of this project topotentially advance one material industry over others. Hence, this option is notrecommended for this study. On the other hand, as indicated in the Introduction, thedevelopment of a manual for seismic design with detailed solutions for characteristicproblems is recommended for others to pursue as it would be consistent with theoptimization that has occurred for gravity load design and could lead to betterperformance in the population of buildings by reducing errors in application by bothengineers and construction workers.

USE OF THE RATIO OF THE BEARING WALLS TO THE FLOOR AREA

ABOVE AS A PRIMARY DESIGN REQUIREMENT

Rules allocating the amount and location of walls in a concrete bearing wall building isan option for consideration. The amount and general location of concrete bearing walls

would be established using the FEMA P-695 methodology (FEMA, 2009) and would bedetermined independently for each SDC. The specific limitation on wall location wouldbe determined consistent with the occupancy of the building. For instance, the walllocations for a residential structure would be different from those for an office building.The archetype design space would need to be selected based on building occupancy andthe resulting rules for wall placement consistent with the analysis results.

Rules-of-thumb consistent with this approach have been used in Chile, a country whosemid- to high-rise residential structures consist almost exclusively of concrete bearing wallsystems. The Chilean approach allocates the percentage of concrete bearing wall on afloor based on the floor area supported above. While this approach allows for a reduction

of wall area over the height of the building, the wall area is typically selected based onthe floor area supported on a lower level and continued throughout the height. Giventheir propensity to use double-loaded corridors in their residential layouts, thelocation/length of the walls is reasonably uniform, resulting in walls that resist similarlevels of earthquake demand.


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Given the wall layout mandated by the occupancy type, and the amount of wall thatwould likely result (two to three times greater that is currently used in the U.S.), thisoption is not recommended for further investigation.

REDUCING DETAILING REQUIREMENTS FOR LOWERR FACTOR

SYSTEMS

The intent of theNEHRP Recommended Provisions is to avoid structural collapse invery rare, extreme ground shaking and to provide reasonable control of damage tostructural and nonstructural systems that could lead to injury and economic orfunctionality losses for more moderate and frequent ground shaking. Under themaximum considered earthquake ground motions (MCE), it is expected that seriousdamage will occur, which might not be economically feasible to repair. This damage iscreated as the structural elements yield, crack, and buckle to dissipate the energyimparted to the structure by the ground shaking.

Experience and applied research have led to many provisions for the detailing ofstructural systems to resist the action of seismic ground shaking. Some elements andsystems are capable of dissipating more energy through inelastic deformation than others,and this fact is one of the fundamental reasons that the seismic response modificationfactor,R, is assigned different numerical values for different systems. The other primaryfactor behind the values of theR factor is that structural systems typically posses a largercapacity to resist horizontal force than computed using standard design methods; thisfactor is normally called overstrength. A factor of less significance is that a few systemsexhibit higher or lower than average amounts of damping in their dynamic response. Thevariation in inelastic deformation capacity leads to the largest differences inR factors.For example, there are threeR factors for seismic force-resisting systems consisting of

reinforced concrete moment resisting frames:

R = 8 for Special frames detailed to deliver a large capacity for many cycles ofinelastic deformation.

R = 5 for Intermediate frames, which have a smaller number requirements fordetailing to avoid the most brittle limit states.

R = 3 for Ordinary frames, which are essentially identical to frames detailed forwind and gravity loads but without consideration of seismic actions

The detailing requirements for systems with highR factors generally involve design

checks to avoid brittle limit states (e.g., tensile fracture of structural steel or compressivecrushing of concrete) and to avoid focusing the inelastic demand in a small portion of thesystem (e.g., the strong-column, weak-beam rule for special moment frames). Many ofthese detailing rules are rooted in the concept of capacity design, in which the structureis constrained by design to perform in certain desirable manners. These detailingrequirements can be very time consuming in engineering practice.


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TheProvisions and ASCE/SEI 7-10 are written to exclude the most brittle systems,generally those with the lowerR factors, from use in the higher seismic design categories,generally locations with the potential for very large seismic ground motions. In somecases, the restrictions apply primarily to tall structures whereas in others, the restrictionsapply to all heights. In some circumstances, this has a significant effect on the cost of

construction, not only because the detailing for ductile behavior leads to the use of morematerial and extra labor, but also because some of the basic structural configurationspermitted for tall structures in high seismic design categories are simply more expensivemethods of construction.

Both of these factors lead to a desire to relax the detailing provisions. The reduction ofengineering labor is properly in the domain of consideration here for simplification of theseismic design provisions. The prime example cited for simplification of this type is thepermission granted to design seismic-force-resisting systems of hot-rolled structural steelwithout regard to any special detailing for seismic energy dissipation, given a value of 3for theR factor. There are desires stated by many to expand the use of such a techniqueby, for example, lowering theR factor for a given system in exchange for increasing thelimit on height for that particular system in a particular Seismic Design Category.

This approach deserves more careful scrutiny than might first be apparent. Intuitionmight imply that design for anR factor of 1 would imply no requirement for ductility oroverstrength. Given that the design procedures include a reduction in the MCE groundmotion as a part of the basic equation for an equivalent design force,R of 1 does actuallyimply some capacity beyond the design capacity. Some individuals cognizant of this factsuggest a value ofR equal to 2/3 as a safe alternative. However, that will not necessarilyassure safe performance and, for some systems, it could also be unnecessarily punitive.Given that the ground motions to be considered exhibit a significant variability in keyparameters and that the capacity of a structure is not known with certainty, a probabilistic

approach is needed. The methodology of FEMA P-695 (FEMA, 2009) provides guidancefor assessing safety, but the amount of work required to do this systematically for a widecategory of structural systems might be overwhelming. Therefore, this option was notselected for consideration at this time.

On the other hand, for a system with a very well defined and limited design space and forwhich behavioral models are well accepted, it is very feasible to derive an acceptablevalue ofR for such an approach. The unfortunate truth is that the categories of systemsfor which the demand for simplification is highest are likely to have a very large numberof likely configurations and be very expensive to validate under a FEMA P-695 approach(FEMA, 2009).


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Development of Simplified

SSDP Framework Report Final

Documents

Transcript of SSDP Framework Report Final