Simplified Seismic Assessment & Seismic Retrofit Guidelines

Simplified Seismic Assessment& Seismic Retrofit Guidelinesfor

Detached, Single-Family, Wood-Frame Dwellings

FEMA P-50 & FEMA P-50-1



FEMA P-50



FEMA P-50-1

FEMA P-50 & FEMA P-50-1Simplified Seismic Assessment

Intent:

Provide a tool that communities and other entities can use to encourage the seismic retrofitting of residential structures, thereby reducing future earthquake losses

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Common Earthquake Vulnerabilities Crawl Space House with Cripple Walls


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Common Earthquake Vulnerabilities Slab-on-Grade House


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Common Earthquake Vulnerabilities Perimeter Post-and-Pier House


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Common Earthquake Vulnerabilities Perime

Hillside House


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Common Earthquake Vulnerabilities Perimeter Post-and-Pier House


Intended Users

• Home inspectors• Home retrofit and remodel contractors• Design professionals• Homeowners• Insurers• Lenders• Building officials• Emergency planners • Others involved in hazard mitigation

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Training Objective: P-50 Seismic Assessment

Introduce methodology to:

Assign a Seismic Performance Grade

Identify seismic retrofit opportunities & priorities

Identify an improved Seismic Performance Grade with seismic retrofit

C+ B+15


Seismic Performance Grade

Seismic Performance Grade A through D-

Conveys generally anticipated seismic performance

Given structural characteristics & geographic location

Relative to overall group of detached wood-framed single-family dwellings

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Training Objective: P-50-1 Seismic Retrofit

Provide a brief introduction to FEMA P-50-1 resources for:

Selection of seismic retrofit measures to implement

Design of seismic retrofit measures

Reassessment of a dwelling

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Webinar Outline

• Part 1: Wood-Frame House Construction

• Part 2: Assessment Using FEMA P-50

• Part 3: Next Steps after the Assessment Using FEMA P-50-1

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Part 1:

Wood-Frame House Construction

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Wood-Frame House Construction

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Common house configurations:

1. Crawlspace w/ cripple wall

2. Basement & foundation wall

3. Slab on grade

4. Perimeter post & pier


Wood-Frame House Construction Crawl Space House with Cripple Walls

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Wood-Frame House Construction Crawl Space House with Cripple Walls


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Wood-Frame House Construction Basement or Foundation Wall House


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Wood-Frame House Construction Basement or Foundation Stem Wall House


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Wood-Frame House Construction Slab-on-Grade House


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Wood-Frame House Construction Perimeter Post-and-Pier House

Footing

Diagonal Brace

Post


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PART 2: Dwelling Assessment

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Dwelling Assessment

The assessor needs:

One hour, approximately

Knowledge of the basics of wood-frame construction, materials, construction techniques

Helpful to have some knowledge of load path (how earthquake loads move through a structure)

Equipment for inspecting attic, crawlspace

FEMA P-50 assessment form

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Intended Use:

Single-family dwelling

Primarily wood-frame construction

Seismic Design Category C through E

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Structural Score:

A. Foundation

B. Superstructure Framing and Configuration

C. General Condition Assessment

D. Nonstructural Elements, Age and Size

E. Local Site Conditions

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Regional Seismic Hazard Score:

F. Regional Seismic Hazard Score


Structural Score

Structural Evaluation Areas:

A. Foundation

B. Superstructure Framing and Configuration

C. General Condition Assessment

D. Nonstructural Elements, Age and Size

E. Local Site Conditions

Structural Score: 100

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Structural Score Terminology

Assessment Items A-1 through E-6 are evaluated

Penalty Points are assigned for each Assessment Item based on the applicable Condition (condition a, condition b, etc.) with the highest number of

Penalty Points

Summed Penalty Points are subtracted from 100 to

give the Structural Score

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Structural Score Terminology

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Assessment Item A-1

Conditions a, b

Penalty Points


Structural Score

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Structural Score

a. Continuous concrete and reinforced masonry exterior footings Continuous perimeter grade beams or tie-beams.

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Structural Score

b. Continuous footings of other materials such as unreinforced brick or stone masonry.Discontinuous footings such as isolated post-and-pier systems, isolated drilled pier systems without tie-beams, and partially continuous perimeter footings

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Structural Score

b. Continuous footings of other materials such as unreinforced brick or stone masonry.Discontinuous footings such as isolated post-and-pier systems, isolated drilled pier systems without tie-beams, and partially continuous perimeter footings

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Structural Score

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Structural Score

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Structural Score

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Structural Score

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1

3


Structural Score

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Structural Score

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Structural Score

7.4

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Structural Score: 92.6


Structural Score

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Structural Score

21.9

Structural Score: 70.7

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Structural ScoreStructural Score: 68.9

1.8

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6.0

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4.0

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Regional Seismic Hazard Score

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http://earthquake.usgs.gov/designmaps/usapp

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1.02102

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http://myplan.calema.ca.gov

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http://myplan.calema.ca.gov/



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http://geohazards.usgs.gov/qfaults/map.php



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4

0

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7.4

21.9

1.86.0

4.041.1

58.9

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7.4

21.9

1.86.0

4.041.1

58.9

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C+

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Basis for Seismic Performance GradesLetter Grades

Expert Opinion

Los Angeles pilot study of 400 dwellings

Damage Ranges

Percent of replacement cost of house

Annual probability of exceedance of 1/500

Reasonable approximation of single event for which owner will have to repair damage

Based on 2012 EQECAT study using modifiers developed for CEA, modeling 7600 combinations of dwelling configuration and seismic design category, and covering a range of seismic events deemed credible by USGS

(Sec. 2.4.5)

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Limits of the Methodology (Sec. 2.5)

Performance for an assigned grade could vary widelydue to limits of assessment

Dwelling construction and resulting earthquake response is inherently variable

Limited portions of the structure are visible without removing finish materials

Variability is inherent in seismic hazard

Owner could choose to perform a more detailed assessment to increase level of knowledge

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Limits of the Methodology

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Improving the Structural Score

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Improving the Seismic Performance Grade

15.073.9

B

21.580.4

B+

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Improving the Seismic Performance Grade

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PART 3: After the Assessment

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Retrofit Regulation

Voluntary Retrofit:

Owner is generally free to

• Choose to implement seismic retrofits

• Select which assessment items to retrofit

• Provided the retrofit does not make the house more vulnerable to earthquake damage

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Retrofit Regulation

Mandatory Retrofit:

• May become mandatory triggered by:

– Alteration or addition to house

– Local retrofit ordinance

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Retrofit Selection

Voluntary Retrofit:

Important to consider available guidance in:

• Selection of assessment items to retrofit

• Design of retrofit measures

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Retrofit Selection

Retrofit objective:

• Reduce damage

• Increase occupant safety

• NOT strengthen the entire house

• NOT avoid all damage

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Retrofit Selection

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Retrofit Selection

FEMA P-50-1 Retrofit Guidance

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FEMA P-50 Assessment Items


Retrofit Design

Guidance available:

• Not a single resource document to provide direction

• FEMA P-50-1 collects information on available methods and resources

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Retrofit Design

Prescriptive: • Construction requirements are prescribed in detail in

text, tables, and/or figures• Generally does not require involvement of a engineer/

architect – can be implemented directly by contractor• Limits exist on the type and configuration of the

building to be retrofitEngineered:• Construction requirements are calculated for particular

building based on specific earthquake loading• Requires involvement of engineer/ architect in addition

to contractor, with greater time and expense• No limits on type an configuration of building

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Retrofit Methods

• Method 1: Prescriptive International Existing Building Code (IEBC) Chapter A3 – cripple wall bracing and anchorage to foundation

• Method 2: Prescriptive retrofit method for nonstructural building elements

• Method 3: Prescriptive retrofit method in accordance with International Residential Code (IRC) requirements for new construction

• Method 4: Engineered retrofit method

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Reassessment of the Seismic Performance Grade

Fundamental to the FEMA P-50 assessment process is the concept that dwellings can be reassessed. This might be done to:

• Revise the Seismic Performance Grade following retrofit

• Better assess items addressed in the assessment form as a “detailed” rather than “simplified” assessment

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The qualified design professional would be:

• An engineer or architect experienced in structural engineering can re-evaluate penalty points assigned towards the Structural Score

• A geotechnical engineer can re-evaluate the assigned Regional Seismic Hazard Score

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Reassessment of the Seismic Performance Grade


Implementation of an Assessment Program

The benefits of reduced economic losses following a major to moderate earthquake extend to the homeowner, but also to many others beyond. Following are some of the persons and organizations that serve to benefit and therefore should consider establishment of plans to implement a program of assessment and retrofit:• Homeowner • Lender• Insurer• Local government

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QUESTIONS OR COMMENTS?

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Simplified Seismic Assessment & Seismic Retrofit Guidelines

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