CFSEI-Design of Non-Structural Members

8/13/2019 CFSEI-Design of Non-Structural Members

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STECH NOTE W105 13 September 2013Cold-Formed Steel Engineers Institute

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Summary:AISI S220, the North American Standard for Cold-Formed Steel Framing - Nonstructural Members,was developed in 2011 to help clearly delineate and eliminate confusion between the requirements for cold-formed steel structural members and nonstructural members. This Technical Note is intended as an introductionto this standard and as an illustrative guide for applying the provisions to the design of nonstructural members.

Disclaimer: Designs cited herein are not intended to preclude the use of other materials, assemblies, structuresor designs when these other designs and materials demonstrate equivalent performance for the intended use;CFSEI documents are not intended to exclude the use and implementation of any other design or constructiontechnique.

TECHNICAL NOTEOn Cold-Formed Steel Construction

Cold-Formed Steel Engineers Institute Washington, DC www.cfsei.org 800-79-STEEL

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DESIGN OF NONSTRUCTURAL MEMBERS

OVERVIEW OF AISI S220

Most typical drywall partitions are specified and con-structed in accordance with the industry or manufactur-ers design tables and would NOT require formal engi-neering input. However, the design tables are based onengineering principles. This Tech Note will summarizethe accepted engineering principles.

AISI S220 (AISI, 2011) was developed in 2011 to helpdelineate and eliminate confusion between the engi-neering priciples and requirements for cold-formed

steel structural members and nonstructural members.As such, provisions formerly in AISI S200 (AISI, 2007)for material, corrosion protection, base steel thickness,product designators, manufacturing tolerances, productidentification member design, member condition, instal-lation, connections, and miscellaneous for nonstruc-tural members were moved to AISI S220.

AISI S220 is based on the premise that the conse-quence of failure for a nonstructural member is lessthan for a structural member and, consequently, per-mits a lower reliability for nonstructural members.

STRUCTURAL VS. NONSTRUCTURALMEMBERS: WHAT DOES

NONSTRUCTURAL REALLY MEAN?

The current standards when defining a nonstructuralmember are focused on wall assemblies as defined asfollows:

ASTM C645 Standard for Nonstructural Steel FramingMembers:

A member in a steel framed wall system, which islimited to a transverse (out-of-plane) nominal loadof not more than 10 lb/ft

2(0.48 kPa), a superim-

posed nominal axial load, exclusive of sheathingmaterials, of not more than 100 lb/ft (1.46 kN/m), ora superimposed nominal axial load of not morethan 200 lbs (0.89 kN).

North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100):

An interior partition wall stud in a composite steelframed interior wall system with sheathing attachedto both flanges and that is limited to a transverse(out-of-plane) nominal load of not more than 10 lb/ft

2(0.48 kPa), a superimposed nominal axial load,

exclusive of sheathing materials, of not more than100 lb/ft (1.46 kN/m), or a superimposed nominalaxial load of not more than 200 lbs (0.89 kN).

However, the nonstructural profile is often used in inte-rior fascia and ceiling construction (Figure A). Thus,during the development of AISI S220 the followingbroader definition of nonstructural was adopted:

North American Stand for Cold-Formed Steel Framing -Nonstructural Members (AISI S220):

Nonstructural Member. A member in a steel-framed system that is not a part of the gravity loadresisting system, lateral force resisting system orbuilding envelope.


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TECH NOTE W105-13 September 2013 2 Cold-Formed Steel Engineers Institute

DESIGN APPROACHES

Historically, the nonstructural wall assembly has beenload tested in accordance with ICC-ES AC86 to definethe strength and stiffness of the wall stud acting withina composite assembly that included the cold-formedsteel wall stud and the gypsum wallboard. Load tablespresented the design capacity of the composite wall

assembly. The load tables were based on a safety fac-tor of 1.5.

Although the composite wall assembly offered eco-nomic advantages, design conditions also often re-quired the nonstructural member to be designed actingnon-composite with the sheathing material. Examplesof non-composite design conditions are ceiling framing,interior fascia framing, or partition walls with sheathingon one side only. Historically, assemblies using the non-composite assembly design approach were designedneglecting the composite-action contribution of the at-tached sheathings based on the design provisions ofAISI S100. Thus, a safety factor of 1.67 or 1.8 was re-

quired.

AISI S220 has addressed the inconsistency in thesafety factors for composite and non-composite assem-blies. S220 is based on the premise that the conse-quence of failure for a nonstructural member is lessthan for a structural member and, consequently, per-mits a lower safety fact or greater phi factor for non-structural members.

EQUIVALENT THICKNESS SECTION

Historically ASTM C645, Table 2, has stipulated mini-

mum section properties for the nonstructural member.Included was the design property, Mn/and the mini-mum base metal thickness.

ASTM C645 Section 4.3 stated:

Members shall be manufactured from steelhaving a minimum thickness, individual meas-urement of 0.0179 in. before application ofprotective coating.

The minimum base steel thickness was given whichwas based on meeting the width-to-thickness ratios ofChapter B of AISI S100.

However, ASTM C645 Section 9.2 permitted nonstruc-tural profiles to have thickness values less than 0.0179in.:

Members that can show third party testing in ac-cordance with ICC-ES-AC86 and conform to thelimiting heights tables in Specification C754 neednot meet the minimum thickness limitation set forthin Section 4.3 or the minimum section propertiesset forth in 8.1

The minimum section properties in Section 8.1, for ex-ample Mn/as listed in Table 2, were computed usingAISI S100 with Fyof 33 ksi.

To achieve more economical design solutions, manu-facturers have developed proprietary cross sectionsthat are thinner than 0.0179 in. but have achieved the

Mn/ required by ASTM C645 by using higher yieldstrength materials. These proprietary cross sectionshave been labeled equivalent thickness, i.e. EQ sec-tions.

MEMBER DESIGN

Because the EQ sections are thinner than 0.0179 in.they do not meet the various dimensional limitationsprescribed by Section B1 of AISI S100. Thus, the provi-sions of Chapters B and C are not directly applicable.

To facilitate design of both the nonstructural sectionsmeeting the Section B1 provisions and the EQ sec-tions, provisions have been adopted into AISI S220 for

both the non-composite and composite assembly appli-cations.

Non-composite assembly

A non-composite assembly is defined as a wall studapplication wherein the design neglects the contributionof the attached sheathing to contribute to either thestrength or stiffness of the wall stud. AISI S220 pro-vides the following options when designing a non-composite assembly:

(i) Design using Chapters A through E of AISIS100 [CSA S136] with:

N= 0.9

N = 1.1 where

= Safety factor per relevant section of AISIS100 [CSA S136]

= Resistance factor per relevant section ofAISI S100 [CSA S136]

To employ the above provision the cross-

section profile must meet all dimensional andductility limitations imposed by AISI S100. Thetraditional nonstructural C-section wall studhaving a thickness equal to or greater than0.0179 in. could utilize this provision. Tradition-ally a 10 percent reduction in the omega factoror 10 percent increase in the phi factor hasbeen permitted for the composite wall assem-blies. This reduction or amplification factor hasbeen extended to the non-structural assemblyin AISI S220.


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STECH NOTE W105 13 September 2013Cold-Formed Steel Engineers Institute 3

For a section not meeting either the dimensional orductility limitations of AISI S100 Section A1.2 re-quires that physical tests and/or rational analysis isto be employed. Thus, AISI S220 requires the fol-lowing when tests alone are utilized to define themembers strength:

(ii) Chapter F of AISI S100 [CSA S136] with:

0 = 1.6

where

0 = Target Reliability Index in accordancewith Section F1.1(b) of AISI S100 [CSA S136]

= Safety factor per Section F1.2 of AISIS100 [CSA S136]

= Resistance factor per Section F1.1(b) ofAISI S100 [CSA S136]

It is noteworthy that the above provision willresult in an omega of approximately 1.5 or aphi factor of approximately 1.0. Thus a safetyfactor consistent with the traditional safetyfactor used for a composite wall assembly isachieved.

For a section not meeting either the dimensional orductility limitations of AISI S100 and rational analy-sis is used per Section A1.2 the safety factor, , is2.0 and the is 0.8. However if tests are used tovalidate the rational analysis, AISI S220 permitsthe following:

If Section A1.2(b) of AISI S100 [CSA S136] is util-ized then supplementary tests are permitted to beperformed and Chapter F of AISI S100 [CSA S136]is permitted to be employed for determination of or , with Pm replaced by Ptest/Pcompute and inaccordance with the provisions above.

In the use of AISI S100 [CSA S136] Chapter F, theprofessional factor, P, shall be the test-to-predictedratio where the prediction is that of the rational en-gineering analysis method selected, Pm is themean of P and VP, the coefficient of variation of P.At least three tests shall be conducted.

Composite Assembly

A composite assembly is defined as a wall stud appli-cation wherein the design reflects the contribution ofthe attached sheathing to either the strength or stiff-ness of the wall stud. AISI S220 provides the followingguidance when designing a composite assembly:

(b) Composite Assembly Design - Assembliesusing a composite assembly design approachshall be designed based on the tests under-taken and evaluated in accordance with Chap-ter F of this standard.

For cold-formed steel nonstructural members in interior

non-load bearing wall assemblies, ICC-ES AC86, Ac-ceptance Criteria for Cold-Formed Steel Framing Mem-bers - Interior Nonload Bearing Wall Assemblies (ICC-ES, 2010), is generally an approved test method.

CONNECTION DESIGN

Connection design is in accordance with AISI S100[CSA S136] or testing in accordance with Section F1 ofAISI S100 [CAS S136]. Additional design guidance isprovided in Chapter D of AISI S220 regarding screwinstallation, stripped screws, spacing and edge dis-tance of screws and attachment of gypsum board.

The standard does not mention that the penetrationtest for screws, i.e. the procedure for evaluating themembers ability to pull the head of a screw below thesurface of gypsum sheathing, is addressed by ASTMC645 Section 10.

ILLUSTRATIVE EXAMPLES

Using fictitious test data for Mtest/Mn the intent of thefollowing example problems is to illustrate the applica-tion of Section B1 of AISI S220. The examples illustratethe application of the various non-composite assemblydesign provisions.

Given: 362S125-152

Design thickness = 0.016 (0.0152/0.95 per AISIS100 Section A2.4)

Fy= 50 ksi

Inside bend radius = 0.030

Edge stiffener =

Mn= 3.26 in.-kips computed per AISI S100,Chapter B and Section C3.1.1

(a) Example 1 (Rational Analysis with Supplemen-tary Tests) - Using AISI S220 Section B1(a)(ii). Forthis situation the design cannot be performed in byAISI S100 Chapters B through E because thewidth-to-thickness ratios violate


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Section B1. Thus, AISI Section A1.2(b), rationalanalysis along with supplementary tests is beingapplied. The safety factor and phi factor are com-puted using AISI S220 Section B1(a)(ii).

Test Mtest Mtest/Mn

1 2.90 0.89

2 2.99 0.92

3 2.95 0.90

Average 0.90

Std Dev. 0.0153

COV 0.0170

Based on the Mtest/Mn ratio with Mn computed using

AISI S100 Chapters B and C as a rational analysis re-sults in an average 10% overestimation of the cross-

section moment capacity. Thus, a more appropriate

rational analysis, based on the test results, is to use

0.90Mnwith the following:

= 1.68

= 0.96

The and were computed using Chapter F of

S100 with 0= 1.6 and Pm of 0.90.

(b) Example 2 (Rational Analysis with Supplemen-

tary Tests) - Using AISI S220 Section B1(a)(ii). For

this situation the design cannot be performed in by

AISI S100 Chapters B through E because the

width-to-thickness ratios violate Section B1. Thus,

AISI S100 Section A1.2(b), rational analysis along

with supplementary tests is being applied. The

safety factor and phi factor are computed using

AISI S220 Section B1(a)(ii).

Based on the Mtest/Mn ratio with Mnwas computed us-

ing AISI S100, Chapters B and C as a rational analysis

results in a 1% underestimation of the cross section

moment capacity. Therefore S100 Chapters B and C

are deemed to be a valid rational analysis.

Test Mtest Mtest/Mn

1 3.28 1.01

2 3.25 1.00

3 3.35 1.03

Average 1.013

Std Dev. 0.0153

COV 0.0151

Because supplementary tests were performed in com-

bination with using Chapter B and Section C3.1.1, a

reasonable rational analysis, the following apply:

= 1.49

= 1.08


S100 with 0= 1.6

(c) Example 3 (Rational Analysis without Supple-

mentary Tests) - Not using AISI S220 Section B1

(a)(ii). For this situation the design cannot be per-

formed in by AISI S100 Chapters B through E be-

cause the width-to-thickness ratios violate Section

B1. Thus, AISI Section A1.2(b) is being applied

without supplementary tests. The safety factor and

phi factor are defined by AISI S100 Section A1.2

(b).

Because supplementary tests were not performed and

a rational analysis using Chapter B and Section C3.1.1

had been adopted, Mn = 3.26 in.-kips and the following

factors, per Section A1.2 of S100 would apply:

= 2.0

= 0.80

(d) Example 4 - (Tests without Rational Analysis)Using AISI S220 Section B1(a)(ii). For this situation

the design cannot be performed in by AISI S100Chapters B through E because the width-to-thickness ratios violate Section B1. Thus, AISISection A1.2(a) is being applied. The safety factorand phi factor are computed using AISI S100Chapter F.


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STECH NOTE W105 13 September 2013Cold-Formed Steel Engineers Institute 5

Test Mtest

1 3.28

2 3.25

3 3.35

Average 3.29

Std Dev. 0.0513

COV 0.0156

The design value is the average tested value of 3.29 in.

-kips with the following:

= 1.51

= 1.06


S100 with 0= 1.6

Figure 1: Nonstructural members framing a fascia assembly


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Primary author of original Tech Note:

Roger Laboube, Ph.D, P.E.,Wei Wen Yu Center for Cold-Formed Steel Structures

Reviewers:

Rahim Zadeh, Technical Director,Steel Stud Manufacturers Association (SSMA)

Pat Ford, Technical Director,

Steel Framing Industry Association

Maribeth Rizzuto, Managing Director,CFSEI

This Technical Note on Cold-Formed Steel Construction is published by the Cold-Formed Steel Engineers Institute (CFSEI). Theinformation provided in this publication shall not constitute any representation or warranty, express or implied, on the part of CFSEI

or any individual that the information is suitable for any general or specific purpose, and should not be used without consulting witha qualified engineer, architect, or building designer. ANY INDIVIDUAL OR ENTITY MAKING USE OF THE INFORMA-

TION PROVIDED IN THIS PUBLICATION ASSUMES ALL RISKS AND LIABILITIES ARISING OR RESULTING

FROM SUCH USE. CFSEI believes that the information contained within this publication is in conformance with prevailing engi-

neering standards of practice. However, none of the information provided in this publication is intended to represent any officialposition of the CFSEI or to exclude the use and implementation of any other design or construction technique.

Copyright 2013, Cold-Formed Steel Engineers Institute Washington, DC www.cfsei.org 1-800-797-8335

References

1. Acceptance Criteria for Cold-Formed Steel Framing MembersInterior Nonload-Bearing Wall Assemblies,ICC-ES AC86, ICC Evaluation Service, Whittier, CA, 2010.

2. North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100-07), Ameri-can Iron and Steel Institute, Washington, DC, 2007.

3. North American Standard for Cold-Formed Steel FramingGeneral Provisions (AISI S200-07), American Ironand Steel Institute, Washington, DC, 2007.

4. North American Standard for Cold-Formed Steel FramingNonstructural Members (AISI S200-11), Ameri-can Iron and Steel Institute, Washington, DC, 2011.

5. Standard Specification for Nonstructural Steel Framing Members, (ASTM C1513-10), ASTM International,West Conshohoken, PA, 2010.

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