Diaphragm Flexibility Requirements for Wood-frame Diaphragms in the 2015 International Building Code

By Gary L Mochizuki, PE, SE, LEED AP1, Philip Line, PE2, Tom VanDorpe, SE3

Determining diaphragm flexibility for wood-frame diaphragms is confusing because the 2015 International Building Code (IBC 2015) contains three different sections relating to the design of diaphragms. Two of these sections reference the detailed provisions given in ASCE 7-10 and SDPWS 2015, which conflict with IBC 2015 Section 1604.4.  

Section 1604.4 states the following: “…A diaphragm is rigid for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is less than or equal to two times the average story drift…”

Section 1613.1 specifies that all structures subjected to earthquake loads shall be designed in accordance with the reference standard ASCE 7-10 Minimum Design Loads for Buildings and Other Structures.

 Section 2306.2 specifies that the 2015 edition of Special Design Provisions for Wind and Seismic (SDPWS 2015) shall be used for design of wood diaphragms.

The Seismology Light Frame Sub-committee of the Structural Engineers Association of California (SEAOC) has reviewed the inconsistencies between IBC 2015 and reference standards and is of the opinion that diaphragm flexibility provisions in ASCE 7-10 and SDPWS 2015 should be used for the design of wood diaphragms in lieu of using IBC 2015 Section 1604.4. The differences between IBC 2015 and reference standards and the rationale for recommended use of ASCE 7-10 and SDPWS 2015 provisions are discussed below.

 

IBC Reference to ASCE 7-10 – Diaphragm Flexibility

 

Per IBC Section 1613.1 Earthquake Loads, structures shall be designed and constructed in accordance with ASCE 7-10. ASCE 7-10 provisions for diaphragm flexibility in Section 12.3.1 require the use of a semi-rigid modeling assumption in which relative stiffness of the diaphragm and vertical force resisting elements are explicitly considered in the design, unless the diaphragm can be idealized as either flexible or rigid:

 

12.3.1 Diaphragm Flexibility…..Unless a diaphragm can be idealized as either flexible or rigid in accordance with Sections 12.3.1.1, 12.3.1.2, or 12.3.1.3, the structural analysis shall explicitly include consideration of the stiffness of the diaphragm (i.e., semirigid modeling assumption).”

 

The use of semi-rigid modeling for purposes of distributing horizontal forces is always permissible under ASCE 7. This method is widely considered rational for use in distributing horizontal diaphragm shear loads to vertical resisting elements.  However, idealizations of diaphragm flexibility are often used to simplify analyses and are considered appropriate provided certain conditions are met. 

 

Wood-frame diaphragms are permitted to be idealized as flexible per Section 12.3.1.1:

 

“12.3.1.1 Flexible Diaphragm Condition. Diaphragms constructed of untopped steel decking or wood structural panels are permitted to be idealized as flexible if any of the following conditions exist:

a. In structures where the vertical elements are steel braced frames; steel and concrete composite braced frames; or concrete, masonry, steel, or steel and concrete composite shear walls.

b. In one- and two-family dwellings.

c. In structures of light-frame construction where all of the following conditions are met:

1. Topping of concrete or similar materials is not placed over wood structural panel diaphragms except for nonstructural topping no greater than 1 1/2 in. (38 mm) thick.

2. Each line of vertical elements of the seismic force resisting system complies with the allowable story drift of Table 12.12-1."

 

Wood-frame diaphragms are permitted to be idealized as flexible per Section 12.3.1.3:

 

“12.3.1.3 Calculated Flexible Diaphragm Condition. Diaphragms …are permitted to be idealized as flexible where the computed maximum in-plane deflection of the diaphragm under lateral load is more than two times the average story drift of adjoining vertical elements of the seismic force-resisting system of the associated story under equivalent tributary lateral load …”

 

ASCE 7-10 does not contain provisions for a wood-frame diaphragm to be idealized as rigid. Within ASCE 7-10, only concrete slab or concrete-filled metal deck diaphragms that meet specific requirements of Section 12.3.1.2 are permitted to be idealized as rigid.

 

Within ASCE 7, the diaphragm flexibility classification (i.e. semi-rigid, idealized as flexible, or idealized as rigid) not only describes a permissible method for distribution of horizontal forces, but also affects consideration of special criteria and associated limitations for torsional irregularities, inherent torsion, accidental torsion, and amplification of accidental torsion. 

 

IBC reference to SDPWS 2015 – Diaphragm Flexibility

 

Per IBC Section 2306.2, wood-frame diaphragms shall be designed and constructed in accordance with AWC SDPWS. In accordance with SDPWS, distribution of shear to vertical seismic force resisting elements based on an analysis where the wood-frame diaphragm is modeled as “semi-rigid”, “idealized as rigid” or “idealized flexible” is permissible. The SDPWS condition where a diaphragm is idealized as rigid is the opposite of the flexible diaphragm condition described in ASCE 7, Section 12.3.1.3:

 

“4.2.5… It shall be permitted to idealize a diaphragm as rigid when the computed maximum in-plane deflection of the diaphragm itself under lateral load is less than or equal to two times the average deflection of adjoining vertical elements of the lateral force-resisting system of the associated story under equivalent tributary lateral load. …”

 

The terms “average deflection of adjoining vertical elements” is the vertical seismic force resisting element (e.g. shear wall) deflection within a story such that the calculation-based flexibility would be either idealized as flexible (per ASCE 7 calculation method) or idealized as rigid (per SDPWS calculation method). This calculation-based criteria for wood diaphragm flexibility originally appeared in 2000 NEHRP Provisions for New Buildings and Other Structures to address special criteria for open front structures with cantilevered diaphragms that were rigid and met other limits for permissible aspect ratio, materials and construction. 

 

Within SDPWS, diaphragm flexibility classification is used to describe methods for distribution of horizontal forces, and is used as a limitation on permissible analysis methods for certain structural configurations. For example, for loading parallel to open side of open front structures with cantilevered diaphragms, it is required that wood diaphragms are modeled as semi-rigid or idealized as rigid and incorporate limits on maximum story drift at structure edges.  An envelope analysis is permitted as an alternative to semi-rigid modeling whereby distribution of horizontal diaphragm shear to each vertical seismic force resisting element is the larger of the shear forces resulting from two separate analyses where the diaphragm is idealized as flexible and the diaphragm is idealized as rigid.

 

IBC Section 1604.4 – Diaphragm Flexibility

 

 Per Section 1604.4 of the 2015 IBC, a diaphragm is rigid if it has a lateral deformation less than or equal to 2 times the average story drift:

 

“1604.4…..A diaphragm is rigid for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is less than or equal to two times the average story drift. Where required by…..”

 

Unlike ASCE 7-10 or SDPWS 2015 provisions which do not restrict use of semi-rigid modeling, the general statement “is rigid” in 1604.4 could be interpreted to require use of a rigid diaphragm assumption for distribution of story shears under certain conditions without the option of semi-rigid modeling or the flexible assumption prescribed in ASCE 7. Additionally, provisions for determination of a rigid diaphragm condition itself does not incorporate “of adjoining vertical elements” after “average story drift” which may be interpreted as including effects of diaphragm deflection in the average story drift calculation. This wording in 1604.4 departs from the more restrictive provision in SDPWS for idealizing a rigid diaphragm because it allows a reduced diaphragm stiffness relative to SDPWS.  Also, wording in 1604.4 allows the use of a rigid diaphragm even if the building has horizontal irregularities, which is in conflict with the more restrictive provision of ASCE 7 limiting use to buildings without horizontal irregularities. Table 1 shows the various code provisions and their associated modeling assumptions.

 

Table 1

Various Code Provisions regarding Diaphragm Modeling Assumptions

Condition

 

2015 IBC

ASCE 7-10

SDPWS 2015

Semi-Rigid

 

-

Default Assumption
[12.3.1]

Must consider stiffness of Diaphragm and LFRS. 

Envelope Method Permitted
[4.2.5]

Rigid

 

Occurs when:
 MDD ≤ 2(ADS)
[1604.4]

Permitted if :
Concrete slab/pan
 &
SPAN/DEPTH ≤ 3
 &
No Irregularities
[12.3.1.2]

Permitted if:
 MDD ≤ 2(ADVE)
[4.2.5]

Flexible

 

-

Permitted when:
 1) 1 or 2 Family Dwelling
or
2) 1.5" Non-structural topping if   drift checked on each line of LFRS,
or
3) MDD > 2(ADVE)
[12.3.1.1]

Not Permitted on Open Front [4.2.5.2]

LFRS – Lateral Force Resisting System

MDD –Maximum in-plane diaphragm deflection

ADS – Average Drift of Story

ADVE – Average deflection of adjoining vertical elements of LFRS

 

Summary

 

Different provisions for diaphragm flexibility and wood diaphragms in operative building codes/standards are a potential source of confusion to designers and plan reviewers. Based on the arguments presented, the SEAOC Seismology Light Frame Sub-committee recommends the following:

 

 Diaphragm flexibility requirements shown in ASCE 7 and SDPWS should be used for wood diaphragms. General provisions of IBC 1604.4 for “rigid” diaphragm and associated distribution of horizontal shear forces should not modify or supersede specific coordinated provisions in the reference standards. This indicates a need for change within the IBC. Because specific provisions of ASCE 7 and SDPWS provide options to the design professional without restriction on use of semi-rigid modeling, Section 1604.4 of the IBC should be revised to remove or revise the conflicting language to conform to ASCE 7 and SDPWS. 


1Gary Mochizuki is Chair of the Seismology Light Frame Sub-committee of the Structural Engineers Association of California (SEAOC) and Senior Research and Development Engineer for Simpson Strong-Tie

2Philip Line is Senior Director, Structural Engineering for the American Wood Council

3Tom VanDorpe is a member of the Seismology Light Frame Sub-committee of the Structural Engineers Association of California (SEAOC) and President for VCA Structural, Inc.

 

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