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Copyright © 2007-2010 American Wood Council.All rights reserved.

American Wood CouncilEngineered and Traditional Wood Products

2008 SDPWS -Diaphragm Deflection Design

Michelle Kam-Biron, PE, SEDirector, EducationAmerican Wood Council

Copyright © 2013 American Wood Council. All rights reserved.

Outline

• Basic Lateral Load Overview

• Code acceptance of 2008 SDPWS

• Diaphragm Deflection Equation

• Diaphragm Deflection Example

• FAQ• Resources

Outline






Load Path“Any system or method of construction to be used shall be

based on a rational analysis in accordance with well-established principles of mechanics. Such analysis shall result in a system that provides a complete load path capable of transferring loads from their point of origin to the load-resisting elements. “ IBC 2009 1604.4

Vertical Horizontal



Vertical Load Path Lateral Load Path

Lateral Loads: National Issue

Wind Map: http://hint.fm/wind/


Wind Hazard

FEMA Website: http://www.fema.gov/safe-rooms/wind-zones-united-states



Lateral Loads: National Issue Lateral Loads: National Issue


http://www.awc.org/pdf/ASCE7-10WindChanges.pdf


Earthquake Hazard

USGS website: http://earthquake.usgs.gov/hazards/products/conterminous/2008/maps/us/5hzSA.5in50.usa.jpg



Wind Lateral Loads

F = PAEffort is devoted to determining

P – wind pressure

Wind Loads (ASCE7-05)1

•Pressures vary in magnitude and direction (+/-)

Seismic Lateral Loads

F = ma Effort is devoted to determining:

a – acceleration

General Lateral Load Path



Load PathFrom Diaphragm to Shear Walls

Ties

ChordT T

ChordC C

w

Diaphragm reaction goes to shear walls

Ties

ChordT T

ChordC C

w

Diaphragm reaction goes to shear walls

Load PathSeismic Design Category C, D, E, & F)Diaphragms shall be provided with continuous ties or struts between diaphragm chords to distribute these anchorage forces into the diaphragms. (ASCE 7 - 10 sec. 12.11.2.2.1)

Outline






Special Design Provisions for Wind and Seismic (ANSI/AF&PA SDPWS-08)

• 2006 IBC– Permitted alternative to

Section 2305– References 2005 SDPWS

• 2009 IBC & 2012 IBC– Mandatory– References 2008 SDPWS

in Section 2305 for lateral design and construction



Load PathFrom Diaphragm to Shear Walls

w

W

vL

v

bX

LetG

vLEAbvL c

vv

n

2)(

188.048

5 3

Deflections (4-term equations)• Diaphragm

bending chord deformation excluding slip

Total b v n c

0.25 v L 1000Ga

SDPWS –unblocked and blocked

shear panel deformation

shear panel nail slip

bending chord splice slip

Four-Term Diaphragm Deflection Equation Three-Term Diaphragm Deflection Equation

SDPWS Chapter 4 - Wood Diaphragms

Applies to flat and folded (roof) diaphragms.



SDPWS Commentary Chapter 4 - Wood Diaphragm

Four-Term Diaphragm Deflection Equation

SDPWS Commentary Chapter 4 - Wood Diaphragm

Three-Term Diaphragm Deflection Equation

Outline






Diaphragm Deflection Equation

Bending Chord Deformation Excluding Slip



Diaphragm Deflection Equation

Bending Chord Splice Slip Shear panel deformation and nail slip4-term 3-term

Comparison – 3-term vs. 4-term


Apparent Shear Stiffness

4-term Deflection



4-term Deflection

Use to calculate en

3-term Deflection

3-term Deflection Comparison – 3-term vs. 4-term



Comparison – 3-term vs. 4-term Comparison – 3-term vs. 4-term






Calculated deflections at 1.4 x vs(ASD) closely match test data for blocked and unblocked diaphragms.

Outline








2008 SDPWS Commentary

Wood Diaphragm Deflection Example Diaphragm Dimensions and Loads

Diaphragm Capacity

SDPWS Table 4.2A

Ga = 14 kips/inNominal/2.0 (ASD)vs = 510 plf/2 vs = 255 plf (ASD)

Diaphragm Dimensions and Loads



Chord Splice

Part 1Calculate number of 16d common nails in the chord spliceDiaphragm chord:Two 2x6 No. 2 DFL

E = 1,600,000 psiG = 0.50

Chord Splice

Part 1Calculate number of 16d common nails in the chord splice

Z′ASD = 226 lb.

Chord Splice

Part 1Calculate number of 16d common nails in the chord splice

Might need to consider Mmax if joint location unknown

Mid-Span Deflection

Part 2 – Calculate mid-span deflection

Use 3-term equation

4.2-1



Mid-Span Deflection

Term 1. Deflection due to bending and chord deformation

Mid-Span Deflection


ASCE 7 requires that seismic story drift be determined using strength level design loads

Strength level design loads

Mid-Span Deflection


Effect of chord splice slip on chord deformation addressed in deflection equation Term 3

Excluding chord splice slip

Mid-Span Deflection


One top plate designed to resist induced axial force –second as splice plate. Engineering judgment. Doubling area yields 0.04”.

Single chord assumption



Mid-Span Deflection

Term 2. Deflection due to shear, panel shear, and nail slip

Ga = 14 kips/in., apparent shear stiffness(SDPWS Table 4.2A)

Mid-Span Deflection

Term 3. Deflection due to bending and chord splice slip

x = 16 ft, distance from the joint to the nearestsupport. Each joint is located 16 ft from the nearest support.

Mid-Span Deflection


Δc = Joint deformation (in.) due to chord splice slip in each joint.

Mid-Span Deflection




Mid-Span Deflection


• 180,000 D1.5 load/slip modulus for dowel type connections per NDS 10.3.6

• Diameter for 16d common NDS Table L4• 180,000 (0.162)1.5 = 11,737 lb/in./nail

Mid-Span Deflection


Constant of 2 used in numerator to account for slip in nailed splices on each side of joint

Mid-Span Deflection


Tension chord slip calculation

Mid-Span Deflection


Assume compression chord butt joints have a gap that exceeds the splice slip, so tension chord slip also used for compression chord

Compression chord and total slip calculation



Mid-Span Deflection

Total mid-span deflection

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Frequently Asked Questions

Q: How do pneumatic nails affect diaphragm capacity?

A: International Staple, Nail, and Tool Association (ISANTA)•ICC ES ESR-1539 - Power-Driven Staples and Nails for Use in All Types of Building Construction

•www.isanta.org•http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-1539.pdf


Q: What deflection limits are shown in the code?

A: Depends on the sheathing material•Brittle Finishes – L/240 – IBC Table 1604.3•Flexible Finishes – L/120 – IBC Table 1604.3•Glass –1/175 of glass edge length or 3/4" - IBC 2403.3



Flexible vs. Rigid Diaphragm

2012 SDPWS Sec. 2.2

Q: What deflection limits are shown in the code?


ASCE 7-10 Section 12.3 Diaphragm Flexibility, Configuration Irregularities, and Redundancy 12.3.1.3 Diaphragms … are permitted to be idealized as flexible where the computed maximum in-plane deflection of the diaphragm … is more than two times the average story drift of adjoining vertical elements of the seismic force resisting system….

DIAPHRAGM 2 x SHEARWALLS

Flexible vs. Rigid Diaphragm


ASCE 7-10 Sec. 12.12 DRIFT AND DEFORMATION

12.12.2 Diaphragm DeflectionThe deflection in the plane of the diaphragm, as determined by engineering analysis, shall not exceed the permissible deflection of the attached elements. Permissible deflection shall be that deflection that will permit the attached element to maintain its structural integrity under the individual loading and continue to support the prescribed loads.


ASCE 7-10 Sec. 12.12 DRIFT AND DEFORMATION

12.12.3 Structural SeparationAll portions of the structure shall be designed and constructed to act as an integral unit in resisting seismic forces unless separated structurally by a distance sufficient to avoid damaging contact as set forth in this section….

More…





Q: Is diaphragm deflection cumulative with shear wall deflection?

A: Yes. Shear walls supporting a horizontal diaphragm would also be evaluated for deflection. Cumulative deflection would then be calculated to determine the maximum anticipated movement to compare with allowables.


Q: Are there provisions to calculate the deflection for a diaphragm that is only partially blocked (i.e. at ends only) or is it proper to base the deflection on the entire diaphragm being unblocked?

A: There are no provisions for a partially blocked diaphragm. Suggest calculating for both a blocked and unblocked diaphragm to determine magnitude of difference and use engineering judgment.


Q: When the nailing pattern in a horizontal diaphragm varies instead of being uniform, how is deflection calculated?

A: One approach is to modify the nail-slip constant in the 4-term equation in proportion to the average load on each nail with non-uniform nailing compared to the average load with uniform nailing. APA’s Diaphragm and Shear Wall Design and Construction Guide (L350) provides an example: www.apawood.org.


Q: Is there any benefit if wood structural panels are glued to the assembly?

A: We are not aware of any added benefit with respect to diaphragm deflection.




Q: Is there a multiplier to use for a long-term consideration due to possible enlargement of nail holes or is that all included in the nail slip deflection calculations?

A: Testing done to verify diaphragm deflection calculations is based on full reverse cyclic loads, so the effects of nail hole enlargement has been addressed.


Q: For large diaphragm, what adjustments to deflection equations need to be made to obtain inelastic diaphragm deflection?

A: ASCE 7 Minimum Design Loads for Buildings and Other Structures, Section 12.8.6 includes an amplification factor Cdwhich is used for story drift and seismic gaps.

Outline






Resources

• http://www.structuremag.org/Archives/2010-3/C-CS-Lines-March10.pdf



Resources

• www.apawood.orgL350

Resources

• http://www.amazon.com/Design-Wood-Structures-ASD-LRFD-ebook/dp/B00386MRG8

Resources

• http://www.amazon.com/Analysis-Irregular-Shaped-Structures-Diaphragms/dp/007176383X#_

Resources

http://www.amazon.com



Upcoming AWC webinar…May 30th

http://www.awc.org/codes/dcaindex.html

Questions?• www.awc.org

– Online eCourses– FAQ’s– Helpdesk

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