Outline 2008 SDPWS - Diaphragm Deflection Design Michelle Kam-Biron, PE, SE Director, Education American Wood Council Copyright 2013 American Wood Council. Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources Outline Load Path Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources Any system or method of construction to be used shall be based on a rational analysis in accordance with wellestablished 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 loadresisting elements. IBC 2009 1604.4 Vertical Horizontal
Vertical Load Path Lateral Load Path Lateral Loads: National Issue Lateral Loads: National Issue Wind Hazard Wind Map: http://hint.fm/wind/ FEMA Website: http://www.fema.gov/saferooms/wind-zones-united-states
Lateral Loads: National Issue Lateral Loads: National Issue Lateral Loads: National Issue Lateral Loads: National Issue Earthquake Hazard http://www.awc.org/pdf /ASCE7-10WindChanges.pdf USGS website: http://earthquake.usgs.gov/hazards/products/conterminous/2008/maps/us/5hzsa.5in50.usa.jpg
Wind Lateral Loads F = PA Effort is devoted to determining P wind pressure Wind Loads (ASCE7-05) 1 Pressures vary in magnitude and direction (+/-) Seismic Lateral Loads General Lateral Load Path F = ma Effort is devoted to determining: a acceleration
Load Path From Diaphragm to Shear Walls Load Path w C Chord Ties C Diaphragm reaction goes to shear walls Seismic 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) w C Chord C Diaphragm reaction goes to shear walls Ties T Chord T T Chord T Outline Special Design Provisions for Wind and Seismic (ANSI/AF&PA SDPWS-08) Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources 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 Path From Diaphragm to Shear Walls Deflections (4-term equations) w Diaphragm Total b v n c W v L v bending chord deformation excluding slip shear panel deformation 3 5vL vl ( cx ) 0.188Len 8EAb 4G t 2b v v 0.25 v L 1000Ga shear panel nail slip SDPWS unblocked and blocked 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.
Four-Term Diaphragm Deflection Equation Three-Term Diaphragm Deflection Equation SDPWS Commentary Chapter 4 - Wood Diaphragm SDPWS Commentary Chapter 4 - Wood Diaphragm Outline Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources Diaphragm Deflection Equation Bending Chord Deformation Excluding Slip
Diaphragm Deflection Equation Bending Chord Splice Slip Comparison 3-term vs. 4-term Shear panel deformation and nail slip 4-term 3-term Comparison 3-term vs. 4-term 4-term Deflection Apparent Shear Stiffness
4-term Deflection 3-term Deflection Use to calculate e n 3-term Deflection Comparison 3-term vs. 4-term
Comparison 3-term vs. 4-term Comparison 3-term vs. 4-term Comparison 3-term vs. 4-term Comparison 3-term vs. 4-term
Comparison 3-term vs. 4-term Comparison 3-term vs. 4-term Comparison 3-term vs. 4-term Outline Calculated deflections at 1.4 x v s (ASD) closely match test data for blocked and unblocked diaphragms. Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources
Wood Diaphragm Deflection Example Diaphragm Dimensions and Loads 2008 SDPWS Commentary Diaphragm Capacity Diaphragm Dimensions and Loads SDPWS Table 4.2A G a = 14 kips/in Nominal/2.0 (ASD) v s = 510 plf/2 v s = 255 plf (ASD)
Chord Splice Part 1 Calculate number of 16d common nails in the chord splice Diaphragm chord: Two 2x6 No. 2 DFL E = 1,600,000 psi G = 0.50 Chord Splice Part 1 Calculate number of 16d common nails in the chord splice Z ASD = 226 lb. Chord Splice Part 1 Calculate number of 16d common nails in the chord splice Might need to consider M max if joint location unknown Part 2 Calculate mid-span deflection Use 3-term equation 4.2-1
Term 1. Deflection due to bending and chord deformation Term 1. Deflection due to bending and chord deformation Strength level design loads ASCE 7 requires that seismic story drift be determined using strength level design loads Term 1. Deflection due to bending and chord deformation Excluding chord splice slip Term 1. Deflection due to bending and chord deformation Single chord assumption Effect of chord splice slip on chord deformation addressed in deflection equation Term 3 One top plate designed to resist induced axial force second as splice plate. Engineering judgment. Doubling area yields 0.04.
Term 2. Deflection due to shear, panel shear, and nail slip Term 3. Deflection due to bending and chord splice slip G a = 14 kips/in., apparent shear stiffness (SDPWS Table 4.2A) x = 16 ft, distance from the joint to the nearest support. Each joint is located 16 ft from the nearest support. Term 3. Deflection due to bending and chord splice slip Term 3. Deflection due to bending and chord splice slip Δ c = Joint deformation (in.) due to chord splice slip in each joint.
Term 3. Deflection due to bending and chord splice slip Term 3. Deflection due to bending and chord splice slip 180,000 D 1.5 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 Constant of 2 used in numerator to account for slip in nailed splices on each side of joint Term 3. Deflection due to bending and chord splice slip Tension chord slip calculation Term 3. Deflection due to bending and chord splice slip Compression chord and total slip calculation Assume compression chord butt joints have a gap that exceeds the splice slip, so tension chord slip also used for compression chord
Outline Total mid-span deflection Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources 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 Q: What deflection limits are shown in the code? 2012 SDPWS Sec. 2.2 Flexible vs. Rigid Diaphragm 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 ASCE 7-10 Sec. 12.12 DRIFT AND DEFORMATION 12.12.2 Diaphragm Deflection The 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 Separation All 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 C d which is used for story drift and seismic gaps. Outline Resources Basic Lateral Load Overview Code acceptance of 2008 SDPWS Equation Example FAQ Resources http://www.structur emag.org/archives /2010-3/C-CS- Lines-March10.pdf
Resources Resources www.apawood.org L350 http://www.amazon.com/design- Wood-Structures- ASD-LRFDebook/dp/B00386 MRG8 Resources Resources http://www.amazon.com/analysis- Irregular-Shaped- Structures- Diaphragms/dp/00 7176383X#_ http://www.amazon.com
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