Wind Resistant Design Considerations for Wood-Frame Structures
Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board.
Bryan Readling, P.E. “The Wood Products Council” is This course is registered with a Registered Provider with The AIA CES for continuing American Institute of Architects professional education. As Continuing Education Systems such, it does not include (AIA/CES), Provider #G516. content that may be deemed or construed to be an approval or endorsement by the AIA of any material of Credit(s) earned on completion construction or any method or of this course will be reported to manner ofhandling, using, AIA CES for AIA members. distributing, or dealing in any Certificates of Completion for material or product. both AIA members and non-AIA ______members are available upon Questions related to specific materials, request. methods, and services will be addressed at the conclusion of this presentation. APA b. 1933 Outline
• Load-path and Continuity • Codes and Reference Documents • Engineered vs. Prescriptive • Changes to WFCM and SDPWS “spid-wiz” • Wind vs. seismic design Course Description
The overall strength of a building is a function of all of the components—roof, walls, floors, and foundation—working together as a unit. This session will provide a top-to-bottom overview of lateral design for wood-frame structures with a focus on wind resistant detailing. Topics will include lessons learned from natural disasters, load path continuity, and updates to the International Building Code affecting structural design. Learning Objectives
• Understand relationship between load path continuity and building performance. • Methods whereby common vulnerabilities can be better resisted through enhanced connection technologies. • Identify common installation errors that affect lateral performance, and specifications to ensure load-path continuity. • Review APA research and testing related to shearwalls with openings. Governing Codes for Wood Design Governing Codes for Wood Design
SEI/ASCE 7-10 . Design Loads . Deflection limits (story drift) . Torsional irregularity Governing Codes for Wood Design
. 2015 NDS Changes . Incorporation of CLT . Addition of terminology for laminated strand lumber (LSL) and oriented strand lumber (OSL) . Lag screw withdrawal values excludes tapered tip . Includes Char rate for CLT and Structural Composite Lumber Governing Codes for Wood Design
2015 SDPWS “SPID-WIZ” (Special Design Provisions for Wind and Seismic)
. Provisions for wood members, fasteners, and assemblies for resisting wind and seismic forces – ASD/LRFD . Reference document in 2015 IBC . Submitted ref. doc. for ASCE 7-16 . Free download: http://www.awc.org Additions to SDPWS
2015 SDPWS: • Wind & Seismic design values separate • Min aspect ratios for wind = seismic • More consistent w/ ASCE 7-10 • Flexible Rigid diaphragm definitions removed • New section - uplift force systems • High-load diaphragm blocking 3x • Studs (2) 2x vs. 3x substitution • Repetitive member factor applied to stiffness – studs up to 24” oc Governing Codes for Wood Design
Wood Frame Construction Manual Prescriptive and Engineered design High wind, seismic, and snow loads
. Added uplift design for Wood Structural Panels . Design loads updated per ASCE 7-10 . 0 – 70 PSF ground snow load . 110 – 195 mph 700 yr./3-sec gust
. Seismic design Cat . A - C Governing Codes for Wood Design
• Wood Frame Construction Manual
90 – 130 MPH High Wind Guides Prescriptive Engineered “Bracing” Shearwalls
International Res. Code International Bldg. Code Limitations: Applications: 3-stories max. Any size/shape Wind < 100 mph* Wind - No limit
Uses Braced wall panels Uses Shear walls without hold downs with hold downs Prescriptive Wall Bracing
• Walls too narrow • Not enough bracing length APA Simplified Bracing Method SR-102 - Supplemental to IRC
Compared to IRC Methods
• Uses thicker continuous wall sheathing (7/16” min.) • Tighter nailing above IRC min. • Allows for narrower segments • Gives partial credit for walls too narrow to qualify in IRC Prescriptive Wall Bracing
Georgia GA Codes Advisory Committee added SR-102 as alternate method Continuous Sheathing – More Durable Wind Resistant Construction More Durable
Survey of Newly Built Houses
Shell Survived Intact Case Study: Pleasant Grove, AL
1998 – Pleasant Grove, AL
2011 - Pleasant Grove, AL More Durable
Roof Sheathing Attachment
Gable end connections
Roof to wall connection
Wall to wall continuity
Wall sheathing attachment
Wall connection to sill plate
Sill plate anchorage Georgia - Disaster Resilient Building Code
. Funded by U.S. Dept. HUD . Task force of stakeholders . Non-mandatory . Intended for local adoption . Establishes performance baseline for wind Stronger and More Durable “Engineered” Load Path
(IBC 2012 1604.4) “Any system of 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. “
Vertical Load Path Lateral Load Path Lateral Load Systems
. Wood Design is Less Intuitive
. More Circuitous Load- Path
. Interruptions are less obvious Lateral Loads (Wind)
Effort is devoted to F = P A determining P – wind pressure
Lateral Loads (Seismic) Most effort often devoted to F = ma determining acceleration Force = (Mass) x (Acceleration) Seismic Performance for Wood Structures
Advantages . Lightweight . Flexible . Highly redundant . Good Balance of Strength and Stiffness . Energy Dissipation through Damping Effect of Systems Resist Tornados?
F-5 Tornado Oklahoma 1999 Facts: • 90% of all tornados are EF2 and below • Damaging winds outside vortex are slower than max. • Unrealistic to protect against EF4, EF5, and some EF3. • Provide recommendations to protect building shell. Percentage of Occurrence
EF-Scale Relative Cumulative Percentage Frequency
EF-0 53.5 % 53.5 %
EF-1 31.6 % 85.1
EF-2 10.7 % 95.8
EF-3 3.4 % 99.2
EF-4 0.7 % 99.9
EF-5 < 0.1 100 Tornado Intensity Along Path Building for High-Wind Resistance in Light-Frame Wood Construction
Roof Sheathing Attachment
Gable end connections
Cladding attachment
Roof to wall connection
Wall to wall continuity
Wall sheathing attachment
Wall sheathing continuity
Wall connection to sill plate
Sill plate anchorage Gable-end Framing
2011 Fayetteville, NC Gable-end Framing
Tie gable end walls back to the structure Gable end truss top chord Tension-tie strap, attach with (8) 10d common nails, each end of strap Roof Trusses
(3) 10d Common nails (typical)
Gable end 2x4 flatwise 2" x 4" continuous lateral brace truss bottom blocking @ 6' on center. Lateral brace chord between truss sized to extend from end wall to bottom chords over 3 interior trusses plus 6". Resisting Pressure on Components and Cladding
Sheath gable end walls with wood structural panels, such as plywood or oriented strand board (OSB)
Gable end truss top 8d Common nails - 4" on chord center perimeter of Wood structural panel panel sheathing 8d Common nails - 6" on Gable end truss center along intermediate vertical web member framing 8d Common nails - 4" on Gable end truss bottom center perimeter of panel chord nailed to the top of the double top plate Components and Cladding Loads Components and Cladding Loads
Requirements for Wall Coverings and Wind Pressures APA publication TT-105 Basic vs. Ultimate wind speed Wind Pressure Resistance (APA form TT-105) Deformed Shank Nails
Larger heads enhance Enhanced pullout is pull-thru resistance achieved with ring or spiral shanks nails for enhanced uplift resistance
Not code required! Forces to Resist: Uplift Roof to Wall Connection
Roof framing to wall connection with hurricane/seismic framing anchor or equivalent connector attached on sheathing side of the exterior walls
Roof framing - trusses or lumber framing
Uplift
Shear
Double top plate Framing anchors with uplift and shear capacity Uplift Connectors on Inside of Wall Structural Screws
Rafter to Top Plate • Must be driven straight into middle of rafter tail. • Wind zone and local building code requirements can be met using code evaluation reports WSP’s Used to Resist Combined Uplift and Shear Eliminate Metal Hardware
. Lower Cost . Less interference . Reduced Construction Time Oversize OSB Wall Sheathing
Sized for 8, 9, 10 ft. walls Eliminates blocking Easy to inspect Less air infiltration More direct uplift and lateral load-path Combined Shear and Uplift
• Wall Sheathing used for Uplift • Metal straps still needed around windows and door openings Rim Board Tension Transferred by Splice Plate
WSP Tension Splice Lumber ½” space Uplift Nailing Wall Framing to Sill Plate Connection
Extend wood structural panel sheathing at bottom of wall to sill plate intersection I-joist
Rim Board
Wall sheathing
Other connections are not shown for clarity Bottom Plate Anchorage
Space 1/2" anchor bolts 32" to 48" on center with 0.229" x 3" x 3" slotted square plate washers at the wall to sill plate intersection
I-joist
Rim Board
Wall sheathing
1/2" anchor bolts at 32" to 48“ on center tie the structure to the foundation
Other connections are Anchor-Bolt Connection to Foundation
• Limited by steel-to- wood bearing area • Allowable stress perpendicular to grain often controls Large Plate Washers
Better uplift resistance! Material Properties of Wood
Very strong parallel to grain Material Properties of Wood
Relatively weak perpendicular to grain
Anisotropic Larger Washer Increases Uplift Capacity
• Large plate washers (3”x3”x0.229”) prevent cross-grain splitting of sill plate – • Required for SDC D, E or F (IBC 2305.3.11) Wood structural panel - uplift Plate washer
Cross grain bending is Sill plate Restrained by Plate Washer Podium Construction
Town Brookhaven – Atlanta, GA Anchoring to Concrete Podium Slab Anchor Connection to Concrete Podium at Steel Embed
¾” Dia. Threaded Stud Anchor Bolt
1” Diameter Steel Pipe Sleeve – pack with epoxy grout.
Steel Embed Plate w/Welded Headed Studs 1” Diameter Steel Pipe Sleeve
•1 Packed with expansive epoxy grout • Increases wood bearing area • Threaded rod welded to steel embed Horizontal Diaphragms
PLAN VIEW Sloped Roofs
Idealize sloped wood roof diaphragms as if they are flat Diaphragm Capacity Table - SDPWS Unblocked Diaphragm Blocked Diaphragm Diaphragm Force Distribution Flexible . Δdiaphragm > 2Δshearwalls . Diaphragm load is distributed to shear walls by tributary area . Diaphragm acts like series of simply supported beams Rigid . Δdiaphragm < 2Δshearwalls . Diaphragm load is distributed to shear walls by wall stiffness . Torsion considered in design . Provides more flexibility for shearwall layout . More complicated analysis Semi-rigid or Enveloped . Ref ASCE 7-10 Prescribed Rigid Wood Diaphragms (SDPWS) • Open front • Cantilevered diaphragms
Torsionally irregular Horizontal Shear Distribution
w
L/2 L/2 Flexible Diaphragm w
Dsw
Flexible .25wL .50wL .25wL
Ddi
L/2 L/2 Rigid - All Walls Identical w
Dsw
Rigid (no .333wL .333wL .333wL Torsion)
L/2 L/2 Horizontal Shear Distribution w
Stiffness 2K K 2K
Flexible .25wL .50wL .25wL
Rigid .40wL .20wL .40wL
Enveloped .40wL .50wL .40wL
L/2 L/2 Prescribed Rigid Wood Diaphragms (SDPWS) 6’-0 exception Torsion Effects Pre-recorded Webinar at www.woodworks/education/online-seminars/ High-Load Blocked Diaphragms
. Minimum depth of framing and blocking 3” nominal SDPWS 4.2.7.1.2 …where 10d nails and higher density nailing required
American Wood Council Shear Transfer from Roof Diaphragm - to Shearwall Force Transfer from Diaphragm to Shearwall
Roof Framing Shear Wall Design
Wood structural panel - Specific specific grade stud species and thickness
Specific nail size and Hold-down spacing anchors requirements anchor bolts SW Changes in 2015 SDPWS
• Add dissimilar material capacities on opposite face as WSP for wind only.
• Double 2x can be used in lieu of 3x where:* • Perimeter nails 2” o.c. • 10d common nails 3” o.c. • Shear capacity > 700 plf in SDC D,E, or F.
* Connection between 2 members designed to transfer shear per NDS Unblocked Shearwalls
• Shear capacity reductions • 16’ Maximum wall height • Based on cyclic testing • Up to 2:1 aspect ratio Overturning of Shearwalls
F G
B C Overturning Forces
Only 0.6 x design dead load can be used to resist overturning from wind or earthquake (IBC 1605.3, ASCE 7 Sec. 2.4) Shearwall Hold-Down Anchors Holddown Anchor Holddown Anchor
• Low-slip fasteners • Pre-deformed base • A plus in seismic loading Holddown Anchor
• Multi-story apps. • Self-tightening • A plus in taller structures Continuous Threaded Rod
Timbers carry Compression forces at holddown Shearwall Capacity - SDPWS Shearwall Minimum Aspect Ratios
h/bs
Minimum width: bs = h/2 exception: 3.5:1 can be used h bs = h/3.5 with penalty (2bs/h) W
bs Combined Uplift & Shear – APA Pub. SR101
Tension Transferred by Studs
Nail pattern at each stud Tension Splice at Rim Joist
Tension splice at horizontal wall sheathing joint
Nail pattern at rim joist Anchor Bolt Spacing
• Anchor bolt @ ea. end of each plate, distance from end = • 7x bolt dia., • or ½ tabulated value, • or 12”, whichever is less • Use 0.229” x 3” x 3” plate washer Site-Built Portal Frame Bracing Methods Portal-frame: Continuous Method
Method CS-PF
Figure R602.10.4.1.1 Portal Frame with Hold Downs Reference: APA Report TT-100 Shearwall Design Methods
Segmented Force Transfer Perforated 1. Aspect Ratio for 1. Code does not 1. Code provides seismic 2:1 provide guidance for specific requirements 2. Aspect ratio up to this method 2. The capacity is 3.5:1, if allowable 2. Different approaches determined based on shear is reduced by using rational empirical equations 2w/h analysis are used and tables Reducing Hold-Down Anchorage
Segmented Shearwalls Continuous Shearwalls Segmented (Traditional) Wood Shear Walls
V
Only full height segments are considered
T = C = v x h v = unit shear v v T C T C Aspect ratio applies to full height segment (dotted) Shear Wall With Openings Force Transfer Around Openings
V Wall • Shear around Pier openings accounted for by strapping or framing • based on a “rational analysis” • H/w ratio defined by wall pier v H H Aspect ratio applies to wall pier segment (dotted) Shear Wall With Opening Force Transfer Around Opening V Hold-downs only at ends Extra calculations and added construction details (connections & blocking) Uses traditional v design values H Shear Wall With Opening – Perforated Shear Wall
• Openings accounted for by V empirical adjustment factor • Hold-downs only at ends
v T t C • Uplift between hold downs, t, at full height segments is required • Limited to: • 980 plf – Seismic • 1370 plf - Wind Shear Capacity Adjustment Shear Capacity Adjustment
Equation for Perforated Shearwalls Shear Capacity Adjustment
Equation for Perforated Shearwalls Suggested References APA Publications and Website
Free APA publications www.APAwood.org A Sampling of APA Publications available at: www.apawood.org
T300 – Glulam connection details E30 – Engineered Wood Const. Guide L350 – Diaphragms and Shear Walls T325 – Roof fastening for wind uplift Y250 – Shear transfer at engineered floors A Sampling of APA Technical Topics - available at www.apawood.org
TT-035 – Corrosion resistant fasteners TT-036 – Glued floors TT-039 – Nail withdrawal TT-070 – Nail pull through TT-045 – Min. nail penetration TT-012 – Overdriven fasteners TT-056 – Power driven fasteners TT-050/051 – Screw withdrawal TT-058 – Slant nailing TT-061- Nailing thin flange I-joists TT-020 – Dowel bearing strength Questions?