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