Hurricanes, Earthquakes & Tornados WOOD DESIGN FOR EXTREME FORCES WoodWorks National Sponsors The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of Completion for both AIA members and non- AIA members are available upon request. This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. Copyright Materials This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without t written permission i of the speaker is prohibited. The Wood Products Council 2011
Learning Objectives Governing Codes for Wood Design At the end of this program, participants will be able to: 1. Explain the fundamental behavior of wood structures especially as it pertains to diaphragms, shear walls, and the connections therein. 2. Identify new technologies such as designing g wood structural panel wall sheathing to resist simultaneous shear and uplift forces. 3. Recognize common failure modes using photographs from APA post-disaster evaluations. 4. Identify the relationship between load-path discontinuity and structural damage by viewing common failure modes. 2006 IBC (International Building Code) Chapter 16 Loads Chapter 23 Wood Governing Codes for Wood Design Governing Codes for Wood Design SEI/ASCE 7-05 Design Loads 2005 NDS Load Resistance Code Adopted ANSI Process 2005 NDS (National Design Specification for Wood Construction)
Governing Codes for Wood Design SDPWS Referenced in IBC 2005 & 2008 SDPWS (Special Design Provisions for Wind and Seismic) Available at: http://www.awc.org/standards/sdpws.html Free download! 2006 IBC Permitted Alternative to IBC 2305 Reference to 2005 SDPWS Provisions for wood members, fasteners, and assemblies for resisting wind and seismic forces ASD/LRFD 2009 IBC 2008 SDPWS Mandatory in IBC 2305 for Lateral Design 2008 Edition - SDPWS Governing Codes for Wood Design WSP Combined Shear and Uplift Unblocked Shearwalls 2-Sided Shearwalls Perforated Shearwalls- SCAF Equation Increased Strength Limit for PSW High-load Diaphragms Wood Frame Construction Manual For One- and Two-Family Dwellings
Prescriptive Wind Standards For One- and Two-Family Dwellings Prescriptive Standards Wood Frame Construction Manual 90 130 MPH High Wind Guides ICC 600-2008 Standard for Residential Construction in High Wind Regions ICC 500-2008 Standard for the Design and Construction of Storm Shelters Introducing: New APA Publication!
(IBC 2006 1604.4) Load Path 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. F = PA Lateral Loads (Wind) Effort is devoted to determining P wind pressure Vertical Load Path Lateral Load Path Lateral Load System Less Intuitive Less Intuitive More Circuitous Interruptions in Load-path less obvious
Lateral Loads (Seismic) Force = (Mass) x (Acceleration) F = ma Effort is devoted to determining: a acceleration Seismic Performance for Wood Structures Fully Sheathed Box Advantages Lightweight Flexible Highly redundant Good Balance of Strength and Stiffness Good Energy Dissipation Hurricane Katrina
Prescriptive Bracing International Res. Code Limitations: 3-stories max. Wind < 100 mph* Uses Braced wall panels without hold downs Engineered Shearwalls International Bldg. Code Applications: Any size/shape Wind - No limit Uses Shear walls with hold downs Why Build to Resist Tornados? * In hurricane regions, < 110 mph otherwise Prescriptive designs for wind speeds over 100 are provided by: Wood Frame Construction Manual American Wood Council IBHS Guidelines for Hurricane Resistant Residential Construction or SSTD-10 F-5 Tornado Oklahoma 1999 Facts Enhanced Fujita Scale 90% of all tornados are EF2 and below 70% of tornados are <110 mph Damaging winds outside vortex are slower than max. Unrealistic to protect against EF4, EF5, and some EF3. Safe rooms are NOT a panacea. Provide recomendations to protect building shell. Tornado loads more unknown than seismic! EF-Scale Tornado description Wind Speed (3-sec gust) EF-0 Gale tornado 65-85 EF-1 Moderate tornado 86-110 EF-2 Significant tornado 111-135135 EF-3 Severe tornado 136-165 EF-4 EF-5 Devastating tornado Incredible tornado 166-200 >200 Description of Expected Damage Minor or no damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken. Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; lightobject missiles generated; cars lifted off ground. Severe damage. Entire stories of well-constructed houses destroyed; trains overturned; trees debarked; heavy cars lifted off the ground and thrown Extreme damage. Well-constructed and whole frame houses completely leveled; cars and other large objects thrown and small missiles generated. Total Destruction. Strong framed, well built houses leveled off foundations and swept away;
Percentage of Occurrence Tornado Intensity Along Path EF-Scale Wind Speed Relative Cumulative (3-sec gust) Frequency Percentage EF-0 65-85 53.5 % 53.5 % EF-1 86-110 31.6 % 85.1 EF-2 111-135 10.7 % 95.8 EF-3 136-165 3.4 % 99.2 EF-4 166-200 0.7 % 99.9 EF-5 >200 < 0.1 100 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
Roof Sheathing Attachment Step Down Trusses More Challenging Connections! Nail roof sheathing with 8d ring shank (0.131" x 2-1/2") or deformed shank nails at 4" on center at panel ends and edges and 6" on center in the intermediate framing 8d Common nails - 6" on center along intermediate framing 8d Common nails - 4" on center at panel ends Roof sheathing Roof framing Fayetteville, NC Gable-end Framing Gable-end Framing Tie gable end walls back to the structure t 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 truss bottom chord 2x4 flatwise blocking between truss bottom chords 2" x 4" continuous lateral brace @ 6' on center. Lateral brace sized to extend from end wall to over 3 interior trusses plus 6". 2011 Fayetteville, NC
Gable-end end Framing Resisting Pressure on Components and Cladding Sheath gable end walls with wood structural t panels, such as plywood or oriented strand board (OSB) 2011 Pleasant Grove, AL 8d Common nails - 4" on center perimeter of panel 8d Common nails - 6" on center along intermediate framing 8d Common nails - 4" on center perimeter of panel nailed to the top of the double top plate Gable end truss top chord Wood structural panel sheathing Gable end truss vertical web member Gable end truss bottom chord Components and Cladding Loads Wind Pressure Resistance (APA form TT-105) Table 1. Maximum wind speed (mph - 3 second gust) permitted for wood structural panel sheathing used as wall covering to meet IRC Table R301.2(2) requirements 1, 2, 3 Minimum Minimum Maximum Wind Speed Minimum Nail Wall Panel Nail Spacing Wood Nominal (mph) Stud Structural Panel Wind Exposure Size Penetration Spacin Edges Field Panel Thicknes Category g Span s (inches (inches -- (inches) (inches) B C D Rating (inches) o.c.) o.c.) 12 110 90 85 6d 24/0 3/8 16 6 6 110 90 85 (0.113" 1.5 12 110 100 90 x 2.0") 24/16 7/16 16 6 6 150 125 110 12 130 110 105 8d 16 6 6 150 125 110 (0.131" 1.75 24/16 7/16 24 or 12 110 90 85 x 2.5") 6 less 6 110 90 85 1. Panel strength axis parallel or perpendicular to supports. Three-ply plywood sheathing with studs spaced more than 16 inches o.c. shall be applied with panel strength axis perpendicular to supports. 2. Table is based on wind pressures acting toward and away from building surfaces per R301.2, lateral bracing requirements shall be in accordance with R602.10.
Deformed Shank Nails Forces to Resist: Uplift Enhanced pullout is achieved with ring or spiral shanks nails for enhanced uplift resistance Larger heads enhance pull-thru resistance Not code required! Roof to Wall Connection Structural Screws Roof framing to wall connection with hurricane/seismic i i framing anchor or equivalent connector attached on sheathing side of the exterior walls Shear Uplift Roof framing - trusses or lumber framing 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 (i.e. Fastenmaster t ESR-1078) Double top plate Framing anchors with uplift and shear capacity
Lateral Load Systems WSP s Used to Resist Combined Uplift and Shear Less Intuitive More Circuitous Interruptions in Load-path less obvious Using WSP to Eliminate Metal Hardware Oversize OSB Wall Sheathing Lower Cost Reduced Construction Time Sized for 8, 9, 10 ft. walls Eliminates blocking Easy to inspect Less air infiltration More direct uplift and lateral load-path
Wall Sheathing used for Uplift Combined Shear and Uplift Metal straps still needed around windows and door openings Tension Transferred by Studs Tension Splice at Rim Joist Nail pattern at each stud Tension splice at horizontal wall sheathing joint Nail pattern at rim joist
Rim Board Tension Transferred by Splice Plate WSP Tension Splice ½ space Lumber Uplift Nailing Wall Sheathing Attachment Fully Sheathed Walls Nail wall sheathing with 8d common (0.131" x 2-1/2") nails at 4" on center in the boundary of wood structural panel wall sheathing and 6" on center in the intermediate studs Sheath all walls with wood structural t panels 8d Common nails at 6" on center at intermediate supports 8d Common nails at 4" on center at panel ends and edges Building paper Even with the loss of wall covering and building paper, continuous plywood and OSB sheathing offers Building paper
Fully Sheathed Walls Hurricane Katrina 2011 - Pleasant Grove, AL 2011 - Pleasant Grove, AL 2011 - Raleigh, NC
Flying Debris 2011 - Raleigh, NC Components and Cladding Loads NC Tornados 4-16-11 Non-Structural Sheathing Reference: Requirements for Wall Coverings and Wind Pressures, APA publication TT-105 2011 - Wilson, NC
Corner Bracing Non-Structural Sheathing 2011 - Wilson, NC Wilson, NC Non-Structural Sheathing Wall Framing to Sill Plate Connection Extend wood structural panel sheathing at bottom wall to sill plate intersection I-joist Rim Board Wall sheathing 2011 - Raleigh, NC Attaching the continuous sheathing directly to the sill plate helps tie the structure above to the foundation below. Other connections are not shown for clarity
2011 - Raleigh, NC Raleigh, NC 2011 - Raleigh, NC Raleigh, NC
Bottom Plate Anchorage Anchor-Bolt Connection to Foundation 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 Limited by steelto-wood bearing area Allowable stress perpendicular to grain often controls 1/2" anchor bolts at 32" to 48 on center tie the structure to the foundation Material Properties of Wood Very strong parallel to grain Material Properties of Wood Relatively weak perpendicular to grain Vocabulary word for today! Anisotropic
Larger Washer Increases Uplift Capacity Foundation Anchorage 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 Restrained by Plate Washer Sill plate 2011 - Raleigh, NC 2011 - Raleigh, NC 2011 - Raleigh, NC
Foundation Anchorage 2011 - Coaling, AL 2011 - Coaling, AL Coaling, AL 2011 - Coaling, AL
2011 - Coaling, AL 2011 - Coaling, AL Square Plate Washer Dramatically Increases Uplift Capacity Combined Uplift & Shear APA Pub. SR101 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 Restrained by Plate Washer Sill plate
Anchor Bolt Spacing for Combined Shear and Wind Uplift from SR-101 Podium Construction Based on Full-scale Test Results Eliminate splitting of bottom plate as a failure mode Town Brookhaven Atlanta, GA Anchoring to Concrete Podium Slab Anchor Connection at Steel Embed
Anchor Connection to Concrete Podium at Steel Embed 1 Diameter Steel Pipe Sleeve ¾ Dia. Threaded StudAnchor Bolt 1 Packed with expansive epoxy grout Increases wood bearing area Threaded rod welded to steel embed 1 Diameter Steel Pipe Sleeve pack with epoxy grout. Steel Embed Plate w/welded Headed Studs Follow the Load E H Sloped Roofs Idealize sloped wood roof diaphragms as if they are flat F PLAN VIEW G
Blocked Diaphragm Unblocked Diaphragm Unblocked Shearwalls Diaphragm (Plan View) w Shear capacity reductions 16 Maximum wall height Based on cyclic testing Up to 2:1 aspect ratio L/2 L/2
Flexible Diaphragm w Rigid - All Walls Identical w sw sw Flexible.25wL.50wL.25wL Rigid (no Torsion).333wL.333wL.333wL di L/2 L/2 L/2 L/2 Flexible v. Rigid w Flexible vs. Rigid Diaphragms Flexible Stiffness Flexible Rigid (no Torsion) 2K.25wL.40wL K.50wL.20wL L/2 L/2 2K.25wL.40wL diaphragm > 2 shearwalls Diaphragm load is distributed to shear walls by tributary area Diaphragm acts like series of simply supported beams diaphragm < 2 shearwalls Rigid Diaphragm load is distributed to shear walls by wall stiffness Diaphragm load is distributed to shear walls by wall stiffness Torsion considered in design Provides more flexibility for shearwall layout More complicated analysis
Prescribed Rigid Wood Diaphragms (IBC 2305.2.5) Prescribed Rigid Wood Diaphragms (IBC 2305.2.5) Open front and Cantilevered diaphragms Torsion Effects Shear Transfer from Roof Diaphragm - to Shearwall
Force Transfer from Diaphragm to Shearwall Overturning of Shearwalls Roof Framing F G B C Shearwall Hold-Down Anchors Holddown Anchor
Holddown Anchor Holddown Anchor Low-slip fasteners Pre-deformed base A plus in seismic loading Multi-story apps. Self-tightening A plus in taller structures Timbers at Base of Holddown Overturning Forces Timbers carry Compression forces at holddown 4x6 through 8x8 size Timbers on first level of 5-story wood 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 Minimum Aspect Ratios w = h/3.5 for wind w = h/2 for seismic exception: 3.5:1 can be used with penalty (2w/h) Specific stud species Shear Wall Capacity Based on: APA wood structural panels of specific grade and thickness h w Hold-down anchors anchor bolts Specific nail size and spacing requirements How about composite action using Adhesives? 40% Increase for Wind Capacity - 2006 IBC 2306.3.2 and 2306.4.1 Theoretically possible! True composite action requires rigid structural adhesives. (Construction adhesives creep under load and do not give 100% composite action.) Structural adhesives are very hard-to-impossible to do successfully in the field. The current shear wall and diaphragm tables are based on a 2.8 min. safety factor Code authors agreed that a 2.0 safety factor is Code authors agreed that a 2.0 safety factor is adequate due to confidence in wind load accuracy - thus a 40% increase in tabulated values
Summing Shear Capacities of Dissimilar Materials Section 2305.3.8 For wind design, adding the gypsum capacity on the inside face of wall is allowed Shear Walls: Wind v. Seismic Wind Design: 40% increased capacity Gypsum strength can be added 3.5:1 max. aspect ratio Seismic Design: Requires 3x framing more often (SDC D-F) 2:1 max. aspect ratio without penalty 3.5:1 permitted with penalty (2w/h) Shear Walls: Wind v. Seismic Given: 7/16 OSB 8d common 3 / 6 edge/field nail spacing Gypsum on opposite face V 225 2.25 H v H 8 Shear Walls: Wind v. Seismic Wind Capacity: V=(450 plf x 1.4+100 plf) x 2.25 = 1640 lb Length of wall For gypsum from table 40% increase for wind WSP capacity from table Seismic Capacity: V=450 plf x 2(2.25 )/8 x 2.25 = 570 lb When less than 2:1 aspect ratio, 2w/h adjustment
Site-Built Portal Frame Bracing Methods Sturd-I-Frame with Hold Downs Reference: APA Report TT-100 Overdriven Fasteners Overdriven Fasteners APA Recommendations If < 20% fasteners overdriven by <1/8", then they may be ignored. If > 20% fasteners overdriven by >1/8", then: Prescriptive add 1 additional fastener for every 2 overdriven Mechanics based re-analyze capacity based on average thickness of panel measured from the bottom of the nail head. (5/8" panel with fasteners overdriven by 1/8" = capacity of 1/2" panel.)
Reference Publication Reducing Hold-Down Anchorage APA Technical Topics: Power-Driven Fastener Considerations, (Form TT-056, 1 page) Discusses some common issues encountered when using power-driven di fasteners. Available only as a downloadable pdf file. Segmented Shearwalls Continuous Shearwalls Shearwall Design Methods Segmented 1. Aspect Ratio for seismic 2:1 2. Aspect ratio up to 3.5:1, if allowable shear is reduced d by 2w/h Force Transfer 1. Code does not provide guidance for this method 2. Different approaches using rational analysis are used Perforated 1. Code provides specific requirements 2. The capacity is determined based on empirical equations and tables IBC 2305.3.3 3 3 IBC2305381 2305.3.8.1 IBC2305382 2305.3.8.2 Segmented (Traditional) Wood Shear Walls (IBC 2305.3) V Only full height segments are considered Max aspect ratio 2:1 for seismic 3.5:1 for wind Current Code design v values based on data v dating back to 1950 s. H H H H Aspect ratio applies to full height segment (dotted)
Shear Wall With Openings Force Transfer Around Openings V Shear around openings accounted for by strapping or framing based on a rational analysis H/w ratio defined by wall pier (IBC 2305.3.7.1) Wall Pier v H H Aspect ratio applies to wall pier segment (dotted) Shear Wall With Opening PerforatedShear Wall (IBC 2305.3.7.2) Openings accounted for by empirical adjustment factor Hold-downs only at ends Uplift between hold downs, t, at full height segments is required Limited to 490 plf X 980 plf Seismic 1370 plf - Wind V H t v H Aspect ratio applies to full height segment (dotted) Shear Capacity Adjustment Shear Capacity Adjustment Equation for Perforated Shearwalls
Shear Capacity Adjustment Suggested References Equation for Perforated Shearwalls Performance Based Seismic Design NEESWood Capstone Tests PBSD offers owners/operators the opportunity to limit business interruption, economic loss, and other consequences for less severe but more probable earthquake hazards Necessitates accurate modeling of structures under seismic loading which for wood light-frame structures, this is complex because the load path is not as discrete
APA Publications and Website For free downloads go to www.apawood.org org and enter the Publications Store 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 A410 Roof retrofitting for wind uplift D485 Corrosion resistant fasteners 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 en 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? This concludes The American Institute of Architects Continuing Education Systems Course Scott Lockyear scott@woodworks.org Bryan Readling bryan.readling@apawood.org Wood Products Council 866.966.3448 info@woodworks.org