2012 wood frame construction manualaug 01, 2013 · hyannis, ma 117 112 122 new port, ri 117 109...
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Copyright © 2013 American Wood Council 1
2012 Wood Frame Construction Manual:Wind Speed and Design Pressure
Determination According to ASCE 7‐10
Presented by:
William L. Coulbourne, PE
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Learning Objectives
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At the end of this program, participants will:
Be able to determine site‐specific wind speeds using ASCE 7‐10
Understand how wind speeds are used for calculating Main Wind Force Resisting System (MWFRS) and Components and Cladding (C&C) loads
Understand how to convert from ASCE 7‐10 back to ASCE 7‐05 wind speeds
Understand how to develop loads from wind speeds
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WFCM
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Basis for this webinar series is 2012 Wood Frame Construction Manual (WFCM)
Basis follows WFCM Prescriptive Provisions (Chapter 3).
Prescriptive provisions are provided for:
Connections
Floor systems
Wall systems
Roof systems
Provisions provide construction details and load tables
WFCM also has engineering design in Chapter 2
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WFCM and IBC
Chapter 16 – Wind Loads Section of IBC
Indicates wind loads are to be determined in accordance with ASCE 7
Exception is residential structures can be designed using the provisions of the WFCM
WFCM can not be used for design of structures located on hills, ridges or escarpments
Chapter 23 – Wood design
Significant coverage of wind design using wood
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WFCM Prescriptive Parameters
Exposure B or C
Mean roof height does not exceed 33 ft.
3 stories
Length and/or width of building < 80 ft.
Joist and rafter span 26 ft.
Loadbearing wall height 10 ft.
Joist, wall stud, rafter spacing max 24 in.
Limitations on shear wall offsets
Use of ASD level wind pressures
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ASCE 7‐10 Wind Speed Maps
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Speeds are for ultimate event
Maps for 3 Risk Categories (I, II, III and IV)
Wind Speed metrics are: 3‐sec peak gust 33 ft (10 m) above ground
Exposure C
Importance Factor is now included in the speeds shown on the maps
www.atcouncil.org/windspeed
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700 Year RP Winds
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140
130
150
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140
130
110
120130150
110
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Comparison of ASCE 7‐10/√1.6 vs. ASCE 7‐05
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Wind Speeds at Selected Locations
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Location 6.1/700V
ASCE 7-05 Exposure C
Exposure C Exposure D Bar Harbor, Maine 97 95 103 Boston, MA 106 103 112 Hyannis, MA 117 112 122 New Port, RI 117 109 119 Southampton, NY 120 110 119 Atlantic City, NJ 114 102 111 Wrightsville Beach, NC 132 119 129 Folly Beach, SC 131 115 125 Miami Beach 145 136 148 Clearwater, FL 128 115 125 Panama City, FL 129 107 116 Biloxi, MS 138 129 140 Galveston, TX 131 119 129 Port Aransas, TX 134 117 127 Hawaii 105 103 112 Guam 170 155 168
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Finding Your Windspeed
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Users should consult with local building officials to determine if there are community-specific wind speed requirements that govern.
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Strength Design Load Combinations
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Wind load factor changed in 2010 Edition:
Old: LF = 1.6
New: Load factor from 1.6 to 1.0; load factor is built
into the MRI for the maps
For ASD design, new load factor is 0.63 (actually it is
0.6), reduced from 1.0
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Converting from old to new (or vice versa)
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ASCE 7‐10 wind speed/√1.6 = ASCE 7‐05 wind speed
ASCE 7‐10 wind pressures*0.6 = ASD wind pressures
Note = an exact equivalent ASD reduction factor = 0.625
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Pressure at Stagnation Point from Bernoulli’s equation, using a standard atmosphere for density =
0.00256 V2
Wind Flow Around Building
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Greater separation angle = greater void between surface & windstream.
Greater void = higher suction (negative pressure).
Increasing roof angle decreases void, thus lowering suction.
At roof angle = separation angle, pressure becomes positive.
Flow Separations
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Wind Forces
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Wind Actions on BuildingsUplift
Roof only
Entire building
Lateral loads (base shear)
Connection between building and foundation
Racking
Pushing building over at the top
Overturning
Pushing building over when connection to foundation fails
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Wind Uplift
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Source: APA
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Source: APACopyright © 2013 American Wood Council
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Base Shear (Sliding)
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Source: APA
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21Source: APA
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Racking
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Source: APA
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Source: APACopyright © 2013 American Wood Council
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Overturning
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Source: APA
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Source: APACopyright © 2013 American Wood Council
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Load Path Through Building
Wind pressure is collected by walls and roof
Pressure is distributed into “diaphragms” at roof and floor levels
Diaphragms take loads into shear walls
Shear walls must be stiff enough to not “rack” and take loads into foundation
Shear walls must be tied down to resist overturning
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Developing Wind Design Pressures
Developing pressures for wind design requires combining:
Meteorological aspects of wind
• Speed
• Turbulence
Interaction of wind with terrain
Aerodynamics
• Interaction of wind with building
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p = Wind Pressure
q = Velocity Pressure (Atmospheric Effects).
G = Gust Effect Factor (Atmospheric & Aerodynamic Effects).
Cp = Pressure Coefficient / Shape Factor (Aerodynamic Effects).
p = q * G * Cp
Basic Wind Equation
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ASCE 7 adds two more factors:
Topographic Factor ‐ Kzt• Hills and Escarpments
Directionality Factor ‐ Kd• 0.85 for all building structures
q = (.00256 V2) KzKztKd
Velocity Pressure
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For buildings with External and Internal Pressure:
qi = Velocity pressure calculated for internal pressure, usually at mean roof height h
GCpi = Internal Pressure Coefficient (+/‐ 0.18 for enclosed conditions)
ASCE 7 calls this Directional Procedure (All Heights)
p = qGCp – qi(GCpi) Eq. 27.4-1
ASCE 7 Basic Wind Equation
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where: qh = velocity pressure at mean roof height h GCpf = external pressure coefficient GCpi = internal pressure coefficient
p = qh[(GCpf) – (GCpi)] Eq. 28.4-1
MWFRS Procedure used in WFCM
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MWFRS Load Case A – ASCE 7
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Used with MWFRS procedure in ASCE 7 and for WFCM
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Site – determine wind speed and exposure
Design based on most extreme exposure expected
Find q (velocity pressure) for variety of windward heights and for h
Determine p (wind pressure) for all surfaces for both + and – internal pressure
Wind pressures act normal to surfaces
Design with the most restrictive pressures
Process for Applying Loads
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Mean Roof Height
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Exposure Categories
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B Suburban, use as DEFAULT unless others apply >60% to 80% of all buildings are in this category
C Open country, 1500 ft creates this category
D Water, including on hurricane coast!
It’s about Flow Characteristics vs. Surface Roughness
Change in ASCE 7-10
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Exposure BSuburban
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Exposure C
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External Pressure Coefficients
ACSE 7-10 Figure 28.4-1
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IBHS – wind tunnel tests
http://www.disastersafety.org/video/videos‐research‐center/
Wind Effects on Buildings
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IBHS Wind Tunnel Test Results
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Example
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For h = 33 ft tall building, 40 ft (windward face) x 20 ft in plan, find:
Roof to wall connection load
Load taken into shear walls on ends of house
Wind speed = 140 mph
Exposure B condition
5:12 roof slope (200 is taken as worst case)
GCpi = +/‐ 0.18 (enclosed condition)
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h Kz V q ASD q GCp wind +Gcpi ‐Gcpi
33 0.72 140 30.7 18.4 ‐0.69 ‐16.0 ‐9.4
GCp lee +GCpi ‐GCpi
33 0.72 140 30.7 18.4 ‐0.48 ‐12.1 ‐5.5
Calculated Roof Pressures
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Converting Pressure to Loads
Wind pressures determined for roofs and walls must be converted to loads
Pressure x tributary area = loads
Loads may be reduced at points in the structure because weight is providing resistance
Correct distribution of the loads is key to accurate design
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Sum Moments to Determine Uplift Load
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Tension (connector load) = 122 lbs
20 ft
33 ft
Tension
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WFCM Roof to Wall Connection
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Using roof pressures from calculation procedure (see Slide 41)
For 20 ft. roof span, connector load is determined by summing moments about one wall/roof joint. Result = 214 lb
Reduce for dead load of roof system: WFCM uses 9 psf as reduction for dead load (90 lb at each wall)
WFCM result = 165 x 0.75 reduction = 124 lb(reduction allowed when 8 ft away from roof edge)
Roof‐Wall Connector Load
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General Lateral Load Path
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MKB10
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Calculated Lateral Pressures
Horizontal roof load distributed to shear walls
Wall pressures distributed to shear walls (windward + leeward)
Total shear wall load distributed along the wall to foundation connection
WFCM result = 218 plf x L/W (40/20) = 436 plf
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Slide 47
MKB10 It seems like this slide could be used in conjunction with slide 21.Michelle Kam-Biron, 8/1/2013
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WFCM ‐Sill Plate to Foundation Connection
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Low‐rise buildings with h ≤ 60 ft. based on Envelope Procedure
Buildings with h ≥ 60 ft. based on Directional Procedure
p = qh[(GCp) – (GCpi)] Eq. 30.4-1
p = q(GCp) – qi(GCpi) Eq. 30.6-1
C&C Pressure Equations
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Components & Cladding
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Walls Roofs
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Questions?
www.awc.org
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Certificates
Instructor: William L. Coulbourne, PE• Sept. 4th 2012 WFCM: Wind Speed and Design Pressure
Determination According to ASCE 7‐10• Sept. 11th 2012 WFCM: Wind Load Distribution on
Buildings – Load Paths • Sept. 18th 2012 WFCM: Connections• Sept. 25th 2012 WFCM: Foundation Design to Resist
Flood Loads and WFCM Calculated Wind Loads• NEW! Nov. 21st Prescriptive Residential Wood Deck
Construction Guide (DCA 6)• NEW! Jan. 16th AWC’s Code Conforming Wood Design
• http://www.awc.org
Copyright © 2013 American Wood Council www.awc.org