2012 Wood Frame Construction Manual€¦ · · 2015-07-172012 Wood Frame Construction Manual:...

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

Copyright © 2013 American Wood Council

2Copyright © 2013 American Wood Council

Copyright Materials

This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written permission of the

speaker is prohibited.

© American Wood Council 2013


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Learning Objectives


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


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


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




140

130

150

140

140

130

110

120130150

110

110

Comparison of ASCE 7‐10/√1.6 vs. ASCE 7‐05

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Wind Speeds at Selected Locations


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


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


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)


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


Pressure at Stagnation Point from Bernoulli’s equation, using a standard atmosphere for density =

0.00256 V2

Wind Flow Around Building



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



17

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


Source: APA


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Source: APACopyright © 2013 American Wood Council

20

Base Shear (Sliding)


Source: APA


21Source: APA


22

Racking


Source: APA


23


24

Overturning


Source: APA


25


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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



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



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


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



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


Used with MWFRS procedure in ASCE 7 and for WFCM



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

34

Mean Roof Height



35

Exposure Categories


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



37

Exposure C



External Pressure Coefficients

ACSE 7-10 Figure 28.4-1



IBHS – wind tunnel tests

http://www.disastersafety.org/video/videos‐research‐center/

Wind Effects on Buildings

40

IBHS Wind Tunnel Test Results



41

Example


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)


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


44

Sum Moments to Determine Uplift Load


Tension (connector load) = 122 lbs

20 ft

33 ft

Tension


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WFCM Roof to Wall Connection



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


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


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



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


Walls Roofs


Questions?

www.awc.org

[email protected]


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THANK YOU!Follow up email with:• SurveyMonkey, presentation links and info. on

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

2012 Wood Frame Construction Manual€¦ · · 2015-07-172012 Wood Frame Construction Manual:...

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