2012 Wood Frame Construction Manual: Copyright Materials Wind Speed and Design Pressure

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

 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

Copyright © 2013 American Wood Council 2

Copyright © 2013 American Wood Council

1

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


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



2

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


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



3

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


700 Year RP Winds



8

4

Comparison of ASCE 7 ‐ 10/ √ 1.6

vs.

ASCE 7 ‐ 05

110

130

140

120

150 130


110

110

140

130

150

140

9

Wind

Speeds

at

Selected

Locations

Location

Bar Harbor, Maine

Boston, MA

Hyannis, MA

New Port, RI

Southampton, NY

Atlantic City, NJ

Wrightsville Beach, NC

Folly Beach, SC

Miami Beach

Clearwater, FL

Panama City, FL

Biloxi, MS

Galveston, TX

Port Aransas, TX

ASCE 7-05

Exposure C

131

145

128

129

138

131

134

97

106

117

117

120

114

132

V

700

Exposure C

95

103

112

109

110

102

119

115

136

115

107

129

119

117

/ 1 .

6

Exposure D

103

112

122

119

119

111

129

125

148

125

116

140

129

127



5

Finding

Your

Windspeed

Users should consult with local building officials to determine if there are community-specific wind speed requirements that govern.


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



6

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


Wind Flow Around Building

13

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

0.00256

V 2



7

Flow

Separations

 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.


Wind

Forces



16

8

Wind

Actions

on

Buildings

 Uplift

 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


Wind

Uplift



Source: APA

18

9


Source: APA

19

Base

Shear

(Sliding)



Source: APA

20

10


Racking

Source: APA

21



Source: APA

22

11


Source: APA

23

Overturning



Source: APA

24

12


Source: APA

25

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



13

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


Basic

Wind

Equation

p = q * G * C p p = Wind Pressure q = Velocity Pressure (Atmospheric Effects).

G = Gust Effect Factor (Atmospheric &

Aerodynamic Effects).

C p

= Pressure Coefficient

(Aerodynamic Effects).

/ Shape Factor



14

Velocity

Pressure

q = (.00256 V 2 ) K z

K zt

K d

 ASCE 7 adds two more factors:

 Topographic Factor ‐ K zt

• Hills and Escarpments

 Directionality Factor ‐ K d

• 0.85

for all building structures


ASCE

7

Basic

Wind

Equation

 For buildings with External and Internal Pressure: p = qGC p

– q i

(GC pi

) Eq. 27.4-1 q i

= Velocity pressure calculated for internal pressure, usually at mean roof height h

GC pi

= Internal Pressure enclosed conditions)

Coefficient (+/ ‐ 0.18

for

ASCE 7 calls this Directional Procedure (All Heights)



15

MWFRS Procedure used in WFCM p = q h

[(GC pf

) – (GC pi

)] Eq. 28.4-1

 where:

 q h h

= velocity pressure at mean roof height

 GC pf

 GC pi

=

=

external internal

pressure pressure

coefficient coefficient


MWFRS

Load

Case

A

– ASCE

7

Used with MWFRS procedure in ASCE 7 and for WFCM



16

Process

for

Applying

Loads

 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


Mean

Roof

Height



34

17

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!

Change in ASCE 7-10

It’s about Flow Characteristics vs. Surface Roughness


Exposure B

Suburban



36

18

Exposure C


External Pressure Coefficients

ACSE 7-10 Figure 28.4-1

37



38

19

Wind

Effects

on

Buildings

 IBHS – wind tunnel tests http://www.disastersafety.org/video/videos ‐ research ‐ center/


IBHS

Wind

Tunnel

Test

Results

39



40

20

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 (20 0 is taken as worst case)

 GC pi

= +/ ‐ 0.18

(enclosed condition)


Calculated Roof Pressures h

33

Kz

0.72

33 0.72

V

140

140 q

30.7

30.7

ASD q GCp wind +Gcpi

18.4

‐ Gcpi

‐ 0.69

‐ 16.0

‐ 9.4

GCp lee +GCpi ‐ GCpi

18.4

‐ 0.48

‐ 12.1

‐ 5.5



21

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


Sum Moments to Determine Uplift Load

33 ft

20 ft

Tension (connector load) = 122 lbs


Tension

44


22

WFCM

Roof

to

Wall

Connection


Roof

‐

Wall

Connector

Load

 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)



45

23

MKB10

General Lateral Load Path


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



47

24

Slide 47

MKB10 It seems like this slide could be used in conjunction with slide 21.

Michelle Kam-Biron, 8/1/2013

WFCM ‐ Sill Plate to Foundation Connection


C&C

Pressure

Equations

 Low ‐ rise buildings with h ≤ 60 ft.

based on

Envelope Procedure p = q h

[(GC p

) – (GC pi

)] Eq. 30.4-1

 Buildings with h ≥ 60 ft.

based on Directional

Procedure p = q(GC p

) – q i

(GC pi

) Eq. 30.6-1



49

25

Components

&

Cladding

Walls Roofs


Questions?

Copyright © 2013 American Wood Council www.awc.org

info@awc.org


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26

THANK YOU!

Follow up email with:

•

SurveyMonkey, presentation links and info. on

Certificates

Instructor: William L. Coulbourne, PE

• Sept.

4 th 2012 WFCM: Wind Speed and Design Pressure

Determination According to ASCE 7 ‐ 10

• Sept.

11 th 2012 WFCM: Wind Load Distribution on

Buildings – Load Paths

• Sept.

18 th 2012 WFCM: Connections

• Sept.

25 th 2012 WFCM: Foundation Design to Resist

Flood Loads and WFCM Calculated Wind Loads

• NEW!

Nov.

21 st Prescriptive Residential Wood Deck

Construction Guide (DCA 6)

•

• NEW!

Jan.

16 th AWC’s Code Conforming Wood Design http://www.awc.org

Copyright © 2013 American Wood Council www.awc.org

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