DETERMINATION OF WIND LOADS ASCE 7-05.pdf

7/21/2019 DETERMINATION OF WIND LOADS ASCE 7-05.pdf

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DETERMINATION OF WIND

LOADS

2009 International uilding

COOs

(IBC) wind

proVisions found

in IBC section

609

have adopted

the provisions

of

ASCE

7-05

MInimum eSJgn Loads

for

and

Other

Structures by

reference. These provisions,

which

appear in

6

of

ASCE

7,

can be

difficolt

to understand, This Codemaster provides

instructions explaming how

to determine

the deslgn wind loads First,

1 through 4 address how

to determine

charactenslics

about

the

slNcture's

and configuration

that direcl1y affect the magnitude of wind loads.

Next

5 addresses

determination

of Importance Factor which is

a

step

common

to

wind

design methods. Next, Step 6 clearly

outlines

the

different methods

that

be

used to

determine

the design wind

loads lor

the structure. Finally, a step

-step format

is

presented to

provide

a

clear explanabon for

the

three most

used design methods

location

V (mph)

location

V (mph)

Hawaii

105

Virgin Islands

145

Puerto Rico

145

American Samoa

125

Guam

170

Notes

1,

Unear interpolation between wind contours is permitted.

2. Islands

and

costal areas outslde

the

last contour shall

use

the last

wind

speed

contour of the costal

area

3. MoontalflOUS

terrain.

gorges,

ocean

promontories

and

special

wtnd regions

shaH be examined for

unusual WInd

conditions.

STEP: 2 DETERMINE EXPOSURE CATEGORY

8

C OR

Tel:

(847) 991-2700

Fax (847) 991-2702

[email protected]

Copyr ight 0 2009 by SKGA

DESCRIPTION

Surface Roughness

B

is

dlaractenzed by

urban and suburban

areas, wooded areas.

or

other terrain with numerous doseIy

spaced obstructions

haVIng

the size of single-family dwellings or

larger, Use of Exposure Category

B

is

limited

to those areas lor

which

tarratn

representative

of

Surface Roughness

B

prevails

in

the upwind directlon

for a distance

of at

least 2600 feet

or 20

limes the height

of the

building or

other

structure, whichevef is

greater,l

Exposure C

applies for all cases where Exposure

B

or Ddoes

,_

Surface Roughness

0

is characterized by flat,

unobstructed

areas and watll ' surfaces outside

hurricane

prone regionS

including smooth mud flats. salt flats,

and

unbrolo:en ice.

Exposure

0 applies where

Surface Roughness

0 prevails

in the

upwind direction fOf

a distance

greatll '

than 5000 fI

or 20

times

the

building height, whichevef

is greater. Exposure 0

extends

into downwind

areas

of Surface Roughness BOf C lor a distance

of

600

fI or

20

limes

the

height of

the

building

or structure,

whichever is greater

CodeMaster

developed

by:

CIII _

A subsidiary 01

SK

Ghosh Associates Inc.

www.skghoshassoclates.CO<T\

ISBN 978-1-936039--01_2

c

B

o

EXP.

For buildings whose mean rooftlelQhlllles

than

or equal

klJO ll.

the upwn:

dislance

may

be reduced tp

1500 tl.

Three Exposure Categones (B, C, and 0) are

defined

in terms

of

the extent and

types

of

Surface Roughness

that

are Upwind of the site Surface Roughness

Categories B,

C, and 0

are

a

classification system

established

to reflect

the

;

characteristics

of ground

surface irregularities.

The more obstrucbons (e.g. trees,

structures,

fences.

etc.)

there

are

on

the

site

upwind

of

the

building,

the

more

the

effects

of

WInd forces are reduced due

to

friction. The Exposure Category (B, C, or

0)

needs to

be determined

based on

the Surface

Roughness Category (IBC

section 1609.4.2,

ASCE

7Section 6.5.6.2),

l Iflidl

is

a function

of the topography,

vegetation, and constructed

lad lties.

and the

Upwind distance

oyer

which

the

Surface Roughness

prevails

The

following

table

may be

used to

determine

the

Exposure Category lor the

site (IBC

Section 1609,4, ASCE 7 Section 6.5.6.3).

The term mean

roof

heighr in

ASCE

7Section 6.2 is

defined

as the average

of

the

roof v height and the height

to

the highest poinl

on

the roof surface, except

for

roof angles of less than or equal 1010 degrees the mean roof height

is

equal to

the

roof

eave height Eave

height

is defined

in

ASCE

7

Section

6.2.

'

''''

'

14 14 15

•

western GuWOf

Mexico

I

Mid and .Northern

AtIanlic

M

; ; ; : ; ; : : : : : : : = : : : = = : : = ~ : U : ~ E I dSlates

; ~ =

DETERMINE BASICWINO

SPEED

(3·5ECONO GUST)

Figure 1609 (ASCE 7 Figure

6-1) shown

below presents basic wind

speeds for

contiguous United

States,

Alaska, Hawaii and other U.S territories. The basic

speeds

shown

reflect

the

peak gust Wind speed recorded within

an

averaging

of approximately 3

seconds at 33

feet

above ground lor Exposure Category

C

Step 2).

The

special

wind

regionS (as shown by

gray

shading

on

the

map)

are

by

IBC section

1609.3

(ASCE 7 Section 6.5.4) to have

their basic

wind

detennined

in

accordance With

the local jUrisdiction requirements and ASCE

section

6.54

Even if

not

located in

a

special wind region, it is a good idea

to

basic wind speed with the

local

jurisdicbon in which

the

structure

wiJI

be



;he wind

design

of a building typicany involves two

aspects· design

of

(1)

the Main

I VInd

Fon; -RBSiSlmg

System (MWFRS) and

(2llhe

Components

and

Cladding

C

C).

Mam WIIld F o t c s R e ~ n g

System

(MWFRS) An assemblage of slructural

elements

aSStgled

10 provide

support

and stabilityfor

the

overan structure, The

system

geneTaUy

receIVeS

wlIld Ioalfmg from more han

one sorface

• Components

end

CJaddmg

(C C): Elements of he buldlng envelope

thaI

do

:)

qualify

as

part of

the MWFRS

Examples

of

components are fasteners, studs,

Jrtins,

and

glrts

and

examples

01 cladding are

wall

and

roof

ooveOngs. curtain

IS

and 0V9fhead doots

NOTE: The

minimum horizontalwind

Jnssure

for

the MWFRS

of,n enclosed

Of partially encloud building 15

10

pst acting

on

the ,rea of the building

rojlCted onlo a vertical

plane normal

to

Ihe assumed wind

direction In

iCcordance with ASCE 7Stctlon 6.1. The minimum design windpressure for

C

C 15

10

acting In eitherdirection normal to the surfac•.

Glazing in

Occupancy Cat89OfY

II,

IH, Of IV buildings located In wind-borne debris

Ions needs to be Plotected as Impact resistant

acconling

to ASCE 7

• 6.5.9.3 over 1)

lhe first

60

ft

height above the

ground and 2) the

first

,0

It above

aggregate surfaceroofs locatedwithIn 1500 ft

of the bulldlng.

:Hustration

of

MWFRS

and

C

C

,

•

,

-

,

•

,

,

,»:

.

,

. 12

,e

..,.

STEP

2 DETERMINE T

OPOGRAHIC

FACTOR

If

there are 1 10 abrupt changes in

the

topography and

the

structure

is

on le'l8l

ground

K.J =

1.0.O\heIwise, determme K.J

=

(1+ / 1 / 2 ~ ) l N h e r e K /(2,

and

are

sellorth InASCE 7

Flg\lre

(pages 45-46).

STEP 3 DETERMINE Ps30 FOR

MWFRS

UseASCE

7

FlQure

6-2

(pages

38-39),

Based on

the

basic WInd speed and the

roof angle,

delern1ine

which IS the Slmpflfied deSIgn 'Mnd

pnlssure

for

Exposure e,

=

30

: and

/

=

1

,0,

Notethat load

Case

1

cooesponds to poSItive

internal pressure and

Load

case

2

10

negauve Internal

pressure The design of the

roof

members or

the MWFRS

IS

Influenced

by

intemal

pressure However.

for

SIITIpIe diaphragm

bUJIcIngsWith

roof

angles

less

Ittan 25

it

can be assumed

thai

the

maXImumUplift. produced by

a

po5Itive

inlemal

pressure. IS

the

mntrolkng

load

case From 25 to 45 both posiove 800 negatIVe internalpressure cases (load

Cases

1

and 2. respectIVely)

need

to bechecked for

the roof,

because heexternal

pressore on

the

W1ndwart roof beoomes positive beyond

a

roof ang le of

appIOXlmalely

25-

-=

).

K

/

plJIJ ASCE

7

Equabon (6-1)

Simplified

design

WInd

pressure.

P., for the MWFRS of Iow-ose

Simple

d I a p h ~

bu\ldIflgS rapresent the

I'Illt

pressures (sum

of

ellltemal and intemaJ) 10

be

applied

10

the

honzonlal and vertical proteebonsof b o i l d l ~

sorta<:es

as

shawn below and

In

Figure 6-2 (page

37),

It IS calculated accxwdance WIth ASCE

7

Secbon

6.4.2.1 For

the honzontal

pressure zooes (Zones A,

e, C.

and 0), fJf Is the

combInabori

of

the 'NlrIdward and

leeward pressures

Notes:

1.

End

zone WIdth =

28,

8 =

10% of least-horiZontat

dimenSIOn or O.4h,

whichever

is

smaller,

bul not less

than

either 4%

of

leasl horizontal

dimension

or

3

II.

2,

For the design of the MWFRS III the longitudinal direction of wioo, use

a=O·

and

locate \he Zone ElF, GIH boundaries at mid-leoglh of the

Idlng

3, load

Case 1and 2

are tobe c:hecked lor

25-

<e<

45 .

Load Case

2

at 25-

is

provided

only

for

interpolalJon

purposes.

4

IIIhe

total

horizontal

loads

on Zones

Band 0 are

negallve, they are to

be

tall:en

aqualla zero.

Note

that

the

load

pattern

shown

in

ASCE

7 Figure

6-2 (page 37) Is

reqwred

to be

applied to each

comer

of

the building In tum as the

reference

comer -

See

ASCE

7 FlQUre

6-10 (page 53). However.

if

a

buiIdiog

IS

doubly

symmetrical, no new Il1foonalion

wiD be produced

as

the building

is

turned 10 make 8 new comer he reference c om er I f a building

Is

syrrmebical about one

axis only,

one 90- bJm

of

lt1e building wi produce

new

InformaIJon

that

needs

to be oooSldered In

design

STEP 5 - DETERMINE PnefJO FOR C C

Use ASCE

7

FlQure 6-3 (pages 42-44) to determme PtrIt» Based on

the

basic wmd speed, tie roof angle. and the effective

area

ofthe component

determine

P d

which

Is

the

net

design

Wind

pressure

fOt B,

=30 11:./ = 1.0, and /(a = 1.0.

The

pressures areglYen

for

zones 1-3 (roof)

and Zones 4 and

5 waJlS)1n1O

wtlICh the SlJrface area of the buildrng Is

d. Ided. as shown Kl

Figure 6-3 (page

41).

DETERMINE NET DESIGN WIND PRESSURE

PnfttFORC C

I

,

,

,

,

,

Flal Roof

•

<

1

and

takes into

account the

reduced

probability

of

maximum Winds

coming

from

any given

direction.

II should be noled that

the

corresponding

wind load factor

(ASCE 7 Sections 2.3

and

2.4) should be

consistent

with the

selection Of K.,. Refer

toASCE 7

Commentary to

Chapter 2 for

further

delails.

How?

ASCE

7 Table 6-4

(page 80)

Wind

Oirectionality

Factor,

(6.5.4.4)

Gable Roof ( 7 < e 45°

STEP-Sy·STEP

PROCEDURE

FOR METHOO 2

AN -vnC -

PROCEOURE

(ASCE

7

SECTION

6.51

Determine

What?

(ASCE 7 SectIon)

Noles'

1. a

=

10%

of the Ieasl honzontal

dimension

or

OAh,

whicl1ever Is smaller,

but

notlessthan

either 4% of

leasthorizontal dimension or

3 ft

2. For hip roofs with e 25°, Zone 3 is to

be

treated as Zooe 2

Note: Steps

1-6 at beginning

of this CodeMaster

should

be

completed

befool proceeding with

the

steps beIcrti

ASCE

7

Equallorl (6-2)

,

,

•

:

·,

P

= Kid IPtdl

.,

,

·,

,

Net design wind pnmure.

P/III

for

the C C of bUildings deSigned using

Method

1 represents

the net

pressures

(sum of

external

and internal) to be

applied normal to each

bUilding

surface as

shown

in ASCE 7 Ffgure 6-3

(page 41) and as ShOWn

below

It IS calculated in ICCOfdance

with ASCE

7

SecIJon 6,4 2.2

I

•

,

H

1Transve

....

)

MWFRSonc- .

-

-

I I r

, ,r

1

Ii

STEP·8y.STEP PROCEDURE

FOR

ASCE 7

METHOO 1:

S,MPLIFIED PROCEOURE (ASCE 7 SEcr , 6.4)

Note steps

1-6

at

beglnnJrlQ of thJs CodeMasler should be compleled before

p r o c e e d l ~ WIth

the

steps below

ThIs SImplified method

IS applk:able to buildHlgs satisfying

all of the

conch\iolls set

forth

In

ASCE

7Secboo

6,4,1. These condlbons

IocllJI1lllhe

following:

•

The

boiIdlng Is ~ n s e smple diaphragm,

and

reguIar-shaped as defned

lt1 ASCE 7

section6.2.

• The

butldlng Is eodosed as

defined

In ASCE

7

Secboo 6.2 rKl conforms

to

the

WlflO.bom8 debris prDVISlOns of

ASCE

7

Sectton

6.5.9.3.

• The

buildingis no a I\eXlble

building as defined

III

ASCE

7 Section

6.2

• The

bUIlding does rlOl have response

c:haracteoslics

making

H

subje<;t to

across Wind IoadIllQ

or

0Ihet assocaated ph.enomena and does not have

a

Site

kx:allon where

upwind

obslructlons

may

wammt

special ronsIderalJon.

•

The buBdlng has an

approximaleJy symmetrical cross section

In each

directlon

with

either a flat

roof or 8

gable or

hlp roof

With

slopes

no

exceeOr1Q

45-.

• The building is exempted from

mionel

load cases

as

iodicated

In

ASCE

7,

FlOUre

6-10,

Note 5 or these torsional load

cases

do

not conlrollhe

design

of any of the MWFRSs (rTl8alllll J the building

is

not

sobfed to

Sllfnfficanl

tmlon).

OelllflTllllEllhe

Height 8nd

Expowre

Al1Juslment

Coebnt

;., In

accordance with

ASCE

7

FlQure 6-2 (page 40).

The tabUlated

Wind

pnlssures

are based on

Exposure

B al

30 II.

heighl The Helghf and

Exposul8

Adjustment Coebnl )..

lakesInto accoont other exposure and helght condi\lOlls.

rST 1 DETERMINE HEIGHT AND EXPOSURE

• _ AOJUSTMENT COEFFICIENT A





This worillS will h1lt'd1llllrldio'lg tIat

SKGA.

SCI ICCa ld Ihs IUthor1 n

IltformatlOll

bul

are not

antrnptlng 10

render tnglllCOnllQ or olhef prof8lllONll

1lIr'oUS. If such

servICea

we 18q1JQd. thIauilianal

of

quIllfltd profeuiDnall

ahooId

be.:lUght SKGA, SCI. ICC and ths IU#lors DISClAIM

IOY..aII

RESPQNSIBIUTY

and lIABILOY

b lCClAC)' of n

tie .......1d

Iht InfOrmIIon

CCXlIIIned in

Ihd

pOOIicIlJon

the fl j

extent

pemIIIad by il l

I8w

The

siu::lule cannot ha e fInj 01 ..

fypeS 01

roofs

. -

....

.-

-

WI1roofsk:Jpe

)

45

d g

The

sInX:tlru cannot r 01 \lit fOOMng

•

Solid

fre&.starldIng

wa s

LlfMat.onson type of

• SoIIIIlgIIS IlnItlUrtI

._-

S T E P

3

DETERMINETOPOGRAPHIC

FACTOR

K l

STEP

4

DETERMINE

NET

PRESSURE COefFICIENT e

ne

fOR M W F R S

AND

C & C

Determine veIodly pre5S1Jre exposure ooe1'rtcienlS. III

accordance With

ASCE 1

5ection

6.566. Nolt flat the sublaipt Z-II lor

any

helghl abOVe

ground

level and he

subsctlpt

of(' II

tot the

mean

roof htlght

For tOe

windward wall of

a structure.

K..IS used, Forleeward wall

and

SldttwaIII,

and

for

\ 1 f\dWard and

K,Is . . . .

Based

on he baslc wn:I speed.

V

. . ......nd

1LIQRa\lOn

pteIaIA.q.

from

IBC

Table 16096.2 PI

S T E P 5 DETERMINE DESIGN WINO PRESSURE p P fOR

MWFRS AND C & C

S T E P

2 DETERMINE VELOCITY PRESSURE EKPOSURE

COEFFICIENT K

z

K

c...::; net preatn ' bM Id on 1G t(;al 'c.

For lie

des9l

rJ

fie

MWFRS

and

C&C .. un oil'll edlrnallI'ld net

f t

based on

.. net

pt - . .a coeftIcieo'l(

c... The IftSSUIt o:JdoInI.

c.... lor walsand roofs ildlIIiIl .lId ttw n 18C

T8bIt 1609

6.2 (2) 'MIn c..may

M¥fI 'In

flan one

YaIIJe.

ltlI

rm'e

. . . . . . . WI Id laid

condilal

u.-t.,

deIign

STEP 1

DETERMINE WINO STAGNATION PRESSURE q,

If '* n rJJ 8ttl.4lt dmges tllIe k 4 X 9 ~ I) WId tIte IN:lft 1\ on M

K . c l O . O h l r w I s e ~ . n e K -r1.,cI< K. whereK

1

¥ldl(

dr, •.

1CM t1

ASCE 7

FIgUte 6-4

(pagel4S--401

p

'

q.K, C.dl

K J I1 C

1&.3'

WInd

pteSSlJrBS

iQ iIAIIed on. end 1\ a cIrecIon

I'CInTlII

m.

II

I>lting..... . .. . ,«d

_ from

11 C_

1609 &

3

. . . ,

_

.. MWfRS '* ' be

. . . . 10 pol

by

iIe

of h i tI Jc tl n

P'Of'C*l

on a pIInt

m

he

8IIUIned

WIld

th::lIon, See ASCE 7Section 6.1 4 b aw .

IBC

SectIon 1609.641 reqUItes consldnllOn ollOftional effects as lndk::ated

In

ASCE

1

FIgUre

69

in the

design

of

he

MWFRS The

exceptlOt'll1

ASCE

7

section 6.5.12.3 permils one-story buIlding,

With h

30 feel to be cleslgned fOt

load

Case

1

and

load

Case

3

only,

load Case 3 requires

15

percenl

of

he wind rxeillJm

on

the Windward and

leeward

waUs ofCase

1 to

ICCOUIlI

for heeffectsdue 10 Vrtnd alOng

he

dl8gOO8t

of

the building.

Oeslgt

wn j

JWesstnlS b C &C csmI be . . . . . 10 PIf DI g In

dncion

normal 10 I1e

arfIoa.

'Mnd

b'.ctt

COlIW•• <r

eIenwlllri

besed

on f1e BIIecIYe WIld

.. .

c:onlIIIed wihn

fie ZO *

I IO wNdl

tie ItJfaoe

area

of

the b u ~ d l O g IS

dl\*lded.

II indiCated

In ASCE

1 figures referenced Itt

IBC ' . . . . '609.62(2).

ASCE

7

5edlon

62 de m 'BUilDING,

SJ'*'l£

DIAPHRAGM- on PI9' 21 as: A buiIdIIg In 'flhdI ball

WIIdwaro

and

IeewW

wind

loacII lit

lhlOllgh lIoor

and

roof iI lV ...

to

.. . . , .

IIWicaI

MWFRS

{It.g

no SIlUCIl.nllCP8lltlorw}

I

A S C ~

7Section

82 oem.. 'BUilDING Of£N CII

Pf9I

21 • AbuIcIn; IfwinQ

eatfI

.. .

II IIaII 80

pert:ent

opIfI. n. oondIlIl:It .. .

b

IK l _

by

ltlI

A.

)

0

SA,

wtoIIt

A. 1IltaI. .

d ClpII'IIfIOIltI' will

.....

1ttCtIYIIlK*Ivt

ulrTWl . . . . . . . in ft· 1m'

A =

III

grt:III

d.1III

flo ' 'tIIflicII A IIIC11nD1sd

••

(m'l

Howo?

(continued)

BI..idngs with h 60

It

p q(GC,,)'qj (GC ,)

Notes on q

• q represents lhe velocity pressure, Which Isthe

ronversion of the basic wind slleltd into

pressure at a specifichelghl

• q ' O . O O 2 5 6 ( K J ( K X K . r ~ V 2 ) ( / ) as defined In

ASCE 7 SectIOn 6.5.10

• For definllions of

q.

qh,

qr, see

ASCE 7

5ecboo 6.3

F:q,GC,A,

wtlere;

C

,

nel force coemctenl from ASCE 7 Figures 6·

21

through 6-

23

(pages 74-76)

I

A, ' projectedarea normal to the

wind

except

where

C,

is specified for

tha

actual lurface

area.

sq,

ft.

F

for

Open

Buildings

And Other

Structures

(6.5.13)

(continued)

p

for Enclosed

lind Partially

Enclosed

Buildings

(6.5.12)

For buidngs the buildllg

needS

b

~ i f y as a

1>

r ASCE

7 5Ialon

e

2 1KJIlOlNG

OR OTHER

ST'RUCl\JRE REGlJLAR-5HAPEO on

21 . A

IbUdng

or

0Nl ' tlNdIft

hlIYt111

flO

~ __ ~ · l ~ ~ ~ ~ _ · _ , , , , , ; ~ = = = = ,

ASCC

1 Sedlon U dIrr.

'B\Jl.OINO

OR OllER

STRUC'Tl..RES RIGlT on

flIDI21

as (II

0IlW1N3n

- . . l d a .. . f r e q l . . c y . ~

_1ilII'_101Hz..

ASCt: 1

CornmIntII,

t8

5.8

P9 I 2lU Illll

JII'OVI6II

'tonnItIon

on

hOw to dteImlinI

li t

01. stn.e:turI

AIIo IBe Sec:tlon

The h8IgIll 0I1 he 1609 6 1 llIrn 1

for

a/I 'II r.d purpclMI 6IftnII1

( ~ 5 1 B l K IQd 1N: l n . one h I

- .

(751e1t 1nIt

•

1 I I d ~ . t l ) .

i ~ ~ l I I t i d I l _ 4

sa.,..ltndlIe,.,

. - : I I

<

..

IleiIt'

7:l

teel

n •

kHUIt wktIllIIb

(..

I

- ,d: lmIII r .: l lIy

QUIIIIeI to I lII

Il t

, MeIlad II1W

..

dlIliIIt

The sln.lctln cannot

be ASlIUCt.n l i l t .

ngdGl ' li t heIflt Ind....

&e\S Vf 10 ~ r t a r r a c ., _ 1m

III:Ii:M •

unIIefy 10

tit

ID

...

= = = ; _ c c .....

ttij;

be

a.. for

d'I

.............

buIlt*tg

JI ..

walle

d

---

PIdIl lXI .U

CI

G C ~

snd GC.

reflect lila

relatIVe

pressures esUmated

to

eXIst

onIhe

eKterlol surfacfl' and

are dep&nClenl upon

Ithe ~ m e t f l c

configuration of lhe

structure including

Ilhe

roof.

These

coefflclents ere nol

applicable to open

buildings

GC

values for C

&

C

alsodependon the

effective WInd area.

which

Is

denned In

ASCE 7

section

5,2

----I

Rigid Buildings-

MW= = --- - -

O

•• l gn W ind (ASCE

7

Sections 6.512.2

1

and

Prollure 65.12.2,2)

All heights: p ' qGC

p

- Q.

(GC

Pt

)

LOIN

'; p ' Q ~ { G C ~ ) - (GC

...»

Ftexl

aulldings-

MWFRS

(ASCE Section 6512 2.3)

P

'

q(

Gt

C,,) - q, (GC

pj

)

I

Parapets

jASCE 7 SectioI16.5.12.2.4)

p' qpGC/Xl

C&C

I

(ASCE 7 Seclion 6.5.124)

Low-rISebuHd'ngs and buildi'lgs with h.5

eo

ft;

p ' qll {GC

a

) - (GC...)J

ASCE 7 F igs 6-11

Ge for

I

hrough 6-17

Components

(pages

55-65)

and Cladding

(6,5.1122)

P

for Enclosed

Ind

PartIally

EncloMd

Buildings

(6.512)

Ge,,/

for

Low

Rise MWFRS

(6,5,1121)

EK1CmJIII

Pressure

Coefficient I

ASCE 7

C, fot MWFRS Figs. 6-6. 6-7,

(65.11.21) and 6-8

Ipages 48-51)

ASCE 7

Fig.

6-10

(pages 53-54)

D.I.rmln.

What? How? Wnll doe. Itmoan?

(ASCE

7 SeclJon)

-- - -

Hllipful Noles:

(continued)

13. SeeASCE

7

Commentary

Section C65.8,

pages 293-294, for

determinationof

I

bUilding period or

its

reciprocal.

building frequency,

(contlOU6d)

4 ASCE

7

(co/llinued)

Commenlaly

Section C62,

pages 281-282.

impHes

thaia

buHding with height

not eKceedlng four

1 t imes the least

horizontaldimension

may

be

considered

rigid.

ASCE 7 Figure 6·5

(page

47)

Foropen buildings.

GCI) renects , ,ow

Intemal

Gel ' 0.00

muchbuild-upof

Pr ur .

pressure

IS

el t ma'ed

C ~ c l . n t .

For partially

10

exf$t Within the

OC.

enclosed buildings.

11n:erlor 01 the build:ng

(6,5111)

GCPf

' +0.55 and

and

Is

dapcndolll

·0,55

upon

the enclosure

For enclosed

I classtflcaUon

buildings. Gel> '

l+O,18 and () 18

2) dynamic

ampUficalion of 'N nd

ror neKible b u ~ d i n g s

G, GdG/for nexlbl.

bulldil l9s)

tak.lnto

accounl the

following

two effects

II

the

direction parallel

10

wind loads

1) wind turbulence

structure Interaction

HelpfulNotes:

1. Where

combined gusl

effectfactors

and

pressure

coefficients

(GCI

GC

pj

•

and GC ,)

are

given in f.gures

lind tables. the

gusteffect

factO\ is

not determined

separately.

2.

In

lieu of the

proceduredefined I

In these sections,

gust effectfactor

may

be

determined

by any rational

procedtJre given in

the

Illcognlzed

Ilteralllre

• For neKible

structures (period

? 1sac.). calculate

G per ASCE 7

Sectlorl6.5.8.2

OUII Effect

Fletor,

G,

Gt

16.'.6)

Topographic

Faclor, Kif

(6572)

Oelermlne

Wh.t? How? What

does

II

me.n?

(ASCE' _Ion,

SeE7 Table 6-3

(page 79) While the bask: wind

It

is

Important 10

speed,

V, represents

notethat the

V,toel1y the w in d s pe e d a t 3 3

subscopt z is lor

Pre•• u.. . any he ightabove fee labove groundfor

Ellposure groundlevel and Exposure Category

C,

Coef'fklentl

(tot ttl sob .

I

-h . II<, Var V

,ach

wind e scnp

IS

represents

he WInd

dlrKtlon), for the m ea n r oof s pe ed a l z or

h

feel

K..

I4t

heigh, Also, the

respectlvelyabo'n

(65.8.6)

velOCIty p r e s s u ~

ground

for the

81lposure co eff ICl en t d . • E

may be

c al cu la te d C7 xposura

In

accordancewith aegory.

Nota 2 toTable6-3.

OnlyappliesIf

K

zt

win Do

grealer

there are abrupt than 1

if

the rr.e

changes in the conditions listed I I

topognlphy. If ASCE 1 SectIOn

Ulef8

areno

abrupt

6,5.7.1

exist.

K

changes

in the

takesInto aCOOUft the

topographyand faCllhal jf a buUdlng

the

structure

is

on sils on the upperhalf

level ground, Kif

= 1.

of an isolated hWI,

If the live conditions ridge, or escarpment,

Il8led In

ASCE

7

I

as a result, the

5ecI1OO

6,5

7 1

bulldlllQ

can

be

eKIIt. then expected to

K

n

(1

+K,

K

1

K:J f

ekperi9l1ce

;and

K

K

2

• and

1<:J

higher

wmd

sPMds

are set r orth l rt thaI Ii 1

ww)

ASeE 7 FlQure 6-4

1

siluatett 'll ItwO l

( pa ge s 45 -4 6) . g rou nd

.

-

• For

rigid

structures (period

<

1

sac.). use

G '

0.85 or calculate

per ASCE 7

Section 6.5.8.1.



STEP.By.STEP

PROCEDURE

FOR THE ALTERNATE

ALL-HEIGHTS

METHOD

(I Be SECTION

1609.6)

L i m i t a ~ o n s

on roof

configuration

(continued)

p

for Enclosed

and

Partially

Enclosed

Buildings

(6.5.12)

F

for Open

Buildings

And

Other

Structures

(6.5.13)

How?

(continued)

Buildings

with h >

60

I.:

p q(GC

p

- qi

GCpi

Notes

on

q

:

• q represents the velocity pressure, which

is

the

conversion of the basic wind

speed

into

pressure at a specific height

•

q O.OO256 K, K

zt

} K

d

Vl} I}

as defined

in

ASCE

7 Section 6.5.10

• For definitions of

q,

qi,

qn,

q see

ASCE

7

Section 6.3

IF=q GC A

Where:

C/ ' net force coefficient from

ASeE

7 Figures 6

21 through 6- 23 (pages 74-76)

A, '

projected area

normal

to the wind except

where C

,

is

specified for the actual surface

alea, sq. ft.

The

structure cannot have any of the types

of roo1s:

• Mullispan

gable

•

Stepped

.

00

• With

roof

sq:.e > 45 dcg

The

sIru::lure ClWlrlOt tle My lhe :::-.;:::= . = ~ - t - I

•

Solid frll&.stanOOg

walls Limitations on type of

•

Solid

S9l5

structure

._-

Detennine velocity

pressure exposure coelficlents, K .

in

accordance

WIth

ASCE 7 Sectton

6.5.6,6,

Note that

the

subscript

z

is

fOf

any

height

above ground

level

and

the

subscript

h

is for the

mean

roof

height.

For the

windward

wall

of

a slNclure,

K is

used. For leeward wall

and

sidewalls.

and for

windward

and

leeward roofs, is

used

L ~ . ~ ' . d l

DETERMINE

TOPOGRAPHIC FACTOR, K

zt

C

=net pressure coefficient based on

[ G X C ~ ) - (GCp,))

For the design of the MYt'FRS and C C, the sum of the external and intemal net

pressures

are

based on the net

pressure coefficient

C .. The pressure

coefficient,

C

,

for

walls

and roofs

is

determined

from IBC

Table

1609,6,2

(2).

'Nhere

C

may

have

more

than

one value,

the

more severe

WInd

klad

condition

is used

in design.

p

=

q. K

C .[I Kzl

IBe Equation

16-34

Wind pressures

are

applied SKTlUItaneously

00,

and in a direction normal to,

aI

building

envelope waf and

roof

surfaces ISlg

the

above equation from IBC section

16096.3 Design wind

forces for

the

MWfRS

camot be less

than

10

psf multiplied

by

the

area

of the

structure proJeCted on

a plane

normal

to the assumed

wind

direction see

ASCE

7

section

61.4

for criIefia.

IBC Secbon 1609,6.4.1

requires

oonslderabon of

torsional

effects as

indicated

In

ASCE 7

Figure

6.9 in

the

design

of

the

MWFRS.

The exception

in ASCE 7

section 6.5.12.3 permits one-story buildings with h.:::: 30 feet

to

be deSigned for

I Load Case 1

and

load

Case 3 only

Load Case

3

requires

75 percent of

the wind

pressures on the WIndward

and

leeward walls of

Case

1 to account

fOf the

effects due

to

Wind along the diagonal

of

the building

Desigl wild presstreS for C

C camot be less than 10 pst acli1g i'I Elllhef cirectia'I

normal

to the surtace. Wnd

pressures

for each amponent

or

cladding

element is

based on the elfedr..oe

wirld

ar9ll

oontamd wrttwlthe zones

into

'Nhich the

surlace

area of the building is divided, as indicated

in

ASCE

7 figures referenced in

~ Tablel609,6.2(2).

Ns'tll'Crl; is

pubIshed

WIlh

the

understaf1ding ltIiI Sl<GA,

SCI. ICC

and the authors

an

supplying llfarmabon but

are not

altempbng to render engineefiog

or

other professional

services

If

suctl

servlC8S

are reqwred.

the

assi5taoce

of qualified

profeSSlOl l8ls

should

be sought.

Sl<GA. SCI. ICC and the authors

DISClAIM

any

and aI RESPONSIBILITY

and LlABlUTYtor the

accuracy

of

and

the

application

of the llformabon contained III this

publicaboo 10 the full

extent

pennilled by the law

If there

are no

changes l

the topogaphy and

the slruclun

is 00

level grcxnl,

K

zl

= 1,0. OlherNise, determioe

K

I

= {1+ K,K:l.KJ f

whem K

K

2

and are set forth

in ASCE 7 FlQUre

6--4

(pages 45-40)

see

ASCE

7

Commentary

C6.6

ASCE

7 Section 6.2

defines BUiLDING, SIMPLE

DIAPHRAGM' on page 21

as:

A building

in

whdl both

windward

and

leeward wind loads

are

transmitted

through floor and

roof diaphragms

10

the

same vertical

MWFRS

(e.g.

no structural separations).

ASCE 7 section 6,2 del\ne$ BUILDING, OPEN

00

page 21 as: A t:M.tilg havilg each WIll

at

least

80

percent open This

rordibon

IS

expressed

lor each WIll

by lhe eq..I8Iion A 08As Where

IA =1oIlII_ r:I opllIWlgS

II

aWIll

that

I8C8MlS

positive

exlllmal

11 l (mlj

=

the

gross

area rt thaI wall I

which A.

is

derJlIfied.

• II' 1m )

For buildings, the buiIdiog

needs

to

quaMy as

a

simple diaphragm buMding.

ASCE

7 Section 6.2

defines BUILOING

OR

OTHER

STRUCTURE, REGULAR-SHAPEO 00 page 21 as: A

building

or

other structure having no unusual

I

geometrical irregulanty

in

s ~ p ~ ; i a l ~ f i l i ~ m ~ . = = = = ~

ASCE

7

Section 6.2 defines BUILDiNG OR OTHER

STRUCTURES. RIGID on

page 21

lIS:

A building

or

St.ructure

reeOs

to be

one

'oII1er

structure

wnos.

fundamental

frequency

is

gl'eater

01

the

following: han or

aqua/to

1Hz

1) Rigid ASCE

7 Commentary C658,

pages 293 and 294,

provides

Information on how

to

determine the

OR

IundamentaI frequency r:Ia

structunl, Also.

IBC

Section

2)

The I'letoht of the

1609,6.1,

Item

1, klf

aI

Wllents and purposes, defines a

structure: ,: 75

feet

ngid

slrUCture as one that has a height::

751eet

and a

and

the helght-lO- height-kHeast \Yldth ratio 4,

So

if the

structure

has a

least

WIdth

ratiO 4

height

75

feet

and a

helght-tlHeast \Yldth rallo

4,

rt

automalK:ally

qualifies

10

use the A1lemate Al-Heights

Method

under tIvs

d1eckliStltem

The s1n.lc1ure c a n o o ~ A,lructure

lhat

is rigid Of meets

the height

and the

s e n s ~ e

to dynamic stendemess l i m ~ above is

unlikely

to be

sensitive

to

effects d y n a ~ m ~ ~ , _ ' , ~ , ~ . _ .

_

The

sln.dure carnJt be

located on

a_

tr

wtidl

- ' ~

lM1g he wake rt

---

peciallXJIlSidel aIO I

Steps 1-6

al

beginning

of

his

CodeMasler should be compleled before

ding with the

steps below

e alternate all-heights method is applicable to

buildings

thaI

satisfy all of

the

set

forth

in IBC

Section 160961

The condltJorlS are:

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