DETERMINATION OF WIND LOADS
The 2009 International Building COOs (IBC) wind proVisions found in IBC section 1609 have adopted the provisions of ASCE 7-05 , MInimum DeSJgn Loads for BuiJdmgs and Other Structures, by reference. These provisions, which appear in Chapter 6 of ASCE 7, can be difficolt to understand, This Codemaster provides step.by·step instructions explaming how to determine the deslgn wind loads First, Steps 1 through 4 address how to determine charactenslics about the slNcture's location and configuration that direcl1y affect the magnitude of wind loads. Next Step 5 addresses determination of Importance Factor which is a step common to all wind design methods. Next, Step 6 clearly outlines the different methods that may be used to determine the design wind loads lor the structure. Finally, a stepby-step format is presented to provide a clear explanabon for the three most I commonly used design methods
~
DETERMINE BASIC WINO SPEED (3·5ECONO GUST)
IBC Figure 1609 (ASCE 7 Figure 6-1) shown below presents basic wind speeds for the contiguous United States, Alaska, Hawaii and other U.S territories. The basic wind speeds shown reflect the peak gust Wind speed recorded within an averaging time of approximately 3 seconds at 33 feet above ground lor Exposure Category C (see Step 2). The special wind regionS (as shown by gray shading on the map) are required by IBC section 1609.3 (ASCE 7 Section 6.5.4) to have their basic wind speed detennined in accordance With the local jUrisdiction requirements and ASCE 7 section 6.54 Even if not located in a special wind region, it is a good idea to confinnthe basic wind speed with the local jurisdicbon in which the structure wiJI be built.
;;;:;";:::::'::=:::==::=~:U":~EIdSlates
~-------; ~-----=--
'"
''''
'00
location Hawaii Puerto Rico Guam
V (mph)
location
V (mph)
105 145 170
Virgin Islands
145 125
American Samoa
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 D)
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 7 Section 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). EXP.
DESCRIPTION
B
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
c
"',_
o
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 B Of C lor a distance of 600 fI or 20 limes the height of the building or structure, whichever is greater
Exposure C applies for all cases where Exposure B or D does
'For buildings whose mean rooftlelQhlllles$ than or equal klJO ll. the upwn:! dislance may' be reduced tp 1500 tl.
,~
'M
• Mid and .Northern AtIanlic
western GuW Of Mexico I
'''''"
'" '''' ,~
,~
'M
'"
140 140 150
The term "mean roof heighr in ASCE 7 Section 6.2 is defined as the average of the roof eave height and the height to the highest poinl on the roof surface, except tha~ 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.
......CIIIA._ Code Master
developed by:
A subsidiary 01 SK Ghosh Associates Inc. www.skghoshassoclates.CO
Tel: (847) 991-2700 Fax (847) 991-2702
[email protected] Copyright 0 2009 by SKGA
;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 Fotcs-Re~ng System (MWFRS) An assemblage of slructural elements aSStgl'led 10 provide support and stability for 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 horizontal wind 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 7 Stctlon 6.1. The minimum design wind pressure for C & C 15 10 ~I acting In either direction normal to the surfac•. Glazing in Occupancy Cat89Of'Y II, IH, Of IV buildings located In wind-borne debris Ions needs to be Pl'otected 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 surface roofs located withIn 1500 ft of the bulldlng. :Hustration of MWFRS and C & C
.-..
12
,e
..,.
STEP
2
DETERMINE TOPOGRAHIC FACTOR
Krr
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'Nhere K" /(2, and ~ are sellorth In ASCE 7Flg\lre ~ (pages 45-46). STEP
3
DETERMINE Ps30 FOR
MWFRS
Use ASCE 7 FlQure 6-2 (pages 38-39), Based on the basic WInd speed and the roof angle, delern1ine p~, which IS the Slmpflfied deSIgn 'Mnd pnlssure for Exposure e, h =30 11:, and / =1,0, Note that 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 bUJIcIings With roof angles less Ittan 25' it can be assumed thai the maXImum Uplift. produced by a po5Itive inlemal pressure. IS the mntrolkng load case From 25" to 45' both posiove 800 negatIVe internal pressure cases (load Cases 1 and 2. respectIVely) need to be checked for the roof, because !he external pressore on the W1ndwartl roof beoomes positive beyond a roof angle of appI'OXlmalely 25-
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 3,
a = O· and locate \he Zone ElF, GIH boundaries at mid-leoglh of the "'Idlng load Case 1 and 2 are to be 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 comer If 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 -
p~ -=
). KJl / plJIJ
ASCE 7 Equabon (6-1)
Simplified design WInd pressure. P., for the MWFRS of Iow-ose Simple dIaph~ bu\ldIf'lgS rapresent the I'Illt pressures (sum of ellltemal and intemaJ) 10 be applied 10 the honzonlal and vertical proteebons of boildl~ 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
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 of the component determine P/d, which Is the net design Wind pressure fOt ~re B, h = 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).
mF~-i
DETERMINE NET DESIGN WIND PRESSURE
~j P"" = "Kid I Ptdl
STEP·8y.STEP PROCEDURE FOR ASCE
7 METHOO 1:
Note steps 1-6 at beglnnJrlQ of thJs CodeMasler should be compleled before proceedl~ WIth the steps below
•
The boiIdlng Is ~-nse, smple diaphragm, and reguIar-shaped as defned lt1 ASCE 7 section 6.2.
,
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 building is no! a I\eXl'ble building as defined III ASCE 7 Section 6.2
•
The bUIlding does r'lOl have response c:haracteoslics making Hsubje<;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.
•
--
•
1
A
;., 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, ).. lakes Into accoont other exposure and helght condi\lOl'ls. OelllflTllllEllhe Height 8nd Expowre Al1Juslment Coebnt,
, ,
•
,»:
Gable Roof (7" < e ~ 45°) Noles' 1. a = 10% of the Ieasl honzontal dimension or OAh, whicl1ever Is smaller, but not less than either 4% of least horizontal 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
•
Determine
What?
How?
Kd < 1 and takes into account the reduced probability of
Flal Roof
(1
.,
'," ·, , •
-
.,
, •, I
(ASCE 7 SectIon)
, ,r"I I r
DETERMINE HEIGHT AND EXPOSURE
"-
,
2AN"-vnC"- PROCEOURE (ASCE 7 SECTION 6.51
H
Ii"
AOJUSTMENT COEFFICIENT
,
,
·,
1Transve.... )
'"
•
STEP-Sy·STEP PROCEDURE FOR METHOO
,
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).
rSTEP •_
..
MWFRSonc-.
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-. •
, I
,
,
ASCE 7 Equallorl (6-2)
ThIs SImplified method IS applk:able to buildH'lgs satisfying all of the conch\iol'ls set forth In ASCE 7 Secboo 6,4,1. These condlbons IocllJI1lllhe following: •
I
PnfttFORC&C
Net design wind pnmure. P/IIlI, 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
S,MPLIFIED PROCEOURE (ASCE 7 SEcr"", 6.4)
,
C& C
DETERMINE PnefJO FOR
Wind
, •
"~ /:
Oirectionality Factor, K d (6.5.4.4)
maximum Winds coming from any given direction. II should be noled that the corresponding ASCE 7 Table 6-4 wind load factor (page 80) (ASCE 7 Sections 2.3 and 2.4) should be consistent with the selection Of K.,. Refer to ASCE 7 Commentary to
Chapter 2 for further delails.
D.I.rmln. What? (ASCE 7 SeclJon)
How?
Oelermlne Wh.t? (ASCE'
How?
_Ion, ~
Hllipful Noles: (continued)
What does II me.n?
(page 79) It is Important 10
II<,
there are abrupt
K zt win Do grealer than 1 if the rr.e
changes in the topognlphy. If
conditions listed ASCE 1 SectIOn
Ulef8 are no abrupt changes in the topography and the structure is on level ground, Kif = 1. If the live conditions
6,5.7.1 exist. K" takes Into aCOOUflt the faCllhal jf a buUdlng
Topographic Faclor, Kif
(6572)
sils on the upper half
of an isolated hWI,
I
-
1
• For rigid structures (period <; 1 sac.). use G '" 0.85 or calculate per ASCE 7 Section 6.5.8.1.
Helpful Notes:
OUII Effect Fletor, G, Gt
16.'.6)
1. Where combined gusl effect factors and pressure coefficients (GCI" GCpj • and GC",) are given in f.gures lind tables. the gust effect factO\'" is not determined separately. 2. In lieu of the procedure defined In these sections, gust effect factor may be determined by any rational procedtJre given in the Illcognlzed Ilteralllre
G, GdG/for nexlbl. bulldil"l9s) tak.lnto accounl the following two effects II'l the direction parallel 10 wind loads
1) wind turbulence structure Interaction 2) dynamic ampUficalion of 'N!nd ror neKible bu~dings
I
STEP 1
ASCE 7 SectIOn 6.5.10 • For definllions of q. q~ qh, qr, see ASCE 7 5ecboo 6.3 F:q,GC,A,
for Open Buildings And Other Structures (6.5.13)
IlnItlUrtI
DETERMINE WINO STAGNATION PRESSURE q,
Based on !he baslc wn:I speed. V from IBC Table 16096.2 PI
2
STEP
wtlere;
F
LlfMat.ons on type of
• SoIIIIlgIIS
• q '" O.OO256(KJ(K"XK.r~V2)(/) as defined In
~11~ . , .
......nd 1LIQRa\lOn pteIaIA. q.
DETERMINE VELOCITY PRESSURE EKPOSURE COEFFICIENT
Cf ", nel force coemctenl from ASCE 7 Figures 6· 21 through 6- 23 (pages 74-76) A, '" projected area normal to the wind except where C, is specified for tha actual lurface area. sq, ft.
Kz K"
Determine veIodly pre5S1Jre exposure ooe1'rtcienlS. 14 ~ 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, For leeward wall and SldttwaIII, and for \lll'1f\dWard and _ _ K,Is ....
I
ASCE 7 Figure 6·5 (page 47) For open buildings. GCI) renects ,",ow much build-up of Gel" '" 0.00 pressure IS elt ma'ed For partially 10 exf$t Within the enclosed buildings. 11n:erlor 01 the build:ng GCPf '" +0.55 and and Is dapcndolll ·0,55 upon the enclosure I classtflcaUon For enclosed buildings. Gel> '" l+O,18 and"() 18
Intemal Pr ... ur. C~cl.nt.
OC. (6,5111)
CI" GC~. snd GC. Pressure reflect lila relatIVe pressures esUmated Coefficient to eXIst on Ihe ASCE 7 eKterlol' surfacfl' and C, fot MWFRS Figs. 6-6. 6-7, are dep&nClenl upon (65.11.21) and 6-8 I the ~metflc (pages 48-51) configuration of lhe ASCE 7 Fig. 6-10 structure including Ilhe roof. These Ge,,/ for Low- (pages 53-54) coefflclents ere nol Rise MWFRS applicable to open (6,5,1121) ASCE 7 Figs 6-11 buildings GC" values for C & C Ge" for through 6-17 also depend on the Components (pages 55-65) effective WInd area. and Cladding which Is denned In (6,5.1122) ASCE 7 section 5,2
STEP
If
STEP 4 _
7 5Ialon e2 . . 1KJIlOlNG OR OTHER ST'RUCl\JRE REGlJLAR-5HAPEO" on ~ 21. A bUdng or 0Nl' tlNdIft hlIY't111 flO ~
=1
MW="'='---'-'-
All heights: p '" qGCp - Q. (GCPt
P for Enclosed Ind PartIally EncloMd Buildings (6.512)
Ftexl (ASCE"
»
aulldings- MWFRS Section 6512 2.3)
Parapets jASCE 7 SectioI16.5.12.2.4)
p'" qpGC/Xl
C&C (ASCE 7 Seclion 6.5.124) Low-rISe buHd'ngs and buildi'lgs with h.5 eo ft; p '" qll ({GCa) - (GC...)J
I
sa.,..ltndlIe,.,
1IId"~.tl). i~~lIItidIl_(4 .-:!II" <; .. IleiIt' ~ 7:l teel n • ~ kHUIt wktIllIIb (.. I - , d:lmIIIr.:llIy QUIIIIieI to IlII Ilt _ _ ~ , MeIlad II1W . . dlIliIIt ~
ASlI'UCt.n lilt. ngdGl" ""'" lit heIflt ~ Ind .... " ' . , _ 1m! III:Ii:M • unIIefy 10 tit ID
The sln.lctln cannot be
...
&el\SjllVf!
10 ~rtarrac
'~,~===;--_-cc ttij; 1N*n~ be
.....
~
a.. for """d'I
--buIlt*tg JI . . walle d
P '" q( Gt C,,) - q, (GCpj )
I
IQd 1N:ln. one hI ' - . ~ (751e1t 1nIt •
~ (~51BlK
~on
"'; p '" Q~ !{GC~) - (GC...
LOIN
DETERMINE
IPIdIl
c...::; net preatn
¥ldl(
dr,•.
'1CM'tt1
lXI".U
ASCE 7 5edlon 62 dem 'BUilDING, SJ'*'l£ DIAPHRAGM- on PI9' 21 as: A buiIdIIg In 'flhdI ball WIlIdwaro and IeewW wind loacII lit ~ lhl'Ollgh lIoor and roof iIlV"... to .. ..,. IIWicaI
For buidngs the buildllg needS b ~ify as a
""" """""" 1>,"""
MWFRS {It.g no SIl'UCIl.nllCP8lltlorw} I
ASC~ 7 Section 82 oem.. 'BUilDING Of'£N
CI'I
A buIcIn; IlfwinQ 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 ullrTWll! ....... in ft· 1m'! A" =III grt:III. . d.1III "flo '" 'tIIflicI'I A,.. IIIC11nD1sd •• (m'l Pf9I 21 •
NET
PRESSURE COefFICIENT
'bMId on ~
ene
!
C& C
!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. lor walsand roofs ildlIIiIl".lId ttw"n 18C T8bIt 1609 6.2 (2) 'MIn M¥fI!!'In flan one YaIIJe. ltlI rm'e ....... WI"Id laid u.-t., deIign
c....
condilal"
STEP
5
DETERMINE DESIGN WINO PRESSURE
c..may
p"P(
fOR
MWFRS AND C & C
1609 6 1 llIrn 1 for a/I"'II r.d purpclMI 6IftnII1
The h8IgIll 0I1he
.............
)
-r1.,cI<'K. whereK1
fOR MWFRS AND
I
~__~·l~"'~'''~'''~_·_",,,,,;~====, ASCC 1 Sedlon U dIrr. 'B\Jl.OINO OR OllER STRUC'Tl..RES RIGlT on flIDI21 as ,,~ (II 0IlW1N3n -..!lda! . . . freql..cy.~ _1ilII'_101Hz.. ASCt: 1 CornmIntII, t8 5.8 P9I 2lU Illll ~ JII'OVI6II "'tonnItIon on hOw to dteImlinI lit ~ ~ 01. stn.e:turI AIIo IBe Sec:tlon
I
Rigid Buildings(ASCE 7 Sections 6.5 12.2 1 and 65.12.2,2)
rJJ 8ttl.4lt dmges tllIe k4X9~I)' WId tI'te IN:lft 1\ on M ~
r ASCE
I
----I
DETERMINE TOPOGRAPHIC FACTOR K.l!
"ASCE 7 FIgUte 6-4 (pagel4S--401
EK1CmJIII
O•• lgn Wind Prollure
'*" n
3
K.cl,O.OhlrwIse~.neK
I
• For neKible structures (period ? 1 sac.). calculate G per ASCE 7
Sectlorl6.5.8.2
for Enclosed lind Partially Enclosed Buildings (6.5.12)
Il'I
ridge, or escarpment, Il8led In ASCE 7 as a result, the 5ecI1OO 6,5 7 1 bulldlllQ can be eKIIt. then expected to Kn '" (1 +K, K1 K:J f ekperi9l1ce ;and K" K2• and 1<:J higher wmd sPMds are set rorth lrt thaI" Ii ,1 ww)! ASeE 7 FlQure 6-4 siluatett 'll ItwO!l (pages 45-46). ground .
The sInX:tlru cannot r 01 \l'it fOOMng • Solid fre&.star'ldIng waijs
Notes on q • q represents lhe velocity pressure, Which Is the ronversion of the basic wind slleltd into pressure at a specific helghl
p
(co/llinued)
._-
• WI1 roof sk:Jpe ) 45 d!lg
p'" q(GC,,)' qj (GC",)
(continued)
....
""""
BI..idngs with h " 60 It
4 ASCE 7 Commenlaly Section C62, pages 281-282. impHes thai a buHding with height not eKceedlng four 1 times the least horizontal dimension may be considered rigid.
7""
Only applies If
(continued)
I
(contlOU6d)
.-·
.""-
--
13. See ASCE 7 Commentary Section C65.8, pages 293-294, for determination of bUilding period or its reciprocal. building frequency,
SeE 7 Table 6-3 While the bask: wind speed, V, represents note that the V,toel1y the wind speed at 33 subscopt 'z' is lor Pre••u... any height above feel above ground for Ellposure ground level and Exposure Category C, Coef'fklentl (tot ttl sob . I -h" . Var K~ V ,ach wind e scnp IS represents !he WInd dlrKtlon), for the mean roof speed al z or h feel K.. I4t heigh," Also, the respectlvelyabo'n (65.8.6) velOCIty pressu~ ground for the 81lposure coeffIClent"d . • E may be calculated C xposura In accordance with a egory. Nota 2 to Table 6-3.
--
The siu::lule cannot ha'le fInj 01 .. ~ fypeS 01 roofs
Howo?
Wnll doe. It moan?
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 !itIJctln P'Of'C*l on a pIInt ~ m!he 8IIUIned WIld th::lIon, See ASCE 7 Section 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 of Case 1 to ICCOUIlI for !he effects due 10 Vrtnd alOng !he dl8gOO8t of the building. Oeslgt wnj JWesstnlS b C & C csmI be . . . . . 10 PIf DIg In . . . dncion normal 10 I1e arfIoa. 'Mnd ~ b'.ctt COlIW••
.-
How?
The structure cannot have any of the • Mullispan gable • Stepped
(continued) Buildings with h p'" q(GCp )
(continued)
-
>
for Enclosed and Partially Enclosed
• With roof sq:.e > 45 dcg
• q'" O.OO256(K,)(Kzt }(Kd )(Vl}(I} as defined in ASCE 7 Section 6.5.10 • For definitions of q, qi, qn, q" see ASCE 7 Section 6.3
Buildings
(6.5.12)
types of roo1s:
.00"""
60 I.:
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
p
~
._-
The sIru::lure ClWlrlOt tle My ,j lhe
Limita~ons
on roof configuration
:::-.;:::='.-=~-t-I - -
• Solid frll&.stanOOg walls • Solid S9l5
Limitations on type of structure
IF=q,GC,A,
Where:
F
for Open
C/ '" net force coefficient from ASeE 7 Figures 621 through 6- 23 (pages 74-76)
Buildings And Other
A, '" projected area normal to the wind except where C, is specified for the actual surface alea, sq. ft.
Structures (6.5.13)
STEP.By.STEP PROCEDURE FOR THE ALTERNATE ALL-HEIGHTS METHOD
(IBe
SECTION
1609.6)
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, K.~ is used
L~!.~!'.dl
DETERMINE TOPOGRAPHIC FACTOR,
Kzt
Note: Steps 1-6 al beginning of !his CodeMasler should be compleled before proceeding with the steps below
If there are no ~ changes il the topogaphy and the slruclun! is 00 level grcxnl, Kzl = 1,0. OlherNise, determioe KlI = {1+ K,K:l.KJ'f whem K1, K2 , and K] are set forth
The alternate all-heights method is applicable to buildings thaI satisfy all of the ooodilions set forth in IBC Section 160961 The condltJorlS are:
in ASCE 7 FlQUre 6--4 (pages 45-40)
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~";ial~fili~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 1 Hz 1) Rigid ASCE 7 Commentary C658, pages 293 and 294, provides Information on how to determine the OR IundamentaI frequency r:I a 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 canoo~"" A,lructure lhat is rigid Of meets the height Iim~ and the sens~1Ve to dynamic stendemess lim~ above is unlikely to be sensitive to effects dyna~m~"~,_',~,~._. The sln.dure carnJt be ~ located on a_ tr wtidl
---"""'~
_
see ASCE 7 Commentary C6.6
IlM1g " he wake rt
speciallXJIlSidel aIO I
ASCE 7 Section 6.2 defines "BUiLDING, SIMPLE For buildings, the buiIdiog DIAPHRAGM' on page 21 as: A building in whdl both needs to quaMy as a windward and leeward wind loads are transmitted simple diaphragm buMding. 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 a WIll that I8C8MlS positive exlllmal ~, 11 fl2 (mlj ~ = the gross area rt thaI wall'I which A. is derJlIfied.
• II' 1m')
C.. = net pressure coefficient based on ~ [(GXC~) - (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 ASCE 7 section 61.4 for criIefia.
see
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 ~Be Tablel609,6.2(2). Ns'tll'Crl; is pubIshed WIlh the understaf1ding ltIiI Sl
