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Portal frames Portal frames are !enerally lo#rise structures$ comprisin! columns and hori%ontal or pitched rafters$ connected &y moment#resistin! connections' connections' (esistance to lateral and )ertical actions is pro)ided &y the ri!idity of the connections and the &endin! sti*ness of the mem&ers$ hich is increased &y a suita&le haunch or deepenin! of the rafter sections' This form of continuous frame structure is sta&le in its plane and pro)ides a clear span that is uno&structed &y &racin!' Portal frames are )ery common$ in fact +,- of constructional steel used in the UK is in portal frame construction' They are )ery e.cient for enclosin! lar!e )olumes$ therefore they are often used for industrial industrial$$ stora!e$ retail retail and and commercial applications as ell as for a!ricultural purposes' This article descri&es the anatomy and )arious types of portal frame and /ey desi!n considerations'
0ulti#&ay portal frame durin! construction
Contents 1hide 1hide22 3 Anatomy of a typical portal frame 4 Types of portal port al frames fram es 5 Desi Desi!n !n cons consider ideratio ations ns o
5'3 Choice of material and section
o
5'4 Frame Frame dimensions dimensi ons
5'4'3 5'4' 3 Clea Clearr span and hei!ht
5'4'4 5'4' 4 0ain frame
5'4'5 5'4' 5 Haun Haunch ch dimen dimension sions s
5'4'6 Positions of restraints 6 Ac Acti tion ons s 6'3 Permanen Permanentt acti actions ons
o
6'4 7aria&l 7aria&le e acti actions ons
o
o
6'3'3 6'3' 3 Ser)i Ser)ice ce loa loads ds
6'4'3 6'4' 3 Impo Imposed sed roof loads
6'4'4 6'4' 4 Sno loads
6'4'5 6'4' 5 8ind acti actions ons
6'4'6 6'4' 6 Cran Crane e acti actions ons
6'4'+ 6'4' + Accid Accidenta entall acti actions ons
6'4'9 6'4' 9 (o&ustn (o&ustness ess
6'4' 6' 4': : Fire 6'5 Com&inations of actions + Frame Frame analysis anal ysis at ULS
0ulti#&ay portal frame durin! construction
Contents 1hide 1hide22 3 Anatomy of a typical portal frame 4 Types of portal port al frames fram es 5 Desi Desi!n !n cons consider ideratio ations ns o
5'3 Choice of material and section
o
5'4 Frame Frame dimensions dimensi ons
5'4'3 5'4' 3 Clea Clearr span and hei!ht
5'4'4 5'4' 4 0ain frame
5'4'5 5'4' 5 Haun Haunch ch dimen dimension sions s
5'4'6 Positions of restraints 6 Ac Acti tion ons s 6'3 Permanen Permanentt acti actions ons
o
6'4 7aria&l 7aria&le e acti actions ons
o
o
6'3'3 6'3' 3 Ser)i Ser)ice ce loa loads ds
6'4'3 6'4' 3 Impo Imposed sed roof loads
6'4'4 6'4' 4 Sno loads
6'4'5 6'4' 5 8ind acti actions ons
6'4'6 6'4' 6 Cran Crane e acti actions ons
6'4'+ 6'4' + Accid Accidenta entall acti actions ons
6'4'9 6'4' 9 (o&ustn (o&ustness ess
6'4' 6' 4': : Fire 6'5 Com&inations of actions + Frame Frame analysis anal ysis at ULS
o
+'3 Plas Plastic tic anal analysis ysis
o
+'4 ;las ;lastic tic anal analysis ysis 9 In#plane frame sta&ilit sta&ility y
o
9'3 Second order e*ects
o
9'4 First#ord First#order er and second#order analysis
o
9'5 Calculation Calcula tion of
o
9'6 Sensiti)ity to e*ects of the deformed !eometry : De Desi si!n !n
o
:'3 Cross#section resistance
o
:'4 0em&er sta&ility sta&ili ty
o
:'5 (after desi!n and sta&ility sta&il ity
:'5'3 =ut#of#plane sta&ility
:'5'4 >ra)ity com&ination of actions
:'5'5 :'5' 5 The upli uplift ft condi conditio tion n
:'5'6 :'5' 6 In plan plane e sta& sta&ili ility ty :'6 Column desi!n and sta&ilit sta&ility y
o
:'6'3 =ut#of#plane sta&ility
:'6'4 :'6' 4 In plan plane e sta& sta&ili ility ty ? Br Brac acin in! ! ?'3 7ertica 7erticall &ra &racin! cin!
o
?'3'3 ?'3' 3 Porta Portalise lised d &ays
?'3'4 Bracin! to restrain lon!itudinal loads from cranes ?'4 Plan &rac &racin! in!
o
?'4'3 ?'4' 3 (estrai (estraint nt to inner @an!es
Con Connec nectio tions ns '3 Column &ases
o
3, (efe (eferen rences ces 33 Further readin! readin ! 34 (eso (esourc urces es 35 Se See e al also so 36 ;ter ;ternal nal lin/s 3+ CPD
1top top22Anatomy
of a typical portal frame
Principal components of a portal framed &uildin! A portal frame &uildin! comprises a series of trans)erse frames &raced lon!itudinally' The primary steelor/ consists of columns and rafters$ hich form portal frames$ and &racin!' The end frame !a&le frame can &e e ither a portal frame or a &raced arran!ement of columns and rafters' The li!ht !au!e secondary steelor/ consists of side rails for alls and purlins for the roof' The secondary steelor/ supports the &uildin! en)elope$ &ut also plays an important role in restrainin! the primary steelor/' The roof and all claddin! separate the enclosed space from the eternal en)ironment as ell as pro)idin! thermal and acoustic insulation' The structural role of the claddin! is to transfer loads to secondary steelor/ and also to restrain the @an!e of the purlin or rail to hich it is attached'
Cross#section shoin! a portal frame and its restraints
Portal framed structures # o)er)ie
1top2 Types
of portal frames
0any di*erent forms of portal frames may &e constructed' Frame types descri&ed &elo !i)e an o)er)ie of types of portal construction ith typical features illustrated' This information only pro)ides typical details and is not meant to dictate any limits on the use of any particular structural form'
Pitched roof symmetric portal fra me >enerally fa&ricated from UB sections it ha su&stantial ea)es haunch section$ hich may &e cut from a rolled section or fa&ricated from plate' 4+ to 5+ m are the
Pitched roof symmetric
most
portal frame
e.cient
Lancashire 8aste
spans'
De)elopment
Portal fra me with internal mezzanine oor =.ce accommoda tion is often pro)ided ithin a
Portal frame ith internal
portal frame me%%anine @oor
structure usin! a partial idth me%%anine @oor' The assessment of frame sta&ility must include the e*ect of the 8aters 0eetin! Health me%%anineE Centre$ Bolton !uidance is (Image courtesy BD !i)en in SCI Structures Ltd. and P44' Crane portal fra me with column brackets 8here a tra)ellin! crane of relati)ely lo capacity up to say 4, tonnes is reuired$ &rac/ets can &e Ged to the columns to support the crane rails' Use of a tie mem&er or
Kloeckner Westok)
ri!id column &ases may &e necessary to reduce the ea)es de@ection' The spread of the frame at crane rail le)el may &e of critical importance to the functionin! of the craneE reuirement s should &e a!reed ith the client and ith the crane manufactur er' Tied portal fra me In a tied portal frame the hori%ontal mo)ement of the ea)es and the &endin! moments in
the columns and rafters are reduced' A tie may &e useful to limit spread in a crane# supportin! structure' The hi!h aial forces introduced in the frame hen a tie is used necessitate the use of second# order softare hen analysin! this form of frame' Monopitch portal fra me A mono pitch portal frame is usually chosen for small spans or &ecause of its
proimity to other &uildin!s' It is a simple )ariation of the pitched roof portal frame$ and tends to &e used for smaller &uildin!s up to 3+ m span' Propped portal fra me 8here the span of a portal frame is lar!e and there is no reuirement to pro)ide a clear span$ a propped portal frame can &e used to reduce the rafter si%e and
Propped portal frame
also the
(e&ottlin! Plant$
hori%ontal
Hemsell
shear at the (Image courtesy of foundations' Metsec plc)
Mansard portal fra me A mansard portal frame may &e used here a lar!e clear hei!ht at mid#span is reuired &ut the ea)es hei!ht of the &uildin! has to &e minimised' Curved rafter portal fra me Portal frames may &e constructed usin! cur)ed rafters$ mainly for architectura l reasons' Because of transport limitations r afters lon!er than 4, m may reuire
splices$ hich should &e carefully detailed for architectura l reasons' The cur)ed mem&er is often model led for analysis as a series of strai!ht elements' >uidance on the sta&ility of cur)ed rafters in portal frames is !i)en in SCI P4?3' Alternati)el y$ the rafter can &e fa&ricated as a series of strai!ht elements' It ill &e necessary to pro)ide purlin cleats of )aryin! hei!ht to achie)e the
cur)ed eternal proGle' Cellular beam portal fra me (afters may &e fa&ricated from cellula r &eams for Cellular &eam portal frame aesthetic reasons or
Hayes !arden centre
hen
(Image courtesy of
pro)idin! lo Kloeckner Westok) n! spans' 8here trans port limitations i mpose reuirement for splices$ they should &e carefully detailed$ to preser)e the architectura l features' The sections used cannot de)elop plastic hin!es at a cross# section$ so only elastic
desi!n is used' 1top2Desi!n
considerations
In the desi!n and construction of any structure$ a lar!e num&er of inter#related desi!n reuirements should &e considered at each sta!e in the desi!n process' The folloin! discussion of the desi!n process and its constituent parts is intended to !i)e the desi!ner an understandin! of the inter#relationship of the )arious elements of the structure ith its Gnal construction$ so that the decisions reuired at each sta!e can &e made ith an understandin! of their implications'
1top2Choice
of material and section
Steel sections used in portal frame structures are usually speciGed in !rade S4:+ or S5++ steel' In plastically desi!ned portal frames$ Class 3 plastic sections must &e used at hin!e positions that rotate$ Class 4 compact sections can &e used elsehere'
1top2Frame
dimensions
Dimensions used for analysis and clear internal dimensions A critical decision at the conceptual desi!n sta!e is the o)erall hei!ht and idth of th e frame$ to !i)e adeuate clear internal dimensions and adeuate clearance for the internal functions of the &uildin!'
1top2Clear span and hei!ht The clear span and hei!ht reuired &y the client are /ey to determinin! the dimensions to &e used in the desi!n$ and should &e esta&lished early in the desi!n process' The client
reuirement is li/ely to &e the clear distance &eteen the @an!es of the to columns the span ill therefore &e lar!er$ &y the section depth' Any reuirement for &ric/or/ or &loc/or/ around the columns should &e esta&lished as this may a*ect the desi!n span' 8here a clear internal hei!ht is speciGed$ this ill usually &e measured from the Gnished @oor le)el to the underside of the haunch or suspended ceilin! if present'
1top20ain frame The main portal frames are !enerally fa&ricated from UB sections ith a su&stantial ea)es haunch section$ hich may &e cut from a rolled section or fa&ricated from plate' A typical frame is characterised &y
A span &eteen 3+ and +, m
An clear hei!ht from the top of the @oor to the underside of the haunch &eteen + and 34 m
A roof pitch &eteen +J and 3,J 9J is commonly adopted
A frame spacin! &eteen 9 and ? m
Haunches in the rafters at the ea)es and ape
A sti*ness ratio &eteen the column and rafter section of approimately 3'+
Li!ht !au!e purlins and side rails
Li!ht !au!e dia!onal ties from some purlins and side rails to restrain the inside @an!e of the frame at certain locations'
1top2Haunch dimensions
Typical haunch ith restraints The use of a haunch at the ea)es reduces the reuired depth of rafter &y increasin! the moment resistance of the mem&er here the applied moments are hi!hest' The haunch also adds sti*ness to the frame$ reducin! de@ections$ and facilitates an e.cient &olted moment connection'
The ea)es haunch is typically cut from the same si%e rolled section as the rafter$ or one sli!htly lar!er$ and is elded to the underside of the rafter' The len!th of the ea)es haunch is !enerally 3,- of the frame span' The haunch len!th !enerally means that the ho!!in! moment at the end of the haunch is approimately eual to the lar!est sa!!in! moment close to the ape' The depth from the rafter ais to the underside of the haunch is approimately 4- of the span' The ape haunch may &e cut from a rolled section often from the same si%e as the rafter$ or fa&ricated from plate' The ape haunch is not usually modelled in the frame analysis and is only used to facilitate a &olted connection'
1top2Positions of restraints
>eneral arran!ement of restraints to the inside @an!e Durin! initial desi!n the rafter m em&ers are normally selected accordin! to their cross sectional resistance to &endin! moment and aial force' In later desi!n sta!es sta&ility a!ainst &uc/lin! needs to &e )eriGed and restraints positioned udiciously' The &uc/lin! resistance is li/ely to &e more si!niGcant in the selection of a column si%e$ as there is usually less freedom to position rails to suit the desi!n reuirementsE rail position may &e dictated &y doors or indos in the ele)ation' If introducin! intermediate lateral restraints to the column is not possi&le$ the &uc/lin! resistance ill determine the initial section si%e selection' It is therefore essential to reco!nise at this early sta!e if the side rails may &e used to pro)ide restraint to the columns' =nly continuous side rails are e*ecti)e in pro)idin! restraint' Side rails interrupted &y for eample roller shutter doors$ cannot &e relied on as pro)idin! adeuate restraint' 8here the compression @an!e of the rafter or column is not restrained &y purlins and side rails$ restraint can &e pro)ided at speciGed locations &y column and rafter stays'
1top2Actions Ad)ice on actions can &e found in BS ; 33 132$ and on the com&inations of actions in BS ; 3,142' It is important to refer to the UK ational Anne for the rele)ant ;urocode part for the structures to &e constructed in the UK'
1top2Permanent
actions
Permanent actions are the self ei!ht of the structure$ secondary steelor/ and claddin!' 8here possi&le$ unit ei!hts of materials should &e o&tained from manufacturersM data' 8here information is not a)aila&le$ these may &e determined from the data in BS ; 33#3# 3152'
1top2Ser)ice loads Ser)ice loads ill )ary !reatly dependin! on the use of the &uildin!' In portal frames hea)y point loads may occur from suspended al/ays$ air handlin! units etc' It is necessary to consider carefully here additional pro)ision is needed$ as particular items of plant must &e treated indi)idually' Dependin! on the use of the &uildin! and hether sprin/lers are reuired$ it is normal to assume a ser)ice loadin! of ,'3,'4+ /"m 4 on plan o)er the hole roof area'
Roof slope,
k !k"#m
%$α
N 5,J
,'9
5,J N N 9,J
,'919, # "5,2
O 9,J
, &mposed loads on roofs
1top27aria&le
actions
1top2Imposed roof loads Imposed loads on roofs are !i)en in the UK A to BS ; 33#3#3162$ and depend on the roof slope' A point load$ Q/ is !i)en$ hich is used for local chec/in! of roof materials and Gin!s$ and a uniformly distri&uted load$ q/$ to &e applied )ertically' The loadin! for roofs not accessi&le ecept for normal maintenance and repair is !i)en in the ta&le on the ri!ht' It should &e noted that imposed loads on roofs should not &e com&ined ith eit her sno or ind'
1top2Sno loads Sno loads may sometimes &e the dominant !ra)ity loadin!' Their )alue should &e determined from BS ; 33#3#51+2 and its UK ational Anne192 the determination of sno loads is descri&ed in Chapter 5 of the Steel Desi!nersM 0anual' Any drift condition must &e alloed for not only in the desi!n of the frame itself$ &ut also in the desi!n of the purlins that support the roof claddin!' The intensity of loadin! at the position of maimum drift often eceeds the &asic minimum uniform sno load' The
calculation of drift loadin! and associated purlin desi!n has &een made easier &y the maor purlin manufacturers$ most of hom o*er free softare to facilitate rapid desi!n'
1top28ind actions 8ind actions in the UK should &e determined usin! BS ; 33#3#6 1:2 and its UK ational Anne1?2' This ;urocode !i)es much scope for national adustment and therefore its anne is a su&stantial document' 8ind actions are inherently comple and li/ely to in@uence the Gnal desi!n of most &uildin!s' The desi!ner needs to ma/e a careful choice &eteen a fully ri!orous$ comple assessment of ind actions and the use of simpliGcations hich ease the desi!n process &ut ma/e the loads more conser)ati)e' Free softare for esta&lishin! ind pressures is a)aila&le from purlin manufacturers' For more ad)ice refer to Chapter 5 of the Steel Desi!nersM 0anual and SCI P56' 8ind loadin! calculator
1top2Crane actions
>antry !irders carryin! an o)erhead tra)ellin! crane The most common form of cranea!e is the o)erhead type runnin! on &eams supported &y the columns' The &eams are carried on cantile)er &rac/ets or$ in hea)ier cases$ &y pro)idin! dual columns' In addition to the self ei!ht of the cranes and their loads$ the e*ects of acceleration and deceleration ha)e to &e considered' For simple cranes$ this is &y a uasi#static approach ith ampliGed loads For hea)y$ hi!h#speed or multiple cranes the alloances should &e specially calculated ith reference to the manufacturer'
1top2Accidental actions The common desi!n situations hich are treated as accidental desi!n situations are
Drifted sno$ determined usin! Anne B of BS ; 33#3#5 1+2
The openin! of a dominant openin! hich as assumed to &e shut at ULS
;ach proect should &e indi)idually assessed hether any other accidental actions are li/ely to act on the structure'
1top2(o&ustness (o&ustness reuirements are desi!ned to ensure that any structural collapse is not disproportionate to the cause' BS ; 3, 142 sets the reuirement to desi!n and construct ro&ust &uildin!s in order to a)oid disproportionate collapse under accidental desi!n situations' BS ; 33#3#:12 !i)es details of ho this reuirement should &e met' For many portal frame structures no special pro)isions are needed to satisfy ro&ustness reuirements set &y the ;urocode' For more information on ro&ustness refer to SCI P53'
1top2Fire
Collapse mechanism of a portal ith a lean#to under Gre$ &oundary condition on !ridlines 4 and 5' In the United Kin!dom$ structural steel in sin!le storey &uildin!s does not normally reuire Gre resistance' The most common situation in hich it is reuired to Gre protect the structural steelor/ is here pre)ention of Gre spread to adacent &uildin!s$ a &oundary condition$ is reuired' There are a small num&er of other$ rare$ instances$ for eample hen demanded &y an insurance pro)ider$ here structural Gre protection may &e reuired' 8hen a portal frame is close to the &oundary$ there are se)eral reuirements aimed at stoppin! Gre spread &y /eepin! the &oundary intact
The use of Gre resistant claddin! Application of Gre protection of the steel up to the underside of the haunch
The pro)ision of a moment resistin! &ase as it is assumed that in the Gre condition rafters !o into catenary
Comprehensi)e ad)ice is a)aila&le in SCI P535'
1top2Com&inations
of actions
BS ; 3,142 !i)es rules for esta&lishin! com&inations of actions$ ith the )alues of rele)ant factors !i)en in the UK ational Anne 13,2' BS ; 3, 142 co)ers &oth ultimate limit state ULS and ser)icea&ility limit state SLS$ althou!h for the SLS$ onard reference is made to the material codes for eample BS ; 35#3#31332 for steelor/ to identify hich epression should &e used and hat SLS limits should &e o&ser)ed' All com&inations of actions that can occur to!ether should &e considered$ hoe)er if certain actions cannot &e applied simultaneously$ they should not &e com&ined' >uidance on the application of ;urocode rules on com&inations of actions can &e found in SCI P594 and$ speciGcally for portal frames$ in SCI P5'
1top2Frame
analysis at ULS
At the ultimate limit state ULS$ the methods of frame analysis fall &roadly into to types elastic analysis and plastic analysis'
1top2Plastic analysis
Bendin! moment dia!ram resultin! from the plastic analysis of a symmetrical portal frame under symmetrical loadin! The term plastic analysis is used to co)er &oth ri!id#plastic and elastic#plastic analysis' Plastic analysis commonly results in a more economical frame &ecause it allos relati)ely lar!e redistri&ution of &endin! moments throu!hout the frame$ due to plastic hin!e rotations' These plastic hin!e rotations occur at sections here the &endin! moment reaches the plastic moment or resistance of the cross#section at loads &elo the full ULS loadin!' The rotations are normally considered to &e localised at plastic hin!esQ and allo the capacity of under#utilised parts of the frame to &e mo&ilised' For this reason mem&ers here plastic hin!es may occur need to &e Class 3 sections$ hich are capa&le of accommodatin! rotations' The G!ure shos typical positions here plastic hin!es form in a portal frame' To hin!es lead to a collapse$ &ut in the illustrated eample$ due to symmetry$ desi!ners need to consider all possi&le hin!e locations'
1top2;lastic analysis
A typical &endin! moment dia!ram resultin! from an elastic analysis of a frame ith pinned &ases is shon the G!ure &elo' In this case$ the maimum moment at the ea)es is hi!her than that calculated from a plastic analysis' Both the column and haunch ha)e to &e desi!ned for these lar!e &endin! moments' 8here de@ections SLS !o)ern desi!n$ there may &e no ad)anta!e in usin! plastic analysis for the ULS' If sti*er sections are selected in order to control de@ections$ it is uite possi&le that no plastic hin!es form and the frame remains elastic at ULS'
Bendin! moment dia!ram resultin! from the elastic analysis of a symmetrical portal frame under symmetrical loadin!
Portal frame analysis softare (!astrak model courtesy of "r#m$le )
1top2In#plane
frame sta&ility
8hen any frame is loaded$ it de@ects and its shape under load is di*erent from the un# deformed shape' The de@ection has a num&er of e*ects
The )ertical loads are eccentric to the &ases$ hich leads to further de@ection
The ape drops$ reducin! the archin! action Applied moments cur)e mem&ersE A ial compression in cur)ed mem&ers causes increased cur)ature hich may &e percei)ed as a reduced sti*ness'
Ta/en to!ether$ these e*ects mean that a frame is less sta&le nearer collapse than a Grst# order analysis su!!ests' The o&ecti)e of assessin! frame sta&ility is to determine if the di*erence is si!niGcant'
1top2Second order e*ects
P#R and P# effects in a portal frame The !eometrical e*ects descri&ed a&o)e are second#order e*ects and should not &e confused ith non linear &eha)iour of materials' As shon in the G!ure there are to cate!ories of second#order e*ects
;*ects of displacements of the intersections of mem&ers$ usually called P# effects' BS ; 35#3#31332 descri&es this as the e*ect of deformed !eometry'
;*ects of de@ections ithin the len!th of mem&ers$ usually called P#R effects'
Second#order analysis is the term used to descri&e analysis methods in hich the e*ects of increasin! de@ection under increasin! load is considered eplicitly in the solution$ so that the results include the P#R and P# effects'
1top2First#order and second#order analysis For either plastic analysis of frames$ or elastic analysis of frames$ the choice of Grst#order analysis or second#order analysis depends on the in plane @ei&ility of the frame$ characterised &y the calculation of the cr factor'
1top2Calculation of cr The effects of the deformed !eometry P# effects are assessed in BS ; 353 31332 &y calculatin! the factor cr$ deGned as
here ! cr is the elastic critical &uc/lin! load for !lo&al insta&ility mode$ &ased on initial elastic sti*nesses ! ;d is the desi!n load on the structure'
cr may &e found usin! softare or usin! an approimation epression +'4 from BS ; 35# 3#31332 as lon! as the frame meets certain !eometric limits and the aial force in the rafter is not si!niGcantM' (ules are !i)en in the ;urocode to identify hen the aial force is si!niGcant' 8hen the frame falls outside the speciGed limits$ as is the case for )ery many orthodo frames$ the simpliGed epression cannot &e used' In these circumstances$ an alternati)e epression may &e used to calculate an approimate )alue of cr$ referred to as cr$est' Further details are !i)en in SCI P5'
1top2Sensiti)ity to e*ects of the deformed !eometry The limitations to the use of Grst#order analysis are deGned in BS ; 3533 1332$ Section +'4'3 5 and the UK ational Anne1342 Section A'4' as For elastic analysis cr 3, For plastic analysis
cr + for com&inations ith !ra)ity loadin! ith frame imperfections$
pro)ided that a the span$ L$ does not eceed + times the mean hei!ht of the columns & %r satisGes the criterion %r" sa4 V %r" s&4 W ,'+ in hich s a and s& are the hori%ontal distances from the ape to the columns' For a symmetrical frame this epression simpliGes to %r W ,'4+L'
cr 3, for com&inations ith !ra)ity loadin! ith frame imperfections for clad structures pro)ided that the sti*enin! e*ects of masonry inGll all panels or diaphra!ms of proGled steel sheetin! are not ta/en into account
1top2Desi!n =nce the analysis has &een completed$ alloin! for second#order e*ects if necessary$ the frame mem&ers must &e )eriGed' Both the cross#sectional resistance and the &uc/lin! resistance of the mem&ers must &e )eriGed' In#plane &uc/lin! of mem&ers usin! epression 9'93 of BS ; 35#3#3 1332 need not &e )eriGed as the !lo&al analysis is considered to account for all si!niGcant in#plane e*ects' SCI P5 identiGes the li/ely critical %ones for mem&er )eriGcation' SCI P5: contains numerical eamples of mem&er )eriGcations'
1top2Cross#section resistance 0em&er &endin!$ aial and shear resistances must &e )eriGed' If the shear or aial force is hi!h$ the &endin! resistance is reduced so com&ined shear force and &endin! and aial force and &endin! resistances need to &e )eriGed' In typical portal frames neither the shear force nor the aial load is su.ciently hi!h to reduce the &endin! resistance' 8hen the portal frame
forms the chord of the &racin! system$ the aial load in the rafter may &e si!niGcant$ and this com&ination of actions should &e )eriGed' Althou!h all cross#sections need to &e )eriGed$ the li/ely /ey points are at the positions of maimum &endin! moment
In the column at the underside of the haunch
In the rafter at the sharp end of the haunch
In the rafter at the maimum sa!!in! location adacent to the ape'
1top20em&er sta&ility
Dia!rammatic representation of a portal frame rafter The G!ure shos a dia!rammatic representation of the issues that need to &e addressed hen considerin! the sta&ility of a mem&er ithin a portal frame$ in this eample a rafter &eteen the ea)es and ape' The folloin! points should &e noted
Purlins pro)ide intermediate lateral restraint to one @an!e' Dependin! on the &endin! moment dia!ram this may &e either the tension or compression @an!e
(estraints to the inside @an!e can &e pro)ided at purlin positions$ producin! a torsional restraint at that location'
In#plane$ no mem&er &uc/lin! chec/s a re reuired$ as the !lo&al analysis has accounted for all si!niGcant in#plane e*ects' The analysis has accounted for any si!niGcant second#order e*ects$ and frame imperfections are usually accounted for &y includin! the eui)alent hori%ontal force in the analysis' The e*ects of in#plane mem&er imperfections are small enou!h to &e i!nored'
Because there are no minor ais mom ents in a portal frame rafter$ ;pression 9'94 simpliGes to
1top2(after
desi!n and sta&ility
In the plane of the frame rafters are su&ect to hi!h &endin! moments$ hich )ary from a maimum ho!!in!M moment at the unction ith the column to a minimum sa!!in! moment close to the ape' Compression is introduced in the rafters due to actions applied to the frame' The rafters are not su&ect to any minor ais moments' =ptimum desi!n of portal frame rafters is !enerally achie)ed &y use of
A cross section ith a hi!h ratio of I yy to I%% that complies ith the reuirements of Class 3 or 4 under com&ined maor ais &endin! and aial compression'
A haunch that etends from the column for approimately 3,of the frame span' This ill !enerally mean that the maimum ho!!in! and sa!!in! moments in the plain rafter len!th are of similar ma!nitude'
1top2=ut#of#plane sta&ility Purlins attached to the top @an!e of the rafter pro)ide sta&ility to the mem&er in a num&er of ays
Direct lateral restraint$ hen the outer @an!e is in compression
Intermediate lateral restraint to the tension @an!e &eteen torsional restraints$ hen the outer @an!e is in tension
Torsional and lateral restraint to the rafter hen the purlin is attached to the tension @an!e and used in conunction ith rafter stays to the compression @an!e'
Initially$ the out#of#plane chec/s are completed to ensure that the restraints are located at appropriate positions and spacin!'
1top2>ra)ity com&ination of actions
Typical purlin and rafter stay arran!ement for the !ra)ity com&ination of actions The G!ure shos a typical moment distri&ution for the !ra)ity com&ination of actions$ typical purlin and restraint positions as ell as sta&ility %ones$ hich are referred to further' Purlins are !enerally placed at up to 3'? m spacin! &ut this spacin! may need to &e reduced in the hi!h moment re!ions near the ea)es' In Xone A$ the &ottom @an!e of the haunch is in compression' The sta&ility chec/s are complicated &y the )ariation in !eometry alon! the haunch' The &ottom @an!e is partially or holly in compression o)er the len!th of Xone B' In Xone C$ the purlins pro)ide lateral restraint to the top compression @an!e' The selection of the appropriate chec/ depends on the presence of a plastic hin!e$ the shape of the &endin! moment dia!ram and the !eom etry of the section three @an!es or to @an!es' The o&ecti)e of the chec/s is to pro)ide su.cient restraints to ensure the rafter is sta&le out#of#plane' >uidance on details of the out#of p lane sta&ility )eriGcation can &e found in SCI P5'
1top2 The uplift condition
Typical purlin and rafter stay arran!ement for the uplift condition In the uplift condition the top @an!e of the haunch ill &e in compression and ill &e restrained &y the purlins' The moments and aial forces are smaller than those in the !ra)ity load com&ination' As the haunch is sta&le in the !ra)ity com&ination of actions$ it ill certainly &e so in the uplift condition$ &ein! restrained at least as ell$ and under reduced loads In Xone F$ the purlins ill not restrain the &ottom @an!e$ hich is in compression' The rafter must &e )eriGed &eteen torsional restraints' A torsional restraint ill !enerally &e pro)ided adacent to the ape' The rafter may &e sta&le &eteen this point and the )irtual restraint at the point of contra@eure$ as the moments are !enerally modest in the uplift com&ination' If the rafter is not sta&le o)er this len!th$ additional torsional restraints should &e introduced$ and each len!th of the rafter )eriGed'
1top2In plane sta&ility o in#plane chec/s of rafters are reuired$ as all si!niGcant in#plane e*ects ha)e &een accounted for in the !lo&al analysis'
1top2Column
desi!n and sta&ility
Typical portal frame column ith plastic hin!e at underside of haunch The most hea)ily loaded re!ion of the rafter is reinforced &y the haunch' By contrast$ the column is su&ect to a similar &endin! moment at the underside of the haunch$ &ut ithout any additional stren!thenin!' The optimum desi!n for most columns is usually achie)ed &y the use of
A cross section ith a hi!h ratio of Iyy to I%% that complies ith Class 3 or Class 4 under com&ined maor ais &endin! and aial compression
A plastic section modulus that is approimately +,- !reater than that of the rafter'
The column si%e ill !enerally &e determined at the preliminary desi!n sta!e on the &asis of the reuired &endin! and compression resistances' 8hether the frame is desi!ned plastically or elastically$ a torsional restraint should alays &e pro)ided at the underside of the haunch' This may &e from a side rail positioned at that le)el$ or &y some other means' Additional torsional restraints may &e reuired &eteen the underside of the haunch and the column &ase &ecause the side rails are attached to the outer tension @an!eE unless restraints are pro)ided the inner compression @an!e is unrestrained' A side rail that is not continuous for eample$ interrupted &y industrial doors cannot &e relied upon to pro)ide adeuate restraint' The column section may need to &e increased if intermediate restraints to the compression @an!e cannot &e pro)ided' The presence of a plastic hin!e ill depend on loadin!$ !eometry and choice of column and rafter sections' In a similar ay to the rafter$ out#of#plane sta&ility must &e )eriGed'
1top2=ut#of#plane sta&ility
If there is a plastic hin!e at the underside of the haunch$ the distance to the adacent torsional restraint must &e less than the limitin! distance Lm as !i)en &y BS ; 35#3# 31332 Clause BB'5'3'3' It may &e possi&le to demonstrate that a torsional restraint is not reuired at the side rail immediately adacent to the hin!e$ &ut may &e pro)ided at some !reater distance' In this case there ill &e intermediate lateral restraints &eteen the torsional restraints If the sta&ility &eteen torsional restraints cannot &e )eriGed$ it may &e necessary to introduce additional torsional restraints' If it is not possi&le to pro)ide additional i ntermediate restraints$ the si%e of the mem&er must &e increased' In all cases$ a lateral restraint must &e pro)ided ithin Lm of a plastic hin!e' 8hen the frame is su&ect to uplift$ the column moment ill re)erse' The &endin! moments ill !enerally &e si!niGcantly smaller than those under !ra)ity loadin! com&inations$ and the column is li/ely to remain elastic
1top2In plane sta&ility o in#plane chec/s of columns are reuired$ as all si!niGcant in#plane e*ects ha)e &een accounted for in the !lo&al analysis'
1top2Bracin!
Bracin! in a portal frame (Image courtesy of W#ll#am &aley 'ng#neer#ng Ltd.) Bracin! is reuired to resist lon!itudinal actions due to ind and cranes$ and to pro)ide restraint to mem&ers' It is common to use hollo sections as &racin! mem&ers' Bracin! arran!ement in a typical portal frame
1top27ertical
&racin!
Common &racin! systems
The primary functions of )ertical &racin! in the side alls of the frame are
To transmit the hori%ontal loads to the !round' The hori%ontal forces include forces from ind and cranes
To pro)ide a ri!id frameor/ to hich side rails and claddin! may &e attached so that the rails can in turn pro)ide sta&ility to the columns
To pro)ide temporary sta&ility durin! erection'
The &racin! may &e located
At one or &oth ends of the &uildin!
8ithin the len!th of the &uildin!
In each portion &eteen epansion oints here these occur'
8here the side all &racin! is not in the same &ay as the plan &racin! in the roof$ an ea)es strut is essential to transmit the forces from the roof &racin! into the all &racin!' An ea)es strut is also reuired
To ensure the tops of the columns are adeuately restrained in position
To assist in durin! the construction of the structure
To sta&ilise the tops of the columns if a Gre &oundary condition eists
1top2Portalised &ays
Lon!itudinal sta&ility usin! portalised &ays 8here it is di.cult or impossi&le to &race the frame )ertically &y con)entional &racin!$ it is necessary to introduce moment#resistin! frames in the el e)ations in one or more &ays' In addition to the !eneral ser)icea&ility limit on de@ection of %"5,,$ here % is the hei!ht of the portalised &ay it is su!!ested that
The &endin! resistance of the portalised &ay not the main portal frame is chec/ed usin! an elastic frame analysis
De@ection under the eui)alent hori%ontal forces is restricted to %"3,,,$ here the eui)alent hori%ontal forces are calculated &ased on the hole of the roof area'
1top2Bracin! to restrain lon!itudinal loads from cranes
Additional &racin! in the plane of the crane !irder If a crane is directly supported &y the frame$ the lon!itudinal sur!e force ill &e eccentric to the column and ill tend to cause the column to tist$ unless additional restraint is pro)ided' A hori%ontal truss at the le)el of the crane !irder top @an!e or$ for li!hter cranes$ a hori%ontal mem&er on the inside face of the column @an!e tied into the )ertical &racin! may &e adeuate to pro)ide the necessary restraint' For lar!e hori%ontal forces$ additional &racin! should &e pro)ided in the plane of the crane !irder'
1top2Plan
&racin!
Plan )ie shoin! &oth end &ays &raced Plan &racin! is located in the plane of the roof' The primary functions of the plan &racin! are
To transmit ind forces from the !a&le posts to the )ertical &racin! in the alls
To transmit any frictional dra! forces from ind on the roof to the )ertical &racin!
To pro)ide sta&ility durin! erection
To pro)ide a sti* anchora!e for the purlins hich are used to restrain the rafters'
In order to transmit the ind forces e.ciently$ the plan &racin! should connect to the top of the !a&le posts'
1top2(estraint to inner @an!es (estraint to the inner @an!es of rafters or columns is often most con)eniently formed &y dia!onal struts from the purlins or sheetin! rails to small plates elded to the inner @an!e and e&' Pressed steel @at ties are commonly used' 8here restraint is only possi&le from one side$ the restraint must &e a&le to carry compression' In these locations an!le sections of minimum si%e 6, Y 6, mm must &e used' The stay and its connections should &e desi!ned to resist a force eual to 4'+- of the maimum force in the column or rafter compression @an!e &eteen adacent restraints'
1top2Connections The maor connections in a portal frame are the ea)es and ape connections$ hich are &oth moment#resistin!' The ea)es connection in particular must !enerally carry a )ery lar!e &endin! moment' Both the ea)es and ape connections are li/ely to eperience re)ersal in certain com&inations of actions and this can &e an important desi!n case' For economy$ connections should &e arran!ed to minimise any reuirement for additional reinforcement commonly called sti*eners' This is ! enerally achie)ed &y
0a/in! the haunch deeper increasin! the le )er arms
;tendin! the ea)es connection a&o)e the top @an!e of the rafter an additional &olt ro
Addin! &olt ros
Selectin! a stron!er column section'
The desi!n of moment resistin! connections is co)ered in detail in SCI P5?'
Ape connection
;a)es connection
Typical portal frame connections Haunched connections
1top2Column
&ases
Typical nominally pinned &ase In the maority of cases$ a nominally pinned &ase is pro)ided$ &ecause of the di.culty and epense of pro)idin! a ri!id &ase' A ri!id &ase ill in)ol)e a more epensi)e &ase detail$ &ut more si!niGcantly$ the foundation must also resist the moment$ hich increases costs si!niGcantly compared to a nominally pinned &ase' If a column &ase is nominally pinned$ it is recommended that the &ase &e modelled as perfectly pinned hen usin! elastic !lo&al analysis to calculate the moments and forces in the frame under ULS loadin!'
The sti*ness of the &ase may &e assumed to &e eual to the folloin! proportion of the column sti*ness
3,- hen assessin! frame sta&ility
4,- hen calculatin! de@ections under ser)icea&ility loads'
1top2(eferences 3'
Z BS ; 33$ ;urocode 3 Actions on structures$ BSI
4'
Z 4', 4'3 4'4 4'5 BS ; 3,4,,4VA34,,+$ ;urocode # Basis of structural desi!n$ BSI
5'
Z BS ; 33#3#3 4,,4 ;urocode 3 Actions on structures' >eneral actions' Densities$ self#ei!ht$ imposed loads for &uildin!s $ BSI
6'
Z A to BS ; 33#3#3 4,,4$ UK ational Anne to ;urocode 3' Actions on structures' >eneral actions' Densities$ self#ei!ht$ imposed loads for &uildin!s$ BSI
+'
Z +', +'3 BS ; 33#3#54,,5VA34,3+ ;urocode 3' Actions on structures' >eneral actions' Sno loads$ BSI
9'
Z A to BS ; 33#3#54,,5VA34,3+$ UK ational Anne to ;urocode 3' Actions on structures' >eneral actions' Sno loads$ BSI
:'
Z BS ; 33#3#6 4,,+ VA3 4,3, ;urocode 3' Actions on structures' >eneral actions' 8ind actions$ BSI
?'
Z A to BS ; 33#3#6 4,,+ VA3 4,3, UK ational Anne to ;urocode 3' Actions on structures' >eneral actions' 8ind actions$ BSI
'
Z BS ; 33#3#: 4,,9 ;urocode 3' Actions on structures' >eneral actions' Accidental actions$ BSI
3,' Z A to BS ;3,4,,4VA3 4,,+ UK ational Anne for ;urocode' Basis of structural desi!n$ BSI 33' Z 33', 33'3 33'4 33'5 33'6 33'+ 33'9 BS ; 35#3# 34,,+VA34,36$ ;urocode 5 Desi!n of steel structures' >eneral rules and rules for &uildin!s$ BSI 34' Z A to BS ; 35#3#34,,+VA34,36$ UK ational Anne to ;urocode 5 Desi!n of steel structures >eneral rules and rules for &uildin!s$ BSI
1top2Further
readin!
Steel Desi!ners[ 0anual :th ;dition' ;ditors B Da)ison \ > 8 =ens' The Steel Construction Institute 4,34$ Chapters 5 and 6
1top2(esources
SCI P44 In#plane Sta&ility of Portal Frames to BS ++,#34,,,$ 4,,3
SCI P4?3 Desi!n of Cur)ed Steel$ 4,,3
SCI P535 Sin!le Storey Steel Framed Buildin!s in Fire Boundary Conditions$ 4,,4
SCI P594 Steel Buildin! Desi!n Concise ;urocodes$ 4,,
SCI P53 Structural (o&ustness of Steel Framed Buildin!s$ SCI$ 4,,3
SCI P56 8ind Actions to BS ; 33#3#6$ SCI$ 4,35
SCI P5: ;lastic Desi!n of Sin!le#span Steel Portal Frame Buildin!s to ;urocode 5$ 4,35
SCI P5? ]oints in Steel Construction 0oment#resistin! ]oints to ;urocode 5$ 4,35
SCI P5 Desi!n of steel portal frame &uildin!s to ;urocode 5$ 4,3+
1top2See
also
Thermal performance
Introduction to acoustics
Steelor/ speciGcation
Steel construction products
Desi!n codes and standards
0em&er desi!n
Concept desi!n
Fa&rication
Braced frames
Alloin! for the e*ects of deformed frame !eometry
0odellin! and analysis
Structural ro&ustness
Structural Gre resistance reuirements
Sin!le storey &uildin!s in Gre &oundary conditions
0oment resistin! connections
Continuous frames
Sin!le storey industrial &uildin!s
(etail &uildin!s
1top2;ternal
Buildin! en)elopes
Desi!n softare and tools
lin/s
Trim&le
1top2CPD
Cate!ory Desi!n
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Analysis and desi!n of portal frames
