Fire Resistanc of Walls and Columns

8/8/2019 Fire Resistanc of Walls and Columns

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li21d dational Research Conseil nationalno. 784 hu nc il Canada de recherches Canadac . 2 A~ A W E D

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FIRE RESISTANCE OF REINFORCEDCONCRETE COLUMNS A N D WALLS

by David E. Allen and Tiam T. Lie

Reprinted, with permission, fromProceedings of the Canadian Structural Concrete ConferenceHeld in Ottawa, Canada, June 1977p. 17 -33

DBR Paper No. 784Division of Building Research

Price 25 cents OTTAWA NRCC 16818


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SOMMAIRE

Des i mm eubles d e g r a n d e h a u t e u r e n b Et on d e c o n s t r u c t i o nr 6 c e n t e c o mp o rt en t d e s murs p o r t e u r s e t d e s p o te a uxr e c t a n g u l a i r e s e n b g t o n p ou va nt c t r e e xp os 6s a u f e u s u rp l u s d 'u n c Ct6 . L es d eg rE s d e r g s i s t a n c e a u f e u d e c e sE l s m e n t s , 6 t a b l i s 2 p a r t i r de p ot ea ux p o r t e u r s c a r r S s e te x p o s g s a u x f la mm es s u r t o u s l e u r s co^ tE s s e m b l e n t ma in-t e n a n t c t r e b e a u c o up t r o p m odgr6s. Des 6 t u d e s a n a l y t i q u e ss u r l e s p r o p r i 6 t 6 s d e s m a t s r i a u x i n f l u e n c g e s p a r l at em p gr a t ur e pe uv en t s e r v i r au c a l c u l d e l a t e mp gr a t ur e ,d e s c o n t r a i n t e s , d e s mouvements e t d e l a r 6 s i s t a n c e d 'u n es t r u c t u r e d u r a n t un i n c en d i e ; c e s 6 t ud e s p eu ve nt a u s s ir em pl ac er l e s e s s a i s c o dt e ux d e t e nu e a u f e u . On ae f f e c t u 6 une t e l l e 6 t u d e d a ns l e c a s d e p o t e au x e t d em ur s e n b g t o n arm 6 e t o n a 6 t a b l i d e n ou ve au x d e g rE s d er g s i s t a n c e a u f e u . L ' a r t i c l e t r a i t e d es e f f e t s del ' i n t e r a c t i o n s t r u c t u r a l e e t d e l ' e f f r i t e m e n t du b 6 t o ns u r l a r g s i s t a n c e a u f e u a i n s i q ue d e l a r e l a t i o n e n t r el e s e xi ge nc es d e r 6 s i s t a n c e a u f e u e t l a p r o t e c t i o n e nc a s d ' i n c e n d i e , e n p a r t i c u l i e r d an s l e s immeubles d eg r a n d e h a u t e u r .

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FIRE RESISTANCE OF REINFORCED CONCRETE

COLUMNS A N D WALLS

B Y David E . Allen and Tiam T . Lie

SYPNOSIS

R ec en t h i g h - r i s e c o n cr e t e b u i l d i n g s c o n t a i n l o ad - b ea r in g w a l l sand r ec t angu la r conc re t e co lumns t h a t may b e s u b j e c t e d t o f i r e onmore t h an o n e s i d e . E x i s t i n g f i r e r a t i n g s f o r t h e s e members, b a s edon a f u l l y l oa d e d s q u a r e column e xp os ed t o f i r e on a l l s i d e s , a p p e a rt o b e e x c e s s i v e l y c o n s e rv a t iv e .

A n a l y t i c a l s t u d i e s b as ed o n tempera ture-dependent mater ia lp r o p e r t i e s c an b e u se d t o c a l c u l a t e t e m p e r at u r e , s t r e s s e s , movements,c o n s t r a i n t f o r c e s and s t r e n g th of a s t r u c t u r e d u ri ng f i r e an d t or e p l a ce c o s t l y f i r e t e s t s f o r de te rm in in g f i r e r e s i s t a n c e . Such as t u d y was c a r r i e d o u t f o r r e i n f o r c e d c o n c r e t e c ol um ns a nd w a l l s a ndnew f i r e r a t i n g s d et e rm i n ed . The p a pe r d i s c u s s e s t h e e f f e c t o fs t r u c t u r a l i n t e r a c t i o n and s p a l l i n g on f i r e r e s i s t a n c e a nd t h er e l a t i o n o f f i r e r e s i s t a n c e r e qu i re me n ts t o t h e g e n er a l pro blem o ff i r e s a f e t y , p a r t i c u l a r l y f o r h ig h - ri s e bu i l d i n g s .

KEYWORDS

f i r e r e s i s t a n c e ; c o n cr e t e; columns; w a l l s ; s t r u c t u r a l i n t e r a c t i o n ;s p a l l i n g ; de si gn c r i t e r i a .


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D . E . A l l en a nd T. T. L ie a r e r e s e a r c h o f f i c e r s w i t h t h e D iv i s io n o fBui ld ing Research , Nat io nal Research Counci l of Canada . D . E . A l l e nh a s b een e ngag ed i n s t r u c t u r a l e n g i n e er i n g r e s e a r c h i n t h e a r e a s o fs t r u c t u r a l s a f e t y and l i m i t s t a t e s d es ig n, f l o o r v i b r a t i on a nd f i r er e s i s t a n c e o f c o n c r e t e s t r u c t u r e s . T.T. L ie h a s b ee n e ng ag ed i nf i r e r es ea rc h i n t he a r ea s o f s t r u c t u r a l f i r e r e s i s t a n c e ands t r u c t u r a l f i r e s a f e t y .

With t h e i n c r e a s i n g us e o f s t r u c t u r a l c o n c r e t e i n h i g h - r i s eb u i l d i n g s t h e r e i s a need f o r c l o s e r s t u d y o f t h e p rob lem o fs t r u c t u r a l f i r e r e s i s t a n c e : what i s r e q u i r e d ? ; what do c o n cr e t es t r u c t u r e s a c t u a l l y p ro v id e? T h i s p ap er d i s c u s s e s t h e q u e s t i o n w it hp a r t i c u l a r r e f e r e n c e t o l o a d - b e a r i n g columns a n d w a l l s .

As i n t r o d u c t i o n a b r i e f r ev ie w f o l l o w s o f how s t r u c t u r a l f i r er e s i s t a n c e r e l a t e s t o t h e g e ne r al problem o f f i r e s a f e t y i nb u i l d i ng s , e s p e c i a l l y h i g h - r i s e b u i l d i n g s .

STRUCTURAL FIRE RESISTANCE A N D FIRE SAFETY OF BUILDINGS

Of fundamenta l concern i s human sa f e t y - oc cu pa nt s may b ek i l l e d o r i n j u r e d d ur i ng f i r e f rom a sp h yx i at i on , h e a t , b u rn s ,s t r u c t u r a l c o l la p se o r p o s si b ly a s a r e s u l t o f a p an ic s i t u a t i o n .A lt ho ug h t h e d e a t h r a t e f ro m f i r e i n Ca nad a ( Tab l e I ) i s a b ou t a no r d e r o f m ag ni tu de l ow er t h a n t h a t f o r c a r a c c i d e n t s , i t i s abou ttwo o r d e r s o f magn itu de h i g h e r th a n t h a t f o r s t r u c t u r a l c o l l a p s e i fc o n s t r u c t i o n a c c i d e n t s a r e e x cl u de d . The p r e s e n t f i g u r e f o r f i r e ,however, r e p r e s e n t s f a t a l i t i e s ca u s ed ma in ly by a s p h y x i a t i o n a ndburns , and thes e a r e co n t ro l l ed more by measu res such a s f i r ed e t e c t i o n , smoke c o n t r o l and s p r i n k l e r s t h an by s t r u c t u r a l f i r e

r e s i s t a n c e . N e v e rt h e l es s , human s a f e t y i n h i g h - r i s e b u i l d i n g s i sc l o s e l y l in k ed w it h s t r u c t u r a l f i r e r e s i s t a n c e .

A s ec on d c o n s i d e r a t i o n i s e co no mi cs ; f i n a n c i a l l o s s e s f ro m f i r edamage can be ve ry l a rg e . Such lo s s e s i nc l ude co n t en t s , p roduc t iona nd b u i l d i n g r e p a i r o r r e pl a ce m en t . The ec on om ic go a l i s t ominimize t h e t o t a l f i r e c o s t , w hich i n cl u de s t h e c o s t o f f i r ep r o t e c t i o n p l u s e x p ec t ed l o s s , e x pe c te d l o s s b e i ng e q ua l t o t h ep r o b a b i l i t y o f f i r e t i me s t h e f i r e l o s s . Because t h e p o t e n t i a l l yl a r g e f i r e l o s s a l s o r e p r e s e n t s a c o n si d er a bl e e x p e nd i tu r e o fe n e rg y, t h e c o s t o f t h a t e ne rg y may a l s o h av e t o b e c o n s i d e r e d i n

t h e f u t u r e .

F i r e p r o t e c t i on i s the means of c o n t r o l l i n g t h e a bo ve -m en ti on edhazards. A p p ro p r ia t e m ea su re s i n c lu d e: f i r e p r e v e n t i o n ; f i r ed e t e c t i o n ; f i r e s up pr es si on ( i n c l u di n g s p r i n k l e r s a nd f i r ef i gh t ing ) ; communica t ion ; evacua t ion , r e f u g e o r re sc ue ; u s e ofb u i l d i n g m a t e r i a l s w it h a l o w c o n t r i b u t i o n t o f i r e g r o w t h ; smokec o n t r o l { p a r t i c u l a r l y f o r h i g h - r is e b u i l d in g s ) ; f i r e c o n t a i n m e n t ;


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s t r u c t u r a l s t a b i l i t y . With t he p o s s i b le e xc ep ti on o f t h e f i r s t two,a l l o f t h e s e a r e i nv ol ve d i n b u i l d in g d e si g n. Design f o r f i r es a f e t y i s t h er e fo r e a m ul t i - d is c i pl i na r y a c t i v i t y . S t r u c t u r a leng i nee r s shou ld concern themse lves p r im ar i l y w i th f i r e containmentand s t r u c t u r a l s t a b i l i t y , o r s t r u c t u r a l f i r e r es i s t an c e .

F i r e con tainment o r compar tmen tat ion i s a f un da me nt al p r i n c i p l eo f f i r e co n t r o l and f i r e s a f e t y . The b a s i c i de a i s t o c o n t r ol t h es p re ad o f f i r e o r t o p r e ve nt f i r e f rom e n t e r i n g r ef ug e a r e a s s ucha s f i r e s t a i r s . F i r e co nta in men t r e q u i r es no t o n l y t h a t t h e r e beno g a ps t h ro u gh w hi ch f i r e c a n s p r e a d , b u t a l s o t h a t a d j a c e n tc om pa rt me nt s n o t h e a t t o i g n i t i o n t em p e ra t ur e . S t r u c t u r a l c o l l a p s eo b v i o u s l y means l o s s o f f i r e c o nt a in m en t , and t h i s i s why s t r u c t u r a lf i r e r e s i s t a n c e a nd f i r e c on ta inm en t a r e c l o s e l y r e l a t e d . F igu re 1i n d i c a t e s how f i r e c on ta in me nt r e du c es f i r e h a z a r d s f o r l a r g eb u i l d i n g s . Wit ho ut f i r e c o nt a in m en t t h e e x p e c t ed l o s s g oe s upr ou gh ly a s t h e s q ua re o f t h e f l o o r a r e a .

F i r e c on ta in me nt i s r e g u l a t e d i n b u i l d i n g c o de s by s p e c i f y i n gminimum f i r e r e s i s t a nc e r a t i n g s f o r t he component s between compar t-m en ts a nd f o r t h e s t r u c t u r e s u p p o r t in g them. The f i r e r e s i s t a n c er a t i n g f o r a ny component i s t h e number o f h o u r s o f s a t i s f a c t o r yp er fo rm an ce d u r in g a s t a nd a rd f i r e t e s t . The r e q u i r e d r a t i n g s a r er e l a t e d t o t h e f i r e l o a ds ( co mb us ti bl e c on t e n tk ) l i k e l y t o oc cu ra nd t o t h e r i s k s and ha za rd s i n v ol v ed i n c a se o f f a i l u r e ; i n t h e

Nat ion al Bui ldi ng Code of Canada (1) they range from 1/ 3 t o 4 hour s ,w i t h 2 h ou rs s p e c i f i e d f o r h i g h - r i s e o f f i c e a nd r e s i d e n t i a lbu i l d i ng s . To ach ieve f i r e con tainment , howeve r, i t i s n o ts u f f i c i e n t t ha t wa l l s and f l oo rs between compar tments have a minimumf i r e r a t i n g . The f i r e may f i n d some o t h e r r o u t e , f o r ex am pl e, froms t o r e y t o s t o r e y v i a windows.

S t r u c t u r a l f i r e r e s i s t a n c e i s a l s o d ef in ed i n bu i ld i ng codes i nt er ms o f h o u r s o f s a t i s f a c t o r y pe rf or ma nc e d u r in g t h e s t a n d a r d f i r et e s t a nd t h e r e q ui r e me n t s a r e t h e same a s f o r f i r e c o nt ai nm en t;e . g . , 2 h o u rs f o r most h i g h - r i s e b u i l d i n g s .

Another d e f i n i t i o n o f s t r u c t u r a l f i r e r e s i s t a n c e i s i n termso f f i r e l o a d . During any f i r e t h e s t r u c t u r e weakens u n t i l aminimum l o a d - b ea r i n g c a p a c i t y ( s t r e n g t h ) i s r e ac h ed a f t e r b u rn - ou to f t h e f i r e l o a d . T h i s minimum s t r e n g t h d e c r e a s e s w i t h i n c r e a s e i nf i r e l o ad and t h e c r i t i c a l f i r e l oa d i s t h a t f o r which t h e a s s oc i a-t e d minimum s t r e n g t h i s e qu a l t o t h e a p p l i e d s t r u c t u r a l l o a d . I t i st h e f i r e l oa d ( c om bu st ib le c o n t e n t s ) j u s t s u f f i c i e n t t o c au ses t r u c t u r a l c o l l a ps e . The f i r e r e s i s t a n c e s a f e t y f a c t o r i s d ef in eda s t h e r a t i o o f t h e c r i t i c a l f i r e l oa d t o t h e ex pe ct ed f i r e l o a d.T hi s d e f i n i t i o n i s more c l e a r l y r e l a t e d t o t h e l i m i t s t a t e o f

s t r u c t u r a l c o l l a p s e an d may e v e n t u a l l y r e p l a c e t h e c on v en t io n ald e f i n i t i o n .


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REQUIRED STRUCTURAL FIRE RESISTANCE

How much s t r u c t u r a l f i r e r e s i s t an ce i s nec ess ary ? The answert o t h i s q u e s t i o n r e q u i r e s a c o n s i d e r a t i o n o f human s a f e t y a ndeconomics (and energy conservat ion) .

With r ega rd to t h e economic a s pe c t , r e s u l t s o f an economic-r i s k s t u d y f o r a t y p i c a l b u i l d i n g ( 2 ) a r e shown i n F i g ur e 2. Theyi n di ca t e t h a t , i n ge ne ra l, s t r u c t u r a l f i r e r e s i s t a n c e i s n o teconomic f o r norma l sma l l bu i l d i ng s , a l t hough i t i s f o r medium o rl a r g e b u i ld i n g s o r f o r b u i l d i n g s w i t h v a l ua b l e c o n t e n t s . Theeconomic f i r e r e s i s t a n c e s a f e t y f a c t o r sh ou ld , i n f a c t , i n c r e a s ew it h t h e s i z e o f t h e b u i l d i n g t o c om pens ate f o r t h e g r e a t l y

i n cr e as e d p r ob a bl e f i r e l o s s i n l a r g e b u i l d i n g s ( F ig u re 1 ) .I n c i d en t a l l y , t he c o s t o f p ro vi di ng f i r e r e s i s t a n c e i n c o n cr e tes t r u c t u r e s i s ve ry s m al l compared t o t h a t f o r o t h e r f i r e p r o t e c t i o nmeasures.

How much s t r u c t u r a l f i r e r e s i s t a n c e i s n ee de d f o r human s a f e t y ?For low b u i l d in g s , t h e s t r u c t u r a l f i r e r e s i s t a n c e r e q u i r ed f o rhuman s a f e t y n ee d o n l y b e o f t h e o r d e r o f 1 / 2 h o u r, o r en oug h t i m et o c l e a r t h e b u i l d i n g co m p le t el y. T h i s a ss um es , o f c o u r s e , t h a td ur in g t h i s t i me t h e r e a r e s t a i r s and e x i t s a d eq u at e ly f r e e o f f i r ean d smoke. For t a l l b u i l d i n g s , ho weve r, t h e s i t u a t i o n i s q u i t e

d i f f e r e n t . E va cu at io n tim e i n c r e a s e s c o n s id e r ab l y i n v e r y t a l l ,h e a v i l y p o p u l a t e d b u i l d i n g s . I n a d d i t i o n , t h e r e a r e p e o p l e who mayn o t b e c a p a b l e o f d e s c e n di n g many s t a i r s u n d e r e merge ncy c o n d i t i o n s .F ur th er mo re , f ir em e n r e q u i r e s a f e a c c e ss f o r f i r e f i g h t i n g a ndr e s c u e . F i n a l l y, i t may b e p r e f e r a b l e t o u s e r e f ug e a r e a s r a t h e rt h a t a t t em p t t o ev a c ua t e t h e b u i l d i n g . F o r t h e s e r e a so n s t h e r es ho ul d b e a low r i s k o f c o l l a p s e , a s su m in g c o mp l et e b u r n - o u t o f a nycompartment. I t a pp ea rs t h a t s t r u c t u r a l f i r e r e s i s t a n c e o f ve r yt a l l s t r u c t u r e s sh ou ld b e g r e a t e r t ha n t h a t r e qu i r ed f o r f i r ec o nt a in m en t a l o n e . B e fo r e a n y c h an g es i n e x i s t i n g r e q u i r em e n t s c a nb e pr o po s ed , h ow eve r, more s t u d i e s a r e r e q u i r e d .

STRUCTURAL FIRE RESISTANCE OF REINFORCED CONCRETE COLUMNS A N D WALLS

E x i s t i n g f i r e r e s i s t a n c e r a t i n g s f o r c o n c r e t e columns a nd w a l l sgiven in Supplement No. 2 , Nat io nal Bui ld i ng Code of Canada ( 3 ) ~r e qu i r e a minimum th i ckn ess fo r members f u l l y exposed to f i r e o fa b ou t 1 2 i n c h e s t o p a s s t h e 2 -h ou r r e q u i r e m e n t f o r m os t t a l l b u i l d -i n g s . T h i s i s u s u a l l y e a s i l y met by colum ns, b u t i t m y be ap ro bl em f o r u n pr o te c te d w a l l s l o c a t e d w i t h i n a f i r e co mp ar tm en t.

F i r e r e s i s t a n c e ha s t r a d i t i o n a l l y b een d e t er m in ed by s t a n d a r df i r e t e s t s of i s o l a t e d members c a r r y i n g s p e c i f i e d l o a d s . I t cana l s o b e d et er mi ne d by s t r u c t u r a l a n a l y s i s , i n t h e same way a s f o rnormal s t r u c t u r a l d e si g n . Us ing t h e p r i n c i p l e s o f h e a t t r a n s f e r , at e m p e r at u r e a n a l y s i s o f t h e member c r o s s - s e c t i o n u n de r s t a n d a r df i r e c o nd i ti o ns i s c a r r i e d o u t . T h i s i s fo l lowed by a s t r e s sa n a l y s i s o f t h e s e c t i o n, t a k i n g i n t o ac c o un t te m pe ra tu r e s t r a i n s ,a p p l i ed l o a ds and s t r e s s - s t r a i n p r o p e r t i e s o f c o n c r e t e ( F i g u r e 3)a nd s t e e l ( F ig u re 4 ) a t h ig h t e m p er a t u re . As may be s e e n i n


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F i g ur e 5, a c olumn o r w al l s e c t i o n i s h ig h l y s t r e s s e d n e ar t h e o u t -s i d e d u ri n g f i r e and c ra ck ed i n t h e c e n t r e . As t h e f i r e p r o g r e s s e s ,t h e o u t e r c o n cr e t e weakens, t h e s t r e s s e s a r e r e d i s t r i b u t e d to war dt h e c e n t r e , and t h e s e c t i o n f a i l s when t h e u l t i ma t e s t r e s s d i s t r i b u -t i o n i s r ea ch ed . Ul t ima te s e c t i o n s t r e n g t h can t h e r e f o r e b e c a lc u -l a t e d f o r any t e mp er at ur e d i s t r i b u t i o n and p l o t t e d a s a f u n c t i o n o ft i me o f f i r e . The f i r e r e s i s t a n c e i s t h e t im e i t t a ke s f o r s t r e n g t ht o b e r ed uc ed t o t h e l e v e l o f t h e a p p l ie d l o a d . F igu re 6 showss t r e n g t h i n t e r a c t i o n c u rv es d u r in g f i r e f o r a t y p i c a l column, f i r er e s i s t a n c e b ei ng o f t h e o r d e r o f 2 h o u rs . (The f i r e r e s i s t a n c e i nF i g u r e 6 i s g r e a t e r i n c om pr ess io n t h an i n b e n di n g b e ca u se t h ec o n c r e t e c o r e h e a t s more s l o wl y th a n t h e r e i n f o r c i n g . )

Buck1 in g e f f e c t s become more impor t an t du r ing f i r e because th eo u t e r l a y e r s weaken f i r s t , e f f e c t i v e l y i n c r ea s i n g t h e s le n d e r ne s so f t h e column o r w a l l . The i n i t i a l c a l c u l a t i o n s f o r s q u ar e columns(4) took buc kl in g i n t o account by t h e ACI method, which inv olv edc a l c u l a t i n g . t he E u le r b u ck l i ng l o a d from an i n t e g r a t e d s e c t i o ns t i f f n e s s a nd m a gn if yi ng t h e moment o r e c c e n t r i c i t y . This methodwas used i n de t e rmin ing th e r a t in gs i n Supp lement No. 2 .

I n more r e c e n t c a l c u l a t i o n s o f w a l l s and t h i n r e c t a n g u l a rcolumns (5) t h e somewhat em pi ri ca l ACI app roac h was abandoned i nf a v o u r o f a more r a t i o n a l l o a d - d e f l e c t i o n b u c k li n g a n a l y s i s ( s e eF igure 7 ) . End r e s t r a i n t i s t ak e n i n t o ac c ou n t by a b uc k li n g l e n g t hf a c t o r k a nd i m p e r f e c t i o n s su ch a s c o n s t r u c t i o n t o l e r a n c e s a ndhoneycombing a r e t a k e n i n t o a c c ou n t by a n i n i t i a l d e f l e c t i o n yo .The r e s u l t s o f s uc h l o a d - d e f l e c t i o n a n a l y s e s a r e shown i n F i g ur e 8f o r d i f f e r e n t f i r e t im es , and s tr e n g t h s a r e pi ck ed o f f a s t h emaximum loads. I n t h i s way s t a n d a r d c u r ve s of s t r e n g t h v e r s u s t i m ea r e c a l c u l a t e d ( F i gu r e s 9 and 101 and t h e f i r e r e s i s t a n c e r a t i n g i sth e t i m e a t w hi ch s t r e n g t h becomes l e s s th a n a p p l i e d l o a d . Forexample, F i gu r e 10 i n d i c a t e s t h a t a f u l l y - l o a d e d 8 - i n c h w a ll 9 f e e th igh , s u b j e ct e d t o f i r e on b ot h s i d e s , h a s a f i r e r e s i s t a n c e o fabout 2 h o u r s compared t o t h e p r e s e n t r a t i n g o f 1 h o u r. Moree x t e n s i v e r e s u l t s a r e c o n t ai n e d i n R e f er e n ce ( 5 ) . TO check t h e s e

c a l c u l a t e d r a t i n g s , a r e s e a r c h program o f column t e s t i n g w i l l b ec a r r i e d o u t i n c o o p er a ti o n w i t h P o r t l a n d Cement A s s o c i a t i o n i n anew f a c i l i t y b e i n g i n s t a l l e d a t t h e Na t io n a l R es ea rc h Co u nc i l.

S t r u c t u r a l f i r e r e s i s t a n c e can a l s o b e c a l c u l a t e d i n terms o ff i r e l o a d by r e p l a c i n g t h e s t a n d a r d f i r e t e m pe r at u r e c u l v e by morer e a l i s t i c c u rv e s t h a t depend on f i r e lo ad and v e n t i l a t i o nc h a r a c t e r i s t i c s ( 6 ) . F i g u r e 11 shows t h e r e s u l t s o f s uc h an i nves -t i g a t i o n , where t h e c r i t i c a l f i r e l oa d ( one t h a t j u s t c au se s f a i l u r e )ranges f rom 2 2 t o 32 kg/m2 o f wood, depend ing on v e n t i l a t i o nc h a r a c t e r i s t i c s , S u c h an approach, which i s of s p e c i a l i m p o r t a n c e

f o r t h e s a f e t y o f v e ry t a l l b u i l d in g s , needs f u r t h e r deve lopment .

Not o n l y i s i t p o s s i b l e t o ca l c u l a t e t h e f i r e r e s i s t an c e o fi s o l a t e d members bu t a comple t e f rame an a l ys i s can b e c a r r i e d o u tb y d e t e r m in i n g member p r o p e r t i e s and t h e i r i n t e r c o n n e c t i o n d u r i n gf i r e . Some c a lc u la t ed s t r u c t u r a l i n t e r a c t i o n e f f e c t s (5 , 6) w i l lb e d i s c u s s e d b r i e f l y .


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E x p l o s i v e p a l l i n g is due p r i m a r i l y t o b ui l d -u p o f s te amp r e s s u r e (7 ) , which c a u s e s t e n s i l e s t r e s s e s [D p i n F i g u r e 1 s) .-ac t i ng ou tward towards the exposed f ace . A secondary cause i s h ighcompressive t empera tu l le s t r e s s (oc i n F igu re 17 ) nea r t h e edge ( seeF i g u r e 5) . The bui ld -up o f s team pre ssu re depends on the anount off r e e w a t e r p r e s e n t , t h e r a t e o f v a p o r i z at i o n , and t h e p e r m ea b i l it yof t h e c o n c r e t e c o v e r. E x p l o s i v e s p a l l i n g w i l l t ak e p la c e a t t h elo ca t i on o f vapo r i za t ion , a t a t e mpera tu re somewhat above 10oC.The p o t e n t i a l f o r s p a l l i n g i n cr e a se s a s t h i s l o c at i on moves inwardd u ri n g t h e f i r e b ec au se t h e r e i s an i n c r e a s e i n f r e e w at e r c o n t en tdue t o condensat ion of inward-f lowing s team and because th er e i smore c o n c r e t e t o r e s i s t o u tw ar d st e am fl ow. A c r i t i c a l d i st an ce i sa bo u t 2 i n c h es . I f s t i r r u p s a r e l oc a t ed c l o s e r t o t h e edge th ant h i s , s p a l l i n g i s c o n t a i n e d . T h i s i s why S up pl em en t No. 2 t o t h eNa t iona l Bu i ld ing Code r e qu i r e s a dd i t io na l mesh f o r l a rg e cover i nc o l u m s .

The po te n t i a l f o r sp a l l i ng depends main ly on moi stu re con ten t ,t y p e o f a g g r e g a t e ( s i l i c a t e s s pa 11 more t h an c a r b o n a t es o r l i g h tw e i g ht s ) , r e s t r a i n t s t r e s s e s , p o r o s i t y and p e rm ea b il it y. F u r t he rs t u d y i s n eed ed t o i d e n t i f y u nd er what c o n d i t i o n s s p a l l i n g o rs p l i t t i n g c an b e s e r i o u s a nd w ha t d e s i g n m ea su re s s h o u ld b e t a k e nt o c o n t r o l t h e p r o b l e m .

CONCLUSIONS

1 . S t r u c t u r a l f i r e r e s i s ta n c e i n l ow -r is e b u il d in g s i s r e qu i re dp r i m a r i l y t o c o n t a i n f i r e s f o r e co no mi c (a nd en e rg y c o n s e r v a ti o n )r e a s o n s . F o r h i g h - r i s e b u i l d i n g s , however, i t i s a l s o e s s e n t i a lf o r human s a f e t y .2 . S t r u c t u r a l f i r e r e s i s t a n c e o f i s o l a t e d columns o r w a ll s c an b ec a l c u l a t e d by h e a t t r a n s f e r a nd s t r u c t u r a l a n a l y s i s u s i n g t empera-t u r e- d e pe n d en t m a t e r i a l p r o p e r t i e s . (New f i r e r a t i n g s a r e c a l c u l a -t e d f o r w a l l s and r e c t a n g u l a r columns l o c a t e d w i t h i n a f i r e c om pa rt -ment. )3. I n t e r a c t i o n s t u d ie s i nd i ca t e t h a t s t r u c t u r a l f i r e r e s i s t a n c e

d ep en ds p r i m a r i l y on s t r u c t u r a l c o n t i n u i t y, i n t e g r i t y and d e t a i l i n g ,an d c on s eq u en t ly t h a t t r a d i t i o n a l c a s t - i n - p l a c e c o n st r u c ti o n i sexpec ted to pe r fo rm much be t t e r du r ing f i r e t han poor ly connec tedp r e f a b r i c a t e d s ys te ms s u s c e p t i b l e t o p r o g r e s s i v e c o l l a p s e .4 . F u r t h e r s t u d y i s needed t o e s t a b l i s h t h e c o n d i t i o n s u n de r whichs p a l l i n g o r s p l i t t i n g w i l l o cc ur e a r l y i n a f i r e and t o i d e n t i f ywhat design measures, i f a n y, s ho ul d b e t a k en t o c o n t r o l the problem.

REFERENCES

1 . Nat ion al Buildin g Code of Canada 1975.

2 . Lie , T.T. , "Optimum Fi r e Re si st an ce o f S t r u c t u r e s ," ASCE, J .S t r u c t u r a l D i v i s i o n , Vol . 98 , N O . ST1, J an ua ry 1972.

3. Supplement No. 2, Na ti on al Bu il di ng Code o f Canada 1975.


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4 . L i e , T. T. a nd A l l e n , D .E ., " C a l c u l a t i o n o f t h e F i r eRes i s t a nce o f Re in fo rced Conc re t e Columns, " Na t iona l Resea rch

Cou nci l , Div is ion of Bui l d ing Research , August 1972.

5 . A l l en , D.E. and L ie , T.T.. "Ca l cu l a t e d F i r e Res i s t a nce o fRei nfo rce d Co ncr ete Walls and Re ct an gu la r Columns." To beP u b l i s h e d .

6 . Al len , D . E . and L i e. T. T., " F u r t h e r S t u d i e s o f t h e F i r eRes is tan ce of Reinforced Con cre te Columns ," Nat iona l ResearchC o u n ci l , D i v i s i o n o f B u i l d i n g R e s e a r c h , J u n e 1 9 74 .

7 . S h o r t e r , G . W . and Harma thy, T.Z . , D i scus s ion o f "F i r eR e s i s t a n c e o f P r e s t r e s s e d C o n c r e t e , " P r o c . , I . C . E . , Vo l . 20, 1 96 1,p . 313 .

T hi s pa pe r i s a c o n t r i b u t i o n f ro m t h e D i v i s i o n o f B u i l d in gResea rch , N a t iona l Resea rch Counc i l o f Canada and i s pub l i she d w i tht h e a p pr ov al o f t h e D i r e c t o r o f t h e D i v i s i o n .

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19/19

Fire Resistanc of Walls and Columns

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