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Notes from a one-day colloquium on progressive collapse
NATIONAL RESEARCH COUNCIL OF CANADA OIVISION OF BUILDING RESEARCl~l
NOTES FROM A ONE-DAY COLLOQUIUM ON PROGRESSIVE COLLAPSE Compiled b y D . A . T a y l o r and W . R . S c h r i e v e r Proceedings No. 1 o f t h e 1 ) i v i s i o n o f B u i l d i n g Kcsearch OTTAWA June 1976
TABLE OF CONTENTS Page No. P r c f n c c Notcs o f Proceedings Chairmanf s Remarks P r e s e n t a t i o n by L . R . Creasy D i s c u s s i o n ( 1 P r e s e n t a t i o n by S . Grans trom D i s c u s s i o n P r e s e n t a t i o n By E . F.P. B u r n e t t P r e s e n t a t i o n by D . E . A l l e n P o t e n t i a l V u l n e r a b i l i t y o f Primary S t r u c t u r a l M a t e r i a l s and Systems (General D i s c u s s i o n )
C o n c r e t e Masonry S t e e l 'Timber
Code Nceds (General D i s c u s s i o n ) C l o s u r e
Appc ndi x
( i ) ( i i ) ( i i i )
( i
1
PREFACE S i n c e t h e p a r t i a l c o l l a p s e o f t h e Ronan P o i n t a p a r t m e n t b u i l d - i n g i n B r i t a i n i n May 1968, t h e q u e s t i o n o f w h a t p r e c a u t i o n s a r e n e e d e d i n Canada t o e n s u r e r e a s o n a b l e s a f e t y a g a i n s t p r o g r e s s i v e c o l l a p s e h a s b e e n r a t h e r c o n t e n t i o u s . I n 1970 t h e A s s o c i a t e Committee o f t h e N a t i o n a l B u i l d i n g Code o f Canada i n t r o d u c e d a n A r t i c l e a n d a Commentary d e a l i n g w i t h p r o g r e s s i v e c o l l a p s e i n t o P a r t 4 o f t h e N a t i o n a l B u i l d i n g Code. To d a t e , h o w e v e r , d i s c u s s i o n o f t h e s e p r o v i s i o n s a n d t h e new r e v i s e d A r t i c l e a n d Commentary i nt l l c 1975 Code h a s a p p a r e n t l y b e e n f a i r l y l i m i t e d .
I n o r d c r t o promote more d i s c u s s i o n among e n g i n e e r s a n d bctwcen t h c I)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 a n d t h e e n g i n e e r i n g community, t h c D i v i s i o n d e c i d e d t o t a k e a d v a n t a g e o f t h e f a c t t h a t two wcll-known e x p e r t s i n p r o g r e s s i v e c o l l a p s e , b l r . L . R . C r e a s y f r o m G r e a t B r i t a i n , a n d D r . S.A. ~ r a n s t r s m f r o m Sweden, would b e a t t e n d i n g t h e American C o n c r e t e I n s t i t u t e c o n v e n t i o n i n B o s t o n 6 t o 11 A p r i l 1975 t o s p e a k a b o u t p r o g r e s s i v e c o l l a p s e . Tney w e r e i n v i t e d t o come t o O t t a w a o n A p r i l 1 4 t h t o p a r t i c i p a t e i n a o n e - d a y c o l l o q u i u m w i t h 2 3 C a n a d i a n e n g i n e e r s . The c o l l o q u i u m l e d t o d i s c u s s i o n s r a n g i n g o v e r a wide a r e a o f i n t e r e s t t o s t r u c t u r a l e n g i n e e r s and i t was a p p a r e n t t h a t t h e n o t c s o f t h e p r o c e e d i n g s c o u l d b e u s e f u l t o o t h e r e n g i n e e r s . Two d r a f t s o f t h e p r o c e e d i n g s w e r e p r e p a r e d and s e n t t o t h o s e p r e s e n t f o r c l a r i f i c a t i o n o f t h e p o i n t s made. T h e n , a s t h e p a r t i c i p a n t s h a d b e e n a s s u r e d t h a t t h e y c o u l d s p e a k i n c o n f i d e n c e , a l l names e x c e p t
t l l o s c o f t h e f o u r main s p e a k e r s were removed f r o m t h e t e x t o f t h c s c n o t c s .
'lllc collocluium was o r g a n i z e d by D . A . T a y l o r a n d c h a i r e d b y W . It. S c l l r i c v c r , 1,oth f r o m t h e B u i l d i n g S t r u c t u r e s S e c t i o n o f D B R . 'I'llcx I ) i vi s i o n w i sllcs t o t!l;lnh a 1 1 t h o s e who p a r t i c i p a t e d and
I'rclxtrcd conliac.nts, and c s l ~ c c i a l l y I ' r o f c s s o r I i . l : . P . R u r n c t t o f t h c Ilni v c r s i t y o f W a t e r l o o f o r Ilis a d v i c c a n d a s s i s t : ~ r l c e . 'I'hc o p i n i o n s e x p r e s s e d i n t h e s e n o t e s a r c n o t n e c e s s a r i l y t h o s e h e l d I)y t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada o r t h e 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 . O t t a w a .Jiinc 19 76 C . B . C r a w f o r d D i r e c t o r , D B R / N R C
AGENDA FOR PROGRESSIVE COLLAPSE COLLOQUIUM, 14 APRIL 19 75 9 : 3 0 A.M. I n t r o d u c t i o n Purpose o f Seminar S h o r t D e s c r i p t i o n o f t h e NBC A r t i c l e 4 . 1 . 1 . 8 SESSION I 9:55 - 11:15 A.M. 9 : 5 5 - 10:40 Talk by L . R . Creasy 10:40 - 11 115 D i s c u s s i o n 1 1 : 1 5 - 11:30 C o f f e e SIISSION 1 1 11:30 A . M . - 1:00 I'.M. --- 1 1 11:30 - 1 2 : l S Talk by S . [;r:instrom 12:15 - 1 1 0 0 D i s c u s s i o n
1:OO - 2:OO Lunch
SESSION 111 2 ~ 0 0 - 2 ~ 4 5 P.M.
2:OO - 2:30 T a l k s by E.F.P. B u r n e t t on
Abnormal Loads and by D . E . A l l e n o n t h e Source o f Canadian S t a t i s t i c s on Abnormal Loading. 2:30 - 2:45 D i s c u s s i o n o f Abnormal Loading ( S a f e t y , S t a t i s t i c s o f Loading, R i s k ) . SESSION IV 2:45 - 5 1 1 5 P.M. D i s c u s s i o n
V u l n e r a b l e Bui l d i n g Types, M a t e r i a l s , S y s t e m s , Uses. P o t e n t i a l Problems w i t h : C o n c r e t e - P r e c a s t l o a d - b e a r i n g p a n e l b u i l d i n g s . - P r e c a s t h o l l o w - c o r e f l o o r s l a b s . - Unbonded p o s t - t e n s i o n e d c o n s t r u c t i o n . - I n s i t u b u i l d i n g s . Masonry - B r i c k a n d b l o c k s t r u c t u r e s .
S t e e l - Beam and column s t r u c t u r e s , t r u s s e s . Timber - Timber r o o f s t r u c t u r e s .
SESSION V 5:15 - 5:30 P.M. C l o s u r e
LIST OF PARTICIPANTS European V i s i t o r s : L . R . Creasy - B r i t a i n S
.
A . ~ r a n strgm
- Sweden U n i v e r s i t y P r o f e s s o r s : E.F.P. B u r n e t t - Waterloo R . G . D r y s d a l e - McMas t e r M.W. Huggins-
Toronto G . S u t e r - C a r l e t o n A . Woinowsky-Krieger - Lava1 C o n s u l t a n t s : P r c c a s t e r s - -Code Comnii t t e e s : A s s o c i a t i o n s o r I n s t i t u t e s : B u i l d i n g O f f i c i a l s : R . Bergmann - Toronto T. A. E l d r i d g e - Montreal P . Roberge - Montreal J . S p r i n g f i e l d-
T o r o n t o G . Adam R . Banks - Con- Force, C a l g a r y - Winnipeg P . K l u t c h e r t - Toronto V . C . Fenton - Masonry F. J . Keenan - Timber D . J . L . Kennedy - S t e e l W.G. Plewes - C o n c r e t e J . W . Cumming - Canadian P r e s t r e s s e d C o n c r e t e I n s t i t u t e M. S h i r l a w - P o r t l a n d Cement A s s o c i a t i o n . N . Douglas-
C i t y H a l l C o r p o r a t i o n o f Ottawa E . Y . Uzumeri - Borough o f North York D.1:. A1 l e n W . (;. I'lcwcs W . R . S c h r i e v e r D . A . T a y l o rNOTES FROM A ONE-DAY COLLOQUIUM ON PROGRESSIVE COLLAPSE h e l d a t DBR/NRC, 14 A p r i l 1 9 7 5
Compilcd by
I ) . A . ' I ' a y l o r a n d W
.
R . S c h r i e v c rCI IAI RMAN' S REMARKS
The Chairman welcomed t h o s e a t t e n d i n g and, i n p a r t i c u l a r , L . K . C r e a s y from t h e U n i t e d Kingdom a n d S . A . ~ r a n s t r s m from Sweden, who h a d b e e n i n v i t e d b y t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada t o p a r t i c i p a t e i n t h e s e m i n a r f o l l o w i n g t h e i r a t t e n d a n c e a t t h e 1 9 7 5 American C o n c r c t e I n s t i t u t e C o n v e n t i o n h e l d i n B o s t o n . H e
e x p l a i n e d t h a t t h e l i s t o f t h o s e i n v i t e d h a d p u r p o s e l y b e e n k e p t s m a l l t o a1 low f o r round- t a b l e d i s c u s s i o n .
'Illc purpose o f t h e s e m i n a r was t o p r o v i d e a f o r u m f o r
di s c u s s i o n ;ind c x 1 ) ~ c s s i o n o f o p i n i o n on t h e s u b j c c t o f p r o g r e s s i vc c o l l ; ~ l ~ s c . I t was h o p c d t h a t t h c n l c c t i n g would l c a d t o rccommcnda- t i o n s t l 1 ; l t would I)c u s c f u l i n d c a l i n g w i t h p r o g r c s s j v c c o l l a p s c i n tllc N a t i o n a l Uuj l d i n g Code
.t
A l t h o u g h t h e i m p o r t a n c e o f t h e s u b j e c t was o b v j o u s t o t h o s c p r e s e n t , ' p r o g r e s s i v e c o l l a p s e ' was w i d e l y i g n o r e d i n p r a c t i c e , a s p o i n t e d o u t i n a p a p e r by D . A . T a y l o r p r e p a r e d f o r s u b m i s s i o n t o t h c C a n a d i a n . J o u r n a l o f C i v i l E n g i n e e r i n g * ( t h i s p a p e r a n d some o t h e r m a t e r i a l was s e n t t o a l l p a r t i c i p a n t s i n a d v a n c e o f t h e m e e t i n g ).
F'erhaps o n e r e a s o n t h c s u b j e c t was s t i l l w i d e l y i g n o r e d i n Canada was t h a t t h e r e h a d n o t b e e n any m a j o r p r o g r e s s i v e c o l l a p s e si 11 o c c u p i e d h i g h - r i s e b u i l d i n g s i n Canada. T h e r e h a d b e e n , h o w e v e r ,
'1' N ; ~ t i o l ~ : r l I i ~ r i l c l i n g Coclc o f Canada 1 9 7 5 , A r t i c l e 4 . 1 . 1 . 5 and (:o~~inic~lt;l ry (: i n Supplclncnt N o . 4 . P u b l . No. NRCC 13989 o f t h c N ; l t i on;^ l I(csc.;~rch Count i 1 o f C a n a d a , O t t a w a .
*
I l r , I . I ' r o g r c s s i v e C o l l a p s e . Can. J . C i v i l E n g . , Vol. 2 , No. 4 , Ilcc. 1975, p . 51 7-529: a l s o T e c h . P a p e r No. 450a c c o r d i n g t o d a t a c o l l e c t e d by DBR, 72 i n c i d e n t s o f l ~ r o g r e s s i v c c o l l a p s e i n t h e 1 0 y e a r s from 1962 t o 1972, h a l f o f them d u r i n g c o n s t r u c t i o n . The Chairman s u g g e s t e d t h a t t h e p r e s e n t t r e n d s i n c o n s t r u c t i o n t o w a r d more e f f i c i e n t u s e o f m a t e r i a l and l e s s c o s t l y s t r u c t u r e s might have l e a d t o some s t r u c t u r e s w i t h l i t t l e i n h e r e n t t o u g h n e s s a n d r e s i s t a n c e t o p r o g r e s s i v e c o l l a p s e .
PRESENTATION BY L . R . CREASY
M r . C r e a s y n o t e d t h a t b u i l d i n g s were becoming more s e n s i t i v e t o a b n o r m a l l y h i g h l o a d s a n d t o e r r o r s i n d e s i g n and c o n s t r u c t i o n . I n a d d i t i o n s o p h i s t i c a t e d d e s i g n s were now t a k i n g a c c o u n t o f t l l r e c - d i m e n s i o n a l e f f e c t s a n d t h e s t r e n g t h o f c l a d d i n g t h u s c r o d i n g m a r g i n s o f ' s a f e t y 1 l ~ i t h e r t o unused i n d e s i g n . N o n s t r u c t u r n l
e l e m e n t s were now s o s l e n d e r and i n s u b s t a n t i a l t h a t tllcy were u n a l ~ l c t o a s s i s t a t a l l i n p r e v e n t i n g d i s a s t e r . O f t e n e l e m e n t s t h a t c o u l d h e l p were i n e f f e c t i v e b e c a u s e t h e c o n n e c t i o n s were i n a d e q u a t e t o c a r r y i m p a c t f o r c e s . B u i l d i n g R e g u l a t i o n s I n t h e a r e a o f b u i l d i n g r e g u l a t i o n s , M r . C r e a s y o u t l i n e d t h e B r i t i s h s y s t e m where t h e ' R e g u l a t i o n s D19' t h e F i f t h Amendment, which was s t i l l i n f o r c e , s e t o u t t h e g e n e r a l f u n c t i o n a l r e q u i r e - m e n t s . The s p e c i f i c ' m a t e r i a l s ' codes o f p r a c t i c e , which p r o v i d e d t h e g e n e r a l r u l e s , were 'deemed t o s a t i s f y ' t h e s e . I n t h e R e g u l a t i o n s D19, t h e d i f f i c u l t p r o b l e m o f t r y i n g t o s p e c i f y a way o f p r e v e n t i n g p r o g r e s s i v e c o l l a p s e was s o l v e d by r e q u i r i n g a check on t h e u l t i m a t e s t a b i l i t y o f t h e s t r u c t u r e i n : ( a ) t h e 'undamaged' s t a t e , i . e . , a b o u t t o c o l l a p s e b u t w i t h a l l members o f t h e frame i n p l a c e ; a n d (b) t h e 'damaged' s t a t e , i
.
e.
,
a b o u t t o c o l l a p s e when some s p e c i f i e d membcr was m i s s i n g ( n o t i o n a l removal).
(A member f o r t h i s p u r p o s e was a s l a b , beam, column, a n d a s e c t i o n o f b e a r i n g w a l l between i n t e r s e c t i n g w a l l s n o t g r e a t e r t h a n 2 1 / 4 t i m e s t h e s t o r e y h e i g h t .) Dl9 s p e c i f i e d t h a t u n d e r (b) t h e s p r e a d o f l o c a l c o l l a p s e had t o b e c o n f i n e d t o 3 s t o r e y s v e r t i c a l l y and 750 f t 2 h o r i z o n t a l l y o r 15 p e r c e n t o f t h e t o t a l f l o o r a r e a . M r . C r e a s y i n d i c a t e d t h a t t h e r e was b a s i c agreement t h a t t h e way o f s a t i s f y i n g t h e f u n c t i o n a l r e q u i r e m e n t was :
(1) t o e n s u r e t h a t t h e means f o r a r r e s t i n g p r o g r e s s i v e c o l l a p s e were s e l f - c o n t a i n e d i n each s t o r e y , (2) and e i t h e r t o d e s i g n t o r e s i s t a s p e c i f i e d c x p l o s i v e p r e s s u r e , - o r ( 3 ) t o t i e t h e whole s t r u c t u r e t o g e t h e r i n o r t h o g o n a l d i r e c t i o n s t o r e s i s t a s p e c i f i e d f o r c e . E x p l o s i v e P r e s s u r e S o l u t i o n P o i n t ( 2 ) , n o t e d M r . Creasy, was f u n d a m e n t a l l y u n s a t i s f a c t o r y and r e q u i r e d o v e r l y heavy components w i t h no a s s u r a n c e t h a t p r o g r e s - s i v e c o l l a p s e would b e p r e v e n t e d . I n a d d i t i o n t h i s s o l u t i o n d i d n o t make e f f i c i e n t u s e o f lcommunity psychology' which a p p a r e n t l y a c c e p t e d l o c a l damage.
T i e d Frame S o l u t i o n
P o i n t ( 3 ) , t h e more e f f i c i e n t s o l u t i o n , depended on members b e i n g p r o p e r l y j o i n e d , n o t n e c e s s a r i l y by heavy moment c o n n e c t i o n s , b u t by l i n k e d - p i n c o n n e c t i o n s a d e q u a t e t o c a r r y t h e r e q u i r e d f o r c e s and p r o v i d e n e c e s s a r y s t a b i l i t y .
P a r t i a l o r Residual S t a b i l i t y
S i n c e Ronan P o i n t (1968), B r i t i s h Codes o f P r a c t i c e had s p e c i f i e d t i e s i n t h e p r i n c i p a l and s e c o n d a r y o r t r a n s v e r s e s p a n d i r e c t i o n s . The t i e f o r c e s , c o n s i s t e n t w i t h t h e c o n c e p t o f p a r t i a l o r r e s i d u a l s t a b i l i t y , were n e c e s s a r y t o p r e v e n t t h e s p r e a d of a l o c a l f a i l u r e when a l o a d - b e a r i n g member had become i n e f f e c t i v e
.
C a r e f u l d e s i g n arrangement o f t h e form o f t h e b u i l d i n g and c a r e f u ld e s i g n o f t h e j o i n t s between components would b e n e c e s s a r y t o b r i d g e o r s t r i n g a c r o s s an e l e m e n t t h a t h a d been removed. However, t h e d c s i g n o f t h e e l e m e n t s f o r b e n d i n g would b e l a r g e l y u n a f f e c t e d . M r . Creasy c o n f i r m e d t h a t t h i s manner o f d e s i g n i n g t o p r e v e n t p r o g r e s s i v e c o l l a p s e would i n c r e a s e t h e c o s t o f t h e b u i l d i n g o n l y nla rg i n a 1 1 y
.
M r . ( : r c : ~ s y showed a s l i d e ( F i g u r e 1)* i l l u s t r a t i n g t h e g e n e r a l c o n t l i t i o n in a n i n t e r n a l bay a f t e r t h e l o s s o f a c e n t r a ls u p p o r t and i 1 l u s t r a t e d how t o compute t h e c a t e n a r y f o r c e s n e c e s s a r y t o keep t h e f l o o r s l a b s , of l e n g t h L , hanging i n p o s i t i o n . When t h e lr~aximum p e r m i t t e d s a g , A , was known, t h e h o r i z o n t a l component, T, o f t h e c a t e n a r y t i e f o r c e a t t h e s u p p o r t was o b t a i n e d a s shown:
W L ~ - maximum moment
:.
T
= - - 2A maximum sag C A T E N A R Y I N P L A N 1 B E A M T I E D O U T S I D E D A M A G ~ D B ~ A M A N D O U T E R C O L U M N I N T E R N A L B A Y O U T S I D E B A Y E M E R G E N C Y M O M E N T ( M ) N O R M A L C O N D I T I O N : W O R K I N G L O A D ( W ) O N S P A N ( L ) E M E R G E N C Y C O N D I T I O N : W O R K I N G L O A D ( 2 W ) O N S P A N ( 2 L ) C R I T E R I A O F P A R T I A L S T A B I L I T Y O N L O S S O F C E N T R A L S U P P O R T C A T E N A R Y ( T = M/, ) 1 ) AQ20°/o L ' LI S E C T I O N A - A C O L L A P S E L O A D,
,
2, W O R K I N G L O A DTo s u s t a i n t h e f o r c e T t h e j o i n t s between t h e s a g g i n g members and t h e s u r r o u n d i n g s t r u c t u r e had t o be c a r e f u l l y d e s i g n e d . For example i n c a s t - i n - p l a c e c o n c r e t e t h e bottom r e i n f o r c e m e n t i n t h e s l a b s h o u l d b e lapped a t l e a s t 1 2 d i a m e t e r s o v e r t h e s u p p o r t , and i n p r e c a s t c o n c r e t e t h e r e i n f o r c e m e n t s h o u l d i n t e r l o c k o v e r t h e s u p p o r t s . O u t e r Bav E v e n t u a l l y t h e c a t e n a r y t i e f o r c e s , e s p e c i a l l y from b a y s n e a r t h e edges o f t h e b u i l d i n g , might show up a s unbalanced f o r c e s a t t h e f a s c i a o r edge beams on t h a t l e v e l . blr. Creasy showed how t l i c s c f o r c e s c o u l d b e c a r r i e d by t h e edge beam a c t i n g a s a h o r i z o n - t a l c a t e n a r y i n - p l a n . I n a d d i t i o n , t h e same edge beam, o r r e i n - f o r c e d edge o f t h e f l o o r s l a b , might have t o c a r r y f o r c e s i n
e l e v a t i o n due t o t h e l o s s o f a w a l l p a n e l . Ile s t r e s s e d t h a t f l o o r s o f p r e c a s t s t r i p components h a d t o b e c a r e f u l l y anchored a t t h e i r e n d s and t h a t when t h i s was p r o p e r l y done such a f l o o r would behave a s i f c a s t - i n - p l a c e . Corner Bay The c o r n e r s o f a b u i l d i n g were v u l n e r a b l e a r e a s b u t i f t h e r e q u i r e m e n t t h a t a l l t h e c o r n e r e l e m e n t s i n c l u d i n g t h e r o b u s t non- s t r u c t u r a l e l e m e n t s b e p r o p e r l y t i e d t o g e t h e r was f o l l o w e d , t h e n t h e p a r t i a l s t a b i l i t y o f a s t r u c t u r e w i t h a damaged c o r n e r would b e a s s u r e d .
M r . Creasy a l s o showed how t h e membrane and c a n t i l e v e r a c t i o n of t h e f l o o r s l a b c o u l d b e used when a c o r n e r column was removed and how a s t r u t a c t i o n c o u l d b e developed by t h e u s e o f a n edge beam o r s t r e n g t h e n e d edge o f t h e s l a b t o accommodate t h e c a t e n a r y
t i e f o r c e r e s u l t i n g from a a t e n a r y a c t i o n o f t h e s l a b when t h e columns a d j a c e n t t o t h e c o r n e r were damaged ( F i g u r e 2 ) . The l o s s o f a c o r n e r , o r t h e columns a d j a c e n t t o t h e c o r n e r was ~ l s u a l l y nlorc s c v c r e than t h e l o s s o f a c o r n e r w a l l panel which c o u l d b c
r e a d i l y ;rccommodatcd u s i n g a wall/beam approach w i t h p r o p e r connections I)etwecn p a n e l s ( F i g u r e 3 ) .
I n f i l l P a n e l s
Diagonal compression o f f u l l i n f i l l p a n e l s o r even s i l l h e i g h t i n f i l l p a n e l s , a s shown i n F i g u r e 4 , would s t a b i l i z e a frame s p a n n i n g o v e r a m i s s i n g column i f t h e beam-to-column j o i n t s and t h e column were a d e q u a t e t o t a k e t h e a x i a l and f l e x u r a l
D A M A G E D ' N E X T - A D J A C E N T ' C O L U M N
VER C O N A D J A C E N T W A L L C A N T I L E V E R O U T S I D E B A Y INNER B A Y T I T C H I N G P A N E L S
FIGURE 4 SILL tlEIGI1T WALL P r e c a s t Wall P a n e l s o f One-Storey H e i g h t P a n e l s o f t h i s t y p e , s a i d M r . C r e a s y , were w e l l s u i t e d , i f p r o p e r l y t i e d t o g e t h e r a n d t o t h e f l o o r s , f o r s p a n n i n g o v e r a m i s s i n g p a n e l a t a c o r n e r l o c a t i o n o r a l o n g a w a l l . The n e c e s s a r y h o r i z o n t a l t i e i n t h e p l a n e o f t h e w a l l t o make beam a c t i o n ~ ' o s s i b l e c o u l d b e p r o v i d e d by a r o d l y i n g i n t h e j u n c t i o n o f t h c f l o o r and t h c u p p e r a n d l o w c r w a l l p a n e l s . 'I'his r e i n f o r c e r n c n t t h c n would b e l i n k e d by l o o p e d b a r s p r o t r u d i n g from t h e e n d s o f t h e v a r i o u s i n t e r s e c t i n g p a n e l s . ' h e l o n g i t u d i n a l b a r was i n t e n d e d t o a c t a s t h e t e n s i o n ocrnhcr o f a s t o r e y - h e i g h t beam s p a n n i n g a s p a c e 2 1 / 4 times t h e s t o r e y Ilci g h t
.
l ' c s t s h a d a p p a r e n t l y c o n f i r m e d t h a t s h e a r between t h e w a l l p a n e l s when a c t i n g as a beam c o u l d be a d e q u a t e l y c a r r i e d by g r o u t e d u n r e i n f o r c e d v e r t i c a l j o i n t s .I n a d d i t i o n , h e n o t e d t h a t t h e s e n s i t i v i t y o f c r o s s - w a l l c o n s t r u c t i o n t o p r o g r e s s i v e c o l l a p s e c o u l d b e reduced by t y i n g t h e p a n e l s , where n e c e s s a r y , t o s u b s t a n t i a l n o n - s t r u c t u r a l p a r t i t i o n s . Load- Bearing Masonry
The t r a d i t i o n a l t y p e o f mu1 t i - s t o r e y masonry used i n B r i t a i n , t h a t o f c e l l u l a r c o n s t r u c t i o n w i t h r e l a t i v e l y s h o r t s p a n s , h a d , i n most c a s e s , enough i n t e g r i t y t o s u r v i v e a g a s e x p l o s i o n . However, t h e modern masonry b u i l d i n g s o f c r o s s - w a l l p l a n w i t h non-load- b e a r i n g f a s c i a w a l l s were p o t e n t i a l l y v u l n e r a b l e t o p r o g r e s s i v e c o l l a p s e i f a c r o s s - w a l l o r e s p e c i a l l y an end w a l l s u f f e r e d l o c a l damage. The u s e o f s p i n e w a l l s and r e t u r n s on u n s u p p o r t e d w a l l s would t e n d t o p r e s e r v e t h e c r o s s - w a l l beyond t h e ' s p i n e ' o r ' r e t u r n '
I t was n e c e s s a r y t h a t t h e f l o o r s l a b s above a damaged s e c t i o n l a t e r a l l y s u p p o r t t h e w a l l s . A l s o , t o e n s u r e t h a t s l a b s would b e a b l e t o span a c r o s s a damaged w a l l by c a t e n a r y a c t i o n , l o o p s i n t h e ends o f t h e s l a b s s h o u l d b e l i n k e d around a l o n g i t u d i n a l r o d i n t h e w a l l . I n a d d i t i o n , M r . Creasy s a i d t h a t i n masonry w a l l s v e r t i c a l r e i n f o r c i n g s p a c e d no f u r t h e r a p a r t t h a n t h e s t o r e y h e i g h t was n e c e s s a r y t o e n s u r e t h a t t h e w a l l s and f l o o r s l a b s would a c t
t o g e t h e r a s a beam.
A s t h e end w a l l was always v u l n e r a b l e , i f c o n t i n u i t y o f t h e s p i n e w a l l i n t h e end bays was q u e s t i o n a b l e , t h e end w a l l s h o u l d
b e framed. I n a framed o r beam-and-column system t h e columns h a d
t o b e w e l l t i e d through t h e f l o o r diaphragm t o t h e r e s t o f t h e s t r u c t u r e
.
Mutual S u p p o r t F a c t o r A s c a t e n a r y a c t i o n o f t h e f l o o r s l a b h a d been c o n s i d e r e d s e p a r a t e l y from c a n t i l e v e r a c t i o n o f w a l l s f o r s i m p l i c i t y , t h e B r i t i s h d e c i d e d , when g i v i n g t h e n u m e r i c a l v a l u e o f t i e f o r c e c a l c u l a t e d f o r c a t e n a r y a c t i o n o f two a d j a c e n t f l o o r s p a n s , t o r e d u c e t h e f o r c e s by a Mutual S u p p o r t F a c t o r o f 2 . (Authors'
Note: T h i s might n o t b e a w i s e t h i n g t o do f o r t h e l a r g e r s p a n s o ff l o o r s used i n North America where c a t e n a r y a c t i o n o v e r two s p a n s might b e i m p r a c t i c a l .)
C o s t s
The i n t r o d u c t i o n o f r e q u i r e d t i e s i n c r e a s e d c o s t s by a
n e g l i g i b l e amount. I n l o a d - b e a r i n g masonry, however, t h e form o f a v u l n e r a b l e b u i l d i n g might have t o b e r e p l a n n e d .
European P r a c t i c e
M r . Creasy q u i c k l y o u t l i n e d a comparison between t h e B r i t i s h a n d Swedish p r a c t i c e s and showed a s l i d e comparing t i e f o r c e s
r e q u i r e d i n m u l t i - s t o r e y c o n s t r u c t i o n ( F i g u r e 5)
.
I n t h e Swedish Code o n l y 1 / 3 o f t h e l i v e l o a d was used i n computing t h e c a t e n a r y f o r c e s s o t h e v a l u e s a r e somewhat l e s s ; however t h e Swedish r e g u l a - t i o n s a p p l i e d t o low b u i l d i n g s and t h e f o r c e s c o n t i n u e d t o i n c r c a s c a f t e r 10 s t o r e y s u n l i k e t h e B r i t i s h t i e f o r c e s . 'Ihe B r i t i s h f o r c e s were c a l c u l a t e d f o r two 1 7 - f t s p a n s , c a r r y i n g a dead l o a d p l u s l i v el o a d o f 150 p s f t o t a l , and w i t h assumed s a g o f 23.6 p e r c e n t o f a s i n g l e span a t 5 s t o r e y s , and 16 p e r c e n t a t 10 s t o r e y s . These v a l u e s were t h e n d i v i d e d by 2, t h e Mutual S u p p o r t F a c t o r , where a p p r o p r i a t e .
General
M r . Creasy ended h i s p r e s e n t a t i o n w i t h t h e r e s u l t s o f e x p e r i - mental work ( n o t g i v e n h e r e ) - and w i t h some g e n e r a l c o n c l u s i o n s .
( i ) Modern b u i l d i n g s a r e g e n e r a l l y formed w i t h l i g h t c l a d d i n g and s u p p o r t e d on a p i n n e d form o f s t r u c t u r e . They have l a r g e l y l o s t t h e ' h i d d e n bonus1 c a r r i e d by t h e t r a d i - t i o n a l b u i l d i n g which p r o v i d e d a c o n s i d e r - a b l e s a f e g u a r d a g a i n s t unexpected c a l a m i t y
.
( i i ) I n t h e s e c i r c u m s t a n c e s , t h e community e x p e c t s t h e s t r u c t u r a l e n g i n e e r t o b u i l d i n t o h i s d e s i g n a r e a s o n a b l e l e v e l o f s a f e - g u a r d a g a i n s t p r o g r e s s i v e c o l l a p s e a r i s i n g from an u n f o r e s e e n h a z a r d . ( i i i ) For t h i s purpose, i t i s n e c e s s a r y t o examine t h e d e s i g n i n b o t h t h e 'undamaged' and damaged's t a t e . ( i v ) I t i s p a r t i c u l a r l y n e c e s s a r y t o e n s u r e t h a t a d j a c e n t components a r e a d e q u a t e l y t i e d t o g e t h e r o v e r an a r e a o f l o c a l f a i l u r e d e s p i t e s e v e r e l o c a l d i s t o r t i o n i n t h e framc, f o l l o w i n g t h e l o s s o f a s u p p o r t.
(v) The c a p a c i t y o f a t i e d s t r u c t u r e t o behave i n t h i s way can b e f o r e c a s t from t h ep r i n c i p l e s o f p a r t i a l s t a b i l i t y , b a s e d on t h e c a t e n a r y shape o f t h e damaged frame. ( v i ) These e n a b l e t h e d e s i g n e r t o d e a l w i t h
p a r t i c u l a r problems from f i r s t p r i n c i p l e s , o r t o deduce numerical v a l u e s f o r a Code o f P r a c t i c e , which a r e a p p l i c a b l e t o t h e common c a s e .
I
I
W O R K I N G L O A D = 1 5 0 p . s . f .c-
S P A N = 1 7 f tI-
I I
I V V I L L )T I E F O R C E S , ( C . P . 1 1 0 ) - 7
1
.,
N U M B E R O F S T O R E Y S
( v i i ) The c o s t o f t h i s l e v e l o f i n s u r a n c e when i n c o r p o r a t e d i n a new d e s i g n i s q u i t e n e g l i g i b l e . D i s c u s s i o n I n t h e d i s c u s s i o n f o l l o w i n g h i s t a l k , M r . Creasy was a s k e d w h e t h e r t h e 750-sq f t a r e a s h o u l d n o t b e r e p l a c e d by an a r e a r e l a t e d t o t h e bay s i z e . H e r e p l i e d t h a t t h e d e s i g n p h i l o s o p h y was t h e
removal of e i t h e r one beam, one s l a b o r one w a l l and n o t necessarily
750 s q f t . N e v e r t h e l e s s 750 f t 2 seemed t o b e a b o u t t h e l i m i t a c c e p t a b l e t o t h e p u b l i c . i I e a l s o e x p l a i n e d t h a t CP 110, P a r t 1 ,
1972, was a recommended code o f good s t a n d a r d p r a c t i c e and was one
o f t h e s t a n d a r d s 'deemed t o s a t i s f y ' t h e B u i l d i n g R e g u l a t i o n s . I n f u r t h e r d i s c u s s i o n M r . Creasy i n d i c a t e d t h a t t h e development o f c a t e n a r y a c t i o n and c a n t i l e v e r i n g o f s l a b s e t c . was used a s a b a s i s f o r a n a l y z i n g t h e consequences o f t h e n o t i o n a l removal o f one
member. I t a p p e a r e d t h a t i n i t i a l l y t h e B r i t i s h approach was based l a r g e l y on t h e assumption o f a g a s e x p l o s i o n and t h a t t h e problem o f b a f f l i n g and p r e s s u r e r e l i e f from window w a l l s was p a r t o f t h e
c o n c e p t . There was c o n s i d e r a b l e d i s c u s s i o n on how t o a c h i e v e adequat-ely tough ( s t r o n g and d u c t i l e ) c o n n e c t i o n s between p r e c a s t p a n e l s by r e i n f o r c i n g l o o p s t h a t c o u l d b e j o i n e d t o g e t h e r ( u s u a l l y by a t i e b a r through t h e l o o p s ) d u r i n g t h e c o n s t r u c t i o n p r o c e s s . Mr. Creasy a l s o s a i d t h a t f u r t h e r r e s e a r c h , e s p e c i a l l y i n t h i s f i e l d would b e v e r y d e s i r a b l e . I n r e p l y t o a q u e s t i o n , M r . Creasy s a i d t h a t i f a g a s e x p l o s i o n removed two o p p o s i t e l o a d - b e a r i n g w a l l s c a t e n a r y a c t i o n o v e r t h r e e s p a n s might b e r e q u i r e d . There was c o n s i d e r a b l e d i s c u s s i o n on t h i s b e c a u s e o f t h e l a r g e i n c r e a s e i n f o r c e s produced. M r . C r e a s y t h e n e x p l a i n e d how t o d e t e r m i n e t h e t i e s r e q u i r e d f o r t h i s from c o n s i d e r - a t i o n of t h e damaged s t a t e .
The problems o f p r o g r e s s i v e c o l l a p s e i n o n e - s t o r e y long-span r o o f c o n s t r u c t i o n , o f p r o g r e s s i v e c o l l a p s e d u r i n g t h e c o n s t r u c t i o n s t a g e , a n d o f t h e need f o r s t r e n g t h e n i n g o f e x i s t i n g b u i l d i n g s were a l s o d i s c u s s e d . Commenting on t h e l a t t e r , M r . Creasy s a i d t h a t t h e B u i l d i n g R e g u l a t i o n s were mainly i n t e n d e d f o r new c o n s t r u c t i o n b u t t h a t some o f t h e B r i t i s h p r e c a s t c o n c r e t e b u i l d i n g s p r e d a t i n g t h e new r e g u l a t i o n s were a c t u a l l y s t r e n g t h e n e d .
One o f t h o s e p r e s e n t , commented t h a t h e t h o u g h t t h e concept o f 'damage volume' was u n n e c e s s a r y . T h i s c o n c e p t , p r e f e r r e d i n Sweden f o r t h e p u r p o s e o f c h e c k i n g l o c a l and g l o b a l s t a b i l i t y , was u s e d t o d e f i n e t h e maximum damage a l l o w e d i n d e s i g n when a l o c a l
f a i l u r e o c c u r r e d . M r . Creasy n o t e d t h a t CP 110, a l t h o u g h deemed t o s a t i s f y B r i t i s h r e g u l a t i o n s d i d n o t , i n f a c t , s p e c i f y any damaged volume.
Another p a r t i c i p a n t wanted t o know w h e t h e r t h e B r i t i s h had any r e g u l a t i o n s c o n c e r n i n g b r a c i n g ( i . e . , i f a p a n e l o f b r a c i n g was removed). M r . Creasy answered t h a t b r a c i n g was covered by t h e d e s i g n c o n c e p t s b u t n o t s p e c i f i c a l l y by t h e r e g u l a t i o n s .
A t h i r d wished t o know how t h e B r i t i s h monitored whether t h e r e g u l a t i o n s on p r o g r e s s i v e c o l l a p s e were b e i n g f o l l o w e d . The answer was t h a t a l t h o u g h t h e y r e l i e d p r i m a r i l y on t h e d e s i g n e r , through t h e honour system, many b u i l d i n g o f f i c i a l s now examined t h e d e s i g n t o s e e t h a t i t conformed w i t h p r o g r e s s i v e c o l l a p s e d e s i g n c o n c e p t s . A q u e s t i o n was r a i s e d a b o u t a p a r t m e n t b u i l d i n g s c o n s t r u c t e d of l a r g e l o a d - b e a r i n g p a n e l s . These b u i l d i n g s depended f o r t h e i r l o n g i t u d i n a l s t a b i l i t y o n t h e s t a i r w a y s and e l e v a t o r s h a f t s a n d f o r t h e i r s t a b i l i t y a c r o s s t h e b u i l d i n g on l a r g e l o a d - b e a r i n g p a n e l s t h a t s t o p p e d e i t h e r s i d e o f a c o r r i d o r . A s h o r t Tee-beam a c r o s s t h e c o r r i d o r c a r r i e d t h e f l o o r s l a b s . S i n c e t h e c e n t r a l c o r r i d o r w a l l s were n o t l o a d - b e a r i n g i t seemed t h a t t h e b u i l d i n g was 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 i f any o f t h e b e a r i n g w a l l s was 'removed'
.
T h i s would o c c u r b e c a u s e o f t h e l a c k o f l o n g i t u d i n a l s p i n e w a l l s and t h e i n a b i l i t y o f t h e b e a r i n g w a l l s t o c a n t i l e v e r f o r w a r d from t h e c o r r i d o r o p e n i n g b e c a u s e o f i n a d e q u a t e s h e a r r e s i s t a n c e a c r o s s t h e c o r r i d o r ( F i g u r e 6).
L O A D - B E A R I N G P A N E L S L O A D - B E A R I N G N O N - L O A D - B E A R I C O R R I D O R W A L L C O R R I D O R E L E V A T I O NFIGURE 6 LARGE PANEL BUILDINGS WIT11 NON-LOAD-BEARING CORRIDOR WALLS
Another i n q u i r y concerned o f f i c e b u i l d i n g s . Most o f them h a d l a r g e open s p a c e s b u t on t h e o t h e r hand no g a s . M r . Creasy sug- g e s t e d t h a t c a t e n a r y and membrane a c t i o n o f t h e s l a b s c o u l d b e u s e d t o advantage i n t h i s c a s e t o p r e v e n t a h o r i z o n t a l o r v e r t i c a l p r o g r e s s i o n o f t h e c o l l a p s e , s h o u l d a l o a d - b e a r i n g element f a i l . P A R T O F W A L L S L I R V I V I N G E X P L O S I O N L O A D - B E A R I N G C R O S S - W A L L S / \ P L A N
FIGURE 7 EXPLOSION REMOVING TWO LOAD-BEARING CROSS-WALLS
A f u r t h e r q u e s t i o n concerned t h e c a s e where an e x p l o s i o n d e s t r o y e d two p a r a 1 l e l l o a d - b e a r i n g c r o s s - w a l l s ( F i g u r e 7) ; how c o u l d t h e s t r u c t u r e above s p a n o v e r such a gap? I n t h i s c a s e M r . C r e a s y i n d i c a t e d t h a t t h e d e s i g n problem was t o s e e t h a t o n l y p a r t o f e a c h o f t h e c r o s s - w a l l s would b e blown away. The problem
c o u l d b e s o l v e d by t h e u s e o f s p i n e w a l l s o r s t r o n g p o i n t s ( l o c a l l y s t r e n g t h e n e d a r e a s t h a t would r e s i s t t h e abnormal l o a d s ) .
M r . Creasy was a s k e d t h e n w h e t h e r p r e c a s t s l a b s c o u l d b e c o u n t e d on f o r two-way a c t i o n . He answered t h a t p r o v i d e d l o n g i t u d i n a l and t r a n s v e r s e t y i n g s t e e l , u s u a l l y i n t h e p l a n e o f t h e f l o o r , c o u l d e f f e c t i v e l y b i n d b a y - s i zed a r e a s t o g e t h e r i n a ' p i c t u r e frame'
e f f e c t ( F i g u r e 8) some measure o f two-way a c t i o n c o u l d b e developed. B A Y T I E STEEL A R O L I N D B A Y , B I N D I N G P R E C A S T S L A B S I N " P I C T U R E F R A M E " E F F E C T P L A N
He p r o c e e d e d t o i l l u s t r a t e , a t t h e r e q u e s t o f a p a r t i c i p a n t , t h e c a t e n a r y s a g c a l c u l a t i o n o v e r s p a n 2L f o r a 1 0 - s t o r e y b u i l d i n g . tie e x p l a i n e d t h e b a s i c c a s e a s f o l l o w s ( r e f e r back t o F i g u r e 5 ) . C a t e n a r y t i e f o r c e CTF = w ( 2 L ) 2 = 3.13wL f o r one-way a c t i o n and 8(.16L ) s a g s a g o f 16 p e r c e n t o f L a l l o w e d . Assuming w = 150 p s f and L = 1 7 f t . t h e n CTF = 3.13 x 150 x 17' = 8000 l b / f t f o r 1-way a c t i o n = 4000 l b / f t f o r 2-way a c t i o n
For a 4 - s t o r e y b u i l d i n g t h e assumed s a g was 23.6 p e r c e n t o f t h e s p a n , L , i . e . , S a g A
-
4 ' o r a b o u t h a l f t h e minimum s t o r e y h e i g h t ..
CTF = 2.12 w L = 5,400 l b / f t 1-way = 2,700 l b / f t 2-way I n r e s p o n s e t o an i n q u i r y a b o u t t h e c o n s t r u c t i o n p h a s e a s i t r e l a t e d t o p r o g r e s s i v e c o l l a p s e , M r . Creasy n o t e d t h a t measures t a k e n t o p r e v e n t p r o g r e s s i v e c o l l a p s e o f t h e f i n i s h e d b u i l d i n g , i n B r i t a i n , a l s o a p p l i e d t o t h e p a r t i a l l y f i n i s h e d s t r u c t u r e under c o n s t r u c t i o n . He a l s o n o t e d t h a t t h e d e s i g n e r h a d t o work i n c l o s e a s s o c i a t i o n w i t h t h e c o n t r a c t o r s . ( I n t h e w r i t t e n d i s c u s s i o n i t was n o t e d t h a t i n Canada t h e s t r u c t u r a l e n g i n e e r was n o t r e s p o n s i b l ef o r c o n s t r u c t i o n . )
PRESENTATION BY
s
.
G R A N S T R ~ MD r . GranstrGm p r e s e n t e d a summary o f t h e p r e s e n t approach i n Sweden. S t a t i s t i c s i n d i c a t e d t h a t between 2 and 5 p e r c e n t o f b u i l d i n g s ( 1 i n 50 t o 1 i n 20) would e x p e r i e n c e some e x c e s s i v e o r abnormal l o a d n o t p r e v i o u s l y f o r e s e e n by t h e Code whereas o n l y 1 i n 10,000 ( 0 . 0 1 p e r c e n t ) would e x p e r i e n c e a dangerous c a s e w i t h - i n t h e r u l e s ( o r d i n a r y o v e r l o a d i n g , e t c . )
.
He f e l t t h a t a ' f a i l s a f e ' b e h a v i o u r was t h e main g o a l . Many f a i l u r e s o r n e a r f a i l u r e s had n o t come t o t h e p u b l i c a t t e n t i o n and t h a t i n o r d e r t o o b t a i n p r o p e r s t a t i s t i c s t h e s e had t o b e t r a c k e d down. D r . Granstr8m showed t h e assembly one o f h i s r e p o r t s * ( p a r t o f which had been*
~ r a n s t r ' d m , S . and C a r l s s o n , M. 1974. Byggforskningen T3:Byggnaders Beteende Vid O v e r p a v e r k n i n g a r (The Behaviour o f
B u i l d i n g s a t E x c e s s i v e Loadings)
.
Swedish I n s t . f o r B u i l d i n g Research, Stockholm.t r a n s l a t e d i n t o E n g l i s h ) which c i t e d many c a s e s o f p r o g r e s s i v e c o l l a p s e i n v a r i o u s p a r t s o f t h e w o r l d . He i l l u s t r a t e d some o f t h e s e w i t h s l i d e s showing c o l l a p s e s i n B a r c e l o n a , ~ o d t h % b , Glasgow, Naples, London ( P o s t O f f i c e Tower), P a r i s , B r u s s e l s , Copenhagen, and Stockholm. A l l c a s e s showed t h e importance o f c o n s i d e r i n g u n e x p e c t e d o r abnormal s i t u a t i o n s . One o f t h e s e c a s c s , i n ~ o d t h ? h , Greenland, i l l u s t r a t e d t h a t t h e c a s c a d e e f f e c t i n a gas e x p l o s i o n c o u l d r e s u l t i n p r e s s u r e s c o n s i d e r a b l y h i g h e r t h a n 5 p s i .
Because i t i s t o o e x p e n s i v e and i n f a c t i m p o s s i b l e t o r e d u c e r i s k s c o m p l e t e l y , t h e e n g i n e e r h a s t o t r y t o r e d u c e damage t o a t o l e r a b l e amount. T o l e r a b l e damage would b e t h a t damage t h a t h a d t o be r e l u c t a n t l y a c c e p t e d b e c a u s e o f p r a c t i c a l l i m i t a t i o n s . The t o l e r a t e d damage would v a r y , t h e n , w i t h t h e n a t u r e and magnitude o f ' e x c e s s i v e ' l o a d i n g and w i t h t h e s i z e and geometry o f t h e b u i l d - i n g . D r . ~ r a n s
trsm
showed an i n t e r e s t i n g t a b u l a t i o n o f damages g i v i n g t h e number o f p e r s o n s k i l l e d , t h e number o f a f f e c t e d room u n i t s and t h e number o f c o l l a p s e d room u n i t s , many o f which were 1 a r g e r t h a n ' t o l e r a b l e f H e mentioned i n p a s s i n g t h a t e x p l o s i o n s due t o s u i c i d e a t t e m p t s were n o t uncommon.F o r a minimum r i s k approach t o d e s i g n D r . ~ r a n s t r E m recom- mended t h a t t h e e n g i n e e r s h o u l d :
( a ) Design p r o p e r l y t o keep damage w i t h i n ' t o l e r a b l e l i m i t s f f o r a g i v e n c a u s e i n most o f t h e c a s e s ; and (b) Design w i t h a w a t c h f u l e y e s o t h a t i f t h e w o r s t happens t h e r e i s a l s o s.ome d e c e n t l i m i t a t i o n on t h e ' n o t t o l e r a b l e ' damage. To r e d u c e damage t o t o l e r a b l e l i m i t s , t h e e n g i n e e r s h o u l d p r o v i d e : S t r u c t u r a l redundancy C a r e f u l l y d e s i g n e d j o i n t s S u b s t a n t i a l deformabi l i t y
The d e s i g n e r s h o u l d always e x p e c t t h e w o r s t and t h i n k i n terms of a damaged s t r u c t u r e
.
I n Sweden t h e b u i l d i n g code r e g u l a t i o n s had a s i m i l a r g o a l t o t h o s e i n t h e N a t i o n a l B u j ~ l d i n g Code o f Canada ( A r t i c l e 4 . 1 . 1 . 8 ) e x c e p t t h a t t h e o n l y b u i l d i n g s now exempted from p r o g r e s s i v e c o l - l a p s e c o n s i d e r a t i o n s a r e s i n g l e f a m i l y d e t a c h e d houses and b u i l d - i n g s where t h e r i s k t o l i f e i s v e r y s m a l l .
I n t h e Swedish code much emphasis i s g i v e n t o t h e p r o v i s i o n o f t i e f o r c e s , which a r e r e q u e s t e d t o b e o f i n c r e a s i n g s t r e n g t h
a s t h e b u i l d i n g s i z e i n c r e a s e s . F u r t h e r t h e r e a r e s p e c i a l r u l e s concerning t h e t o t a l s t a b i l i t y o f t h e b u i l d i n g demanding a s a t i s - f y i n g s t a b i l i t y even i f a l l members w i t h i n an a r b i t r a r i l y p l a c e d damage volume a r e o u t o f a c t i o n . Because o f t h i s r u l e
-
which h a s e x c e p t i o n s - s t a b i l i t y r e s e r v e s a r e o b t a i n e d t h a t c o n s i d e r a b l y reduce t h e r i s k o f t o t a l f a i l u r e . The s i z e o f t h e 'damage volume' a s w e l l a s of t h e t i e f o r c e s i s i n c r e a s e d with t h e s i z e o f t h e b u i l d i n g i n o r d e r t o e q u a l i z e t h e r i s k f o r t h e i n h a b i t a n t o f a s m a l l and a b i g b u i l d i n g .Research i n Sweden a t t h e moment i s f o c u s e d on v a r i o u s t y p e s o f o r d i n a r y c o n s t r u c t i o n and a s t u d y o f t h e importance o f t i e f o r c e s . Ilr. ~ r a n s t r b ' m showed s l i d e s of some model t e s t s o f load- b e a r i n g panel b u i l d i n g s with p r e c a s t f l o o r s l a b s . I n t h e s e models
t i e s were p l a c e d i n two o r t h o g o n a l d i r e c t i o n s with one always
d e s i g n a t e d a s t h e main l o a d - b e a r i n g d i r e c t i o n . The models i l l u s t r a - t e d t h a t t i e s t e e l and t h e c e l l u l a r s t r u c t u r e o f a b u i l d i n g c o u l d c a u s e f o r c e s t o s p r e a d o u t around a zone o f l o c a l damage i . e . , e s t a b l i s h a l t e r n a t e p a t h s . The Swedish had r e l i e d h e a v i l y on B r i t i s h experiments i n t h i s a r e a . D r . ~ r a n s t r g m s a i d , and now
recommended t h a t t o c a r r y f o r c e s i n t h e main l o a d - b e a r i n g d i r e c t i o n , t h e r e i n f o r c i n g b a r s g r o u t e d i n t o t h e hollow c o r e s o f t h e s l a b s be looped around t h e t r a n s v e r s e t i e s t e e l a s shown i n F i g u r e 9:
G R O U T I C U L T TO P L A C O L L O W C O R E S L A
.
.
..
.
B E A R I N G W A L L OR B E A MI l J l 1 $1 LOOPED T I l i S T E E L INTERLOCKED WIT11 'I'IIANSVEIISli Sl'EliL A'T CONNECTION O F P R E C A S T
D i s c u s s i o n
The f i r s t q u e s t i o n concerned w h e t h e r h o r i z o n t a l s t e e l l y i n g i n t h e j o i n t s between p r e f a b r i c a t e d f l o o r e l e m e n t s was e f f e c t i v e .
M r . C r e a s y s a i d t h a t i t was i n e f f e c t i v e u n l e s s t h e r e was i n t e r l o c k - i n g w i t h t h e s t e e l i n s i d e t h e members. The b a r s would p u l l o u t o f t h e i n t e r - s l a b j o i n t s . D r . ~ r a n s t r b ' m p o i n t e d o u t t h a t t h i s r e i n - f o r c i n g s t e e l was d e s i g n e d u s i n g s t a t i c l o a d s when some form of dynamic l o a d i n g r e a l l y s h o u l d b e c o n s i d e r e d .
The Chairman asked a b o u t a n o t h e r t y p e o f p r o g r e s s i v e c o l l a p s e t h a t h a d o c c u r r e d i n Canada: t h e l a t e r a l s c r e a d o f c o l l a p s e o f roof members i n low b u i l d i n g s . D r . Granstrom r e p l i e d t h a t t h e r e were examples t h a t p r o t e c t i o n a g a i n s t t h i s c o u l d b e a c h i e v e d u s i n g
t ? ~ e s p a c e frame r o o f c o n c e p t .
PRESENTATION BY E.F.P. BURNETT
D r . B u r n e t t spoke a b o u t a r e s e a r c h p r o j e c t h e h a d been i n v o l v e d w i t h a t t h e N a t i o n a l Bureau of S t a n d a r d s where an e x t e n s i v e
s t a t i s t i c a l s t u d y o f abnormal l o a d s , had been i n p r o g r e s s f o r a b o u t
3 y e a r s . I n c l u d e d i n t h e s t u d y were p r e s s u r e , i m p a c t , s t a t i c and d e t r a c t i v e l o a d i n g s . Although t h e s t a t i s t i c s were c r u d e t h e y d i d show t h a t many s o - c a l l e d abnormal l o a d s o c c u r r e d w i t h s u r p r i s i n g f r e q u e n c y t o d u r i n g t h e l i f e o f t h e b u i l d i n g ) . I n p a r t i c u l a r t h e r i s k o f a gas e x p l o s i o n i n a h i g h - r i s e b u i l d i n g w i t h 100 d w e l l i n g u n i t s u s i n g gas was 1 0 ' ~ assuming a 5 0 - y e a r l i f e o f t h e b u i l d i n g . The f r e q u e n c y o f bomb e x p l o s i o n s was a l s o v e r y h i g h , i n some a r e a s e q u a l l i n g t h e f r e q u e n c y o f t h e d e s i g n wind. D r . B u r n e t t produced some i n t e r e s t i n g s t a t i s t i c s g a t h e r e d i n t l o l l a n d by L i g t e n b e r g on c a u s e s o f c o l l a p s e ( n o t n e c e s s a r i l y p r o g r e s s i v e ) and on t h e r e l a t i v e f r e q u e n c y o f t h e i r o c c u r r e n c e : " o r d i n a r y " o v e r l o a d i n g 1 f a u l t y d e s i g n 2.5 i m p a c t , c o l l i s i o n 2.5 - 5 e x p l o s i o n 10 wind 10 f i r e 75 I n summarizing, D r . B u r n e t t concluded t h a t p r o g r e s s i v e
c o l l a p s e due t o abnormal e v e n t s was a problem worth c a r e f u l s t u d y and r e f e r r e d t o a p u b l i c a t i o n c o n t a i n i n g more d a t a (ACI SP 4 8 ) * .
*
American C o n c r e t e I n s t i t u t e , S p e c i a l P u b l i c a t i o n No. 48, I n d u s t r i a l i z a t i o n i n C o n c r e t e B u i l d i n g C o n s t r u c t i o n . D e t r o i t , Michigan, 48219.PRESENTATION BY D.E. ALLEN
D r . Allen reviewed some s t a t i s t i c s on t h e i n c i d e n c e o f p r o g r e s - s i v e c o l l a p s e and abnormal l o a d s o b t a i n e d from Canadian newspaper c l i p p i n g s and t h e E n g i n e e r i n g News-Record f . Although no p a s t Canadian s t r u c t u r a l f a i l u r e s c o u l d b e blamed p r i m a r i l y on l a c k o f d e s i g n requirements t o guard a g a i n s t p r o g r e s s i v e c o l l a p s e , s t a t i s t i c s i n d i c a t e d t h a t about 15 t o 20 p e r c e n t o f c o l l a p s e s were p r o g r e s s i v e
( h a l f o f t h e s e o c c u r r e d d u r i n g c o n s t r u c t i o n ) , and t h a t t h e r e was a s i g n i f i c a n t i n c i d e n c e of abnormal l o a d s o r weaknesses c a u s i n g s t r u c t u r a l damage. Because o f t h e change t o forms o f c o n s t r u c t i o n w i t h l e s s s t r u c t u r a l i n t e g r i t y t h a n i n t h e p a s t , ( f o r example, t o h i g h - r i s e u n r e i n f o r c e d masonry w i t h p r e c a s t f l o o r s l a b s ) t h e r e was a need f o r r e q u i r e m e n t s t o guard a g a i n s t p r o g r e s s i v e c o l l a p s e .
Before t h e d i s c u s s i o n proceeded t o a review of t h e s i t u a t i o n f o r c l i f f e r e n t m a t e r i a l s , a number o f t h o s e p r e s e n t e x p r e s s e d s u r p r i s e t h a t p r o g r e s s i v e c o l l a p s e s o c c u r r e d s o f r e q u e n t l y and somc doubt e x i s t e d a s t o how good t h e American s t a t i s t i c s were. The s t a t i s t i c s were a p p a r e n t l y based on an abnormal e v e n t c a u s i n g damage o f more t h a n $1000 which was a very low t h r e s h o l d f i g u r e f o r s t r u c t u r a l damage.
POTENTIAL VULNERABILITY OF PRIMARY STRUCTURAL MATERIALS AND SYSTEMS
(General D i s c u s s i o n ) Concrete
One o f t h e p a r t i c i p a n t s s t a t e d t h a t w h i l e t h e r e was no problem w i t h i n s i t u r e i n f o r c e d c o n c r e t e c o n s t r u c t i o n t h e r e was a problem i n (a) unbonded, pos t-:tensioned c o n s t r u c t i o n , e s p e c i a l l y i n f l a t p l a t e f l o o r s l a b s , and (b) i n p r e c a s t hollow c o r e s l a b s . I n t h e
former t h e moment and s h e a r c a p a c i t y might b e completely l o s t i f a tendon f a i l e d . I n t h e l a t t e r , and i n p a r t i c u l a r w i t h e x t r u d e d hollow-core s l a b s , t h e connection t o t h e w a l l s and between t h e
s l a b s i n t h e l o n g i t u d i n a l and t r a n s v e r s e d i r e c t i o n was v e r y
d i f f i c u l t and t h e r e was some c o n t r o v e r s y about t h e type o f welded corlnections t h a t c o u l d be made a t t h e f l o o r - t o - w a l l j o i n t s . Could wcld p l a t e s be provided a l o n g t h e l o n g s i d e s o f e x t r u d c d hollow-
c o r e s l a b s ? The answer was t h a t a p p a r e n t l y they c o u l d be and t h a t
$ A l l e n , D . E . and S c h r i e v e r , W . R . 1972. P r o g r e s s i v e C o l l a p s e ,
Abnormal Loads and B u i l d i n g Codes. P r o c . o f t h e Am. Soc.
C i v . Eng. National Meeting on S t r u c t . Eng., C l e v e l a n d ,
t h e e x t r u d e d s l a b s c o u l d b e made s a t i s f a c t o r y f o r most a p p l i c a t i o n s w i t h a few c o n n e c t i o n changes. However some o f t h o s e p r e s e n t had
t r i e d t o o b t a i n s l a b s w i t h weld p l a t e s , w i t h o u t s u c c e s s . M r . Creasy commented on t h e Bison system o f p r e c a s t ( b u t n o t e x t r u d e d o r
p r e s t r e s s e d ) h o l l o w - c o r e s l a b s i n which f u l l - s c a l e t e s t s had 1)rovcn t h a t i t p r o v i d e d comparable r e s i s t a n c e t o i n s i t u c o n c r c t c whcn looped r e i n f o r c i n g b a r s , g r o u t e d i n t o t h e hollow c o r e s wcrc 1 i n k o d a b o u t t h e t r a n s v e r s e r e i n f o r c i n g , a s shown i n F i g u r c 9 . I n g e n e r a l , he f e l t t h a t p r e c a s t c o n c r e t e c o u l d b e made t o p r o v i d e performaricc comparable t o i n s i t u c o n c r e t e . One d e s i g n e r a l s o r e f e r r e d t o h o l l o w - c o r e s l a b c o n s t r u c t i o n w i t h s t e e l i n s e r t e d i n t h e l o n g i t u - d i n a l j o i n t s b u t t h e r e was c o n s i d e r a b l e doubt t h a t any a l t e r n a t e p a t h o r c a t e n a r y a c t i o n c o u l d be developed by t h i s t y p e o f c o n s t r u c -
t i o n a s i t was c u r r e n t l y u s e d .
Another d e s i g n e r r a i s e d t h e q u e s t i o n o f punching s h e a r i n f l a t s l a b c o n s t r u c t i o n t o which M r . Creasy r e p l i e d t h a t r e i n f o r c i n g t h r o u g h t h e column was very i m p o r t a n t . One o f t h e most i m p o r t a n t d e t a i l i n g changes f o r i n s i t u c o n c r e t e was t h e c o n t i n u a t i o n o f
bottom s t e e l i n beams and s l a b s o v e r s u p p o r t s o r through columns, o r t h e p r o v i s i o n o f a welded o r hooked anchorage a t t h e s e l o c a t i o n s . T h i s would e n s u r e some d e g r e e o f c a t e n a r y a c t i o n and would t i e t h e s t r u c t u r e t o g e t h e r g e n e r a l l y i n a d d i t i o n t o improving u l t i m a t e s h e a r b e h a v i o u r .
Masonry
A p a r t i c i p a n t fami 1 i a r w i t h t h e d e s i g n o f masonry s t r u c t u r e s , a s k e d whether u n r e i n f o r c e d masonry c o u l d b e used f o r h i g h - r i s e c o n s t r u c t i o n r e g a r d l e s s o f t h e e a r t h q u a k e zone. Some f e l t t h a t i t was a b s o l u t e l y n e c e s s a r y t o u s e v e r t i c a l r e i n f o r c i n g s t e e l ,
e s p e c i a l l y a t t h e c o r n e r s o f each bay, o t h e r s d i d n o t . M r . Creasy s a i d t h a t c e l l u l a r masonry c o n s t r u c t i o n ( r e l a t i v e l y s m a l l rooms e n c l o s e d by masonry w a l l s ) was common i n Europe and t h a t i t o f t e n i n c l u d e d v e r t i c a l t i e s . Another spoke a b o u t t h e t y p e o f c o n s t r u c - t i o n t h a t i n v o l v e d Hambro j o i s t s ( i . e . , open web s t e e l j o i s t s w i t h a 2 1 / 2 - i n . t o p f l a n g e o f i n s i t u c o n c r e t e ) ; h e f e l t t h a t a good c o n n e c t i o n between f l o o r s and masonry w a l l s was a c h i e v e d i n t h i s t y p e o f cons t r u c t i o n
.
I t was s u g g e s t e d t h a t masonry s t r u c t u r e s designed f o r zone 2 and 3 e a r t h q u a k e s would r e q u i r e r e l a t i v e l y i n e x p e n s i v e changes t o p r e v e n t p r o g r e s s i v e c o l l a p s e . M r . Creasy a g r e e d t h a t t h i s was probably t r u e and t h a t a l l t h a t was r e q u i r e d was a t t e n t i o n t o d e t a i l s .
S t e e l
One p a r t i c i p a n t , f a m i l i a r w i t h t h e b e h a v i o u r o f s t e e l s t r u c t u r e s f e l t t h a t , i n g e n e r a l , s t e e l s t r u c t u r e s c o u l d b r i d g e o v e r a column removed by impact s i n c e t h e c o n n e c t i o n s c o u l d very
l i k e l y s u p p o r t t h e c a t e n a r y f o r c e s . There might b e a problem i n l a r g e - s p a n r o o f s on o n e - s t o r e y assembly b u i l d i n g s because, w i t h t h e f a i l u r e of one t r u s s , continuous c r o s s - b r a c i n g c o u l d b r i n g down a d j a c e n t t r u s s e s by a t w i s t i n g a c t i o n . For t h e s e i t might bc b e s t t o conncct p a i r s o f t h e l a r g e - s p a n t r u s s e s t o g e t h e r i n o r d c r t o I imi t t h c f a i l u r e t o one p a i r o f t r u s s e s r a t h e r t h a n t h e who1 c r o o f . Another p a r t i c i p a n t d i s c u s s e d a long-span t r u s s s p a c c frame t h a t would b e s t a b l e and d i v o r c e d from i t s neighbours e x c e p t f o r c o n n e c t i o n s v i a t h e p u r l i n s and roof decking. I f one o f t h e s e c o l l a p s e d i t would b e very u n l i k e l y t h a t o t h e r t r u s s e s would b e p u l l e d down.
D r . ~ r a n s t r s m s a i d t h a t t h i s had a l s o been d i s c u s s e d i n Sweden b u t t h a t he d i d n o t c o n s i d e r c o n n e c t i n g p a i r s o f t r u s s e s t o g e t h e r a c c e p t a b l e . I t was d e s i r a b l e t o p r o v i d e t h e major members w i t h s u f f i c i e n t s t r e n g t h t o c a r r y t h e remaining l o a d i n c a s e one o f t h e a d j a c e n t members f a i l e d . I n a d d i t i o n h e noted t h a t diaphragm a c t i o n f o r d i s t r i b u t i n g wind f o r c e s and p r o v i d i n g l a t e r a l r i g i d i t y would be h a r d t o a c h i e v e w i t h independent t r u s s e s o r p a i r s of
t r u s s e s d e l i b e r a t e l y f l a n k e d by ' p l a n e s o f weakness'
.
Another comment was t h a t c a t e n a r y a c t i o n o f t h e p u r l i n s a n d / o r decking might t h w a r t t h e i n t e n t o f t h e p l a n e s o f weakness.One b u i l d i n g t y p e t h a t caused some d i s c u s s i o n was t h e l i g h t l o w - r i s e b u i l d i n g with c o n c r e t e s l a b s having very l i t t l e anchorage a t t h e w a l l - t o - f l o o r c o n n e c t i o n s .
Concern was a l s o e x p r e s s e d about t h e type o f b u i l d i n g t h a t had s t e e l beams and columns, j o i n e d by h e a d e r c o n n e c t i o n s , t o c a r r y v e r t i c a l l o a d s and l i g h t composite s t e e l d e c k - c o n c r e t e f l o o r s t o c a r r y l a t e r a l l o a d s t o an i n s i t u c o n c r e t e c o r e . These b u i l d i n g s would have very l i g h t wind o r e a r t h q u a k e b r a c i n g i n t h e e x t e r n a l w a l l s and would seem t o o f f e r l i t t l e r e s i s t a n c e t o p r o g r e s s i v e
c o l l a p s e s h o u l d one o f t h e l i g h t columns f a i l . D r . ~ r a n s t r g m mentioned t h a t t h e y were a l s o concerned a b o u t t h i s t y p e o f c o n s t r u c t i o n i n Sweden.
Onc e n g i n e e r , concerned with t h e d e s i g n o f l i g h t t i m b e r
s t r u c t u r e s , f e l t t h a t t h e i r o n l y problem was t h e s t a b i l i t y o f l i g h t t r u s s c s J u r i ng e r e c t i o n because o f t h e i r ' f l i m s i n e s s
.'
He a l s o thought t h a t i n some c a s e s t h e decking c o u l d by i t s c a t e n a r y a c t i o nh e l p t o p r e v e n t h o r i z o n t a l s p r e a d of t h e c o l l a p s e s h o u l d one o f t h c t r u s s e s f a i l . Another f e l t t h a t t h e r e was no problem w j t h heavy
t i m b e r s t r u c t u r e s .
I n t h e o p i n i o n o f a n o t h e r p a r t i c i p a n t , t h e most i m p o r t a n t i s s u e s were d e s i g n f o r c o n s t r u c t i o n l o a d s and e r r o r s and d e s i g n t o minimize t h e r i s k o f p r o g r e s s i v e c o l l a p s e due t o f i r e . Unfortu- n a t e l y t h e colloquium was behind s c h e d u l e and a l t h o u g h t h e cons t r u c -
t i o n a s p e c t s were touched on from time t o time, t h e t o p i c o f f i r e was n o t d i s c u s s e d a g a i n .
CODE NEEDS (GENERAL DISCUSSION)
One o f t h o s e p r e s e n t commented t h a t t h e e x p r e s s i o n ' e x c e s s i v e loading' was p r e f e r a b l e t o 'abnormal l o a d i n g . ' He was a g a i n s t any s p e c i f i c design p r e s s u r e such as t h e 5 p s i f i g u r e q u o t e d i n t h e 5 t h Amendment and thought t h a t t h e a l t e r n a t e p a t h method w i t h n o t i o n a l removal was p r e f e r a b l e . One o f t h e c o n s u l t a n t s thought t h a t t h e p r e s e n t NBC A r t i c l e 4 . 1 . 1 . 8 was f a r t o o vague and t h a t t h e e n g i n e e r c o u l d n o t d e s i g n p r o p e r l y w i t h i t . Me was emphatic i.n r e q u e s t i n g more s p e c i f i c i n f o r m a t i o n f o r t h e d e s i g n e r s and
s u g g e s t e d t h a t a s a f i r s t s t e p t h e Code s h o u l d d e f i n e more c l e a r l y t h e c o n d i t i o n s t h a t must b e c o n s i d e r e d .
D r . ~ r a n s t r b l m s t a t e d t h a t computer c a p a c i t y was now a v a i l a b l e and c o u l d b e used t o s e a r c h f o r some o f t h e t y p i c a l t r o u b l e a r e a s i n t h e a n a l y s i s o f damaged b u i l d i n g s .
Concern was e x p r e s s e d f o r t h e b u i l d i n g o f f i c i a l s by one p a r t i c i p a n t who a l s o asked f o r more s p e c i f i c r e q u i r e m e n t s i n t h e
Code. He thought t h a t t h e n o t i o n a l removal o f one member was a
good requirement
.
The vagueness o f t h e Code was r e g r e t t e d by one o f t h e c o n s u l - t a n t s (who e x p r e s s e d p r e f e r e n c e f o r a s p e c i f i c a t i o n approach i n t h e n e x t Code)
.
He a l s o f e l t t h a t i t would have been p r e f e r a b l e n o t t o have t h e p r e s e n t commentary a t a l l because by d e f a u l t i t s a n c t i o n e d t h e 5 p s i e x p l o s i v e p r e s s u r e formerly used i n t h e U . K .Ilc 3 1 5 0 thought t h a t any requirement t o design f o r c o n s t r u c t i o n
h a z a r d s was i m p r a c t i c a l . Another c o n s u l t a n t s u g g e s t e d a t t h j s p o i n t t h a t d e s i g n and c o n s t r u c t i o n phases s h o u l d be e n t i r e l y s e p a r a t e and t h e l i a l j i l i t i e s d i v o r c e d . Many d i s a g r e e d with t h i s .
There seemed t o b e a consensus t h a t t h e A s s o c i a t e Committee o f t h e NBC s h o u l d make a d e c i s i o n on how t o proceed w i t h p r o g r e s - s i v e c o l l a p s e very soon, f o r t h e 1977 NBC, and t h a t n o t i o n a l removal o f a member was t h e most a c c e p t a b l e i d e a . The i d e a o f a d e s i g n p r e s s u r e such a s 5 p s i was n o t favoured by most o f t h o s e p r e s e n t b u t a t t h e same time some wished t o have a minimum amount o f t y i n g s t e e l recommended.
