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Planning for codification of window glass research
Dalgliesh, W. A.
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Planning for Codification of
Window Glass Research
I
by W.A. Dalgliesh
Reprinted from
"Hurricane Alicia: One Year Later"
Proceedings of the Specialty Conference
American Society of Civil Engineers
Galveston, TX, August 16- 17, 1984
p. 168-177
(DBR Paper No. 1347)
Price $2.00
NRCC 25297
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B I B L ~ T H Z Q U ~ E
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Rmh.
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C N R C- IC13T-:,
IBien que l'on utilise de plus en plus le verre comme materiau de revetement, beaucoup de concepteurs connaissent ma1 son comportement structurel, en particulier sa sensibilitg au taux et 3 la d u d e d'application des charges. Avant 1979, les fabricants de vitres recommandaient l'emploi de telle ou telle Bpaisseur pour une surface donnee en se basant presque exclusivement sur les rgsultats d'essais standard de rgsistance mais, cette annee-la, on a commence
a
utiliser une s6rie de tableaux tenant aussi compte du calcul des contraintes.. - - Un coml*g -- .a ligr 0 b.> as d u f 11 d 'I s
e
1':
a-' a ton de Ses anta
: hode e la vent r de -I 3 ~rie les BUX aesReprinted from Proceedings o f Specialty Conference "Hurricane Alicia" - Aerospace Div.. EM Div. and ST D ~ v . / A S C E /
Galvesron, Texas, August 16-1 7, 1984
PLANNING FOR CODIFICATION OF WINDOW GLASS RESEARCH
W.A. D a l g l i e s h *
A b s t r a c t
The growing u s e of g l a s s a s a c l a d d i n g m a t e r i a l f i n d s many d e s i g n e r s u n f a m i l i a r w i t h i t s s t r u c t u r a l behaviour, which i n c l u d e s a s e n s i t i v i t y t o r a t e and d u r a t i o n of loading. P r i o r t o 1979,
manufacturers' recommended g l a s s t h i c k n e s s e s f o r any d e s i r e d a r e a were based almost e n t i r e l y on t h e r e s u l t s of p r e v i o u s l o a d t e s t s , b u t i n t h a t y e a r a s e t of c h a r t s r e l y i n g a l s o on s t r e s s a n a l y s i s was introduced.
A Canadian committee i s working on design g u i d e l i n e s w i t h t h e f o l l o w i n g o b j e c t i v e s : t o c l a r i f y d e c i s i o n s l e a d i n g t o c o n s i s t e n t s a f e t y by a p p l y i n g l i m i t s t a t e s d e s i g n concepts; t o r e l a t e f l u c t u a t i n g wind p r e s s u r e s t o t h e s p e c i f i e d one-minute p r e s s u r e r e f e r r e d t o i n design c h a r t s ; and t o review p r o g r e s s i n supplementing e m p i r i c a l d e s i g n by s t r e s s a n a l y s i s and s t a t i s t i c a l s t r e n g t h t h e o r y a s s o c i a t e d w i t h s u r f a c e flaws.
The i n f l u e n c e s of a s p e c t r a t i o and weathering remain
c o n t r o v e r s i a l ; c a r e f u l l y designed experiments w i l l b e r e q u i r e d t o a s s i s t i n q u a n t i f y i n g t h e i r e f f e c t s f o r design.
Background
Glass has gained wide a c c e p t a n c e among a r c h i t e c t s a s t h e c l a d d i n g m a t e r i a l f o r major b u i l d i n g p r o j e c t s throughout North America. In
f u n c t i o n i n g a s c l a d d i n g , window g l a s s c o n t r i b u t e s t o t h e l o o k of t h e b u i l d i n g , c o n t r o l s t h e t r a n s m i s s i o n of l i g h t , h e a t and sound, and e x c l u d e s r a i n and wind. These a r e a l l important concerns f o r t h e a r c h i t e c t , and i t i s n o t s u r p r i s i n g t h a t s t r u c t u r a l requirements a r e n o t uppermost among them. However, t h r e e i n c r e a s i n g l y important f a c t o r s make window d e s i g n t o r e s i s t wind p r e s s u r e more t h a n j u s t a r o u t i n e checking e x e r c i s e : t h e u s e of l a r g e panes; exposure t o s e v e r e winds a t upper s t o r i e s and wind-borne m i s s i l e s lower down; and t h e consequences of f a i l u r e ( i n j u r y from broken g l a s s , damage t o b u i l d i n g and c o n t e n t s ) (1).
The development and d i s s e m i n a t i o n of i n f o r m a t i o n on t h e s t r u c t u r a l b e h a v i o r of g l a s s h a s n o t k e p t p a c e w i t h i t s a p p l i c a t i o n ; s t r u c t u r a l e n g i n e e r s t e n d t o avoid m a t e r i a l s w i t h b r i t t l e f a i l u r e mechanism8 and v a r i a b l e b r e a k i n g s t r e n g t h s , and i n any c a s e t h e y a r e *Head, 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 , D i v i s i o n of B u i l d i n g Research,
National Research Council of Canada, Montreal Road, Ottawa, ( h t a r i o , Canada KIA 0R6.
WINDOW GLASS RESEARCH 169
lore concerned w i t h t h e d e s i g n of t h e s t r u c t u r e t h a n of t h e cladding. The a r c h i t e c t t e n d s t o r e l y mainly on t h e manufacturer f o r s t r u c t u r a l a s w e l l a s performance p r o p e r t i e s , and a l t h o u g h c h a r t s r e l a t i n g l a t e r a l p r e s s u r e s t o g l a s s t h i c k n e s s and a r e a a r e provided, t h e y f a l l s h o r t of a complete a n a l y s i s of t h e wind l o a d i n g problem.
Most c h a r t s a r e based l a r g e b on t h e r e s u l t s of many t e s t s t o f a i l u r e of new g l a s s samples expos& t o p r e s s u r e i n c r e a s i n g e i t h e r c o n t i n u o u s l y o r i n small s t e p s w i t h a one-minute "dwell" a t e a c h s t e p (2). The b r e a k i n g p r e s s u r e t e n d s t o b e p r o p o r t i o n a l d i r e c t l y t o t h i c k n e s s t o about t h e 312 power and i n v e r s e l y t o a r e a , b u t even f o r
I
t h e same t h i c k n e s s and a r e a , r e s u l t s f o r a n n e a l e d g l a s s have as t a n d a r d d e v i a t i o n of 20-25 p e r c e n t . I n 1979, however, one manufacturer (3) brought o u t c h a r t s based on t h e maxirmm s u r f a c e s t r e s s , which made a s p e c t r a t i o an a d d i t i o n a l important c o n s i d e r a t i o n . A t t e n t i o n was a l s o drawn t o s t r e n g t h d i f f e r e n c e s between new g l a s s and i n - s e r v i c e g l a s s .
Advances have been made i n d e s c r i b i n g f l u c t u a t i n g wind p r e s s u r e s s i n c e t h e time most of t h e aforementioned g l a s s t e s t i n g was done (4).
T h i s has l e d t o renewed i n t e r e s t i n t h e e f f e c t s of l o a d d u r a t i o n and v a r i a b i l i t y on g l a s s s t r e n g t h , and t o a reassessment of t h e r e s i s t a n c e of g l a s s t o wind load. Improved s t r e s s a n a l y s i s methods u s i n g
computers have made i t p r a c t i c a l t o a t t e m p t en u n d e r s t a n d i n g of how g l a s s f a i l s under p r e s s u r e . A f a i l u r e p r e d i c t i o n model h a s been proposed i n c o r p o r a t i n g : Weibull's s t a t i s t i c a l t h e o r y of s t r e n g t h ; s t r e s s c o n c e n t r a t i o n caused by s u r f a c e flaws; and l o a d r a t e e f f e c t s o f s t r e s s c o r r o s i o n . B e t t e r i n f o r m a t i o n a b o u t t h e v a r i a b i l i t y of l o a d s
on t h e one hand, and of g l a s s s t r e n g t h on t h e o t h e r , r a i s e t h e p o s s i b i l i t y of a c h i e v i n g more c o n s i s t e n t l e v e l s of s a f e t y a g a i n s t c l a d d i n g f a i l u r e s , whether of g l a s s o r any o t h e r m a t e r i a l .
Research on window g l a s s and wind l o a d s does n o t l e a d
a u t o m a t i c a l l y t o improved d e s i g n and c o n s t r u c t i o n p r a c t i c e ,
\
owever. Before new information becomes p a r t of a c c e p t e d s t a n d a r d s of b o d p r a c t i c e , t h e consequences of change s h o u l d b e examined. andp r a c t i c a l l y o r i e n t e d r e s e a r c h must be balanced a g a i n s t t h e accurmlated weight of e x p e r i e n c e i n t h e f i e l d . A n e c e s s a r y f i r s t s t e p i s t o make t h e new information a v a i l a b l e t o t h e decision-makers: d e s i g n e r s , owners, b u i l d e r s , i n s p e c t o r s , m a t e r i a l s s u p p l i e r s . h e way t o approach t h i s problem of technology t r a n s f e r i s t o a t t e m p t t h e
d r a f t i n g of new d e s i g n g u i d e l i n e s by b r i n g i n g t o g e t h e r r e s e a r c h e r s and decision-makers t o s e e what new knowledge is ready f o r i n c o r p o r a t i o n w i t h c u r r e n t p r a c t i c e .
A committee of t h e Canadian General Standards Board (CGSB) i s p r e p a r i n g a document t o b e c a l l e d "The S t r u c t u r a l Design of G l a s s i n Buildings". I n p l a n n i n g f o r t h i s s t a n d a r d , t h e committee i n t e n d s t o a p p l y t h e u n i f y i n g p r i n c i p l e s of l i m i t s t a t e s d e s i g n , t o examine t h e t r a d e - o f f s between s i m p l e and complex r e p r e s e n t a t i o n s of wind p r e s s u r e and g l a s s s t r e n g t h , and t o s u g g e s t p r i o r i t i e s f o r f u r t h e r r e s e a r c h and t e s t i n g .
170 HURRICANE ALICIA
Limit S t a t e s Design
Limit s t a t e s d e s i g n o f f e r s a common approach t o meeting t h e b a s i c
r e q u i r e m e n t s of t h e s t r u c t u r e : s a f e t y a g a i n s t c a t a s t r o p h i c f a i l u r e ,
and s a t i s f a c t o r y performance of i t s i n t e n d e d f u n c t i o n s (5). Owing t o
u n c e r t a i n t y i n h e r e n t i n t h e p r e d i c t i o n of b o t h wind l o a d s and g l a s s s t r e n g t h , a b s o l u t e s a f e t y can n e v e r be a s s u r e d ; r a t h e r , an a c c e p t a b l y
low r i s k of f a i l u r e d u r i n g t h e u s e f u l l i f e of t h e s t r u c t u r e i s t h e
d e s i g n e r ' s goal. Defining a c c e p t a b l e r i s k i s p a r t i c u l a r l y d i f f i c u l t
whenever t h e consequences of f a i l u r e i n c l u d e i n j u r y t o humans, b u t d e c i s i o n s implying r i s k l e v e l s have a l r e a d y been made i n e x i s t i n g
d e s i g n p r a c t i c e ; i t remains only t o d i s c o v e r , through p r o b a b i l i t y
a n a l y s i s , what t h e y a r e i n s p e c i f i c i n s t a n c e s and then s t r i v e f o r u n i f o r m i t y i n g e n e r a l l y a p p l i c a b l e r u l e s .
The p a r t i a l f a c t o r method f o r checking s t r u c t u r a l adequacy a g a i n s t c o l l a p s e i s one way of c a r r y i n g o v e r d e c i s i o n s a b o u t r i s k l e v e l s from one design problem t o a n o t h e r , o r from one s o l u t i o n t o
a n o t h e r f o r t h e same problem (e.g., u s e of a c r y l i c r a t h e r t h a n g l a s s ) .
The f a c t o r e d r e s i s t a n c e must e q u a l o r exceed t h e e f f e c t of t h e f a c t o r e d l o a d s , and f o r u n i f o r m i t y , l o a d f a c t o r s and r e s i s t a n c e f a c t o r s can be chosen t o c o n t r i b u t e approximately e q u a l l y t o t h e
d e s i r e d r e d u c t i o n i n r i s k of f a i l u r e . T h i s i s n o t t h e c a s e i n c u r r e n t
p r a c t i c e ; design wind p r e s s u r e s , a t l e a s t nominally. may have about an even chance of b e i n g t h e maximtm f o r t h e l i f e o f t h e s t r u c t u r e . T h e r e f o r e , t h e e n t i r e burden of s a f e t y seems t o r e s t w i t h what might
be c a l l e d a r e s i s t a n c e f a c t o r i n t h e g l a s s d e s i g n c h a r t s . A r c h i t e c t s
a r e informed ( 2 ) t h a t an a d d i t i o n a l f a c t o r may be r e q u i r e d i n some
a p p l i c a t i o n s t o p r o v i d e a d d i t i o n a l s a f e t y ; t h i s can b e l i k e n e d t o t h e importance f a c t o r of t h e p a r t i a l f a c t o r method.
There may be more than one u l t i m a t e s t a t e r e q u i r i n g checking; wind-borne m i s s i l e s , s u c h a s roof g r a v e l , impose a d i f f e r e n t s o r t of l o a d i n g than wind a l o n e , and t h e f a i l u r e mode can be q u i t e
d i f f e r e n t ( 6 ) . A t p r e s e n t , o n l y p u r e l y e m p i r i c a l guidance i s
a v a i l a b l e t o cope w i t h t h i s a s a d e s i g n problem. Another approach
would be t o f o r b i d t h e placement of l o o s e o b j e c t s , l i k e roof g r a v e l , w i t h o u t adequate p r o t e c t i o n a g a i n s t displacement by s t r o n g winds.
S e r v i c e a b i l i t y l i m i t s t a t e s t y p i c a l l y r e f e r t o b e h a v i o r a t lower
l o a d l e v e l s , and may r e q u i r e a d i f f e r e n t s t r u c t u r a l a n a l y s i s t h a n f o r
u l t i m a t e l i m i t s t a t e s . D e f l e c t i o n l i m i t a t i o n s f o r a e s t h e t i c r e a s o n s
might be based on e x p e c t e d y e a r l y maximum l o a d s , and t h e c a l c u l a t i o n
of d e f l e c t i o n s would be based on e l a s t i c a n a l y s i s r a t h e r than l o a d
t e s t s ( a l t h o u g h some n o n - d e s t r u c t i v e t e s t i n g c o u l d b e u s e d t o confirm t h e a n a l y s i s ) .
S e p a r a t i o n of t h e s a f e t y f a c t o r i n t o a l o a d f a c t o r and a
r e s i s t a n c e f a c t o r i s one of t h e u n i f y i n g p r i n c i p l e s of l i m i t s t a t e s
design. It is assumed t h a t t h e l o a d and i t s a s s o c i a t e d v a r i a b i l i t y
can be d e a l t w i t h once f o r a wide r a n g e of d e s i g n s i t u a t i o n s , and t h a t
t h e l o a d f a c t o r s h o u l d be independent of t h e s t r u c t u r e . The
r e s i s t a n c e f a c t o r i s assumed t o depend o n l y on t h e m a t e r i a l and t y p e
WINDOW GLASS RESEARCH 17!
n o t hold f o r wind p r e s s u r e s a c t i n g upon g l a s s . The same problem i s I d e a l t with in l i m i t s t a t e s design f o r wood by a modification f a c t o r on, r e s i s t a n c e t o account f o r i t s dependence on l o a d i n g h i s t o r y o r I
duration.
I
Loading History and Glass S t r e n g t h
The depeudence of g l a s s s t r e n g t h on d u r a t i o n ef load as w e l l aa
i t s magnitude i s handled i n c u r r e n t design p r a c t i c e by s p e c i f y i n g
1
s t r e n g t h in t e r n of a uniform wind l o a d of one minute d u r a t i a n . T h e 1 a r c h i t e c t r m s t a r r i v e a t an e q u i v a l e n t o n e - d n u t e l o a d 8 8 specified i n t h e design c h a r t s f o r rrelecting g l a s s t h i c k n e s s t o r e s i s t windp r e s s u r e (2.3). Before peak p r e s s u r e s came i n t o wide u s e f 3r c l a d d i n g design, p r e s s u r e s batled on t h e " f a s t e s t mile" wind speed i n t h e k i t e d Statea gave about the r i g h t v a l u e d i r e c t l y , and s i n c e t h a t time i t h a s be- customary simply t o uae t h e h i g h e r peak p r e s s u r e without
m o d i f i c e t l m . The r e s u l t i a an i n c r e a s e d margin of e a f e t y againat
1
where s p e d a l wind t u n n e l t e a t s are used t o provide r a t h e rp r e c i s e e s t i m a t e s of c l a d d i n g p r e s s u r e s , one occaaicmally f inde
~
extremely l a r g e n e g a t i v e p r e s s u r e e of very s h o r t d u r a t i o n a c t i n g on s m a l l a r e a s of cladding. T h i s h a s l e d t o SOW concern a h to v e r - d e s i p ~ of g l a s s s t r e n g t h . For e i n g l e , c o n s t a n t load
a p p l i c a t i c m e , converrrion of magnitude from one d u r a t i r m t o afiother h a a
1
been baaed on t h e following e m p i r i c a l l y d e r i v e d expreseion:,
Rt i s the m a d m u m t a a i m s t r e s s a t t h e break o r i g i n f o r the load c a u s i n g f a i l u r e i f t h e s t r e e s (and t h e l o a d ) l a s t s f o r t secmdah f o r t failute occurs; Rho is f o r a d u r a t i o n of 60 s e c m d a , and n i s
found by experiment t o b e - a b o u t 15-17 f o r annealed a o d a l i m e g l a s s .
The m i t u a t i m i s more complicated i f t h e l o a d causing stress in t h e g l a e s v a r i e s w i t h ti*, a s i s t r u e f o r f l u c t u a t i n g wind preeeures.
A simple mathematical model h a s been proposed t o d e s c r i b e t h e e f f e c t on breaking s t r e n g t h of g l a s s exposed t o v a r y i n g l a t e r a l p r e s s u r e s a s f o l l o w s (7):
In t h i s equation, R[x] r e p r e s e n t s any a r b i t r a r y function of time, which i s r a i s e d t o t h e power n and i n t e g r a t e d o v e r t h e
time-to-failure, t. I f R[x] i s made equal t o Rt and i s constant during t h e time i n t e r v a l t , Equation 2 reduces t o t h e same expression a s Equation 1.
Equation 2 can be shown t o a g r e e w i t h experimental r e s u l t s i n which s t r e s s i n small g l a s s beams was increased a t a constant r a t e u n t i l f a i l u r e occurred. However, i t should be understood t h a t t h e e f f e c t of l o a d d u r a t i o n can only be demonstrated on a s t a t i s t i c a l b a s i s ; t h e e f f e c t w i l l n o t be apparent u n l e s s t h e averages of s e v e r a l samples a t
172 HURRICANE ALICIA
each of s e v e r a l widely-spaced l o a d i n g d u r a t i o n s o r r a t e s a r e compared.
Equation 2 may r e q u i r e m o d i f i c a t i o n i f a p p l i e d t o t h i n p l a t e s s u p p o r t e d on a l l f o u r s i d e s , i n which t h e r e l a t i o n between l o a d and stress v a r i e s w i t h l o c a t i o n on t h e p l a t e and may be non-linear. For s q u a r e o r n e a r l y s q u a r e p l a t e s ( s m a l l a s p e c t r a t i o ) , t h e maxiuum s t r e s s a t f a i l u r e o c c u r s a l o n g t h e d i a g o n a l and n e a r t h e c o r n e r s , where s t r e s s s t a y s l i n e a r l y r e l a t e d t o load. For long, narrow p l a t e s
( l a r g e a s p e c t r a t i o ) , maximum stress a t f a i l u r e is i n t h e c e n t r a l a r e a and i s approximately p r o p o r t i o n a l t o t h e 0.75 power of load. In t h i s c a s e , i f Equation 2 were r e - w r i t t e n t o a p p l y t o l o a d i n s t e a d of
i
s t r e s s , an exponent of 0.75 t i m e s n would seem appropriate.1
S t r e n g t h Test Data and F a i l u r e P r e d i c t i o n ModelsBefore 1979, f a i l u r e p r e d i c t i o n models c o n s i s t e d of c h a r t s r e l a t i n g t h i c k n e s s t o l a t e r a l p r e s s u r e and a r e a . These c h a r t s summarize t h e r e s u l t s of t e s t s t o f a i l u r e of a wide r a n g e o f window s i z e s . The c h a r t s themselves can b e summarized by a s i n g l e e q u a t i o n , w i t h some r a t h e r minor v a r i a t i o n s i n t h e c h o i c e of t h e c o e f f i t i e n t C and t h e exponent a: Equation 3 s t a t e s t h a t t h e p r e s s u r e causing f a i l u r e i n a p l a t e i s d i r e c t l y p r o p o r t i o n a l t o i t s t h i c k n e s s r a i s e d t o t h e power a and i n v e r s e l y p r o p o r t i o n a l t o i t s a r e a A. The l a r g e s t c o l l e c t i o n of t e s t d a t a r e p o r t e d i n t h e l i t e r a t u r e (8) i n c l u d e s 119 sets of more t h a n 20 samples e a c h , w i t h t h i c k n e s s r a n g i n g from 2.5 t o 19 mm (0.09 t o 0.75 i n c h ) and a r e a from 0.6 t o 19 m2 (6 t o 200 f t 2 ) , of annealed s h e e t , p l a t e , and some f l o a t g l a s s . The c o e f f i c i e n t of v a r i a t i o n o f b r e a k i n g p r e s s u r e s w i t h i n sets tended t o b e a b o u t 25 p e r c e n t , b u t when t h e a v e r a g e s f o r each s e t were f i t t e d t o Equation 3 w i t h an exponent of 1.54, t h e r a t i o of p r e d i c t e d t o a c t u a l b r e a k i n g p r e s s u r e s had a c o e f f i c i e n t of v a r i a t i o n of o n l y 16 p e r c e n t ( s e e F i g u r e 1 (9)).
PREDICTED LOAD CAPACITY, kPa RATIO: OBSERVED I PREDICTED F i g u r e 1. Relatitm 5etween observed f a i l u r e l o a d and t h a t p r e d i c t e d
WINDOW GLASS RESEARCH 173
Although e f f i c i e n t
i n
summarizing test d a t a , t h e p w e r lawr e l a t i m r e v e a l s very l i t t l e about t h e mechanism of f a i l u r e . A more
p o v e r f u l model can be based on t h e concept of randomly d i s t r i b u t e d I
s u r f a c e flaws a c t i n g a s tress c o n c e n t r a t o r s of v a r y i n g s e v e r i t y . Tha s t r e s e concentratirm f a c t o r (SCF) at a f l a w dependa on t h e a h a r p n e ~ ~
of t h e r a d i u s at t h e flaw t i p ; normal c o r r o a i m i n the presence of w a t e r vapor t e n d s t o b l u n t t h e t i p and r e d u c e t h e SCF, b u t c o r r o s i o n under high tensile s t r e s s d w s t h e opposite. The s t r e s s c o r r o s i m
t h e o r y e x p l a i n s t h e dependence of g l a s s s t r e u g t h
on
t h e ti- v a r i a t i m 11
of stress. A break t e n d s t o o r i g i n a t e i n a h i g h l y s t r e s s e d r e g i o n ,
but n o t always where t h e s t r e s s i s maxipum, and t h e s u r f a c e s t r e s s a t
i
t h e break is n o t always t h e same.
I
B r o w (7) suggested a f u l l p r o b a b i l i s t i c model based on U c i b u l l ' s s t a t i s t i c a l t h e o r y of s t r e n g t h of m a t e r i a l s (10) i n an a t t e m p t t o
r e e o l v e apoarent d i s c r e p m c i e s i n t h e r e s u l t s of v a r i o u s
manufacturers' t e a t s . U s model incorporated s t r e s s a n a l y s i s , s t r e a s
c o n c e n t r a t i o n s due t o f l a w s , and stress c o r r o s i o n t h e o r y , but h i s e x a a ~ l e s were l i m l t e d t o s q u a r e p l a t e s . This model was extended by Beasm (11) t o i n c l u d e t h e e f f e c t s of a s p e c t r a t i o , making e f f i c i e n t use of f i n i t e d i f f e r e n c e ceclmiques (12) t o r e l a t e l a t e r a l p r e s s u r e e t o s u r f a c e s t r e s s e s .
A d i f f e r e n t approach was taken by T s a i and Stewart (13) and by
Moore (14) who used n o n l i n e a r f i n i t e element stress a n a l y s i s t o o b t a i n
m a x i m s u r f a c e s t r e a p as a f u n c t i o n of p l a t e geometry md a p p l i e d pressure. Moore a l s o i n c o r p o r a t e d an a r e a c o r r e c t i m procedure, and adopted b o t h t h e r a t e e f f e c t r e l a t i o n of Equation 2 and t h e Weibull d i s t r i b u t i o n found i n Brown's method.
A cormwn f e a t u r e of the models baaed bn s t r e s s a n a l y s i s f e t h e
a i g n l f i c m t i n f l u e n c e of a s p e c t r a t i o on breaking s t r e n g t h . Uufortlinately, t h e r e is l i t t l e s u p p o r t from e x i s t f n g d a t a f o r t h e a s p e c t r a t i o e f f e c t s p r e d i c t e d by t h e s e models, and t h e a g r e e r e n t between c a l c u l a t e d and observed breaking s t r e n g t h s , i f anything, t e n d s
t o be s l i g h t l y b e t t e r f o r t h e simple p w e r law f i t i n v o l v i n g only t h i e h e s e and a r e a , than f o r any of t h e more complicated models. The
l a t t e r w i l l n o doubt improve i n t h e i r e x p l a n a t o r y p a u e r w i t h f u r t h e r t e a e a r c h ; r e c e n t work by walker and W r (15) t o e x t e n d t h e
Beaam (1.1) model t o account f o r edge breaks i n g l a s s l o u v r e 8 supported only m t h e two s h o r t aiderr l o o k s promising.
Wind Pressure and One-minute Loading
In i t s most complete form, t h e f a i l u r e p r e d i c t i o n model besed o n
s t r e s s a n a l y s i s a t t e m p t s t o account f o r
a l l
knllw f a c t o r s c o n t r i b u t i n g t o b r i t t l e f r a c t u r e : s u r f a c e c c m d i t i m snd s t r e s s e s ( e i t h e r r e s i d u a l o r r e s u l t i n g from h e a t t r e a t m e n t ) ; p l a t e g e o w t r y , i n c l u d i n g t h e tendency f o r s t r e n g t h t o d e c r e a s e w i t h i n c r e a s i n g area; s t r e s s c o r r o s i o n i n t h e p r e s e n c e of moisture l e a d i n g t o s t r e n g t h dependence cn l o a d i n g h i s t o r y , The l a t t e r phenomenon b r i d g e s t h e s e p a r a t i o n between l o a d and r e s i s t w c e t h a t l i m i t s t a t e s design s e e k s t o c r e a t e . Research by Reed and Simiu (16) suggests t h a t 6 i e ; n l f f c w t e r r o r s raay11 74 HURRICANE ALICIA
f l u c t u a t i n g p r e s s u r e s a r e r e p l a c e d by a c o n s t a n t p r e s s u r e computed a c c o r d i n g t o Equation 2.
Although c a r e may be r e q u i r e d i n making t h e conversion from f l u c t u a t i n g p r e s s u r e t o an e q u i v a l e n t c o n s t a n t load, i t i s s t i l l p r e f e r a b l e t o i g n o r i n g t h e m a t t e r completely. Much remains t o b e done, b u t a p r e l i m i n a r y s t u d y (17) concludes t h a t t h e peak g u s t p r e s s u r e of one o r two seconds d u r a t i o n h a s a b o u t t h e r i g h t magnitude t o be t r e a t e d a s t h e o n e - m i n u t e l o a d i n g e q u i v a l e n t of a o n e h o u r storm. A s f o r t h e extreme n e g a t i v e peaks sometimes e n c o u n t e r e d i n wind t u n n e l t e s t i n g , r e d u c t i o n s of u p t o 25 p e r c e n t may b e j u s t i f i e d . D e f l e c t i o n s and Load-sharing i n Sealed h i t s
I n s u l a t i n g g l a s s a s s e m b l i e s w i t h two o r more panes s e p a r a t e d by s p a c e r s t o c r e a t e s e a l e d a i r s p a c e s e x p e r i e n c e d i f f e r e n t i a l p r e s s u r e s when b a r o m e t r i c p r e s s u r e o r a i r t e m p e r a t u r e d i f f e r s from what i t was when t h e a s s e m b l i e s were s e a l e d . Barometric p r e s s u r e and t e m p e r a t u r e vary w i t h t h e movement of weather systems and t h e change'of s e a s o n s ; i n a d d i t i o n . barometric- p r e s s u r e d r o p s a b o u t 1 0 kPa p e r 1000-m r i s e i n e l e v a t i o n . A t e m p e r a t u r e d e c r e a s e of h a s a b o u t t h e same e f f e c t a s a r i s e of 1 kPa i n b a r o m e t r i c p r e s s u r e .
I f t h e panes a r e l a r g e and r e l a t i v e l y f l e x i b l e , even l a r g e changes i n p r e s s u r e a n d t e m p e r a t u r e a r e r e a d i l y accommodated w i t h n e g l i g i b l e p r e s s u r e d i f f e r e n c e s on t h e panes. On t h e o t h e r hand, s m a l l e r panes have been kncun t o b r e a k d u r i n g v e r y c o l d weather, even w i t h o u t any added wind e f f e c t . It s h o u l d be remembered t h a t such p r e s s u r e d i f f e r e n c e s may b e s u s t a i n e d f o r s e v e r a l h o u r s , f o r which Equation 1 would i n d i c a t e a s u b s t a n t i a l l y i n c r e a s e d e q u i v a l e n t one-minute p r e s s u r e . Barometric and t e a p e r a h r e e f f e c t s can be c a l c u l a t e d r a t h e r e a s i l y p r o v i d e 3 mean d e f l e c t i o n s of e a c h pane can b e r e l a t e d t o t h e d i f f e r e n t i a l p r e s s u r e s a c t i n g on i t
--
a s i d e b e n e f i t of a f a i l u r e p r e d i c t i o n model based on s t r e s s a n a l y s i s .U n s e t t l e d I s s u e s
The f i r s t t a s k of t h e CGSB committee was t o l a y o u t t h e v a r i o u s o p t i o n s f o r producing s t a n d a r d s of good p r a c t i c e i n t h e s t r u c t u r a l design of g l a s s i n b u i l d i n g s , given t h e c u r r e n t s t a t e of knowledge a b o u t wind p r e s s u r e s on t h e one hand, and a b o u t b r i t t l e f r a c t u r e of g l a s s on t h e o t h e r . The l i m i t s t a t e s design philosophy p r o v i d e s a convenient framework f o r comparing t h e r e l i a b i l i t y of c u r r e n t p r a c t i c e , in g l a s s design t o what is a c c e p t a b l e i n o t h e r design s i t u a t i o n s .
.
Although normally a s i m p l e model f o r r e s i s t a n c e c a l c u l a t i o n i s 'p r e f e r a b l e , t h e more complex ones based on s t r e s s a n a l y s i s s h o u l d have g r e a t e r d e s c r i p t i v e c a p a b i l i t y i n t h e l o n g run. In a d d i t i o n , a g r e a t e r v a r i e t y of s u p p o r t c o n d i t i o n s and d e s i g n systems can be accommodated i n a c o n s i s t e n t manner.
C a r e f u l l y designed experiments w i l l be r e q u i r e d t o i n v e s t i g a t e t h e a s p e c t r a t i o e f f e c t s p r e d i c t e d by t h e s t r e s s a n a l y s i s models. Thus f a r , i n s u f f i c i e n t a t t e n t i o n h a s been p a i d t o t h e phenomenon of
WINDOW GLASS RESEARCH 175
f r a c t u r e o r i g i n s o c c u r r i n g a t e d g e s f o r p l a t e s w i t h low a s p e c t r a t i o s . T h i s may have a b e a r i n g on t h e a s p e c t r a t i o e f f e c t a s w e l l .
Although it is c l e a r t h a t average s t r e n g t h of g l a s s reduces w i t h
I
a g e and w e a t h e r i n g , t h e e x t e n t t o which t h e lower t a i l of t h e d i s t r i b u t i m of s t r e n g t h s is a f f e c t e d is n o t s o e a s i l y seen. By comparison t o t h e number of t e s t s done on new g l a s s , s t r e n g t h d a t a a p p l i c a b l e t o g l a s s t h a t h a s been i n s e r v i c e f o r some t i m e a r e very s c a r c e and s h o u l d b e b u i l t u p w i t h o u t f u r t h e r d e l a y .The i n t e r a c t i o n between l o a d i n g h i s t o r y and g l a s s r e s i s t a n c e would l i k e l y n o t have been s o c l o s e l y examined were i t n o t f o r a t t e m p t s t o develop r a t i o n a l models f o r d e s i g n , and t h a t development s h o u l d c o n t i n u e . A s a r c h i t e c t s and e n g i n e e r s a r e accustomed t o u s i n g computers a s d e s i g n a i d s , methods r e q u i r i n g e x t e n s i v e computations s h o u l d n o t b e r e j e c t e d on t h o s e grounds a l o n e . C h t h e o t h e r hand, u s e r s a r e e n t i t l e d t o a s s u r a n c e t h a t new and complex methods have a sound, l o g i c a l b a s i s and t h a t t h e r e h a s been a d e q u a t e c a l i b r a t i o n and v e r i f i c a t i o n a g a i n s t more f a m i l i a r methods.
Acknowledgements
Much of t h e r e s e a r c h mentioned was c o o r d i n a t e d and c a r r i e d o u t by members of t h e Committee on Window G l a s s Research c h a i r e d by
J.E. Minor, Texas Tech U n i v e r s i t y , whose c o n t r i b u t i o n s a r e g r e a t l y a p p r e c i a t e d . The ongoing work of t h e CGSB committee on T e s t i n g and S t r u c t u r a l Design of G l a s s i n B u i l d i n g s , c h a i r e d by A.G. Davenport, p r o v i d e d t h e f o c u s f q r t h e paper; t h e i r a s s i s t a n c e i s g r a t e f u l l y acknowledged, a l t h o u g h t h e views e x p r e s s e d h e r e a r e n o t n e c e s s a r i l y t h o s e of t h e committee.
This paper is a c o n t r i b u t i o n from t h e Division of B u i l d i n g Research, N a t i o n a l Research Council of Canada, and i s p u b l i s h e d w i t h t h e approval of t h e D i r e c t o r .
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T h i s p a p e r , w h i l e being d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g Research, remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n . D i v i s i o n of B u i l d i n g R e s e a r c h . N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a , O n t a r i o , K1A OR6. Ce document e s t d i s t r i b u 6 s o u s forme de t i r e - & p a r t par l a D i v i s i o n d e s r e c h e r c h e s en batiment. Les d r o i t s d e r e p r o d u c t i o n s o n t t o u t e f 01s l a p r o p r i € t € d e 1 '€ d i t e u r o r i g i n a l . Ce document n e p e u t t t r e r e p r o d u i t en t o t a l i t 6 ou e n p a r t i e s a n s l e consentement d e l ' g d i t e u r . Une l i s t e d e s p u b l i c a t i o n s d e l a D i v i s i o n p e u t O t r e o b t e n u e en e c r i v a n t 3 l a S e c t i o n d e s p u b l i c a t i o n s . D i v i s i o n d e s r e c h e r c h e s e n b a t i m e n t , C o n s e i l n a t i o n a l de r e c h e r c h e s Canada, Ottawa. C n t a r i o . K1A 0R6.
