Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Report (National Research Council of Canada. Division of Building Research), 1954-11-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=85780df4-45a2-443a-80bc-fc09271dfd95 https://publications-cnrc.canada.ca/fra/voir/objet/?id=85780df4-45a2-443a-80bc-fc09271dfd95
NRC Publications Archive
Archives des publications du CNRC
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.4224/20377604
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
A Philosophy on Loading Tests
A P H I L O S O P H Y ON L O A U I S G
TESTS
Zsn2ld Boyd Gorey andWilliam Robert S c h r i e v s r
A p a p e r p r e p a r e d f o r s ~ b m i s s i o n t o t h e Conference on "The C o r r e l a t i o n of S t r e s s e s and bisplaeernent by C a l c u l a t i o n and/or L a h s r a t c r y ~ x p e r‘merits",
t o be h e l d i n London, England,195.5,
ReportMc,
58
o f t h e D f v L s i o r i o f B u i l d i n g Research Ottawa November1754
SYNOPSIS
The purpose and scope of s t r u c t u r a l l o a d i n g t e s t s i s reviewed and c o n t r a s t e d a g a i n s t t h e s p e c i f i c a t i o n s f o r l o a d i n g t e s t s c o n t a i n e d i n p r e s e n t b u i l d i n g codes, which a r e s u m a r i z e d i n a t a b l e . The g r e a t d i v e r s i t y of t h e s e s p e c i f i c a t i o n s p o i n t s t o a l a c k of b a s i c p r i n c i p l e , F a c t o r s which should be c o n s i d e r e d i n t h e f o r m u l a t i o n of l o a d i n g t e s t s p e c i f i c a t i o n s a r e l i s t e d and d i s c u s s e d i n d e t a i l , Peading t o a new approach based on t h e p r e s e n t s t a t e of knowledge, which g i v e s p r o p e r weight t o t h e s e f a c t o r s f o r a g i v e n
s t r u c t u r e and a g i v e n s e t of c o n d i t i o n s . Three main t y p e s of l o a d i n g t e s t s a r e suggested: an a c c e p t a n c e t e s t , a r a t i n g t e s t , and a r e s e a r c h t e s t . A method of a r r i v i n g a t t h e magnitude of' t h e t e s t l o a d i s suggested. The c r i t e r i a t o be used i n a s s e s s i n g s t r u c t u r a l adequacy from t h e r e s u l t s of a l o a d i n g t e s t a r e d i s c i ~ s s e d .
TABLE OF CONTENTS
I. I n t r o d u c t i o n
...
1...
I1
.
Review of B u i l d i n g Code Loading T e s t s p e c i i ' i c a t i o n s3
111. Purpose and Scope o f Loading T e s t s
...
3
...
I V.
F a c t o r s t o b e C o n s i d e r e d i n F o r m u l a t i n g Loading T e s t S p e c i f i c a t i o n s...
4
...
A.
C l a s s i f i c a t i o n o f S t r u c t u r e s5
.
B I n t e n d e d Use o f S t r u c t u r e...
5
...
C.
M a t e r i a l s o f C o n s t r u c t i o n 6...
D-
S t a n d a r d o f Workmanship7
...
E
-
D e s i g n Loads and A c t u a l Loads7
F
-
Overdesign f o r S p e c i a l Reasons...
8
G
-
Types of F a i l u r e...me
8
H
-
F a c t o r o f S a f e t y...
9 I.
R i s k o f F a i l u r e...a...
9J
-
D u r a t i o n o f Loading...
1 0 K-
Repeated Loading and V i b r a t i o n...
11...
V. A N e w Approach t o Loading T e s t s 11...
V I.
A P r o p o s a l on t h e F o r m u l a t i o n o f T e s t L o a d i q s 1 2...
V I I . C r i t e r i a f o r A s s e s s i n g S t r u c t u r a l Adequacy 1 6...
a ) S t i f f n e s s Requirement 17 b ) S t r e n g t h Requirement...
18
...
c ) Other C o n s i d e r a t i o n s18
V I I I . C o n c l u s i o n s...
1 9 Acknowledgements...
20 R e f e r e n c e s...
20A
PHILOSOPITY OW L O A C I N GThSTS
by D. B,
Dorey
and W,
R,Schriever
Every b u i l d i n g o f f i c i a l and p r o b a b l y many owners of b u i l d i n g s , e n g i n e e r s , and a r c h i t e c t s have e n c o u n t e r e d t h e need o f a s s e s s i n g a s t r u c t u r e o r s t r u c t u r a l component by means o t h e r t h a n c a l c u l a t i o n and e x p e r i e n c e , i . e , by a c t u a l l o a d i n g . T h i s may be t h e c a s e when, f o r any r e a s o n , t h e r e i s doubt
a b o u t t h e s t r u c t u r e a l adequacy of a c o n s t r u c t i o n o r when a s t r u c t u r e c o u l d n o t r e a d i l y o r a c c u r a t e l y be s u b j e c t e d t o a n a l y s i s . I n c o n d u c t i n g and e v a l u a t i n g a l o a d i n g t e s t many q u e s t i o n s a r i s e :
What s h o u l d t h e magnitude o f t h e t e s t l o a d be? I s it a d e q u a t e t o s t a t e t h e t e s t l o a d s i m p l y a s a m u l t i p l e o f t h e d e s i y l i v e l o a d ? What c r i t e r i a should be u.sed t o judge t h e s u c c e s s o r f a i l u r e of t h e s t r u c t u r e i n p a s s i n g t h e t e s t ? Should it be a c e r t a i n maximum d e f l e c t i o n , a minimum r e c o v e r y o f d e f l e c t i o n a f t e r l o a d i n g , o r b o t h , o r s h o u l d i t b e m e r e l y t h e a b i l i t y o f t h e s t r u c t u r e t o s u s t a i n t h e t e s t l o a d f o r a c e r t a i n t i m e w i t h o u t s i g n s o f damage? What s h o u l d t h e d u r a t i o n of t h e t e s t l o a d a p p l i c a t i o n be2 l f i a t o t h e r d e t a i l s of t h e l o a d i n g p r o c e d u r e s h o u l d be s p e c i f i e d ? Most b u i l d i n g c o d e s c o n t a i n s p e c i f i c a t i o n s on l o a d i n g t e s t s and d e s c r i b e t o v a r y i n g d e g r e e s t h e t e s t p r o c e d u r e , b u t t h e r e e x i s t s a l a c k o f agreement i n t h e r e q u i r e m e n t s , The inadequacy o f e x i s t i n g l o a d i n g t e s t s p e c i f i c - a t i o n s h a s c a u s e d i n c r e a s i n g c o n c e r n i n t h e f i e l d o f c i v i l and s t r u c t u r a l e n g i n e e r i n g , b o t h i n p r a c t i c e and i n r e s e a r c h work. The a u t h o r s b e l i e v e t h a t t h e s e s p e c i f i c a t i o n s a r e n o t i n k e e p i n g w i t h t h e p r e s e n t s t a t e of knowledge, A number o f e x i s t i n g l o a d i n g t e s t s p e c i f i c a t i o n s found i n u s e i n v a r i o u s p a r t s of t h e world were reviewed by t h e a u t h o r s , For e a s e o f r e f e r e n c e t h e c h i e f r e q u i r e m e n t s of t h e s e s n e c i f i c a t i o n s have b e e n summarized i n t a b - t ~ l a r form
11. REVIEW OF BUILDING
CODE
L O A U I N G TEST-
SPBCIFICATIONSB u i l d i n g codes and b u i l d i n g by-laws a v a i l a b l e t o t h e a u t h o r s have been checked f o r t h e i r c l a u s e s on l o a d i n g t e s t s . I n examining Table I one w i l l n o t i c e t h e d i f f e r e n c e s i n p r i n c i p l e , d e t a i l , and p r o c e d u r e f o l l o w e d i n t h e s e s p e c i f i c a - t i o n s , Some s p e c i f i c a t i o n s on l o a d i n g t e s t s such a s t h e one i n t h e 1941 E d i t i o n of t h e N a t i o n a l B u i l d i n g Code of Canada, d e a l w i t h c o n c r e t e s t r u c t u r e s o n l y , and use merely t h e r e c o v e r y of d e f l e c t i o n a f t e r t h e t e s t a s a c r i t e r i o n f o r a c c e p t a n c e , T h i s Code does n o t s t a t e a maximum a l l o w a b l e d e f l e c t i o n o r a minimum age f o r t h e c o n c r e t e b e f o r e t h e t e s t i s s t a r t e d ,
Many c l a u s e s d i f f e r e n t i a t e between a " s t r e n g t h t e s t t ' and
a
"performance t e s t ' ' ( s e e 3 r d column of Table I )a l t h o u g h t h e use of t h e s e t e r m s i s n o t always t h e same i n a l l c a s e s , I n t h e t e s t s r e f e r r e d t o a s " s t r e n g t h t e s t s " , a l l t h e c r i t e r i a (no s i g n s of f a i l u r e , d e f l e c t f o n and
-
r e c o v e r y ) have been used by one code o r a n o t h e r , G e n e r a l l y speaking, t h e term "performance t e s t t ' i s a p p l i e d t o a check on d e f l e c t i o n , under a l o a d v a r y i n g from 1
x
l i v e l o a d t o 1$x
l i v e l o a d+
x
dead l o a d . Some codes r e f e r t o a "workmanship t e s t h , under 2x
l i v e l o a d , w i t h r e q u i r e m e n t s i n terms of no s i g n s of f a i l u r e coupled w i t h a maximum d e f l e c t i o n and a minimum r e c o v e r y i n c a s e t h e d e f l e c t i o n( a s c a l c u l a t e d from a c c e p t e d e n g i n e e r i n g f o r m u l a e ) i s ex- ceeded. The B r i t i s h Standard Code of P r a c t i c e i s t h e o n l y code making a c l e a r d i s t i n c t i o n between a s t i f f n e s s and s t r e n g t h t e s t , u s i n g d e f l e c t i o n
-
and r e c o v e r y a s c r i t e r i a i n t h e f i r s t c a s e , and r e c o v e r y i n t h e second, It should a l s o be n o t e d t h a t n o t a l l t e s t s a p p l y t o a l l m a t e r i a l s ,Froni t h i s r e v i e w of which t h e above i s a summary o n l y , t h e a u t h o r s conclude t h a t a fundamental and thorough r e v i e w of a l l t h e f a c t o r s e n t e r i n g i n t o l o a d i n g t e s t s on s t r u c t u r e s i s n e c e s s a r y b e f o r e any s i g n i f i c a n t improvement i n t h e i r s p e c i f i c a t i o n can be a c h i e v e d ,
PURPOSE AND SCOPE OF L O A D I N G TESTS
It i s a p p a r e n t t h a t many f a c t o r s must be con- s i d e r e d i n t h e f o r m u l a t i o n of a p r o p e r l o a d i n g t e s t s p e c i f - i c a t f o n , It would t h e r e f o r e seem u s e f u l and n e c e s s a r y t o
o u t l i n e f i r s t t h e purpose and scope of l o a d i n g t e s t s and t h e i r r e l a t i o n t o d e s i g n p r i n c i p l e s .
We may d i s t i n g u i s h two main t y p e s of l o a d i n g t e s t s , depending on t h e i r purpose. There i s f i r s t t h e
" a c c e p t a n c e t e s t s ' which i s made on a completed s t r u c t u r e o r a p a r t t h e r e o f t o d e t e r m i n e t h e a c c e p t a b i l i t y of t h a t
S e c o n d l y , t h e r e i s t h e t e s t which i s c a r r i e d t o f a f l u r e e i t h e r a s a " r a t i n ! ? t e s t " t o d e t e r m i n e t h e a e c e p t a b i l f t y of a c o n u a e r c i a l l y produced t y p e o f c o n s t r u c t i o r ; . o r a s a I I r e s e a r c h t e s t " t o examine t h e s t r u c t u r a l ' u e h a v i o u r o f a g i v e n t ; ~ p e of c o n s t r u < r . t i o n . Two d i s t i n c t a s p e c t s of -the p e r f o r m a n c e s h o u l d a l s o be k e p t i n mind, s t r e n g t h ( o r s a f e t y a g a i n s t f a i l u r e ) and s t i f f n e s s ( o r a b s e n c e of o b j e c t i o n a b l e d e f l e c t i o n s u n d e r o r 1 0 s ) I n some c a s e s a s t r u c t u r e d e s i g n e d f o r a r e q u i r s d s t r e n g t h w i l l a u t o m a t i c a l l y e x h i b i ~ a s a t i s f a c t o r y s t i f ' i ' n e s s ; i n c t h e r c a s e s i t w i l l n o t , It i s e s s e n t i a l t h a t an a d e q u a t e b a l a n c e b e f o u n d between t h e r e q u i r e m e n t s f o r d e s i g n and f o r l o a d i n g t e s t s , o r a p r c p e r l y d e s i g n e d s t r u c t u r e e i t h e r may n o t p a s s t h e l o a d i n g t e s t o r t h e l o a d i n g t e s t may s a n c t i o n a n u n d e r - d e s i g n e d s t r u c t u r e , T h i s shows t h e i m p o r t a n c e of t h e c h o i c e of magnitude of t h e v a r i o u s t e s t l o a d i n g s , It a l s o i m p l i e s t h a t a n improvement o r r e f i n e m e n t i n l o a d i n g t e s t s p e c i f i c a t - i o n s must keep p a c e w i t h t h e s t a t u s of d e s i g n s p s t i f i c a t i o n s , b u t , on t h e o t h e r hand, c a n n o t f o r g e ahead t o o f a r , I n o t h e r words, t h e f a c t o r s c o n s i d e r e d i n f o r r n u l a t i n g t e s t l o a d i n g s must b e i n b a l a n c e w i t h t h e f a c t o r s c o r l s i d e r e d i n t h e d e s i g n , F u r t h e r m o r e , a l o a d i n g t e s t s p e c i f i c a t i o n must b e s i m p l e enough t o be a d m i n i s t e r e d w i t h o u t undue d i f f i c u l t y , An i m p o r t a n t d i f f e r e n c e between t h e a c c e p t a n c e t e s t and t h e r a t i n g o r r e s e a r c h t e s t i s t h a t i n t h e f o r m e r , one i s n o r m a l l y n o t c o n c e r n e d w i t h t h e economy o f t h e d e s i g n . The a c c e p t a n c e t e s t s e r v e s m e r e l y t o e s t a b l i s h t h a t a minlmum o f s t r e n g t h o r s t i f f n e s s i s p r o v i d e d by t h e s t r u c t u r e , w h e r e a s t h e r a t i n g t e s t o r r e s e a r c h t e s t , by e x p o s i n g t h e l o a d f a c t o r o f s a f e t y throu,;h f a i l u r e , shows up o v e r d e s i g n and i n t h i s way p r o v i d e s t h e d s s i g n e r w i t h a d i r e c t t o o l f o r economic d e s i g n ,
I V . FACTORS TO
BZ
CONSIDEREX 11; FOilPXJLATING LOA;IIRG TEST SPIiC IFICAT IONS-
G e n e r a l l y s p e a k i n g , t h e f a c t o r s t c be c o n s i d e r e d I n f o r m u l a t i n g t e s 5 l o a d i n g s f o r s t r u c t u r e s v a r y w i t h t h e t y p e of c o n s t r u c t i o n and w i t h t h e i n t e n d e d u s e o f t h e s t r u c t u r e . I f , f o r i n s t a n c e , t h e t y p e o f c o n s t r u c t i o n i s one which may r e s p o n d t o o v e r l o a d i n g i n such a way a s t c endanger human l i v e s by f a i l u r e w i t h o u t w a r n i n g , s u c h a s t r u c t u r e s h o u l d be s u b j e c t e d t o a r i e o r o u s l o a d i n g t e s t s c h e d u l e i n v o l v i n g a l l p o s s i b l e
a l t e r n a t i v e s i n l o a d i n g , b e f o r e i t i s a c c e p t e d . The g r e a t e r t h e number o f f a c t o r s e n t e r i n g i n t o t h e ciesign a s s u m p t i o n s , t h e more i m p o r t a n t i t i s t o know how t h a t s t r u c t u r e w i l l r e a c t t o t h e f o r c e s a c t i n i ; upon f t d u r i n g i t s p e r i o d o f u s e f u l s e r v i c e , and t h e more e s s e n t i a l i t i s t o a r r i v e a t t h e p r o p e r t e s t
It is not uncommon for the authorities responsible
for a loading test to be hampered by the lack o f important
information necessary for the proper conduct of the test. For
example, a loading test may be required o n an existing building
for which there are no longer any detailed plans, before the
approval of the building officials is given for a change in
o
c
cup
ancy
.
CLAS
SIF'IC
A T I O N O FSTHUCTURI2S
There are two main groups into which the
structures referred to in this paper may be classified.
Structures of the first group are relatively simple and
straightforward to design by the equations of statics and are
therefore called statically determinate. The second, and
more complex, group are classified as statically indeterminate
and include all other structures for the design of which
certain assumptions must be made before a theoretical analysis
is possible.
When the complexity of the calculations discourage
efforts of theoretical analysis, the designer often finds it
necessary to resort to other methods,
In
such instances,
aloading test is a desirable and important means of assessing
the strength and stiffness of a stnlcture.
It is important that the authorities responsible
for the conduct of a loading test should r e ~ o g n i z e
that most
statical-ly indeterminate structures display
areserve of
strength after initial relaxation occurs at the joints, Under
such conditions of apparent overstressins at the joints, the
recovery of deflection will be poor, yet the structure rnay
still be safe,
In
other words,
if'loading was cont-lnued,,
under
most circumstances, further loading would be required to
produce failure, unless a shear failure, say, was impending.
Under similar circumstances of overstressing, however, collapse
would most likely occur in a statically determinate structure
without any appreciable increase i n loading,
B.
INT3TJCRE
USE OF
STlilJCTUKEWhen clearly defined,,
the intended use of a
structure will normally indicate what the strength and stiff-
ness of that structure should be.
A functional structure must
meet both a standard of strength and a standard of stiffness
but the standard of strength may be so high that the stiffness
requirements are achieved without special attention or vice
versae
The consequences of failure depend upon the use
and occupancy, In respect to safety, the risk involved is
far greater i n structures designed for pu.blic use than it
i si n s t r u c t u r e s d e s i g n e d f o r dead s t o r a g e , The d e s f g n e r must, t h e r e f o r e , govern h i s d e s i g n accordin,yly. I n r e s p e c t t o e f f i c i e n c y , t h e d e s i g n e r lnust. a l s o produce a d e s i g n which, when c o n s i d e r e d i n r e l a t i o n t o occupancy and u s e , w i l l not deform t o t h e e x t e n t of c a u s i n g damage t o d e c o r a t i v e f i n i s h e s , It would. n o t only be d e s i r a b l e t o d e s i g n a s t r u c t u r e f o r a g i v e n l o a d l i m i t b u t a l s o t o g i v e i t a " d e s i g n l i f e " , such a s 150 y e a r s . U n f o r t u n a t e l y , however, t h r o u g h a l a c k of l o a d - t e s t s t a t i s t i c s and e x p e r i e n c e i n t h e performance of s t r u c t u r e s under v a y i o u s t y p e s of l o a d i n g , i t i s d i f f i c u l t t o a s s i g n a p e r i o d of time t o t h e i i f e of a
s t n x c t u r e . U n t i l such t i m e a s s u f f i c i e n t knowledge i s accumul- a t e d , t h e u s e f u l l i f e of a s t r u c t u r e i s governed by t h e l o a d s , o r , more g e n e r a l l y t h e i n t e n d e d u s e of t h e s t r u c t u r e , C. MATERIALS OF CONSTRUCTION
-
The s t r e n g t h and s t i f f n e s s of n s t r u c t u r e d u r i n g i t s p e r i o d of i n t e n d e d u s e o r " d e s i g n l i f e " cannot be d i s - a s s o c i a t e d from t h e q u a l i t y of t h e m a t e r i a l s t h a t go i n t o t h e c o n s t r u c t i o n , The p h y s i c a l and chemical p r o p e r t i e s of a g i v e n m a t e r i a l determine t h e use f o r which t h a t m a t e r i a l i s mosts u i t e d . A b r i t t l e m a t e r i a l , f o r i n s t a n c e , may cause a sudden c o l l a p s e , whereas a d u c t i l e m a t e r i a l may deform e x t e n s i v e l y b u t s t i l l r e s i s t t h e f o r c e s t e n d i n g t o produce f a i l u r e , Other m a t e r i a l s a r e a f f e c t e d by changes i n m o i s t u ~ e c o n t e n t b o t h i n t h e i r l o a d - c a r r y i n g c a p a c i t y and i n t h e i r s u s c e p t i b i l i t y t o d e t e r i o r a t i o n . S t e e l i s s u s c e p t i b l e t o c o r r o s i o n i n c e r t a i n environments, u n l e s s p r o t e c t e d , I h e n c o r r o s i o n t a k e s p l a c e , expansion may occur i n c o n n e c t i o n s whic.h may s t r e s s a p p r e c i a b l y t h e connecting media and promote premature f a i l u r e , C o r r o s i o n of even t h e s i m p l e s t s e c t i o n s of s t e e l , u n l e s s p r o t e c t i v e
measures arle t a k e n , i s d e t r i m e n t a l t o t h e f r s t r e n g t h d u r i n g t h e " d e s i g n l i f e " of a s t r u c t u r e . The danger o r f a i l u r e i s reduced when we c o n s i d e r an e x a c t d u p l i c a t e of a mild s t e e l
s t r u c t u r e i n an i d e n t i c a l environment i f t h e second s t r u c t u r e i s c o n s t r u c t e d of s t a i n l e s s s t e e l , The second s t r u c t u r e
should, t h e r e f o r e , n o t be p e n a l i z e d a s s e v e r e l y i n a l o a d i n g t e s t .
The p h y s i c a l and chemical p r o p e r t i e s of t h e m a t e r i a l form a b a s i s f o r f o r e c a s t i n g t h e t y p e of f a i l u r e t o be expected; t h a t i s , t h e c o l l a p s e of a s t r u c t u r e may be t h e r e s u l t of a p h y s i c a l breakdown of a b r i t t l e o r d u c t i l e m a t e r i a l by o v e r s t r e s s i n g , o r t h e r e s u l t of chemical decom- p o s i t i o n followed by an e v e n t u a l p h y s i c a l breakdown by o v e r - s t r e s s i n g of t h e reduced s e c t i o n s . F a i l u r e s may a l s o o c c u r i n f a s t e n i n g s o r bonding of m a t e r i a l s l e a d i n g t o c o l l a p s e by t h e same p r o c e s s e s ,
An e q u a l l y i m p o r t a n t c o n s i d e r a t i o n i n r e s p e z t t o t h e s t r e n g t h a n d s t i f f n e s s o f a s t r u c t u r e i s t h e s t a n d a r d o f worhmanship m a i n t a i n e d d u r i n g f a b r i c a t i o n , T h e r e a r e rcany s t n l c t u r e s wllere t h e r e i s r e l a t i v e P y no c a u r e f o r c o n c e r n b e c a u s e o f t h e n a t u r e o f t h e c o n s t r u c t i o n w h i l e , o n t h e o t h e r h a n d , t h e r e a r e numerous s t r u c t u r e s where t h e s t r e n g t n a n d s t i f f n e s s p r e r e q u i s i t e s a r e a c h i e v e d o n l y by good workmanship and c l o s e s u p e r v i s i o n . I.Jorl~manship 1 s p a r t i c u l . a r l y i r n p o r t e n t i n t l i e field o f c o n c r e t e c o n s t r u c t i c n , an1 no l e s s i m p o r t a n t i n t h e f i e l d of wood col s t r u c t i o n T h c s e who a r e d i r e c t l y
c o n c e r n e d w i t h c o n c r e t e work a r e e v e r c o n s c i o u s o f i t s v a r i a b i l i t y , V a r i a b i l i t y i s r e c c g n i z e d i n good d e s i g n a s b e i n g s o m e t h i n g w h i c h c a n ~ o t b e c o m p l e t e l y overcome b y c l ~ s 6 s u p e r v i s i o n . Very o f t e n , e v e n u n d e r c l o s e s u p e r v i s i o n , r e i n f o r c i n g r o d s a r e n o t p l a c e d i n t h e i r c o r r e c t p o s i t i o n , C o m p o s i t e c o n s t r u c t i o n s u s u a l l y r e q i ~ i r e a h i g h s t a n d a r d of workmanship d u r i n g f a b r i c a t i o n . T h i s d o e s n o t mean t h a t e v e r y s t r u c t u r e s h o u l d b e p e n a l i z e d d u r i n g l o a d i n g t e s t s f o r i n f e r i o r workmanship b u t t h a t e x t r a c a u t i o n s h o u l d b e e x e r c i s e d where t h e q u a l i t y o f t h e worlimanshfp may s e r i o u s l y a f f e c t t h e p e r f o r m a n c e o f a s t r u c t u r e e i t h e r d i r e c t l y o r i n d i r e c t l y d u r i n g i t s " d e s i c ~ n li f e " ,
E . DESIGN LOADS ANT/ ACTUAL LOADS
The recommended d e s i g n l o a d i n g s a r e t h e minimum v a l u e s t h a t a d e s i g n e r i s a l l o w e d t o u s e w h e r e a l o c a l b u i l d i n g c o d e a p p l i e s , T h e s e v a l u e s a r e a c c e p t e d i n a r c h i t e c t u r a l a n d e n g i n e e r i n g J e s i - n a s b e i n g r e p r e s e n t a t i v e o f a c t u a l c o l ~ d i t i o n s o f u s e and o c c u p a n c y , The e n g i n e e r a n d a r c h i t e c t know, h c w e v e r , t h a t t h e r e a r e many i n s t a n c e s w h e r e t h e s e v a l u e s a r e e x c e e d e d u n d e r
unus
121 c i r c ~ l m s t a n c e s , w h e r e a s i n o t h e r I n s t a n c e s t h e dcr1i.n loacli11 s are n e v e r r e a c h e d , From s t u d i e s w h i c h h a v eb e e - m
-
-
:'.loor I . o a d i n g s , f o r i n s t a n c e , it h a s b e e n d e t e r m i n e d t l ~ a t a s t h e a r e ? o f a f l o o r i s i n c r e a s e d , t h e m a g n i t u d e o f t h e l o a d i n y p e r u n i t a r e a d e c r e a s e s , S m a l l a r e a s n a y be r e q u i r e d t o s u s t a i ~ n e a r l y t w i c e t l l e recommended d e s i p l o a d i n 5 w h e r e a s l a r g e r a r e a s may h a v e t o w i t h s t a n d l o a d i n g s o f a v a l u e l e s s t t a n t h o s e recomriegded i n t h e l o c a l b u i l d i n g c o d e , The c o n t e n t s o f l i b r a r i e s , w a r e h o u s e s , a n d o f f i c e b u i l d i n g s o f t e n i n d u c e l o a d i n ~ s w h i c h a r e i n e x c e s s o f assumed d e s i g n l o a d i n g s , It would t h e r e f o r e a p p e a r r e a s o n a b l e t o s u g g e s t t h a t a p p r o p r i a t e r e c o g n i t i o n b e ~ i v e n t o t h e p r o b a b l e d e g r e e o f o v e r l o a d i n g a n y g i v e n s t r u c t u r e niay h a v e t o s u s t a i n when c o n s i d e r i n g t h e m a g n i t u d e o f t h e t e s t l o a d i n g , R e c c p i t i o n o f t h i s p r o b a b l e o v e r l o a d i n z i n r e s p e c t t o recommended d e s i g n l o a d i n g s m i g h t a p p l y n o t o n l y t o new s t r u c t u r e s where d o u b t i s e x p r e s s e d as t o t h e i r s t r u c t u r a l a d e q u a c y b u t a l s o t o s t r u c t u r e s w h e r e a c h a n g e i n u s e o r o c c u p a n c y i s r e q u e s t e d , A c t u a l l y t h i s r e c o s n i t i o n i s t o a c k n o w l e d g e t h a t i t i s i m p o s s i b l e t o p o l i c e a l l b u i l d i n g s c o n t i n u o u s l y f o r c h a n g e s i n u s e o r o c c u p a n c y ,F. OVEREESIGN FOR SPI?;CIAL RLRSONS
O v e r d e s i g n i s n o t u n u s u a l , p a r t i c u l a r l y i n s t r u c t - u r e s i n which t h e y i e l d s t r e s s would o t h e r w i s e b e r e a c h e d o r
exceeded d u r i n g c o n s t r u c t i o n . T h i s a p p l i e s more s p e c i f i c a l l y t o ~ u i l d i n g s d e s i g n e d f o r s m a l l l i v e and dead l o a d s . Unusual s o i l c o n d i t i o n s may w a r r a n t o v e r d e s i g n a s a s a f e g u a r d a g a i n s t d i f f e r e n t i a l s e t t l e m e n t . Changes i n t e m p e r a t u r e and m o i s t u r e c o n d i t i o n s i n some a r e a s may j u s t i f y o v e r d e s i g n i n s of a
s t r u c t u r e . A r e a s s u b j e c t e d t o e a r t h q u a k e s may n e c e s s i t a t e
s p e c i a l c o n s i d e r a t i o n of t h e more v u l n e r a b l e s t r u c t u r e s . Over- d e s i g n f o r t h e r e a s o n s j u s t mentioned d o e s n o t mean t h a t such s t r u c t u r e s s h o u l d b e e x p e c t e d t o g i v e any added p e r f o r m a n c e d u r i n g a l o a d i n g t e s t u n l e s s t h i s p e r f o r m a n c e i s s p e c i f i e d a s
a r e q u i r e n e n t and i s r e l a t e d t o t h e u s e and occupancy. G. TYPIGS OP FAILURE It i s h e l p f u l a t t h i s p o i n t t o c l a r i f y s e v e r a l c o n c e p t i o n s which a r e o f t e n c o n f u s e d when r e f e r r i n g t o s t r u c t u r a l f a i l u r e s by c l a s s i f y i n g t h e t y p e s of f a i l u r e t h a t may o c c u r . F a i l u r e s I n s t r u c t u r e s c a n b e c l a s s i f i e d i n t o t h r e e p o s s i b l e t y p e s : a ) F a i l u r e w i t h o u t warning; b ) F a i l u r e o c c u r r i n g a f t e r y i e l d i n g h a s t a l t e n p l a c e a t a c e r t a i n l o a d ; c ) F a i l u r e t o a s t a t e of u n s e r v i c e a b i l i t y a t a c e r t a i n l o a d b u t s t i l l s a f e , A t f i r s t s i g h t , t h e above c l a s s i f i c a t i o n may a p p e a r t o b e a n a t t e m p t t o o v e r s i m p l i f y t h e a c t u a l c o n d i t i o n s u n d e r which f a i l u r e o c c u r s b u t when t h e q u e s t i o n of r i s k and t h e c o n s e q u m c e s o f f a i l u r e a r e c o n s i d e r e d , t h i s means o f r e f e r e n c e seems j u s t i f i e d .
Under t h e s e t y p e s o f f a i l u r e , t h e f i r s t t y p e l i s t e d i s t h e most d a n g e r o u s when human l i v e s a r e i n v o l v e d . T h i s t y p e o f f a i l u r e comnlonly o c c u r s i n s t r u c t u r e s composed of b r i t t l e m a t e r i a l , o r m a t e r i a l s . It would t h e r e f o r e b e p r o p e r t o assume t h a t , b e f o r e a c c e p t a n c e i s g i v e n t o a s t r u c t u r e o f t h i s g r o u p , f o r p u b l i c u s e f o r i n s t a n c e , e x t r e m e c a u t i o n must b e e x e r c i s e d . The second t y p e o f f a i l u r e c a l l s f o r c a r e f u l a t t e n t i o n b u t s u c h f a i l u r e s a r e n o t a s l i a b l e t o b e a s d a n g e r o u s a s t h e f i r s t t y p e . T h i s t y p e of f a i l u r e g i v e s a w a r n i n g of' impending f a i l u r e and s u f f i c i e n t t i m e may e l a p s e f o r t h e o c c u p a n t s t o r e a c h s a f ' e t y b e f o r e t o t a l c o l l a p s e of t h e s t r u c t l ~ r e o c c u r s , . Under s u c h c i r c ~ m s t a n c e s , t h e r i s k i n v o l v e d i s n o t so y e a t .
The thirad t y p e of' f a i l u r e i s t h e l e a s t h a z a r d o u s of t h e t h r e e . A s t r u c t u r e which w i l l behave I n t h i s manner under c r i t i c a l loadin:; c o n d i t i o n s i s s a i d t o be d u c t i l e , o r tough, lidhere e x t r a p r e c a u t i o n s a g a i n s t c o l l a p s e a r e n e c e s s a r y , t h i s t y p e of s t r ~ c t u r e i s p r e f e r r e d . The d i s t o r t i o n of t h e p h y s i c a l shape under c r i t i c a l l o a d i n g s may r e n d e r t h e s t r u c t u r e u n s e r v i c e a b l e ; b u t t h e s a f e g u a r d a g a i n s t coniplete ancl a b r u p t c o l l a p s e e l i m i n a t e s much of t h e r i s k common t o t h e f i r s t two t y p e s ,
IrJhen r e f e r r i n g t o t h e t y p e s of f a i l u r e s t h a t may o c c u r i n s t r u c t u r e s under c r i t i c a l l o a d i n g s , t h e q u e s t i o n of
" f a c t o r of' s a f e t y " a z a i s e s , T h i s e x p r e s s i o n , u n l e s s c l e a r l y d e f i n e d a s t o which c a s e i t a p p l i e s , can mean e i t h e r " s t r e s s f a c t o p of s a f e t y " o r " l o a d f a c t o r of s a f e t y " ; t h e r e f o r e , a d i s t i n c t i o n must be made between t h e two e x p r e s s i o n s . I f , f o r i n s t a n c e , we a r e concerned w i t h t h e t o t a l "load - r h i c h F: complex ~ t ~ u c t u r e can s u s t a i n w i t h o u t c o l l a p s i n g t h e n , e x c e p t i n r a r e c a s e s , t h e use of t h e " s t r e s s f a c t o r of s a f e t y " i s m i s l e a d i n g . On t h e o t h e r hand, t h e e x p r e s s i o n " l o a d f a c t o r of s a f e t y " e x p p e s s e s p r e c i s e l y e i t h e r t h e t h e o r e t i c a l o r a c t u a l l o a d t h a t a s t r u c t u r e i s e x p e c t e d t o s u p p o r t o r w i l l s u p p o r t a s a s i v e n r a t i o of t h e d e s i g n l o a d i n g . It would t h e r e f o r e a p p e a r t o b e ' a n i m p o r t a n t s t e p i n t h e r i g h t d i r e c t - i o n t o i n t r o d u c e t h e u s e of ''load f a s t o r of s a f e t y M i n l o a d i n g t e s t s p e c i f i c a t i o n s , I. R I S K Ol? FAILURE The r i s k of f a i l u r e i s a n o t h e r f a c t o r which r 2 q u i r e s f u r t h e r e l a b o r a t i o n and may be s a i d t o be d i r e c t l y a s s o c i a t e d w i t h t h e Consequence of p a r t i a l o r complete c o l l a p s e of a s t r u c t u r e i n s e r v i c e . P a r t i a l o r complete c o l l a p s e of a s t r u c t u r e tnay be t h e r e s u l t of poor d e s i g n o r t h e r e s u l t of any number of f a c t o r s , sone of which have been used e a r l i e r i n t h i s p a p e r , The t e r m " r i s k of f a i l u r e " i n t h i s p a p e r does n o t mean t h e p o s s i b l e darnage t o a s t r u c t u r e I n any form, such a s damage of i n t e r i o r f i n i s h e s , b u t I s i n t e n d e d t o mean t h e p o t e n t i a l danger t o human l i v e s and v a l u a b l e p r o p e r t y d u r i n g
i t s p e r i o d of u s e f u l s e r v i c e ,
The r i s k of f a i l u r e i s r e l a t e d , among o t h e r t h i n g s , t o t h e t y p e of f a i l u r e , That i s , a f a i l u r e o c c u r r f n g w i t h o u t warning i s much more s e r i o u s i n t h e c a s e of a g r a n d s t a n d
s t r u c t u r e s u p p o r t i n g a l a r g e crowd of s p e c t a t o r s t h a n i t i s i n t h e c a s e of a s t r u c t u r e u s e d f o r dead s t o r a g e remote from any p u b l i c o r p r i v a t e g a t h e r i n g s , It f o l l o w s , t h e r e f o r e , t h a t t h e r i s k of f a i l u r e may be much l e s s when t h e " l o a d f a c t o r of s a f e t y " i s t h r e e a g a i n s t f a i l u r e of t h e t h i r d t y p e t h a n i t i s when t h e " l o a d f a c t o r of s a f e t y s ' i s f f v e a g a i n s t f a i l u r e of t h e f i r s t t y p e ,
J.
DURATION OF LORDIIJGG e n e r a l l y , most e l a s t i c s t r u c t u r e s s u b j e c t e d t o l o n g - t e r m l o a d i n g e x h i b i t a tendency toward c o n t i n u e d r e - d i s t r i b u t i o n of s t r e s s e s a f t e r f n i t i a l l o a d i n g , accompanied by slow, but measureable, i n c r e a s e s i n d e f o r m a t i o n , T h i s tendency, Iiowever, may n o t be of s u f f i c i e n t sonsequence t o w a r r a n t undue concern u n l e s s t h e s t r u c t u r e h a s been p o o r l y d e s i g n e d , i n whfch c a s e , p r o g r e s s i v e d e f o r m a t i o n s may l e a d t o e v e n t u a l c o l l a p s e of t h e system.
Long-term l o a d i n g c o n s i d e r a t i o n s a r e of such s i g n i f i c a n c e i n wood c o n s t r u c t i o n , f o r i n s t a n c e , t h a t
a
d e c r e a s e i n s t r e n g t h i s allowed f o r i n t h e b ~ o r k i n g s t r e s s e s of t h e m a t e r i 3 i , Conversely when t h e maximum l o a d i s ofs h o r t d u r a t i o n , advantage i s sometimes t a k e n c f t h e i n c r e a s e d s t r e n g t h c h a r a c t e r i s t i c by i n c r e a s i n g t h e a2lowable working s t r e s s e s .
P r o g r e s s i v e d e f o r n a t f o n under long-term l o a d i n g i n an i m p r o p e r l y d e s i g n e d s t r u c t u r e can l e a d t o a d i s r u p t i o n of a t t a c h e d f i n i s h e s , T h i s form of secondary f a i l u r e may be caused by c r e e p i n t h e m a t e r i a l o r through s l i p p a g e between m a t e r i a l s of composite c o n s t r u c t i o n . It i s , t h e r e f o r e ,
i m p o r t a n t t o r e c o g n i z e any p o s s i b l e weaknesses of t h i s k i n d i n 8 s t r u c t u r e and t a k e a p p r o p r i a t e s t e p s t o b r i n g such f a u l t s t o l i z h t d u r i n g a l o a d i n g t e s t .
The q u e s t i o n t h e n a r i s e s a s t o how long-term l o a d i n g can be r e p r e s e n ~ e d i n a t e s t of s h o r t d u r a t i o n . T h i s q u e s t i o n can only be answered i n terms of a l l o w a b l e working s t r e s s e s ; when a long-term l o a d i n g c o n d i t i o n h a s n o t been c o n s i d e r e d i n t h e s e l e c t i o n of a l l o w a b l e working s t r e s s e s , t h e n an i n c r e a s e may be made i n t h e magnitude of t h e t e s t
l o a d i n g a s a means of b r i n g i n g o u t any h i d d e n weakness.
Hepeated l o a d i n g and v i b r a t i o n s a r e o t h e r f a c t o r s worthy of a t t e n t i o n i n t h e f o r m u l a t i o n of l o a d f n g t e s t s , New developments i n d e s i g n i n d i c a t e a t r e n d toward t h e u s e of
n a t e r i a l s vshich, under s t a t i c t e s t s , have h i g h s t r e n g t h , T h i s t r e n d r e s u l t s i n a r e d u c t i o n of t h e o v e r a l l l o a d f a c t o r of s a f e t y . I n a s t r u c t u r e designed f o r s t a t i c l o a d s , t h i s
tendency would appear t o be j u s t i f i e d . U n f o r t u n a t e l y , , however, a r e d u c t i o n i n t h e l o a d f a c t o r of s a f e t y may be c a r r i e d t o o f a r , f o r , a l t h o u g h t h e worl-ring s t r e s s e s have been i n c r e a s e d i n r e s p e c t t o s t a t i c l o a d s , t h e f a t i g u e s t r e n s t h of t h e m a t e r i a l remains r e l a t i v e l y t h e same, For i n s t a n c e , one s t r u c t u r e may n e v e r be s u b j e c t e d t o r e p e a t e d l o a d i n g s a l t h o u g h t h e second
l o a d i n g s , The second s t r u c t u r e may f a i l from f a t i g u e i n t h e m a t e r i a l , o r m a t e r i a l s . T h e r e f o r e , t h e second s t r u c t u r e , w h i l e i d e n t i c a l t o t h e f i r s t , should be p e n a l i z e d d u r i n g a l o a d i n g t e s t b e c a u s e of t h e f a c t t h a t f a t i g u e may o c c u r and a p p r e c i a b l y
s h o r t e n t h e i n t e n d e d " d e s i g n l i f e " . V , A NEW APPROACH T O L O A D I N G TESTS
-
It h a s been shown t h a t , i n most i n s t a n c e s , t h e e x i s t i n g f u l l - s c a l e loadine; t e s t s p e c i f i c a t i o n s l a c k c l a r i t y and a d e q u a t e r e c o g n i t i o n of t h e v a r i o u s f a c t o r s , i n a c c o r d w i t h t h e p r e s e n t s t a t e of knowledge, T h i s f a c t may be con- firmed by a c r i t i c a l r e v i e w of e x i s t i n g s p e c i f i c a t i o n s , The v a r i a t i o n shown i n t h e magnitudes of t e s t l o a d f n g s s u g g e s t s t h a t an a r b i t r a r y f i g u r e h a s u s u a l l y been s h o s e n t o s u i t t h e p e r i o d when l o a u i n g t e s t s were i n t r o d u c e d , It a p p e a r s t h a t t h e r e i s no d e f i n e d p r o c e s s by which t h e magnitude of t h e t e s t l o a d i n g i s d e r i v e d , o t h e r t h a n b y assuming a c e r t a i n m u l t i p l e of t h e d e s i g n l o a d , p l u s a p o r t i o n of t h e dead l o a d , b u t o n l y i n some c a s e s , whether s u i t a b l e i n a l l c i r c w n s t a n c e s o r n o t , A t a t i m e when economy p l a y s so i m p o r t a n t a r o l e i n e n g i n e e r i n g and a r c h i t e c t u r a l d e s i s n , t h i s method of pronouncing judgement on s t r u c t u r a l adequacy must be q u e s t i o n e d ,
Modern t o o l s of r e s e a r c h and co-ntemporary know- l e d g e a f f o r d almost u n l i m i t e d p o s s i b i l . i t y f o r s u i t a b l e p r e c i s e n e s s i n d e t e r m i n i n g t h e h a b i t s of a s t r u c t u r e u n d e r - going a l o a d i n g t e s t . P r e s e n t s p e c i f i c a t i o n s p r c p e r l y s t r e s s t h e importance of d e f l e c t i o n , and r e c o v e r y a f t e r u n l o a d i n g , b u t f a i l t o a g r e e on t h e a l l o w a b l e d e f l e c t i o n s , o r on t h e amount of r e c o v e r y d u r i n g a g i v e n l a p s e of t i m e a f t e r u n l o a d i n g , T h i s d i s a g r e e m e n t may be due t o t h e d i f f e r e n c e s of t h o u g h t on t h e magnitude of t h e t e s t l o a d i n g , o r may be t h e r e s u l t of d i f f e r e n c e s of o p i n i o n on t h e purpose of t h e t e s t ,
The s t r e n g t h and s t i f f n e s s of a s t r u c t u r e does n o t n o r m a l l y depend upon one f a c t o r a l o n e , b u t on a number of f a c t o r s such a s p r o p e r t i e s of t h e m a t e r i a l - workmanship, t y p e of s t r u c t u r e , , and k i n d of l o a d f n g . It i s d i f f i c u l t i n d e e d t o s e p a r a t e any one f a c t o r and t e s t f o r t h a t f a c t o r a l o n e ,
when o t h e r f a c t o r s a p p l y a s w e l l , The a u t h o r s b e l i e v e , however, t h a t
i t
i s p o s s i b l e t o a r r i v e a t a l o a d i n g t e s t s p e c i f i c a t i o n which, based on a q u a n t i t a t i v e assessment of t h e s i g n i f i c a n c e of such f a c t o r s i n t h e s t r e n g t h and s t i f f n e s ? of a s t r u c t u r e , w i l l g r e a t l y a s s i s t t h o s e d e l e g a t e d t o t h e t a s k of making up an a p p r o p r i a t e magnitude f o r t h e t o t a l t e s t l o a d i n g t o be a p p l i e d t o a s p e c i f i c s t r u c t u r e , whether t h e s t r u c t u r e be simple o r complex. The q u e s t i o n t h e n a r i s e s a s t o how a l o a d i n g t e s t might be made up t o i n c l u d e a l l t h e p e r t i n e n t f a c t o r s r e l a t i v e t o a s s e s s i n g t h e s t r u c t u r a l s u f f i c i e n c y of a s t r u c t u r e , T h i s q u e s t i o n cannot be answered d f r e c t l y w i t h o u t r e f e r r i n g a g a i nt o t h e d i f f e r e n c e between t h e t y p e s of l o a d i n g t e s t s which may b e c a r r i e d o u t ,
When a s t r u c t u r e i s t o b e t e s t e d , t h e r e a r e t h r e e d i s t i n c t t e s t s whicli may b e condllcted: a n a c c e p t a n c e t e s t ; a r a t i n g t e s t ; and a r e s e a r c h t e s t , Each t e s t w i l l h a v e i t s own p u r p o s e and s p e c i f i c r e q u i r e m e n t s , b u t a l l t h r e e t e s t s b e a r a c l o s e r e l a t i o n s h i p t o e a c h o t h e r , The a c c e p t a n c e t e s t wit;h i t s a s s o c i a t e d r e q u i r e m e n t s would b e a t e s t designed p r i m a r i l y t o a s s i s t b u i l d i n g o f f i c i a l s w i t h t h e i r d i f f i c u l t t a s k o f c a r r y i n ? o u t
--
a d h o i t e s t s on s t r u c t u r e s when t h e r e i s r e a s o n t o beP1ev-e t h a t t h e y a r e f a u l t y , i n c a p a b l e o f a c c e p t e d a n a l y t i c a l d e s i g n , o r when a c h a n z e i n occupancy i s r e q u e s t e d f o r a n o l d e r s t r u c t u r e , The p u r p o s e of t h e a c c e p t a r i c e t e s t , when t h e p e r f ' o r m a n ~ e o f a s t r u c t u r e i s q u e s t i o n a b l e , would be t o p r o v e t h e p r e s e n t q u a l i t y o f t h e c o n s t r u c t i o n and g i v e a r e a s o n a b l e a s s u r a n : e o f s a f e t y f o r t h e f u t u r e u n d e r normal u s e , The m a _ , n i t u d e o f t h e t e s t l o a d i n g would b e made u p , a s w i l l be e x p l a h e d i n more d e t a i l , byc o n s i d e r i n g a l l t h e f a c t o r s m e n t i o n e d e a r l i e r , The r a t i n g t e s t would be d e s i g n e d , p r i m a r i l y , t o a s s i s t t h e b u i l d i n g o f f i c i a l i n a s s i ; n i n g a r a t i n 2 t o new m a t e r i a l s and t o new t y p e s of c o n s t r u c t i o n , An i m p o r t a n t f e a t u r e o f t h e r a t i n g t e s t woulu be t o a l l o w f o r t h e v a r i z t i l i t g i n s t r e i i g t h and s t i f f n e s s o f t h e z p e c i n e n s ; t h a t
i s , a s t h e number o f specimens r e p r e s e n t e d by one t e s t specimen d e c r e a s e s , t h e t e s t l o a d i n 2 d e z r e a s e s , The r e s e a r c h t e s t would b e a s c i e n t i f i c i n v e s t i g - a t i o n o f t h e p e r f o r m a n c e , ancl r e l a t e d i n f o r m a t i o n , u n d e r l o a d i n g s , u p t o , o r i n c l u d i n , ; c o l l a p s e of t h e s t r u c t u r e t o c o r r e l a t e a c t u a l s t r e s s e s an~3 displacements w i t h t h o s e o b t a i n e d by c a i r , u l e t , i o n , o r o t l ~ e r w i s e , i l l t h e ciesi-gn, The r a t i n g t e s t , i f c a r r i e d t o d e s t r u c t i o n , and t h a r e s e a r c h t e s t would have a s one of' t h e i r o b j e c t i v e s t h e e s t a b l i e h i n g of' a l o a d f a c t ; o r of s a f e t y .
VI.
W - he rrla_i;nitude 01' t h e t e s t l o a d i n e , i n a n a c c e p t a n c e
t e s t may b e i ~ a d e up o f t h r e e p a r t s which, when added t o g e t h e r , make up t k e t o t a l t e s t l o a d t o b e a p p l i e d t o a ~ i v e n s t r u c t u r e ,
- .
The f i r s t p a r t r.,iolxld c o n s i s t o f t l ~ e t o t a r l i v e l o a d p l u s any
n c c e r s a r g d e a d l o a u a G d i t i o n s , The second p a r t would t e t h e overlclad p o r t i o n e:cpressed a s a p e r c e n t a g e o f t h e t o t a l l i v e l o a d . T l l c t'z1ir.d p a r t would be an a d d i - t i o n t i l p o l . t i o n , e x p r e s s e d a s t h e sum o f i n d i v i d u a l p e r c e n t a g e s d e r f v e d from a c o n s i d e r a t - i o n of' tlie f a c t o r s which d i s t i n g u i s h one s t r u c t u r e from a n o t h e r ,
rl l l i i s proceclure c a n b e niore r e a d i l y u n d e r s t o o d by r e f e r r i n g t o
The f i r s t p o r t i o n of t h e t e s t l o a d i n g i n d i c a t e d 311
by t h e l e t t e r "A i n F'ig, 1, wo:xld b e 1 C ' O p e r cen.t of' t h e d e s i g n l i v e l o a d p l u s any a d d i t , i c n a l dead l o a d which mag n o t b e s u p p o r t e d by t h e s t r u c t u r e a t t,he t i m e of t h e t e s t , I f th.e t e s t i s r e q u i r e d t;o a p p r o v e f i zl.lo.ngs i n u s e o r
occupancy o f an e x i s t i n g s t r u c t u r e , , p o r t i o L
"A" of t h e t e s t
l o a d would c o n s i s t of1CO
p e r ceiit of t h e a s s l g n e d l i v e l o a d only., u . n l e s s r e n o v a t i o n s impose a n a d d i t i o n a l dead l o a d .The second p o r t i o n of t h e t e a t l o a d i n g i n d i c a t e d by t h e l e t t e r
"B"
i n F i g , 1 would. be a c e r t a i n p e r c e n t a g e o f t h e l i v e l o a d and would v a r y b ~ i t h tihe ~ i s k of f z i l u c : e . The p e r c e n t a g e of t h e l i v e l o a dto
b e c h o s e n c o u l d h a v e t h r e e p o s s i b l e v a l u e s f o r any one u s e o r o c c a p a n c y ? and would be d e t e r m i n e d by t n e t y p e o f P a i 1 u . r ~ whic'h wouid l i k e 1 , y o c c u r rin t h e s t r u c t u r e u n d e r stxidy, F i g u r e 2 sl~ows t h r e e g r a p h s showing t e n t a t i v e p e r c a n t a g e o v e r l o a d s p l o t t e d a g a i n s t u s e and occupancy. Tke u s e and occupancy o r d i n a t e s v a r y Pn % h s t 3 . i r e c t i o n ofi n ~ r e a s i n : ~ : r i s k ; t h a t i s , from a s t r u c t u r e u s e d f o r dead. s t o r a g e t o a p u b l i c b u i l d i n , l ~ , The t l ~ r e e c u r v e s shown c o r r e s p o n d t o t h e t y p e s of f a i l u r e e x p e c t e d , and i n c r e a s e i n s e v e r i t y of p e r c e n t a g e of o v e r l o a d f r o m t y p e ( c ) t o t y p e ( a j , The p e r c e n t a g e of o v e r l o a d t o be a p p l i e d t o a g i v e n s t r u c . t u r e would b e a s c e r - t a i n e d by f o l l o w i n g t h e c u r v e a p p r o q r i a i e t o t h e t y p e o f f a i l u r e e x p e c t e d t o t l i e p o i r i t wherae t h e C U F L : i 3 3 r o s s e ~ a h o r i z o n t a l l i n e t h r o u g h t h e a p - p r o p r i a t e occupancy arid u s e , The p e r c s n t a g e of o v e r l o a d i s t h e n i n d i c a t e d by a v e r t i c a l l i n e t h r o u g h t h i s p o i n t on t h e c u r v e ,
The t h i r d p o r t i o n of t h e t e s t l o a d i n s i n d i c a t e d by t h e l e t t e r
" c "
i n F i g , 1 would b e t h e summation of p e r c e n t a g e s of l i v e l o a d a p p l - i c a k l e t o t h e s t r n c t u r e t o be t e s t e d , These p e r c e n t a g e s would be s e l e c t e d from p r e d e t e r n ~ i n e d v a l u e s which would f o r m a p a r t of l o a d i n ; t e a t s p e c i f i q a t i o n ? and would p r o v i d e f o r t h e u n c e r t a i n t i e s i n h e r e n t i n t h e c h a r a c t e r and d e s i g n o f a s t r w c t u r e Sn t e r m s o f i n c r e a s e d t e s t Poa l i n g , Such p e r c e n t a g e s would h e t h e r e s u l t of r e c o s n 2 z i n g s u c h f a c t o r s asworlurlanship, d e t e r i o r a t i o n , impact l o a d i n g , and f a t l g u e , The i n f l u e n c e o f t h e s e p e r c e n t a g e s i n f o r m u l a t i n g t h e t e s t l o a d i n g v,rould g i v e weight t o t h e f a c t o r s p e c u i i a r t o t h e g i v e n s t r u c t u r e , I n o t h e r words, t l i e number of a p p l i z a b i e p e r c e n t a g e s of l i v e l o a d would v a r y w i t h t h e t y p e of c o n s t r u c t i o n , T e n t a t i v e v a l u e s f o r t h e s e p e r c e n t a g e s a r e z i v e n i n T a b l e I1 b u t t h i s i s i n c l u d e d f o r i l i u s t r a t i v e p u r p o s e s o n l y , The t o t a l a c c e p t a n c e t e s t l o a d f n g t o be a p p l i e d t o a s t r u c t u r > e would b e t h e sum of p a r t s A ,
B, and
C, P a r t Awould be t h e a s s i p e d l i v e l o a d p l u s any a d d i t i o n a l dead l o a d , P a r t B wou-ld c o n s i d e r t h e r i s k o f f a i l u r e and t h e t y p e of f a i l u r e r e s u l t i n g from t h e breakdown of m a t e r i a l , P a r t C would d i s t i n g u i s h one t y p e of c o n s t r u c t i o n from a n o t h e r , and t h e t o t a l t e s t l o a d , o r summation o f p z r b t s A , 3 , and C , would a s s e s s t h e q u a l i t y o f t h e c o n s t r u c t i o n and t h e s u i t a b i l i t y of t h e s t r u c t u r e t o i t s environment
E-c to W E-c W U F5 4 E-l PI W U v 4 0 u\ 0 cr\ a3 LA- O
m
F;' 0 d -C,s
Cc 4= U3 c 0 U al 4J d 03 0 PI E 0 vThe r a t i n g t e s t would d i f f e r from t h e a c c e p t a n c e t e s t i n t h a t t h e magnitude o f t h e r a t i n g t e s t l o a d would
depend upon t h e number o f specimens r e p r e s e n t e d by one t e s t specimen, I f ever>y specimen were t e s t e d , t h e n t h e two t e s t s would be i d e n t i c a l , e x c e p t t h e t t h e r a t i n g t e s t might be
c a r r i e d t o f a i l u r e a f t e r a s t u d y i s made o f t h e h a b i t s o f t h e t e s t specimen u n d e r t h e r a t i n g t e s t l o a d , I n o t h e r words, t h e r a t i n g t e s t would f o r m a l i n k between t h e a c c e p t a n c e t e s t and t h e r e s e a r c h t e s t , The r a t i n g t e s t might b e c o n d u c t e d o n t h e s i t e o r i n a t e s t i n g l a b o r a t o r y , I f t h e t e s t specimen r e p r e s e n t e d more t h a n one o f i d e n t i c a l d e s i z n , a p r o b a b i l i t y f a c t o r e x p r e s s e d i n t e r m s o f a n a p n r o p r i a t e p e r c e n t a g e o f t h e l i v e l o a d would be added t o p a r t C o f t h e a c c e p t a n c e t e s t l o a d i n g t o p r o v i d e f o r t h e v a r i a t i o n i n t h e s t r e n g t h o f a g i v e n number o f specimens, The d e r i v a t i o n o f t h i s f a c t o r would b e d p t e r m i n e d by l o a d i n g t e s t s t a t i s t i c s on s i m i l a r
m a t e r i a l s and c o n s t r u c t i o n s , T e n t a t i v e v a l u e s f o r p e r c e n t a g e s of l i v e l o a d t o b e added t o p a r t C o f t h e a c c e p t a n c e t e s t f o r t h e number of specimens r e p r e s e n t e d by one t e s t , a r e ,yiven i n T a b l e 111, a g a i n f o r i l l u s t r a t i v e p u r p o s e s o n l y . TABLE 111 R A T I N G TEST P e r c e n t a g e s o f l i v e l o a d t o b e added t o P a r t C of a c c e p t a n c e t e s t l o a d i n g M a t e r i a l S t e e l R e i n f o r c e d C o n c r e t e P r e s t r e s s e d C o n c r e t e S t r u c t u r a l Timber Composite C o n s t r u c t i o n No, of Specimens t o P e r c e n t a g e of b e t e s t e d L i v e Load 1 i n 1 ( a c c e p t a n c e t e s t ) 0 1 i n
3
1 0I
i n 1 0 20 1 i n 1 0 0 25 1 i n 1 ( a c c e p t a n c e t e s t ) 0 1 i n3
1 i n 1 03-5
1in
1 0 0 3033
1 i n 1 ( a c c e p t a n c e t e s t ) 0 1 i n3
1 i n 1 015
30 1 i n 1 0 033
1 i n 1 ( a c c e ~ t a n c e t e s t ) 0 1 i n ( a c c e p t a n c e t e s t ) 1 i n3
1 i n 1 0 1 i n 1 0 0The n e x t i m p o r t a n t a s p e c t of a l o a d i n g t e a t i s
how t o d e c i d e i f a s t r u c t u r e h a s p a s s e d t h e a c c e p t a n c e o r r a t i n 2 t e s t . I n t h i s r e s p e c t we c a n d e a l w i t h t h e a c c e p t a n c e t e s t and t h e r a t i n g t e s t i n t h e same manner, e x c e p t f o r t h e t e a t l o a d . F o r t h e r a t i n g t e s t a h i i ; h e r t e s t l o a d would b e u s e d d e p e n ~ i i r q on t h e nmber o f specimens t e s t e d a s e x p l a i n e d p r e v i o u s l y and t h e r a t i n g t e s t might b e c a r r i e d t o f a i l u r e . It w i l l b e r e c a l l e d t h a t t h e s t r u c t u r e must meet o n e o r b o t h o f t h e r e q u i r e m p ~ t . . ~ o f s t r e n g t h . and s t i f f n e s s . G e n e r a l l y s p e a k i n s t h e s t r e n g t h r e q u i r e m e n t i s met i f , a f t e r t h e p r o p e r c h o i c e of t h e t e s t l o a d i n g , , t h e s t r u c t u r e d o e s n o t show e v i d e n c e of' e x c e s s i v e s t r a i n s ( o r s t r e s s e s ) and/or o f imminent i n s t a b i l i t y , T h i s means t h a t t h e s t r a i n s d e v e l o p e d must b e rneasured a t a l l c r i t i c a l p o i n t s a n d , i n c o n p r e s s i o n members and o t h e r p l a c e s of p o s s i b l e
i n s t a b i l i t y , c l e f o r n a t i o n s must be d e t e r m i n e d . I n soire s t r ~ . ~ c t u r e c , s u c h a s c o m p l i c a t e d frarnenorks and thin-webbed
members, t h l s may c a l l f o r e l a b o r a t e i n s t r ~ m e n t a t i o n and may t h e r e f o r e , n o t b c p r a c t i c a l i n t h e c a s e of t h e normal a.cceptance t e s t b e c a u s e o f t h e complica1;ions and t h e c o s t i n v o l v e d . and
b e c a u s e o f t h e p o s s i b l e i n a c c e s s f ' o i l i t y of t h e c r i t : i . c e l p o i n t s i n t h e f i n i s h e d s t r u c t u r e . C o n s e q u e n t l y , f o r t h e a c c e p t a n c e t e s t , o t l ~ e r means of' a s s e s s i n g s t r u c t u r a l adeq?.;acy f r o m t h e s t r e n g t h p o i n t o f view must be o b t a i n e d , u s i n g
a ) deflection ( o r more g e n e r a l l y d e f o r m a t i o n ) ; b j r e c o v e r y o f d e f l e c t i o n a f t e r u n l o a d i n g ; c ) s l g n s of damage o r d i s t r e s s ;
o r a c o m b i n a t i o n of t h e s e a s a c r i t e r i o n ,
Anot.her q u e s t i o n which must b e d e c i d e d i s w h e t h e r t h e same t e s t i o a d i n g s h o u l d be ilsed f o r s t r e n g t h and s t i f f n e s s , T h i s may n o t seem t o o i m p o r t a n t
at f ' i r s t , a s u s u a l l y a s a t i s -
f a c t o r y s t r u c t u r e s h o u l d n o t r e a c h y i e l d c o n d i t i o n s i n t h es t r e n c t l ~ t e s t so t h a t we c a n a s s m e t h a t d e f o r m a t i o n s a r e
p r o p o r t i o n a l t o l o a d . Thus i t would seem o f l i t t l e consequence t h e d e f o r n ~ a t i o n s a r e rneasured a t th.e cissign l i v e l o a d o r a t t h e t e s t l o a d i n g , T h i s i s not hotue-ver q u i t e c o r r e c t , One i s u s u a l l y p a r t i c u l a r l y i n t e r e s t e d i n t h e m a g n i t u d e of t h e d e l ' l e c t i o l i s a t a l o c d i n s c o r r e s p o n d i n s t c t h e l i v e l o a d . The c l e ~ i g n s ~ e c i f ' i c a t i o n s o f t e n s t a t e a maximum d e f l e c t i o n u n d e r des_i.gn l i v e l o s i ? , A t t h i s ~ O R C ~ t h e s t r u c t u r e 'being t e s t e d s h o ~ ~ l d a l s o b e checked f o r t h e a b s e n c e o f m i n o r damal;;ea, s u c h a s c r a c l r s i n p l a s t e r e d cef 1 i n ~ ; ; s o r wal.is, a s sixth c r a c k s might juz t i f a i a i : ) l y o c c u r a t tine h:i.i:her loa.<'i.in:;s lised f o r t h e
s t r e n g t h . t e s t .
The a u t h o r e b e l i v e t h a t no one c r i t e i - + i o n ? b u t a
corn! i n a t i o n o f 1~axirnu.m def l e c t i o ~ aiitl minimum r e c o v e r y c o u p l e d w i t h t h e a b s e n c e of any s i q n s of' d i s t r e s s i n t V L c main s t r u c t u r a l
p a r t s under t h e t e s t l o a d i n g , should b e used a s a s t r e n g t h r e q ~ . i r e m e n t , whereas t h e s t i f f n e s s r e q u i r e m e n t s t ~ o u l d c o r l s i s t of a max.i.rjum d e f l e c t i o n and t h e absence of any s i g n s of damage under l i v e l o a d .
a ) S t i f f n e s s Requirement
Again t h e a u t h o r s r e a l i z e t h a t t h e a c t u a l v a l u e s of acceptable d e f l e c t i o n s and o t h e r d e f o r m a t i o n s must be
chosen by t h e a p p r o p r i a t e a u t h o r i t y , I n o r d e r t o i;ive an example of t h e t y p e of r e q u i r e m e n t s u ~ ; g e s t e d , howevep, a c c e y t a b l e d e f l e c t i o n s and d i s p l a c e m e n t s f o r d w e l l i n g con- s t r u c t i o n a r e s i v e g i n Table I V ,
TABLE I V
ACCGPTANCE
AND
H A T I N G TESTSTIFFNESS HEQUIL-~ET.IEI~T FOli LWIALLII:G C O N E T ~ : ~ I C T I O T J Under t h e P o r t i o n
A
of t h e t e s t l o a d t h e d e f l e c t i o n s and d i a - placements shown i n t h i s t a b l e s h a l l n o t be exceeded w i t h i n24
h o u r s n o r s l l a l l t h e r e be any c r a c k i n g of p l a s t e r o r o t h e r f i n i s h e s , F l o o r s C e i l i n g s no a r c h . f i n i s h Walls and p l a s t e r f i n i s h P a r t i t i o n s no a r c h . f i n i s h Roofs p l a s t e r f i n i s hOther Components, Elements o r S t r u c t u r e s
% Not a p p l i c a b l e , o r depending on s p e c i a l purpose of s t r u c t u r e a s determined by a p p r o p r i a t e a u t h o r i t y .
b ) S t r e n g t h Requirement
As a l r e a d y mentioned, t h e r e c o v e r y of d e f l e c t i o n a f t e r a p p l i c a t i o n of t h e t o t a l t e s b loadin(z i s n o r m a l l y t h e most simple rneans of d e t e c t i n g whether o r n o t t h e s t r u c t u r e h a s exceeded t h e y i e l d p o i n t . (Some c o m p l i c a t i o n s i n t h i s
r e g a r d w i l i be d e a l t w i t h l a t e 1 .) I f , l~owever, t h e o b s e r v a t i o n of r e c o v e r y and of p o s s i b l e s i g n s of f a i l u r e i s not c o n s i d e r e d t o be s u f f i c i e n t l y i n d i c a t i v e of s a t i s f a c t o r y s t r u c t u r a l
performance, t h e b u i l d i n g o f f i c i a l should r e q u i r e s t . r a i n and/or d e f o r m a t i o n meaxuremants t o be made by a c c e p t e d methods a t c r i t i c a l p o i n t s of t h e s t r u c t u r e t o p r o v e t h a t no e x c e s s i v e s t r a i n s and/or 3i:zn.s of i n s t a b i l i t y a r e o c c u r r i n g a t t h e s e p o i n t s ,
The t o t a l t e s t l o a d should be s u s t a i n e d by t h e s t r u c t u r e b e i n g t e s t e d f o r a rninirnum of
24
h o u r s witho11.t any s i ~ n s of s t r u c t u r a l f a i l u r e . A f t e ~ rel;ioval of t h e t e s t l o a d t h e r e c o v e r y baithill24 h o u r s from t h e b e ~ i i ~ n i n g
of tl2.e un- 1i:ading sllould be a t l e a s t75
p e r c e n t of t h e d e f l e c t i o n under t h e t e s t l o a d ,I f , on t h e o t h e r hand, t h e s t r u c t u r e does n o t
p a s s t h i s t e s t b u t shows a minimum r e c o v e r y of s a y
60
p e r c e n t , r e t e s t i n g shoulc?. be allowed. Some of t h e r e s i d u a l d e f l e c t i o n a f t e r t h e f i r s t l o a d i n g normally i s of no s e r i o u s consequencef t
a s i t i s due t o beddin;; down" a t s u p p o r t s , j o i n t s , e t c , The r e t e s t i n g should be s t a r t e d a s soon a s p o s s i b l e a f t e r t h e f i r s t t e s t i n g and t h e t o t a l t e s t l o a d should a g a i n be a p p l i e d f o r a t l e a s t
24,
h o ; ~ . r s . The r e c o v e r y w i t h i n t h e f o l l o w i n g 24 h o u r s from t h e be::inning of t h e u n l o a d i n g should be a t l e a s t ,say
90
pel7 c e n t of t h e d e f l e c t i o n measured d u r i n g t h e second l o a d i n s .The above-mentioned r e c o v e r y f i g u r e s might of c o u r s e have t o be adapted t o different t y p e s of c o n s t m ~ c t i o n , t a k i n g i n t o account such f a c t o r s a s c r e e p of t h e m a t e r i a l
under l o a d , I f t h e m o u n t of c r e e p which i s e s t i m a t e d t o
d e v e l o p i n t h e s t r i ~ c t u r e under s e r v i c e c o n d i t i o n s , i s c o n s i d e r e d t o be of consequence, t h e b u i l d i n < { o f f i c i a l might r e q u i r e t h a t t h e d e f o r n i a t i o n s be measured a t i n t e r v a l s under l o a d t o p e r m i t p l o t t i n ( ; of t h e time d e f l e c t i o n curve and t h a t t h e l o a d i n g p e r i o d be extended beyond i t s u s u a l l e n g t h ,
c ) Other C o n s i d e r a t i o n s
A number of d e t a i l s of t h e t e s t i n g p r o c e d u r e
should a l s o be s p e c i f i e d by t h e b u i l d i n g o f f i c i a l . Among t h e s e a r e t h e r e q u i r e d r e f i n e m e n t and a c c u r a c y of t h e measurement of d e f o r m a t i o n , t h e minimum and maximum age of a new s t r u c t u r e a t t h e time o f ' t e s t i n g , and s p e c i a l c o n d i t i o n s f o r t h e r a t i n g
t e s t . B a s i c a l l y t h e r e q u i r e m e n t s f o r t h e r a t i n g t e s t corr.espond t o t h o s e f o r t h e a c c e p t a n c e t e s t . The specimens f o r t h e r a t i n g
