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L'Original bridge demolition
NATIONAL RESEARCH COUNCIL CANADA
DIVISION O F BUILDING RESEARCH
L' ORIGNAL BRIDGE DEMOLITION by R . C r a w f o r d I n t e r n a l R e p o r t No. 3 1 2 of t h e D i v i s i o n of Building R e s e a r c h OTTAWA M a r c h 1 9 6 5
P R E F A C E
T h e c o n s t r u c t i o n of f u l l - s c a l e s t r u c t u r e s solely f o r the p u r p o s e of d e s t r u c t i v e t e s t i n g i s u s u a l l y s o costly t h a t l i m i t e d r e - s e a r c h of t h i s kind i s e v e r c a r r i e d out. Consequently, i t b e c o m e s i m p o r t a n t t o t a k e advantage of the opportunities which do a r i s e when s t r u c t u r e s no longer of u s e become available f o r s u c h t e s t s . The Division h a s always encouraged the exploitation of s u c h opportunities and h a s p a r t i c i p a t e d when a p p r o p r i a t e .
T h e c a s e now r e p o r t e d involved the demolition of a d i s -
used b r i d g e by the D e p a r t m e n t of Highways of O n t a r i o , and t o the g r e a t c r e d i t of those c o n c e r n e d , it w a s decided t o u s e the demolition o p e r a t i o n a s a n e x p e r i m e n t a l p r o j e c t in ways that would be productive of u s e f u l engineering information. T h e s t r u c t u r e itself w a s not well suited t o useful l o a d - t e s t i n g , but i t w a s c o n s i d e r e d worthwhile t o m o n i t o r a i r - b l a s t aild ground v i b r a t i o n s during the blasting o p e r a t i o n . T h e Division w a s privileged t o p a r t i c i p a t e in the p r o j e c t , and a s s u m e d p r i m a r y r e s p o n s i b i l i t y f o r a i r -blast m e a s u r e m e n t s , and a s s i s t e d t h e O n t a r i o R e s e a r c h Foundation with the monitoring of ground v i b r a t i o n s . T h e information obtained f r o m t h i s p a r t of t h e p r o j e c t f o r m s the s u b j e c t of t h i s r e p o r t . T h e a u t h o r , a p h y s i c i s t having s p e c i a l i n t e r e s t i n
ground vibration m e a s u r e m e n t s , and a m e m b e r of the Building P h y s i c s Section of the Division, h a s subsequently, f o r p e r s o n a l r e a s o n s , r e - t u r n e d t o G r e a t B r i t a i n t o take up employment in h i s s p e c i a l field t h e r e . Ottawa
M a r c h 1 9 6 5
N. B . Hutcheon A s s i s t a n t D i r e c t o r
LtORIGNAL BRIDGE DEMOLITION by
R . C r a w f o r d
At L I O r i g n a l n e a r Hawke s b u r y , Ontario, the D e p a r t m e n t of Highways of Ontario planned t o d e m o l i s h a d i s u s e d highway b r i d g e . The method of demolition was t o u s e explosives t o s h a t t e r t h e columns of the bridge causing the bridge deck t o f a l l a s a unit t o the ground. A s t h i s bridge was in the country, well away f r o m built-up a r e a s , i t w a s ideally
suited f o r explosive t e s t p r o c e d u r e s .
The m a i n i n t e r e s t of the Division of Building R e s e a r c h w a s in the a i r b l a s t f r o m the demolition of t h e m a i n b r i d g e s t r u c t u r e . M e a s u r e - mept of the a i r - b l a s t p r e s s u r e s w a s undertaken a t t h r e e locations f o r the p u r p o s e of seeing whether c o n s t r u c t i v e i n t e r f e r e n c e with a consequent build-up of p r e s s u r e would o c c u r . Of s e c o n d a r y c o n c e r n t o the National
R e s e a r c h Council, and the m a i n i n t e r e s t of the O n t a r i o R e s e a r c h F o u n - dation, w a s t h e monitoring of the ground vibration a s s o c i a t e d with the b l a s t .
GENERAL
F i g u r e 1 shows the layout of the a r e a including the b r i d g e , Highway 17, and a n observation bunker built by the D e p a r t m e n t of High- ways f r o m which p e r s o n n e l could watch and photographs of the b r i d g e b l a s t could be t a k e n . A i r - b l a s t m e a s u r e m e n t s w e r e taken at locations C , D, and E , e a c h 90 ft f r o m the b r i d g e , and ground v i b r a t i o n s w e r e monitored a t locations A and B which w e r e 134 f t and 560 f t , r e s p e c t i v e l y , f r o m t h e e a s t end of t h e b r i d g e . The complete o b s e r v a t i o n s a r e given in Table I.
The b r i d g e c r o s s e d a section of d i s u s e d railway t r a c k a t a n oblique angle. T h e b r i d g e columns, 20 f t high and 20 f t a p a r t , w e r e
c o n s t r u c t e d of r e i n f o r c e d c o n c r e t e . F o r t h i s demolition operation, holes w e r e d r i l l e d v e r t i c a l l y down e a c h column f r o m t h e bridge d e c k . In the s m a l l e r columns ( 2 it by 2 ft in section) one hole was drilled down the c e n t r e ; in t h e h e a v i e r columns that supported the middle span two o r t h r e e holes w e r e u s e d . T h i r t e e n holes w e r e d r i l l e d in e a c h abutment.
EXPERIMENTAL PROCEDURE FOR THE AIR-BLAST MEASUREMENTS S t a t h a m d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r s w e r e u s e d t o
m e a s u r e the a i r - b l a s t p r e s s u r e s , and t h e final output was displayed on a d i r e c t - w r i t i n g g a l v a n o m e t e r r e c o r d e r , giving a p e r m a n e n t r e c o r d . T h e t r a n s d u c e r s w e r e mounted 4 ft above the ground l e v e l on s t e e l s t a k e s d r i v e n into t h e ground. A s h o r t length of tubing was connected to the dynamic side of the t r a n s d u c e r s o t h a t t h e s t a k e would shield the t r a n s d u c e r f r o m fly r o c k . T o t h e s t a t i c 'side a 400-ft length of tubing was attached and l e d away f r o m t h e b r i d g e t o k e e p a r e f e r e n c e a t m o s p h e r i c p r e s s u r e .
CALIBRATION SHOT
Two d a y s b e f o r e t h e m a i n b l a s t a single column w a s b l a s t e d t o d e t e r m i n e what the loading c h a r g e should be f o r the m a i n s h o t . T h i s f i r s t shot c o n s i s t e d of a single hole with 10 s t i c k s , e a c h m e a s u r i n g 1 1/8 in. by 8 in. of 75 p e r cent s t r e n g t h CIL " F o r c i t e " in t h e column p e d e s t a l and 1 s t i c k e v e r y 2 f t in t h e length of the column. F r o m t h e way t h a t t h i s column b r o k e , t h e CIL r e p r e s e n t a t i v e decided t o leave the c h a r g e in t h e p e d e s t a l t h e s a m e but t o i n c r e a s e the weight of explosive in e a c h hole in the c o l u m n s t o about 5 lb of dynamite, using s t i c k s 2 in. in d i a m e t e r . The shot p a t t e r n w a s a r r a n g e d using P r i m a c o r d 1 5 - m i l l i s e c o n d d e l a y s , with a t o t a l of 16 d e l a y p e r i o d s and a t o t a l c h a r g e of about 260 l b . No c h a r g e s w e r e placed in the bridge deck. The layout of the b r i d g e columns and shot p a t t e r n i s shown in F i g u r e 2.
F o r the c a l i b r a t i o n s h o t , p r e s s u r e t r a n s d u c e r s w e r e s e t up 4 5 f t and 90 f t f r o m the column. F i g u r e 5 shows that o v e r p r e s s u r e s of 0 . 2 5 p s i and 0.50 p s i w e r e r e c o r d e d at t h e two positions. F r o m t h e s e an e s t i m a t e f o r t h e p r e s s u r e s f r o m the m a i n b l a s t w a s m a d e and i n s t r u m e n t s w e r e s e t accordingly. The decaying 50 c p s wave t r a i n on t h e p r e s s u r e r e c o r d i s due t o tube waves in the s h o r t , 3-ft length of tubing attached t o the dynamic side of the t r a n s d u c e r . It w a s thought t h a t by reducing the tube length t o 2 o r 3 i n . the tube waves would not be a p p l i c a b l e , but a s w a s s e e n f r o m the m a i n bridge shot in F i g u r e
9 ,
they w e r e s t i l l p r e s e n t , although t h e frequency had i n c r e a s e d t o about 500 c p s . Despite themasking effect of t h e tube waves, t h e o v e r p r e s s u r e s c a n s t i l l be obtained f r o m the r e c o r d s .
MAIN BRIDGE SHOT
The m a i n b r i d g e shot ( F i g u r e
9 )
w a s s u c c e s s f u l and t h e b r i d g e d e c k f e l l as a unit t o t h e ground once the columns w e r e s h a t t e r e d . F i g u r e s 3 and 4 show t h e b r i d g e b e f o r e and a f t e r t h e m a i n s h o t . F l y r o c k was not e x c e s s i v e beyond a d i s t a n c e of 100 f t , but c l o s e r t o t h e b r i d g e t h e r e w a s c o n s i d e r a b l e evidence of p i e c e s of c o n c r e t e varying in s i z e f r o m c o n c r e t e chips t o p i e c e s 3 and 4 l b in weight.The m a x i m u m a i r p r e s s u r e s m e a s u r e d a t locations C , D and E w e r e 0 . 31 p s i , 0 . 4 4 p s i and 0 . 39 p s i , r e s p e c t i v e l y . It w a s thought that the
combination of the millisecond d e l a y s plus the t i m e l a g c a u s e d by the
s e p a r a t i o n of the columns might c o n s t r u c t i v e l y i n t e r f e r e and c a u s e a build- up of p r e s s u r e . The p r e s s u r e s a t e a c h m e a s u r i n g location w e r e affected by s e v e r a l f a c t o r s including the different d i s t a n c e s f r o m shot point t o r e c e i v e r , v a r i a t i o n s in d i r e c t i o n and t h e t i m e sequences involved. The c e n t r a l
location D r e c e i v e d m o r e e n e r g y f r o m the a i r b l a s t than the o t h e r two locations b e c a u s e of the proximity of the heavily-loaded m a i n c o l u m n s . B e c a u s e the p r e s s u r e p u l s e s f r o m e a c h shot w e r e v e r y s h o r t in d u r a t i o n ( c o m p a r e d t o the d e l a y p e r i o d ) and b e c a u s e of the complicated layout of the t r a v e l paths f r o m column t o r e c e i v e r , no p r e s s u r e build-up o c c u r r e d .
It should be noted that a t a d i s t a n c e of 90 f t the p r e s s u r e s i n - volved in t h i s operation would not be high enough t o c a u s e d a m a g e t o g l a s s windows. A t h r e s h o l d of around 1 . 0 p s i i s usually a c c e p t e d f o r d a m a g e t o
g l a s s . In t h i s i n s t a n c e , t h e fly r o c k would have been of m o r e consequence. It would a p p e a r t h a t b e c a u s e of the unwanted tube waves a p p e a r i n g on the p r e s s u r e r e c o r d s , the S t a t h a m p r e s s u r e t r a n s d u c e r , a t l e a s t when u s e d in the fashion r e p o r t e d h e r e , d o e s nat give a t r u e r e p r e s e n t a t i o n of the p r e s s u r e wave. A microphone technique would probably have been m o r e s u c c e s s f u l . W a t e r , d i r t , and eventually s t o n e s and c o n c r e t e c a u s e d the t h r e e t r a n s d u c e r s t o go off s c a l e a f t e r the s h o t . In fact, although
the t r a n s d u c e r s t h e m s e l v e s w e r e well p r o t e c t e d , one of t h e m w a s s e v e r e l y damaged by a d i r e c t hit on the cable connected t o i t .
GROUND MOTION
Ground motion was monitored a t 134 f t and 560 f t f r o m t h e e a s t end of the b r i d g e on t h e roadway embankment using MB velocity t r a n s d u c e r s . The m a i n bridge shot caused peak velocity amplitudes of 0. 63 i n . / s e c in t h e longitudinal d i r e c t i o n a t 134 f t and 0.053 i n . / s e c i n the v e r t i c a l d i r e c t i o n a t
560 ft. It i s interesting to note that t h e r e w a s a low frequency component of about 3 c p s in t h e ground wave. T h i s i s unusually low f o r m o s t blasting
o p e r a t i o n s . T h e ground n e a r the b r i d g e w a s bouldery t i l l , muddy and a l m o s t waterlogged i n p l a c e s although the embankment w a s quite f i r m .
F i g u r e s 6, 7 and 8 a r e r e c o r d s t h a t w e r e taken a t locations
A and B when the O n t a r i o R e s e a r c h Foundation g r o u p w e r e setting off
s m a l l c h a r g e s ( 2 l b and 11 l b ) in the ground n e a r the o b s e r v a t i o n bunker t o d e m o n s t r a t e how the e s t i m a t i o n of vibration l e v e l s f r o m c h a r g e s i z e w a s made using c a l i b r a t i o n s h o t s and monitoring techniques. Locations A and B w e r e approximately 575 and 930 f t , respectively, f r o m the s h o t s . T h e s e r e c o r d s not only show how well t h e wave shape is r e p e a t e d f o r the
3 s h o t s but a l s o i n d i c a t e t h a t the o b s e r v e d gronnd v e l o c i t i e s a r e in r e a s o n - able a g r e e m e n t with t h e p r e d i c t e d v a l u e s f o r t h e 11 -1b c h a r g e . T a b l e I shows t h e o b s e r v e d ground v e l o c i t i e s and t h e p r e d i c t e d v a l u e s . T h e high l e v e l of background showing on t h e r e c o r d s i s due t o e l e c t r i c a l g r o u n d loop p r o b l e m s e x p e r i e n c e d a t high s e n s i t i v i t y i n w e t w e a t h e r .
ACKNOWLEDGEMENT
T h e p h o t o g r a p h s ( ~ i g u r e s 3 and 4) w e r e m a d e a v a i l a b l e b y t h e c o u r t e s y of C a n a d i a n I n d u s t r i e s L i m i t e d , M o n t r e a l .
T A B L E I
OBSERVATIONS O F AIR BLAST AND GROUND VIBRATION
Shot L o c a t i o n Distance, P e a k P r e s s u r e , P e a k G r o u n d Velocity f t p s i i n . / s e c V e r t i c a l Longitudinal Single p i e r shot 4 5 0 . 50 Single p i e r s h o t 9 0 0. 2 5 0 . 3 7 0 . 2 1 Main b r i d g e s h o t C 9 0 0 . 3 1 Main b r i d g e s h o t D 9 0 0 . 4 4 Main b r i d g e s h o t E 9 0 0 . 3 9 Main b r i d g e shot A 1 34 0 . 3 6 0. 6 3 Main b r i d g e shot B 560 0 . 0 5 3 0 . 0 4 5 C a l i b r a t i o n s h o t s R e c o r d 1 ( 2 l b ) R e c o r d 1 ( 2 l b ) B 9 30 0 . 0 1 0 0 . 0 0 6 R e c o r d 2 ( 2 l b ) A 57 5 0 . 0 2 2 0 . 0 1 1 R e c o r d
L
( 2 l b ) B 9 30 0 . 0 1 2 0 . 0 0 7 R e c o r d 3 ( 1 1 l b ) A 57 5 0. l 0 2 ( 0 . 138)::' 0 . 0 6 0 ( 0 . 070);: R e c o r d 3 ( 1 1 14) B 9 3 0 0. 064(0. 074)':< 0 . 0 4 0 ( 0 . 044):::*
Values in p a r e n t h e s e s a r e p r e d i c t i o n s u s i n g the g r o u n d v e l o c i t i e s f r o m t h e 2 -1b c h a r g e s a n d a s s u m i n g g r o u n d velocity p r o p o r t i o n a l t o(E)
2/3. The d i s t a n c e i s c o n s t a n t f o r t h i s s e t of o b s e r v a t i o n s a n d d o e s not e n t e r into the c a l c u l a t i o n s .F i g u r e 4
About 2663 l'u of "Forcite" ( 7 5 p e r c e n t ) w e r e employed t o b r i n g down t h i s 285-%$ o v e r p a s s , a p r o j e c t c a r r i e d out h y t h e O n t a r i o D e p a r t m e n t of High- ways p r i m a r i l y t o study the e f f e c t s s f high exphosives in demolition w o r k of t h i s kind. The D e p a r t m e n t w a s a s s i s t e d by e x p l o s i v e s t e c h n i c a l s e r v i c e
r e p r e s e n t a t i v e s s f Canadian I n d u s t r i e s L i m i t e d and monitoring of b l a s t w a s c a r r i e d out by the O n t a r i o R e s e a r c h Foundation a r ~ d the National R e s e a r c h Council. ( P h o t o g r a p h c o u r t e s y s f Canadian I n d u s t r i e s L i m i t e d , M o n t r e a l .
