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ROAD RESEARCH LABORATORY
Min is t ry o f T ranspor t
RRL REPORT LR 127
THE W I R E - R O P E S L O T T E D - P O S T
C R A S H B A R R I E R
by
V. J. Jehu and I. B. Laker
CROWTHORNE
ROAD RESEARCH LABORATORY
1967
-CONTENTS
Abstract
I. Introduction
2. Barrier
3. Test procedure
4. Instrumentation
5. Results
6. Synopses of development tests
6. I Tests 22-25
6. 2 T e s t 26
6. 3 T e s t s 27 a n d 28
6 . 4 T e s t s 29 a n d 30
6 . 5 T e s t 31
o
8.
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I0.
6.6 Tests 32-36
6.7 Test 37
6.8 Test 38
6.9 Test 39
Discussion of development tests
Synopses of motorway tests
8. 1 Tests 40-43
8.2 Test 44
8.3 Test 45
8.4 Test 46
8.5 Test 47
Discussion of motorway tests
Synopses of further development tests
i0. 1 Test 70
10. 2 Test 73
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Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.
This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
11.
12.
13.
C o n c l u s i o n
A c k n o w l e d g e m e n t s
R e f e r e n c e s
P a g e
9
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Q CROWN C O P Y W R I G H T 1967
E x t r a c t s f r o m the t ex t m a y be r e p r o d u c e d
p r o v i d e d the s o u r c e is a c k n o w l e d g e d
s
THE WIRE-ROPE SLOTTED-POST CRASH BARRIER
ABS TRAC T
An efficient form of crash barrier developed by the Road
Research Laboratory involves the use of tensioned horizontal
members carried on relatively weak vertical supports. This
report describes a series of tests carried out on wire rope
barriers of this type in order to select the most suitable
detailed design. Twenty-eight dynamic tests were carried out
o n a l t e r n a t i v e d e s i g n s o f b a r r i e r in w h i c h t h e r o p e s r e s t e d i n v e r t i c a l s l o t s c u t i n t o t h e t o p s of w e a k I - s e c t i o n m i i d s t e e l p o s t s . M o s t of t h e t e s t s w e r e c a r r i e d o u t w i t h c a r s w e i g h i n g 3 0 0 0 lb , b u t i n s o m e t e s t s c o n c e r n e d w i t h o p t i m u m r o p e h e i g h t s s m a l l v e h i c i e s a n d t a l l v e h i c i e s w e r e u s e d . I m p a c t s p e e d s r a n g e d f r o m 17 to 67 m i l e / h a t n o m i n a l a n g l e s
t o t h e l i n e o f t h e b a r r i e r o f 10 d e g . a n d 20 d e g .
A f u r t h e r e i g h t t e s t s w e r e m a d e on a i e n g t h o f b a r r i e r e r e c t e d o n t h e 15 f t . w i d e c e n t r a i r e s e r v a t i o n o f a n u n o p e n e d m o t o r w a y , to d e t e r m i n e h o w b a r r i e r d e f l e c t i o n f o r a s p e c i f i c i m p a c t w a s i n f l u e n c e d b y t h e l e n g t h of r o p e b e t w e e n a n c h o r a g e s , t h e r o p e t e n s i o n , a n d t h e p o s t s p a c i n g . In t h i s s e r i e s o f t e s t s c a r s w e i g h i n g 3 0 0 0 lb s t r u c k t h e b a r r i e r a t
a b o u t 60 m i l e / h a n d 20 d e g .
T h e o p t i m u m d e s i g n i s a c o m p r o m i s e b e t w e e n t h e r e q u i r e m e n t s f o r s m a i i a n d t a l i v e h i c l e s . I t c o n s i s t s of t w o _3 in d i a m e t e r s t e e l r o p e s r e s t i n g o n e u p o n t h e o t h e r a t a 4 h e i g h t o f 25 in . i n v e r t i c a l s i o t s 4½ in . d e e p c u t i n t o t h e t o p s o f 3 x I½ in . I - s e c t i o n s t e e i p o s t s s p a c e d 8 f t . a p a r t . T h e r o p e s a r e t e n s i o n e d to 3 0 0 0 l b . a t 5 0 ° F a f t e r b e i n g p r e - s t r e t c h e d , a n d e a c h r o p e i s a n c h o r e d e v e r y 2000 f t . in s u c h a w a y t h a t a c o l l i d i n g v e h i c I e r e l e a s e s t h e c o n n e c t i o n b e t w e e n r o p e a n d a n c h o r a g e . T h e b a r r i e r i s i n t e n d e d f o r u s e o n c e n t r a i r e s e r v a t i o n s a t l e a s t 15 f t . w i d e , a n d w i t h h o r i z o n t a l r a d i i o f c u r v a t u r e n o t l e s s t h a n 2800 f t . T h e s u r f a c e o f t h e c e n t r a l r e s e r v a t i o n s h o u l d b e h a r d e n e d in
o r d e r t o c o n t r o I t h e h e i g h t o f t h e r o p e s .
I. INTRODUCTION
I m p a c t t e s t s c a r r i e d o u t a t t h e R o a d R e s e a r c h L a b o r a t o r y on c a b l e a n d c h a i n I i n k b a r r i e r s i n t e n d e d f o r t h e c e n t r a I r e s e r v a t i o n s o f h i g h - s p e e d r o a d s a r e d e s c r i b e d i n a n e a r I i e r r e p o r t . 1 In a l i t h e s e b a r r i e r a r r a n g e -
ments the cables were clamped in pairs either side of a central line of
weak steel posts. The tests showed that in low angle collisions the posts
may bend without the cables being released, resulting in the cables being
forced down almost to the ground. When this occurs in high-speed
impacts the vehicle may be overturned by the damaged barrier.
This note describes the development of a wire rope barrier
which ensures that the ropes separate from the bending posts, whatever
the angle of impact. Chain-link mesh is not used in the modified barrier,
since the tests described in the earlier report showed that it was not
beneficial to the vehicle-barrier response.
2. BARRIER
E s s e n t i a l l y t h e w i r e r o p e b a r r i e r c o n s i s t s o f s t e e l I - s e c t i o n p o s t s a b o u t 30 in . in h e i g h t , w i t h s l o t s c u t i n t o t h e t o p s o f t h e p o s t s . T h e p o s t s a r e a r r a n g e d w i t h t h e i r w e a k a x e s a l o n g t h e l i n e o f t h e r o a d s o t h a t ' t h e y c a n
3 b e e a s i l y k n o c k e d d o w n . In t h e s l o t s a r e t w o ~ in. d i a m e t e r w i r e r o p e s o f 3 x 7 ( 6 / 1 ) c o n s t r u c t i o n . T h e y r e s t o n e u p o n t h e o t h e r a t a p r e d e t e r - m i n e d h e i g h t a b o v e g r o u n d . T h e e n d s o f t h e r o p e s a r e a n c h o r e d a t g r o u n d l e v e l , t h e c o n n e c t i o n s t o t h e a n c h o r a g e s b e i n g s u c h t h a t t h e y a r e r e l e a s e d b y a c o l l i d i n g v e h i c l e . T h e c o n c r e t e p o s t f o o t i n g s a r e p r o v i d e d w i t h a s o c k e t to a d e p t h o f 18 in . i n w h i c h t h e p o s t i s i n s e r t e d a n d f r o m w h i c h i t c a n b e w i t h d r a w n a n d r e n e w e d b y h a n d in t h e e v e n t o f d a m a g e to t h e f e n c e . T h e e f f e c t on t h e p e r f o r m a n c e o f t h e b a r r i e r s o f p o s t s t r e n g t h a n d s p a c i n g , t h e d e p t h o f t h e p o s t s l o t , t h e r o p e h e i g h t , t e n s i o n , a n d l e n g t h b e t w e e n a n c h o r a g e s w e r e a l l i n v e s t i g a t e d i n t h e s e a r c h f o r t h e m o s t s a t i s f a c t o r y d e s i g n . T h e t h r e e s i z e s o f 1 - s e c t i o n s t e e l p o s t m a t e r i a l u s e d w e r e 2¼ x 1 i n . , 2½ x l i n . , a n d 3 x 1½ i n . , a n d t h e i r r e s p e c t i v e s e c t i o n m o d u l i w e r e 0 . 5 3 in . 3 a n d 0. 0 8 6 in . 3 0 . 6 4 in . 3 a n d 0. 09 in 3 1. l l in. 3 a n d 0. 17 in. 3
3. TEST PROCEDURE
Initial testing of specific parameters of the barrier was carried out with
manned vehicles at speeds of about 30 rnile/h. This was followed by more
severe testing with unmanned vehicles which were remotely controlled by radio.
To attain the required speeds over the available approach distance of 220 yds. each test vehicle was connected to two towing vehicles by
means of a long steel cable, which was automatically released when the
test vehicle was close to the barrier. The build-up of speed of the crash
vehicle was assisted by switching on the ignition and engaging top gear,
so that during the tow its own engine was running. A mercury-filled
U - t u b e , m o u n t e d on t h e w i n g of t h e v e h i c l e s h a t t e r e d u p o n c o n t a c t w i t h t h e b a r r i e r , s w i t c h i n g o f f t h e e n g i n e . T h e r a d i o l i n k e n a b l e d a c o n t r o l l e r o v e r l o o k i n g t h e l i n e o f a p p r o a c h to o p e r a t e t he s t e e r i n g of the v e h i c l e , a n d a l s o to a p p l y t h e b r a k e s a f t e r t he v e h i c l e l e f t t he b a r r i e r .
Additional testing on a long length of a prototype barrier was
carried out on an unopened motorway. In this instance the crash vehicle
was push-started after engaging top gear and switching on the ignition,
so that it gathered speed solely under its own power. A radio link with
a following vehicle enabled the crash car to be steered into the slow
lane of a three-lane carriageway, and then swerved across the carriage-
way to strike the central reservation barrier.
4. INSTRUMENTATION
In a l l t h e t e s t s t h e o u t p u t f r o m s e i s m i c m a s s a c c e l e r o m e t e r s m o u n t e d c l o s e to t h e c e n t r e o f g r a v i t y of t h e v e h i c l e , w a s f e d to a m u l t i - c h a n n e l r e c o r d i n g g a l v a n o m e t e r u n i t i n s u l a t e d a g a i n s t s h o c k a n d v i b r a t i o n . The a c c e l e r o m e t e r s h a d n a t u r a l f r e q u e n c i e s v a r y i n g b e t w e e n 22 c y c l e s / s e c . a n d 4 4 c y c l e s / s e c . , t h e m o s t s e n s i t i v e a c c e l e r o m e t e r s h a v i n g the l o w e s t f r e q u e n c i e s . A d d i t i o n a l l y v e h i c l e s in t he r e m o t e l y c o n t r o l l e d t e s t s w e r e e q u i p p e d w i t h a r a d i o r e c e i v e r , s t e e r i n g m o t o r a s s e m b l y , a n d a p n e u m a t i c r a m t o o p e r a t e t h e b r a k e p e d a l . An a n t h r o p o m o r p h i c d u m m y w a s a l s o s e c u r e d in o n e o f t h e f r o n t s e a t s w i t h f u l l s a f e t y h a r n e s s .
Camera coverage for the manually driven and towed vehicle
tests included two 200 frame/sec, cameras mounted about 40 ft. above
the barrier, and documentary and high-speed cameras at other vantage
points. A 4 ft. grid was painted on the road surface for reference
purposes. In the motorway tests camera coverage was lessprecise
because of the uncertainty of the location of the impact point. Cameras
were mounted in two vehicles which preceded the test car, one in each
carriageway, and some overhead films were obtained from a helicopter.
These data were sometimes supplemented by films from cameras stationed
at the roadside, and directed at the expected point of impact.
The deceleration-time traces were converted into digital values
at incremental intervals of time by means of Benson-Zehner equipment.
The films were analysed using a back projection system, to derive
wherever possible the positions and attitudes of the vehicle relative to
the barrier throughout the impact, including both roll angle and heading
angle. The combined information from accelerometers and films was
processed on a Pegasus computer. Thus lateral and longitudinal decelera-
tions were computed relative to the vehicle axis allowing for roll angle
only, and then relative to the barrier line taking into account both roll
and heading angles, in order to derive the forces on the barrier. The
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v a l u e s of d e c e l e r a t i o n w e r e a I s o i n t e g r a t e d o n c e to o b t a i n r e d u c t i o n s in the v e l o c i t y o f t h e . v e h i c l e , a n d t w i c e to f i n d t h e d i s t a n c e s t r a v e l l e d . T h e d i s t a n c e s t h u s d e r i v e d w e r e t h e n c r o s s - c h e c k e d w i t h t h o s e m e a s u r e d d i r e c t l y f r o m t h e f i l m s .
5. R E S U L T S
E i g h t e e n t e s t s n u m b e r e d 2 2 - 3 9 w e r e c a r r i e d o u t a t t he C r o w t h o r n e t e s t t r a c k d u r i n g the d e v e l o p m e n t s t a g e of t h e b a r r i e r . T e s t s 2 2 - 3 i w e r e d e v o t e d to f i n d i n g an o p t i m u m a r r a n g e m e n t of p o s t s t r e n g t h , p o s t s p a c i n g , a n d s l o t d e p t h f o r a b a r r i e r w i t h r o p e s a t a s i n g l e h e i g h t a b o v e g r o u n d . In t e s t s 3 2 - 3 8 an a r r a n g e m e n t of r o p e s a t t w o h e i g h t s w a s i n v e s t i g a t e d . A r o p e - t o - a n c h o r a g e c o n n e c t i o n d e s i g n e d to b e r e l e a s e d u p o n v e h i c l e i m p a c t w a s p r o v e d in t e s t 39.
A f u r t h e r e i g h t t e s t s n u m b e r e d 4 0 - 4 7 w e r e c a r r i e d ou t on t h e m o t o r w a y s i t e to e x a m i n e the r e l a t i o n s h i p b e t w e e n b a r r i e r d e f l e c t i o n a n d the l e n g t h o f r o p e b e t w e e n a n c h o r a g e s , a n d to s t u d y t h e e f f e c t on the v e h i c l e - b a r r i e r r e s p o n s e o f t h e s o f t e a r t h c e n t r a l r e s e r v a t i o n .
F i n a l l y t w o t e s t s n u m b e r e d 70 a n d 73 w e r e c a r r i e d ou t a t C r o w - t h o r n e w i t h a b a r r i e r s i m i l a r t o t h a t i n v e s t i g a t e d on t he m o t o r w a y , b u t s t i f f e n e d b y t h e u s e of s t r o n g e r p o s t s . -
D e t a i l s o f a l l t he t e s t s a r e c o n t a i n e d in F i g s 1 - 2 8 w h i c h i n c l u d e t he i m p a c t s p e e d a n d a n g l e , t h e v e h i c l e - b a r r i e r r e s p o n s e , t he m e a s u r e d v e h i c l e d e c e l e r a t i o n s , a n d t h e d e r i v e d b a r r i e r l o a d i n g s . T h e b a r r i e r c o n s t r u c t i o n s a r e a l s o s p e c i f i e d in t h e s e d i a g r a m s . P l a t e s 1 - 1 0 i l l u s t r a t e s o m e a s p e c t s o f the v e h i c l e - b a r r i e r r e s p o n s e s , a n d t h e s e q u e n c e o f e v e n t s i s d e s c r i b e d in the f o l I o w i n g s y n o p s e s o f t h e t e s t s .
6. SYNOPSES OF DEVELOPMENT TESTS
6. 1 T e s t s 22 - 25
The vehicle-barrier response for low angle impacts was studied, the speed being progressively increased until in Test 25 the barrier was struck at 62 mile/h and I0 deg., the impact condition which had proved to be critical with the cable and chain-link barrier. The slotted-post arrangement worked well in all these low angle impacts.
6. 2 T e s t 26
With 3 in. l o n g s l o t s in t h e p o s t s a n d a r o p e h e i g h t o f 27 in . t h e i m p a c t a n g l e w a s i n c r e a s e d f r o m 10 d e g . to 20 d e g . , w h i l e r e t a i n i n g
the i m p a c t s p e e d a t a b o u t 60 m i l e / h . T h e b a r r i e r b e h a v e d s a t i s f a c t o r i l y .
4
6. 3 Tests 27 and 28
An attempt was made to find out how barrier deflection was
affected by the length of rope between anchor points. Using similar
impacts barrier lengths of 100 ft. and 790 ft. were tested. At the same
time the length of the post slots was increased from 3 in. to 6 in. Very
little difference in barrier deflection was observed for the minor impacts
used, which could not be made more severe because of the likelihood of
striking the end anchorage of the I00 ft. test section. High speed films
revealed a tendency for the ropes to be retained in the 6 in. slots as
the posts bent over, so that in all further tests except test 70 a slot
length of 4½ in. was used.
6.4 Tests 29 and 30
These high speed tests at 20 deg. with a medium-sized car and
a small van showed that the barrier arrangement of 2 I- 4 x 1 in. I-section
posts with 4½ in. slots and a rope height of 27 in. was satisfactory for
these vehicles.
6 . 5 T e s t 3 1
T h i s t e s t s h o w e d t h a t t h e b a r r i e r c o n s t r u c t i o n o f t e s t s 29 a n d 30,
w h i c h w a s s u c c e s s f u l f o r a c a r a n d a s m a l l v a n , w a s i n e f f e c t i v e a g a i n s t
a c o m m e r c i a l v e h i c l e w i t h a f r o n t b u m p e r a t t h e s a m e h e i g h t a s t h e
w i r e r o p e s , t h e l a t t e r b e i n g r u n o v e r b y t h e v e h i c l e .
6. 6 Tests 32-36
These tests were devoted to an examination of a barrier
arrangement with ropes at heights of 30 in. and 25 in., the purpose of
the higher rope being to redirect vehicles with high front bumpers. The
lower rope was arranged on the side of the barrier remote from impact,
since in this position there was a possibility that the rope might be
carried down with the posts. The tests were all performed at low speeds
with vehicles ranging from a small van to a 3½ ton truck.
The barrier with ropes at two heights performed satisfactorily
in all these low speed impacts, but it was noted in test 32 that the upper
rope slid over the front of the small van and rubbed against the windscreen
pillar. In test 35 tension in the ropes was increased from I000 Ib to
5000 ib in the lower rope and 3850 Ib in the upper rope, to see if the
barrier was thereby noticeably stiffened. Comparison of the result with
that of test 33 showed that the increased tension in the ropes had little
effect on the barrier deflection for a comparable impact. However to
ensure that the ropes would not sag in hot weather a static rope tension
of 3000 ib was applied in all further tests. For the ropes used a tension
of 3000 ib at 50°F is reduced to 1500 ib at 100°F, and increased to
4500 ib at 0°F. This variation in tension should have little effect on
5
barrier behaviour.
6.7 Test 37
The barrier with ropes at heights of 30 in. and 25 in. was sub-
jected to a more severe blow than hitherto by striking it with the 3½ ton
truck at 42 mile/h and 18 deg. The truck was redirected satisfactorily
by the upper rope, but the lower rope was run down.
6.8 T e s t 38
T o c o m p l e t e t h e i n v e s t i g a t i o n o f t h e b a r r i e r w i t h r o p e s a t t w o h e i g h t s an o p e n s p o r t s c a r w a s d r i v e n a g a i n s t i t a t 6 4 m i l e / h a n d 24 d e g . O w i n g to a f a u l t i n t h e r a d i o e q u i p m e n t s t e e r i n g c o n t r o l o f t h e v e h i c l e w a s l o s t d u r i n g t h e a p p r o a c h r u n . T h e c a r c o l l i d e d w i t h t h e b a r r i e r i n a d v a n c e of t h e i n t e n d e d i m p a c t p o i n t , a n d w a s e v e n t u a l l y d e s t r o y e d w h e n i t s t r u c k a h e a v y f i x e d o b j e c t . B e f o r e t h i s h a p p e n e d h o w e v e r t h e c a r t r a v e l l e d f a r e n o u g h w i t h i n t h e b a r r i e r t o s h o w t h e u p p e r r o p e s l i d e o v e r t h e f r o n t o f t h e c a r , d e s t r o y t h e b o l t e d - o n t y p e w i n d s c r e e n , a n d t h e n c o n t a c t t h e n e c k o f t h e d u m m y o c c u p a n t o f t h e c a r . T h i s o c c u r r e d in s p i t e o f the f a c t t h a t t h e l o w e r r o p e r e d i r e c t e d t h e c a r a s i n t e n d e d .
6.9 Test 39
T h i s t e s t w a s c a r r i e d o u t t o p r o v e a g r o u n d a n c h o r a g e a r r a n g e m e n t d e s i g n e d s o t h a t a v e h i c l e r u n n i n g i n t o i t w o u l d r e l e a s e t h e c o n n e c t i o n b e t w e e n r o p e a n d a n c h o r a g e , a n d h e n c e n o t f o u l t h e d o w n r o p e . T h e c o n n e c t i o n c o n s i s t e d in h o o k i n g t h e c l o s e d e y e t e r m i n a t i o n o f t h e r o p e o v e r a s p i g o t f a c i n g i n t o t h e b a r r i e r . T h e c a r s t r u c k t h e b a r r i e r i n a d v a n c e o f t h e a n c h o r a g e a t 55 m i l e / h a n d 18 d e g . T h e c l o s e d e y e w a s f o r c e d o f f t h e s p i g o t , t h e u n a n c h o r e d r o p e r e d i r e c t e d t h e v e h i c l e , a n d t h e r i s i n g r o p e o f t h e c o m p l e t e a n c h o r a g e s y s t e m w a s r u n d o w n a s i n t e n d e d w i t h o u t d e t r i m e n t t o t h e p a s s a g e o f t h e v e h i c l e . T h i s a n c h o r a g e d e s i g n h a s b e e n i n c o r p o r a t e d in t h e s p e c i f i c a t i o n o f t h e w i r e r o p e b a r r i e r , e x c e p t t h a t t h e b a r r i e r l e n g t h o v e r w h i c h e a c h r o p e d e s c e n d s to g r o u n d l e v e l h a s b e e n i n c r e a s e d f r o m 8 f t . t o 16 f t .
7. DISCUSSION OF DEVELOPMENT TESTS
The tests on the barrier with ropes at heights of 30 in. and 25 in. showed
that only the upper rope was effective in redirecting those commercial
vehicles with high bumpers, the lower rope being run over by the vehicle.
At the other extreme of the range of vehicles, it was concluded that
while the lower rope would be effective against the small vehicle, the
upper rope would prove a formidable danger especially to the occupants
of open sports cars. Because of the difficulty of designing a rope barrier
which would be effective for all vehicles it was decided to abandon the
two-height arrangement of ropes in favour of a single rope height which
would be effective against all cars and many commercial vehicles, and
which would not endanger those large vehicles whose bumpers were not
caught under the ropes. The trend in design of commercial vehicles
appears to be towards lower bumper heights, and therefore towards
extending the range of vehicle types against which the single height rope
barrier will be effective. This change means that one must expect very
large vehicles to run over the barrier.
The construction details of the single height rope barrier which
emerged from the development tests were as follows:- post material
2½ x I in. I-section, post slots 4½ in., rope height 27 in., and rope
tension 3000 lb. This construction was investigated further under actual
motorway conditions.
8. SYNOPSES OF M O T O R W A Y TESTS
8. 1 Tests 40-43
The object of these tests was to study the effect of length between
anchorages on the deflection of the barrier, and to examine the influence
on the vehicle behaviour of the soft earth of the central reservation. A
total length of 5000 ft. of the single height rope barrier was installed on
the central reservation of a motorway nearing completion, and remote
control impacts carried out on various anchored lengths of barrier.
Barrier lengths of 2000, 1000, 500 and 5000 ft. were struck in
this order, at angles of 20 deg. and at speeds within the range 50-60
mile/h. The vehicle was always smoothly redirected, but encroachment
on the opposite carriageway occurred in all cases except that of the
500 ft. length. Earth from the central reservation was thrown on to the
road by the scrubbing action of the vehicle tyres during the redirection
process, but this was not detrimental to the vehicle behaviour. For
vehicles smaller than the Vanguard cars used in the motorway tests
rope height is critical, and since the height of the ropes above soft
ground is difficult to control, it was concluded that the barrier should
only be used where the central reserve was hardened.
No correlation between length and barrier deflection was found
in these tests, even allowing for the differences in the severity of
impact. To eliminate any effect of the initial stretch of the new ropes
in subsequent tests, and to stiffen the barrier, the ropes were
cyclically prestretched eight times to a load of 8 tons after test 43.
8.2 Test 44
Prestretched ropes 2000 ft. between anchor points, and with a
static tension of 3000 Ib were struck at 62 mile/h and 20 deg. Maximum
barrier deflection was 106 in., the vehicle momentarily encroaching
upon the opposite carriageway a distance of 16 in. during the redirection
process. This successful test was used as a datum for the three subsequent m o t o r w a y t e s t s .
8. 3 T e s t 45
The s a m e 2 0 0 0 f t . l e n g t h o f b a r r i e r w a s s t r u c k a t 5 8 . 5 m i l e / h and 22 d e g . , b u t w i t h the s t a t i c t e n s i o n in t h e r o p e s i n c r e a s e d f r o m 3000 to 6000 l b . T h e r e s u l t i n g b a r r i e r d e f l e c t i o n of 114 in . w a s l i t t l e d i f f e r e n t f r o m t h a t o f t e s t 44, t a k i n g i n t o a c c o u n t the s l i g h t l y m o r e s e v e r e l a t e r a l b l o w d e l i v e r e d in t e s t 45 . S i n c e i n c r e a s i n g t h e s t a t i c t e n s i o n f r o m 3000 to 6 0 0 0 lb . h a d l i t t l e e f f e c t on b a r r i e r d e f l e c t i o n f o r a g i v e n i m p a c t , t he 3 0 0 0 lb . v a l u e w a s a d o p t e d in a l l s u b s e q u e n t t e s t s , b e c a u s e it e a s e s t h e e r e c t i o n an d m a i n t e n a n c e t a s k s .
8.4 Test 46
A 4 0 0 0 f t . l e n g t h of b a r r i e r w a s s t r u c k - a t 62 m i l e / h a n d 19 d e g . the i n i t i a l r o p e t e n s i o n b e i n g 3000 lb . B a r r i e r d e f l e c t i o n w a s 159 i n . , c o m p a r e d w i t h 106 in. f o r t h e 2 0 0 0 ft . l e n g t h o f b a r r i e r o f t e s t 44 . S i n c e the two t e s t s w e r e a l m o s t i d e n t i c a l e x c e p t f o r b a r r i e r l e n g t h , t he r e s u l t s s u g g e s t t h a t d o u b l i n g t h e l e n g t h b e t w e e n a n c h o r a g e s i n c r e a s e s t h e b a r r i e r d e f l e c t i o n b y a b o u t o n e h a l f .
8.5 Test 47
The effect of the post spacing on barrier deflection was assessed
by removing every other post in a 2000 ft. length of barrier to increase
the post spacing from 8 ft. to 16 ft. In all other respects the barrier was
the same as in test 44. The vehicle struck the barrier at 63. 9 mile/h
and 18 deg~, and the maximum deflection was 168 in. Hence doubling the
post spacing increased barrier deflection by about the same amount as
doubling the length of ropes between anchorages.
9. D I S C U S S I O N O F M O T O R W A Y T E S T S
The m o t o r w a y t e s t s s h o w e d t h a t w i t h the b a r r i e r m o u n t e d a l o n g t h e c e n t r e of the 15 ft. m e d i a n s t r i p a 3000 lb . v e h i c l e s t r i k i n g t he b a r r i e r a t 60
m i l e / h and 20 d e g . w o u l d e n c r o u c h i6 in . on to t he o p p o s i t e c a r r i a g e w a y if the r o p e s w e r e a n c h o r e d e v e r y 2000 f t . H a l v i n g t h e l a t e r a l r e s i s t a n c e o f f e r e d b y t h e p o s t s i n c r e a s e d b a r r i e r d e f l e c t i o n b y a b o u t 50 p e r c e n t . H e n c e by i n c r e a s i n g the l a t e r a l r e s i s t a n c e o f t h e p o s t s t h e b a r r i e r w i l l be a p p r e c i a b l y s t i f f e n e d . T h i s c a n b e d o n e b y r e d u c i n g t h e p o s t s p a c i n g o r b y u s i n g s t r o n g e r p o s t s , t he l a t t e r b e i n g t h e c h e a p e r m e t h o d . T h e p o s t s u s e d in the m o t o r w a y t e s t s w e r e 2½ x 1 in , I - s e c t i o n w i t h s e c t i o n r n o d u l i of 0. 64 in 3 a n d 0. 09 in. 3 B y u s i n g 3 x l½ in. I - s e c t i o n p o s t s
with section moduli of i. ii in. 3 and 0. 17 in. 3, the lateral strength of
the barrier would be almost doubled. If this is done the effect of the
stronger posts on the behaviour of a small lightweight vehicle striking
the barrier at high speed becomes important.
Two possibilities exist as follows:-
i) the rope height of 27 in. may be too high with the stronger
posts which for a given collision loading will not bend as
readily as the weaker posts;
ii) the stronger posts may cause the small vehicle to spin out
of the barrier when it strikes at high speed and low angle.
These effects were examined in tests 70 and 73.
10. S Y N O P S E S O F F U R T H E R D E V E L O P M E N T T E S T S
10. 1 T e s t 70
T w o r o p e s 7 9 0 f t . l o n g w e r e l a i d o n e u p o n t h e o t h e r in 6 i n . d e e p
s l o t s c u t i n t o t h e t o p s o f 3 x 1½ i n . I - s e c t i o n s t e e l p o s t s , t h e h e i g h t to t h e c e n t r e o f t h e l o w e r r o p e b e i n g 27 in . S t a t i c r o p e t e n s i o n w a s 3000 l b . a n d t h e p o s t s p a c i n g w a s 8 f t . A m i n i - v a n w a s s t e e r e d i n t o t h e b a r r i e r a t 6 4 . 5 m i l e / h a n d 20 d e g . T h e r o p e s s l i p p e d o v e r t h e f r o n t of t h e v e h i c l e a n d l o d g e d a g a i n s t t h e w i n d s c r e e n p i l l a r . In t h i s p o s i t i o n t h e r e d i r e c t i n g f o r c e a c t i n g o n t h e v e h i c l e w a s w e l l a b o v e i t s c e n t r ' e o f g r a v i t y , w i t h t h e r e s u l t t h a t t h e v e h i c l e r o l l e d a w a y f r o m t h e b a r r i e r a n d o v e r t u r n e d . H e n c e f o r t h e r o p e b a r r i e r w i t h t h e s t r o n g e r p o s t s to b e s u c c e s s f u l a g a i n s t s i n a i 1 v e h i c l e s t h e r o p e h e i g h t m u s t be l o w e r t h a n
27 in.
i0. 2 Test 73
The details of the barrier were the same as in test 70 except that
the rope height was reduced from 27 in. to 25 in. and the post slots were
4½ in. deep. A mini-van struck the barrier at 62. 6 mile/h and 12 deg.,
and was successfully redirected with no indication of any spin-out effect.
Hence stiffening the barrier by'using 3 x 1½ in. I-section posts necessi-
tates lowering the rope height from 27 in. to 25 in. in order to safely
redirect the smallest vehicles.
1 1. C O N C L U S I O N
The following facts relating to the impact performance of a wire rope
barrier emerge from the completed programme of tests:-
I) A ready detachment of ropes from posts, irrespective of the
z)
3)
4)
s)
angle and speed of impact, is obtained by supporting the ropes
in vertical slots cut into the tops of I-section steel.
A barrier with ropes at two heights, in which the lower rope is
intended to reflect small vehicles and the upper rope is meant
to contain large vehicles, has the disadvantages that the upper
rope is a serious hazard to the small vehicle and the lower rope
may be run down by the large vehicles.
Ropes arranged at a single height to reflect the small vehicle
will also redirect the large vehicle provided that they lodge in
the soft bodywork above the front bumper. Where the ropes are
well below the height of the centre of gravity of the large vehicle,
it may overturn in a severe collision. Vehicles with bumpers at
or above the rope height will run over the barrier, but will not
be overturned in the process.
Angles of reflection of the vehicle from the barrier are less
than one-half the impact angles.
Since the rope height is critical the surface of the median must
be hard and flat.
T h e m a i n f e a t u r e s o f t h e o p t i m u m d e s i g n o f w i r e r o p e b a r r i e r a r e a s f o l l o w s : -
1 1 P o s t s 3 x 15 in . I - s e c t i o n m i l d s t e e l a t 8 f t . s p a c i n g , w i t h 4 X i n . v e r t i c a l s l o t s in t h e t o p s o f t h e p o s t s , ancl a l e n g t h o f 18 i n . i n s e r t e d i n
s o c k e t s f o r m e d in c o n c r e t e f o o t i n g s .
3 Ropes 2 ropes, ~ in. diameter 3 x 7 ordinary lay, ultimate strength
37, 500 Ib, arranged one on top of the other at a height of 25 in. to the
centre of the lower rope.
Rope tension Static tension of 3,000 Ib at 50°F. Before installation each
rope is cyclically loaded 8 times to about 8 tons to remove initial
stretch.
Anchorages Each rope is separately anchored every 2000 ft., the
connection between rope and anchorage being such that it is released
by a colliding vehicle.
The barrier is intended for use on central reservations at least
15 ft. wide, and for which the horizontal radii of curvature are not less
than 2, 800 ft.
l0
12. ACKNOWLEDGEMENTS
The members of the research team who took part in this investigation
were: -
V.J. Jehu, Z.C. Pearson, I.B. Laker, and C. Blarney.
13. REFERENCE
l. JEHU, V.J. and I. B. LAKER. The cable and chain-link crash
barrier.
Ministry of Transport, R.R.L. Report LR 105 Crowthorne,
1967 (Road Research Laboratory).
ll
Plate I
Car striking barrier in Test 25
Plate 2
Car striking barrier in Test 29
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12
Plate 3
Cable groove in Min ivan in Test 30
Plate 4
Pantechnicon cross ing barrier in Tes t 31
13
A.
Plate 5
Lorry striking barrier in Test 37
Plate 6
Upper rope fracturing windscreen in Test 38
14
Plate 7
Car driving through knock-off anchorage in Test 39
Plate 8
Path of vehicle in Test 45 (motorway)
15
Plate 9
Minivan driving under ropes in Test 70
Plate 10
Minivan leaving barrier in Test 73
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