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

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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.

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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

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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

3

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

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12

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Plate 4

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13

A.

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Plate 6

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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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