2383

loss of human lives. It reflects on the

covered by load or safety factors are

reliability of the design procedures as

revealed in weaknesses after construc-

well as on the quality of construction.

tion. Geotechnical investigations are

Use of poor construction materials and

very important for the design of bridge

inadequate design

assumptions are

foundations.

generally this suspects. Designers and

This paper describes a case history of

contractors have to be extra cautious

failure of three reinforced concrete

while designing

and constructing

(RC) highway bridges built across

bridges to resist the effects of natural

Wadi El Nagah watercourse in the

hazards like floods, earthquakes and

northeastern part of Libya. Two out of

landslides in addition to normal traffic

three bridges collapsed and one suf-

loads. Studies have been made in re-

fered damage that could be repaired.

cent years to understand the pattern of

All these bridges have been victim of

occurrence, of these natural phenome-

heavy floods in the wadi, which caused

na statistically, attempting to compute

severe damage due to scour, erosion

probability of their future occurrence.

and undermining of the soil below the

The cumulative effect of various as-

foundations of intermediate piers and

sumptions and approximations intro-

the abutments. In all cases, parts of the

duced at the analysis and design stage

approach road embankments were also

of a bridge which are not sufficiently

washed away.

Description of the Bridges

Bridge A was badly damaged by the

The location of three bridges built

flood water. One abutment collapsed

across Wadi El Nagah are shown in

and the beams at that end of the span

Fig. 38. The catchment area of the

were twisted. Foundation blocks of the

wadi across which these bridges were

intermediate piers were exposed. Part

built consists of hilly terrain. Mean

of foundation soil of the far end abut-

annual rainfall precipitation in this

ment eroded away, leaving the founda-

region is around 400-600 mm. Almost

tion block partially suspended. Expan-

all the precipitation occurs during the

sion joints between the concrete deck

winter months, from October to

sections over the intermediate supports

January of each year.

widened considerably due to distortion

Bridge A, built about three decades

and the lateral displacement of the

ago, consisted of three simply

bridge decks.

supported spans of pre-stressed

The old multispan RC bowstring girder

reinforced

concrete beams

supported

Bridge B totally collapsed. Its two

on two central piers and two end RC

intermediate plain concrete supporting

abutments. The foundation of all these

piers overturned sideways along with

supporting

elements consisted of

the foundation blocks and moved

shallow block foundations. Bridge B

downstream to Bridge C.

was a bowstring girder RC bridge

The new Bridge C did not suffer any

supported on two massive intermediate

appreciable structural damage although

plain concrete pillars and anchored at

scouring of the wadi bed, accompanied

both ends to earth fill embankments.

by undermining of the soil below the

Bridge C, part of the new coastal

foundation blocks of the vertical piers

highway, consisted of an RC portal

and the east RC abutment did occur,

frame with propped

overhangs.

leaving them partially suspended (Fig.

Vertical supporting members of the

3). The approach connection to the

portal are shear wall type piers 10 m

bridge deck from the east was

high and 0.7 m X 6.0 m in cross

destroyed due to the collapse of the

section. The foundations of these piers

approach concrete slab caused by

consisted of RC blocks 10 mX 5 mX

erosion of the earth fill at the back of

1.5 m resting on plain concrete mats of

the abutment. This bridge has been

300 mm depth and located 4 m below

repaired and opened to traffic.

the planned wadi bed level. The RC

CAUSES OF DAMAGE

abutments of this bridge were em-

bedded in the approach road embank-

ments at both ends. Gabion protection

Scour of the wadi bed, undermining

was provided as per the design.

the soil below the intermediate piers

Damage Assessment

and the abutments' foundation blocks,

and the erosion of the approach road

embankments during a heavy flood,

were the causes of the damage and col-

lapse of these bridges. This is not the

Bridge C, has been rehabilitated. The

first time such damage has occurred.

main problems in rehabilitating Bridge

"Bridges are vulnerable to natural

C were to support the foundation

hazards ranging from hurricanes to

blocks of one intermediate pier and

earthquakes. But scour is the problem

one abutment, and to provide a new

that has caused more bridges to Mil

reinforced

concrete

approach

road

than all of rest combined. One study...

slab.

concluded that among 86 bridge fail-

The repair work had to be planned

ures [in the USA] from 1961 to 1976,

very carefully in order not to disturb

48 were due to floods. Out of 48, 46

the structural safety of the other parts.

were due to bridge scour" [1]. Another

of the bridge. The sequence of the re-

recent survey revealed 494 of the 823

pair work consisted of removing loose

bridge failures in the USA from 1951

and eroded soil from below one side of

to 1988 were primarily the result of

the foundation block at a time,

scour of foundation material [2].

thorough compaction of the base and

In addition, some other circumstances

measuring any deflection of the sus-

aggravated the damage and led to col-

pended part of the foundation block.

lapse of one of these bridges. The dam-

This part was then block concreted.

ages occurred partly due to shortcom-

Similarly, the other side of the pier was

ings of the design and partly due to ig-

prepared and block concreted.

norance of the effects of a temporary

To ensure proper contact between old

road embankment across the wadi

and the new concrete blocks, epoxy

about 500 m upstream from Bridge A

mortar was injected between them.

[3]. This temporary road embankment

Then the central part below the foun-

acted like an earthfill dam across the

dation block was cleared of all soil de-

wadi, creating an artificial lake up-

posits. Precast concrete beams were in-

stream. Due to heavy floods, this dam'

serted on the prepared base. Finally,

gave way, releasing a flood with a fast

the end sections were grouted, thus

moving flow of water about 12 m high,

providing another foundation slab to

as observed from the water marks on

the existing foundation block. A

the soffit of the deck of Bridge C, re-

similar procedure was adopted for the

sulting in erosion and undermining

repair of the abutment foundation. The

scour of the wadi bed and the side em-

depth of the new foundation system

bankments. The damage to Bridge A

was based on scour depth calculations.

was severe due to its undesirable loca-

After carrying out all necessary repairs

tion at a bend in the wadi, as shown in

for rehabilitation, the wadi bed and

Fig. 38.

approach

road

embankment

slopes

Rehabilitation

were suitably protected. This bridge

has now been operational for two

years. The owner was advised to

Two of the three bridges on Wadi Al

dismantle Bridge A, so as to provide a

Nagah were abandoned and the third,

clear path

for

the

wadi stream ap-

proaching Bridge C. To date, this dam-

- Location of a bridge near a bend in a

aged bridge has not been removed.

stream should be avoided.

Lessons

-

There is a need for research to es-

tablish

the

relationship

between

- Bridge planning and design is not

flow depth, flow velocity and total

only a job of structural engineer, but is

scour depth for actual conditions in the

the joint responsibility of a team of

field.

structural, hydrology and geotechnical

References

engineers. Structurally well

designed

bridges have failed as a result of

[1] MURILLO, J. A. The Scourge of

hydraulic conditions, primarily due to

scour of foundation material.

Scour. ASCE Civil Engineering, July

- The uncertainty of collecting proper

1987, pp. 66-69.

hydrological data, the probability of

[2] HUBER, F. Update: Bridge Scour.

occurrence of future severe storms and

ASCE

Civil

Engineering,

September

their effect on the bridge system

1991, pp 62-63.

require

advance preparation for all

[3]

MALLICK,D.V.;

ELWAFATHI,

eventualities.

A.M.

-

Initial evaluation of scour vulnera-

Damage Study of Three Reinforced

bility of streambed material is es-

Concrete Bridges over Wadi El Nagah,

sential. Due to the stochastic nature of

Libya. Conference on Our World in

the

hydraulic

parameters

involved

Concrete & Structures, Vol. VI

in bridge design, appropriate scour

(1987),

Singapore, 25-26 August

countermeasure

programmes, like

1987, pp 50-66.

installing riprap, guide banks to protect

abutments

and

embankments and

sheet piling along the face of piers

and/or abutments, must be clearly

planned in advance.

A NEW FOOTBRIDGE, AUSTRIA

At the site selected for a new foot-

Harald Egger,

bridge, the banks of the Mur River in

Prof. Dr Hermann Beck, Research

Graz. Austria, have an elevation dif-

Assist.

ference of about 2.2 m. The designers

Univ. of Technology Graz, Graz,

felt that a simple straight beam in-

Austria

clined across the river at this point

DESIGN

would

be aesthetically

unsatisfactory.

They therefore opted for spanning the

river with a beam that was slightly ele-

vated at its

centre and horizontally

member in relation to the centre of the

ing out from the centre to the supports,

bridge. On the left side of the bridge its

where they are eccentrically connected

top follows the top edge of the stiffen-

to the stiffening member. This spread-

ing member; on the right side, starting

ing is a consequence of the geometrical

from the centre of the bridge, it inter-

configuration of the bearing structure

sects with the lateral surfaces of the

and design considerations. Stability of

prismatoid, parallel to its bottom

the stiffening member is also improved

edges. The compound stiffening mem-

by this expansion and by the eccentric

ber is also asymmetrical. Its character-

connections, the latter also reducing

istic cross section is shown in Fig. 40.

deflection of the stiffening member.

To achieve the required stiffness for

the entire compound bearing structure,

145 mm thick bands of grade Fe 510

steel were used for the tensioning

member. These relatively heavy bands

were attached to the stiffening member

at the quarter points of the central

bridge span and supported against

wind loads. The connection of the

tensioning bands to the compression

member shown in Fig. 41, which

Fig. 40

illustrates both the solution originally

required by the invitation to tender and

The walls of the trilateral prismatoid

the final method employed by the

are 15 mm thick over its entire length,

contractor. Behind the supports, the

but its longitudinal braces - an

bands were anchored to angle cleats

additional flange plate welded where

that were laterally welded to the sleeve

the bridge deck begins to descend -

plates of the stiffening member. The

were adapted to the asymmetry of the

steel bands were stretched in place, but

stiffening member, as was the quality

not prestressed.

of the plate used. The trilateral

The stiffening member rests on four

prismatoid is reinforced by transverse

slender free-standing columns. It is

bulkheads placed 2.2 m from each

fixed to one column, and longitudinal-

other, corresponding to the folds of the

ly movable but transversally fixed to

cover plate of die bridge deck. The

the other three so that all four columns

entire structure is sealed airtight. All

may contribute to the transmission of

parts, whether assembled in the shop

wind loads. In addition, the entire

and at the site, were joined by welding.

bearing structure resting on the

In order to make sufficient allowance

columns is protected against transverse

for floodwaters, the tensioning mem-

wind attack from below.

ber underneath the bridge has a very

For reasons of time and cost, the bear-

flat design, with the elements spread-

ing structure of the bridge, which put

In Denmark the Association of Con-

studied. Nearly 40 % of the companies

sulting Engineers (FRI) has rejected

in the survey had implemented ISO

the ISO standard, arguing that it does

9001 and were certified in accordance

not cover all the critical elements of a

with it. Additional companies had be-

knowledge-based service. As a conse-

gun to implement the standard. This

quence of this position, dialogue be-

tendency is substantiated by a survey

tween ISO and FRI has ceased.

of the European Construction Institute

Only one consultancy had been certi-

(ECI), 52 % of whose members -

fied in accordance with ISO 9001. The

contractors, consultants and clients -

majority of companies have instead es-

replied that they were certified, while

tablished a quality assurance system

11 % were in the process of

based on the paragraphs in the standard

implementing the standard, and 16%

that

seemed

relevant,

supplementing

expressed the wish to do so.

them as required. Several companies

have

likewise

produced

cross

references to the ISO standard, as

some clients have demanded a quality

assurance system in accordance with

the ISO standard. The standard is,

therefore, used - but always as a refer-

ence.

Doubt in Germany

In Germany, the ISO standard has only

recently been introduced and until now

only a few contractors have im-

plemented it. As yet, no engineering

Fig. 42

consultancy

has

done

so.

Several

consultants stated that they could not

In conversations with English consul-

understand the necessity of the ISO

tants it was not unusual to hear that

standard. The general opinion of those

they had/in fact, no expectations for

questioned was that the way the Ger-

substantial positive effects from imple-

man construction industry and the DIN

menting the standard. It is seen as a

standards are related makes the ISO

necessary evil, one which enables con-

standard superfluous.

sulting engineers to qualify for projects

Activity in England

where the client demands the ISO

certificate. The standard and the quali-

ty assurance system it promotes are

The attitude towards the ISO standard

thus seen as a dubious formality,

(BS 7550) in England differs dramati-

useful only as a marketing tool.

cally

from

the

other

two countries

The real value of ISO certification

Given the scepticism of so many of the

seems limited. Both clients and consul-

respondents in the study, it is reason-

tants who were surveyed agreed that

able to question if the ISO standard is

ISO 9001 certification does not actual-

indeed applicable for engineering con-

ly ensure better quality, but only that

sultancies. The standard was drawn up

certain

documented

guidelines

had

for manufacturing companies with tan-

been followed.

gible end-products. As the standard

In England, a small industry has grown

only covers the critical processes for

up around the standard. Around 6000

that type of production, it is not certain

persons now work on implementing,

that it is suitable for knowledge-based

certifying and maintaining quality as-

services like an engineering con-

surance systems.

sultancy. The critical processes are not

No Extra Fee

necessarily the same, so the question

is: How relevant is the standard for

consultancy. This can be illustrated in

Is a client willing to pay extra for this

Fig. 42.

extra initiative? A survey carried out

The differences between product-based

by Prof. Peter Barrett, Salford Univer-

and knowledge-based endeavors are

sity, England, shows that only 3% of

considerable. The main reason for this

clients take quality assurance into ac-

is the cognitive and iterative aspects of

count when they select consultants. 95

consultancy. The process is difficult to

% of the cornpanies and clients sur-

forecast

and

schematicise, which

veyed did not expect a higher price for

makes it problematic to assure quality.

the services of a certified company.

The process of the work of an

Many employees still repudiate the

engineering consultancy is illustrated

quality assurance system, and many

in Fig. 43.

companies are still not working consci-

entiously with the system. There is, for

example, no widespread information

about the costs of ISO-certified quali-

fy. In all three countries, only the ex-

penses of internal and external audits

are registered. Only in Denmark are

expenses such as compensation and

reparations

registered. In

all

three

Fig. 43

countries, however, there is no existing

standard

as

an alternative

to the

ISO

Revision of ISO 9000

9000 standard.

Is ISO

9001

Applicable

for

The complications stemming from the

Engineering Consultancies?

iterative

work

process has been con-

when it started to revise the ISO 9000

The ISO 9001 standard, in its revised

standard. The standard has been split

form, will be a reasonable basis for an

up into four main areas:

effective quality assurance system. The

-

Hardware

standard is a good starting point for a

-

Software

continuous

quality optimisation,

and

-

Processed materials

thus a good basis for Total Quality

- Service, including engineering con-

Management (TQM), etc. Therefore, it

sultancies.

is disappointing that no one has been

The structure of the standards ISO

able to support this initiative and in

9001, 9002, 9003 and 9004 is undergo-

that way participate in the preparation

ing considerable change. ISO 9004

of the standards and guidelines which

will, for instance, be used for contrac-

will have considerable importance for

tual situations in the future. When the

engineering consultancy in the coming

revised standards are published at the

years.

end of 1996, the companies who have

Reference

certified according to the existing

standards have to re-certify within 12

[1] LAUSTSEN, J. Quality Assurance

months, a process that represents con-

and the ISO 9001 Standard in

siderable effort and time.

Consulting

Engineering

Companies.

Too Late?

M.

Sc.

research

project

at

Loughborough Univ. of

Technology,

Dept of Civil Eng., Longborough, UK,

Engineering

organisations

unfortu-

September, 1994.

nately have not been able to influence

the coming revised ISO standards for

service, but one initiative has been

made through the establishment of a

EFCA Task Force (European Federa-

tion of Engineering Consultancies As-

sociation).

Regrettably,

there are