CHOICE IN BRIDGE DESIGN

Speaker: Malcolm Douglas, B.Sc.,C.Eng., F.I.C.E., F.I.Struct.E., Member, on 9 November 1998
The speaker started by showing and explaining the mechanics of his favourite rural bridge, built of logs used as cantilevers clamped in heavy stone abutments (Fig. 1). It is in the Karakoram mountains of Pakistan. If designed now by University students as a course project, it would, he said, probably and less suitably be a cable-stayed bridge of modern materials.
On the appearance of bridges, he referred to the proportions of the Golden Section as applied to bridges, but expressed a preference for Professor Leonhardt's Guidelines as the main source on aesthetically pleasing proportions of concrete bridges. Generally, an odd number of spans look best. It is important that arches and their abutments should be sufficently thick to give the impression of stability even if thinner sections would be strong enough. The aesthetic impact and apparent lightness of concrete bridges are sensitive to the proportions and placing of overhangs on the sides of deck cantilevers and spans, as well as of any supporting haunches at the tops of piers.
Turning to reinforced concrete bridges, the field of his main experience, Malcolm Douglas sketched the principal types, characterised by::
-spans of beams simply supported on piers;
-continuous beams extending over a number of piers;
-cantilevers, often with suspended central spans.
The second of these could be more economical in materials than the other types, but necessarily relied on great rigidity in the foundations of the piers; choice of type was often determined by the strength and nature of the ground and the expected severity of floods. Bridge superstructures are normally supported by bearings, pin joints or fully fixed connections with expansion joints to allow thermal and other movements between them. He explained the subtleties of the thermal movements of a curved bridge with rigid beam/column connections which he had designed. To illustrate these general principles, we were shown most interesting slides of a number of prestressed bridges designed by him, at Inverness, Southampton, Nottingham and elsewhere.
We were then privileged to hear about Malcolm Douglas’ work as a consultant on designing and building a new road in Ethiopia through extremely rugged terrain intersected by gorges about 1000 metres deep between high plateaux. He described the factors that had to be taken into account in the conceptual design of all bridges (Table 1) and of these bridges in particular, which had to survive high peak water levels and fierce attacks on piers by turbulent flood waters carrying heavy boulders with them.

For all conceptual design we need:
Information on:
Road alignment
Bridge location
Profile of valley
Maximum flood levels
Source of masonry stone
Access to the site
Availability of sand, gravel, cement
Availability and quality of reinforcing steel
Availablity of structural steelwork
Experience of, and equipment owned
by, the contractors
Availability of specialist piling contractors

And to consider:
Proportions
Slenderness
Lightness
Proportions of span and height
Proportions of supporting pier and volume of bridge deck
Directions of lines or edge of structure
Repetition of equal elements
Refinement of form
Optical illusion

Table 1 Factors affecting the conceptual
design of bridges

At Lalibella, where tourists are now flocking to see the churches carved out of the solid rock, the local clients’ requirements had to be found out and discussed in deciding with them upon a sympathetic design (Fig.2) for this part of the new road opening up that region of the huge country. To take the road over another river, the Tekeze, there was a choice between two possible routes, an old mule track across a wide valley and river, or across a narrow gorge half a mile away which had advantages provided that some problems, including particularly fierce flood conditions, could be overcome. Malcolm Douglas enthralled the meeting in describing, in characteristically matter-of fact terms, how those problems were solved in designing and building the haunched, three-span bridge across that gorge high above the bed of the river.(Fig. 3)
This meeting gave those attending a taste of the quiet satisfaction to be obtained by engineers from the creation and building of socially useful works which are both sound in function and aesthetically pleasing.
John Coates