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Chapter 6: The development of bridge design practice

Bridges Branch and later Bridges Engineering Division of the Department undertook a vast amount of work of a complex technical nature. The work is described in the great canyons of files in St Christopher House. Work instigated by or taken on board by the Department was carried out by Department staff, consultants, research departments, contractors, Local Government and Road Construction Unit staff. Inevitably the account is inequitable in the personalities it mentions and those it omits. This has nothing to do with merit since the way people get embroiled in events is to a degree random. All who served Bridges Branch and Bridges Engineering Division and all who now serve their successor, Structural Design and Maintenance Division of the Highways Agency are meritorious and this chapter is dedicated to them.

The main source for this chapter is a short history of Bridges Branch and Bridges Engineering Division contributed to the Archive by Ron Bridle, a former Chief Highway Engineer, Philip Lee and Shan Shanmugan; it also draws on material from contributions to the Archive by Colin Jones on foundations and Jack Parker, another former Chief Highway Engineer, on tunnels as well as other contemporary sources.

Acknowledgment is also due to Dr John Miles of Cardiff University for his contribution on bridge aesthetics and Barry Mawson of Gwent Consultancy for his contribution on bridge assessment. Ben Sadka helped by providing a clear understanding of the changes wrought by the creation of the Highways Agency and Alan Pickett of the Highways Agency was a constant and invaluable source of help and advice. Thanks are due also to many other colleagues who read early drafts of this chapter and provided advice and assistance.

The narrative follows a chronological sequence for the main developments in bridge design and construction with additional material on foundations and tunnels presented separately.

The simplest structural form is a ‘simply supported’ bridge deck. It spans between supports and the forces and moments it must bear are easily solved by consideration of equilibrium alone, that is all the vertical and horizontal forces and the moments occurring must balance each other.

A ‘continuous bridge’ is continuous over the piers, there is no joint in the beam but there is a joint between the beam and the supporting pier. It cannot be solved by considering equilibrium alone. An additional equation is needed to solve the reactions at the piers. The structure is termed redundant. A three span continuous bridge is one degree redundant. There are four unknown reactions at the piers and abutments but only three equations of equilibrium. Five unknowns would mean two degrees of redundancy and so on.

A ‘portal frame’ (as the frame around a doorway) is three degrees redundant where its feet (the springings) are fixed, that is they will resist rotation. It is three degrees redundant because the horizontal thrust and the moments at the feet are unknown and beyond the reach of equilibrium considerations.

An ‘arch’ form is easily envisaged. It may have three hinges, a form which is statically determinate, that is it is not redundant. Alternatively it may have two hinges, generally at the springings, when it is one degree redundant, or it may be wholly stiff with no hinges and three degrees redundant like the portal with no hinges.

A ‘cantilever and suspended span’ is again as described. In the three span version the simply supported approach span cantilevers beyond the pier and a simply supported span is provided between the ends of the cantilever. The Forth Railway Bridge is probably the most famous example.

The forms can be increased in span by the use of cable stays anchored to a tower which is the vertical continuation of the pier.

Finally, for the greatest spans a suspension bridge form is used, wherein all the support strength is vested in the cable from which the deck is hung. However, the deck must provide the function of a stiffening girder to prevent the cable changing shape under various loadings and creating the kind of oscillations seen in South American rope bridges.

These forms may be built in reinforced concrete, prestressed concrete or steel, the three main materials of the motorway era. Concrete can carry large compressive stresses but is assumed to be unable to carry tension. Reinforced concrete uses steel bars placed in the concrete to carry any tension arising. Prestressed concrete is simply precompressed in such a way that tension arising from loading is carried as a lessening of existing compression. Steel can carry both high compression and tension.

The deck of a bridge can be constructed in a number of ways. It may be a solid slab of concrete poured over reinforcement between shutters. A slab can also be constructed using precast beams of many kinds. Holes may be formed through the slab in a variety of ways. Such decks are very stiff in torsion, that is they do not twist easily and are capable of spreading loads on one carriageway across the deck.

The deck may also be made up of separate beams with a slab across their tops. The beams may be made of steel, reinforced or prestressed concrete and may be of I form, box form or latticed girders in steel. I-beams with no bottom slab are at the other extreme from slab decks and are less able to spread load transversely. However they also provide less resistance to ground movement and are very useful in areas of mining subsidence.

Many varieties of structures can be created from this basic kit. Practice is the business of designing safe, but economic, structures according to the best knowledge, the standards set and the exploitation of the findings of research and development which continually add to the revision of practice.

Within the Chief Highway Engineer Command, Bridges Engineering Division (BE) collated innovation, information and research findings and translated the whole into practice, through the issue of Technical Memoranda and Advice Notes and by prompting the assembly of relevant British Standards. Therefore, the history of BE reflects the developments and changes that took place in the practice of bridge engineering during the motorway era. BE drew on consultation with the bodies listed in the chapter on technical policy in Volume 1, but also on the immense talents within the industry through the eminent bridge engineers who served on the Bridge Consultative Committee. Too numerous to list, they helped to ensure that the course was set fair and that what was done and enacted was acceptable to professionals, the industry and consumers and was in the public interest.

It is inevitable that the consequences of some events are fundamental, but go without public or political comment. In other cases events attract a plethora of public comment.
Medway bridge

Development of Bridge Design PracticeAt a luncheon in the sixties, Oleg Kerensky, a Partner in the firm of Freeman Fox and Partners and an outstanding figure in the bridge world, was chatting to Richard Dimbleby the well known commentator. Oleg Kerensky’s enthusiasm for what he was doing soon got the better of him. ‘You know’ he said ‘on the M2 we are building, across the Medway the longest cantilever and suspended span bridge in Europe. You should make a programme about it!’. ‘Why’ asked Dimbleby ‘is it falling down?’.

This true anecdote is somewhat prophetic given later problems for Freeman Fox and Partners following the collapses of box girder bridges. It is, unfortunately, events which become a cause celebre, and mistakes which turn into them, that attract most attention. It is also true that a great deal is learnt from well publicised failures or other incidents under the pressure of reacting to them. However, as we shall see, it is not only or even mainly dramatic events that led to important advances in practice. As results from R&D were put into practice, they brought about significant economy and radically advanced practice; also there is a steady if undramatic improvement in practice through feedback and experience.

Detailed engineering, and particularly mathematics, are eschewed in this chapter; the technical record accounts, exhaustively, for the detail. In all cases, for better understanding, the technical problems are described in lay terms.

The following subjects are covered in the printed document (Volume 2):

INTRODUCTION

The fifties: Bridges Branch
The Volume of Work
Media Impact
The Cement and Concrete Association
Highway Bridge Loading

THE SIXTIES AND SEVENTIES: BRIDGES ENGINEERING DIVISION

Mining Subsidence
Tinsley Viaduct
Other Issues of the Sixties
The Seventies
Technical Approval Procedures
Box Girder Problems and the Merrison Committee
Loddon Bridge and the Bragg Committee
Other Issues of the Seventies

THE EIGHTIES AND NINETIES

Bridge Maintenance
Bridge Assessment
The Highways Agency
Bridge Appearance

FOUNDATIONS AND TUNNELS

Foundations
Tunnels

CONCLUSIONS