"Pedestrian Bridges" by Andreas Keil

This new book on the design of footbridges is part of Detail’s Practice series, a line of guidance notes aimed mainly at architectural designers. Pedestrian Bridges (Detail, 112pp, 2013, also available in a German edition) [amazon.co.uk] is aimed at bridge designers generally, with plenty of material aimed at engineers as well as architects.

The lead author, Andreas Keil, is a prominent bridge designer with Schlaich Bergermann und Partner, responsible for several highly innovative designs, such as the Passerelle La Défense in Paris. Although this book includes many SBP designs as illustration, it is a general text rather than one which examines in detail the author’s own experiences.

The book has two obvious predecessors. The first is FIP’s Guidelines for the Design of Footbridges, edited by Keil's colleague Mike Schlaich; and the second is Footbridges by Schlaich with Ursula Baus. Schlaich and Baus’s book is not a design guide, but an extended review of examples of interesting structures from around the world, although in its attempts to explain and understand the work of widely-varied designers, it does offer considerable assistance and inspiration to designers of footbridges. The FIP Guidelines, on the other hand, cover very similar territory to Pedestrian Bridges, so I'll return to them in a bit.

Keil’s book attempts to be comprehensive, with sections on functional requirements, statics and dynamics, materials, structural types, finishings, economics, special bridges, and a short gallery of project examples. This is the engineer’s approach to bridge design, placing the options for response to functional demands at the centre stage. Architectural issues such as aesthetics, context, the pedestrian’s experience of the bridge, form a persistent subtext throughout.

The chapter on functional requirements addresses issues which are frequently set out in national standards, such as minimum width, clearances, ramp gradients, staircase geometry, and parapet height and strength. In the UK, the relevant standard is BD 29, which is freely available online. Most of Keil’s commentary is, however, specific to Germany (with some reference to other international requirements), and I think it’s mildly unfortunate that in an English translation the opportunity has been lost to better internationalise the text. The wide variation in specifications around the world is a worthwhile topic, as it illuminates the arbitrariness of many individual requirements, and helps arm the creative designer if they should ever dare to test the boundaries of regulation.

The same issue is apparent in the section on static and dynamic performance (including aerodynamics). Although the book briefly compares the different loading standards, the key reference for pedestrian-induced vibration is the German HIVOSS approach, which is far from the only available guidance, and which has been reported as being more conservative and less versatile than the British approach (embodied in the National Annex to the Eurocodes). Despite industrial quantities of technical paperwork on the subject, dynamic analysis of pedestrian bridges has been a complex and specialist area, riddled by incompatible and sometimes incomprehensible research. The designer seeking guidance in this area will undoubtedly need to seek more in-depth advice.

The book’s chapter on design and construction is primarily a discussion of different structural forms, but opens by “reviewing and clarifying the tasks involved”, a consideration of the constraints to be documented and analysed before the opportunities for design can begin to be explored. Constraints include matters such as the site topography, geotechnical conditions, and the required bridge useage. This section is quite brief, which is a shame as this is a key area which is covered inadequately in most books on bridge design. Constraints arise not only from the site itself but from the client’s remit (including budget), from the demands of communities and stakeholders, from local contractor capability and resource, and from environmental limitations. For me, a thorough analysis of design constraints is a vital first step in creative design, and there is much more to be said than Pedestrian Bridges provides space for.

The discussion of different structural forms is thorough and clear, and as with the rest of the book, well-illustrated. As you might expect, Keil’s understanding of structural behaviour is acute, and the many diagrams and photographs both help explain the text and expand it by offering examples of inspirational design. As well as the more obvious forms, the book gives generous space to stress ribbon bridges, and to a discussion of the particular issues for curved bridges (although this is one case where the generally fine translation falls, with a confusing discussion of “carding” moments, which it took me some time to deduce meant torsion moments).

One question which readers may ask is whether this is a book aimed at the novice or at the initiate. I think its audience is clearly the former, but it contains much that will be of interest to the latter. I found a great deal that was new to me in some parts of the book (I won’t embarrass myself by saying which parts!) Other parts also helped to encapsulate and put structure to things that I have learned as a designer, but never allowed to coalesce from their varied and fragmentary project landscape.

Although Pedestrian Bridges covers very similar territory to the FIP Guidelines, there are some key differences. Keil's extensive discussion of materials and structural forms is entirely absent from the earlier document, which focuses primarily on performance requirements rather than design opportunities. Keil's book also has a useful section on economic aspects, with a survey of the costs of 22 footbridge examples given for comparison (not the first such effort). Both books offer a selection of case studies, with more in the FIP book, but no overlap between them.

In summary, for anyone inexperienced in pedestrian bridge design, this is a very helpful publication. Even experienced designers should find it a useful addition to their bookshelf. It is very well illustrated throughout, and clearly written and translated.