CityCyclingEdinburgh was launched on the 27th of October 2009 as "an experiment".
IT’S TRUE!
CCE is 17years old!
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One solution might be to replace the wooden top-chord with an equivalent custom steel beam (I-beam or box beam). With appropriate jacking, this could be carried out with the rest of the bridge in place and without modifying the supporting pillars.
I'm not a civil engineer, but naively, this seems like the lowest cost, quickest, and least wasteful solution. It also follows the concept of the "repair economy"
tl;dr: Good in theory, and the bridge would be stronger overall, but would need some careful thought!
A steel top chord would be stiffer (and heavier), so would deflect less under the same compressive loads. I think my main concern would be the joints. The all-wood bridge's connections have wooden bearing faces; a well-fitted mortise and tenon joint only needs dowels for restraining, not load carrying, and you don't usually put a joint like that into tension if you can help it. Since the wood itself is also somewhat ductile and can crush slightly, it allows for load sharing across the structure where the forces are slightly different in each member. I noticed that the diagonal braces in our bridge are not cut with tenons; the mortise is in the chord and the brace is simply fitted directly into it. Quicker to make, certainly, but possibly removes too much material from the chord.
Make the top chord out of steel makes those joints a timber-steel hybrid. The overall forces in the bridge might change by only a modest amount, and the forces in a truss are really a function of geometry. The problem as I see it would be the local bearing forces, which could increase substantially. This would shift the weakness from the top chord itself to the diagonal braces, and their tenons and dowels.
Using a tube or RSJ would usually mean attaching the existing diagonal braces to it with bolts, gusset plates and brackets, which means those fasteners are now carrying the load into the grain of the brace, potentially leading to more cracking. If the braces were instead fitted into a steel (or wooden socket inserted into steel) equivalent of the mortise then you can preserve the bearing function that the original joint had.
Recall that our bridge is a shallow Warren Truss with additional verticals. Generally half of the diagonal braces are in compression and half are in tension. The vertical members are helpful in a wooden bridge because you have quite long lengths of unsupported wooden beam between the diagonal members' joints, so inter-joint bending of the top chord can be reduced. But, verticals are also unhelpful in a wooden bridge because one end has to occupy the same space as two diagonal braces' ends, which means you have even more dowels or other fasteners in much the same place, and at the same time you're carving out even more wood from the beam (top or bottom chord) to form the mortise, which means the beam itself has less strength in either compression (top) or tension (bottom).
Thing is, we also once built huge trestle bridges to carry railways. Apart from the fact that they felled enormous numbers of trees to build one, there is a lot of redundancy in the design, and any failed members could usually be removed and replaced singly without having to shut or dismantle half the bridge.
I mean, really though? It's only 45 metres. Maybe CEC should just build a nice stone arch bridge like we used to do, use the exercise as skills preservation, and be done with it.
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But it also raises some important new questions.
❓ Cracking in the bridge was first identified in 2017 and continued to be monitored. Yet there are gaps in the formal inspection record, including no Principal Inspection since 2017, despite the Council briefing saying inspections were carried out in accordance with the relevant standards.
❓ We also remain unconvinced that all realistic alternatives to complete replacement have been properly explored. The Council has concluded that repair is too uncertain and a temporary bridge impractical, but some of the specialist advice behind that conclusion appears to have come from informal discussions rather than a full written options appraisal.
❓ And remarkably, no proper assessment appears to have been made of how many people use this bridge or the impact its loss has on the surrounding community before decisions with potentially multi-million-pound consequences are being taken.
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