Bridge construction cost per square metre is the figure business cases ask for and the figure least able to answer the question. Published comparisons illustrate the problem rather than solving it: a European sample of concrete bridges has been reported at somewhere around €1 200 per square metre, while individual landmark pedestrian structures have been reported at more than twenty times that.
A metric with that kind of spread is not measuring the same thing in each case. Understanding what the denominator leaves out explains the spread and, more usefully, explains when the figure can be trusted.
What the denominator is
Deck area is length multiplied by width, and it is chosen as a denominator because it is the one dimension of a bridge that is always known early. That is its entire justification. It is not chosen because cost scales with it.
Most of what a bridge costs has no relationship to deck area at all. Foundations are driven by the loads and by what is underneath the ground, not by how wide the deck is. Craneage is driven by the weight of the heaviest lift and by what can reach the site. Approvals are driven by what is being crossed. Approach ramps, which on a raised crossing can exceed the span in both tonnage and cost, are usually excluded from the deck area entirely.
The narrow bridge penalty
This is the specific distortion that matters to anyone buying a footbridge, and it works against the cheaper option.
Take two crossings over the same gap, one two metres wide and one four. The abutments are similar, the foundations are similar, the lift is similar, the design work is similar. The wider structure costs somewhat more in total, because it uses more material, but it has twice the deck area. Divided out, the narrow bridge shows a per-square-metre figure close to double the wide one, while costing less in absolute terms.
A business case that compares options on this metric will therefore prefer the wider structure and record it as better value, which is exactly backwards. Pedestrian and cycle bridges are narrow by nature, and they are systematically penalised by a metric designed around road structures.
What else the figure quietly excludes
Published per-square-metre figures rarely state their scope, and the scope varies enormously between sources. Some include design and approvals; most do not. Some include the substructure; many cover only the deck and superstructure. Approach works, land acquisition, utility diversions and temporary traffic management are almost never included.
Inflation and currency add a further layer, since a figure from a project completed several years ago in another country is not directly transferable. None of this makes published data useless, but it does mean a figure without a stated scope, date and country cannot be used to check a quotation.
When the metric does work
Per-square-metre cost is genuinely useful in one situation: comparing structures of the same type, the same width class and the same scope, within one organisation’s own records. A municipality that has built six footbridges of similar configuration has a benchmark that means something, because all the excluded variables are roughly constant across the set.
It stops working the moment any of those hold constant no longer: a different width, a different clearance, a different ground condition, a different scope boundary. Used across those boundaries it reports differences in the denominator as differences in value.
Better figures to ask for
Three alternatives carry more information and cost nothing extra to produce.
The first is cost by work package: superstructure, substructure, approaches, installation, professional fees. This shows immediately where a project’s money actually is, and it makes offers comparable even when their scope differs, because the missing packages are visible as gaps rather than hidden in a total.
The second is cost per metre of span for a stated width band. It keeps the geometry that actually drives cost and removes the width distortion, which makes it far more honest for pedestrian structures than area-based figures.
The third is whole-life cost over a stated period, which is the only figure that captures what a structure commits an owner to after handover. Its weakness is that it depends on assumptions about maintenance intervals, which is an argument for requiring those assumptions to be stated rather than for avoiding the metric.
How to use it without being misled
Treat a per-square-metre figure as a sanity check with wide tolerances rather than as a target. If a quotation lands within the range of comparable structures, that is mildly reassuring. If it lands well outside, that is a prompt to ask which packages are in or out, not evidence of a good or bad price.
Above all, do not use it to compare options within one project, because the option with the smaller deck will always look worse. Compare those on total outturn cost with the scope boundary fixed, which is the same discipline that makes footbridge quotations comparable in the first place.
Before the number goes into the business case
A benchmark that varies by a factor of twenty-five between published projects is telling you that the denominator is wrong for the question. Deck area is convenient rather than meaningful, and its convenience is precisely why it persists.
Our engineers will produce a package-by-package cost structure for a crossing at feasibility stage, so that a business case can be built on figures that respond to the project’s actual decisions rather than on an average taken from structures that have nothing in common with it.