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How test loading can avoid investing billions in replacing bridges Asset Management Civil Engineering Works Conservation

Are these civil engineering works truly at the end of their life?

A substantial number of the tens of thousands of bridges in the Netherlands mainly owned by regional authorities are ageing. Billions would likely have to be invested in the coming years to replace the bridges and viaducts to make them compliant with updated Dutch legislation. But is replacement really necessary? Are these civil engineering works truly at the end of their life? What kinds of loads can they still take? Test loading is a way of making the most of the structure and making substantial savings on asset management.

The load limit of bridges is often unknown because of a shortage of data, and it is sometimes tricky to determine the capacity. The upshot is that recalculations tend to be based on conservative assumptions of such matters as the forces that affect the structure, the material data and the geometry of foundations and abutments. As these uncertainties must be factored into the standard calculations responsibly, the recalculation often fails to maximise the load capacity of the bridge.

A decision to replace or renovate a bridge often results in the unnecessary destruction of capital

An on-site examination can sometimes reduce the uncertainties and thus increase the demonstrable load capacity. The problem is that such examinations are frequently expensive, cause considerable inconvenience in the surroundings and often result in just a limited reduction of the knowledge gaps. Therefore, they do not always yield the expected gain in the load capacity. A recalculation cannot determine the ultimate capacity of the bridge. A decision to replace or to renovate a bridge is taken too quickly, based on flawed grounds, thus resulting in the unnecessary destruction of capital.

All uncertainties in modelling and knowledge could be eliminated by determining in real life how much load a bridge can take before it starts to exhibit signs of failure. Test loading is a way of utilising the maximum potential of the structure.

How does test loading work?

Before the bridge is subjected to the test load, it is examined structurally so as to record its condition and to obtain a minimum of data (e.g. dimensions). The abutments must not give rise to suspected problems with the foundation, and the deck of the bridge must not exhibit any signs of damage indicative of overloading. Test loading determines the moment capacity of the bridge deck. Prior to test loading the shear force of the deck is worked out mathematically, based on geometry and status, to guarantee that it is not determinative during test loading.

By placing a load incrementally in the centre of the span, it is possible to cause a controlled moment in the bridge deck. This moment is compared with the moment in the bridge deck as a result of the normative load including safety factors whereby any reduction factor is determined. By measuring the deflection of the bridge deck with great accuracy at every incremental load, it is possible to determine whether the deck springs back to its original position after the load is removed from the bridge (elastic deflection). The bridge deck is then capable of taking the load safely. If during an incremental load it is observed that the deck no longer springs back in its entirety, it means that plastic deformation has occurred and the test is stopped. The incremental step is chosen in such a way that the plastic deformation has no adverse effects on strength and durability. In addition to the deflection, the abutments are monitored precisely, with a view to subsidence. If subsidence is observed, it again constitutes a reason to stop the test.

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Test loading in short!

Identifying true load capacity in half a day

The advantage of test loading a bridge is that the demonstrable load capacity can be determined quickly and with minimal inconvenience. Uncertainties in modelling no longer play a role, so the maximum load capacity is determinable. There is no need for extensive testing (or destructive testing) of the bridge. Through this technique Iv-Infra is able to demonstrate the actual load capacity of a bridge in half a day, with only limited inconvenience to local people and road users, thus avoiding the need for investments running into billions.
Would you like to know more about the possibilities for your project? Wouter will be pleased to share ideas about your engineering issues. Contact him by email or call​ 088 943 3200.
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