InQuik Engineering: How to remove dead load stress on the concrete in bridge beams

December 6, 2018by admin

 

Bruce Mullaney
Inventor of the InQuik Bridging System

For thousands of years, humans have used beams to support loads. But the beams don’t just support the load of whatever is put on top of them – they must also support their own weight. In the case of the lintels at Stonehenge, the only load they support is their own, and they need to be thick enough to do so (too thin and the rocks would snap under their own massive weight). The bottom of the unsupported section of the beam experiences tension forces as gravity tries to pull it downwards, and if the beam isn’t strong enough, this can result in cracks, fractures and eventually failure. Structural engineers must ensure that beams are big enough and strong enough to take both the live and dead loads.

One strategy to reduce the effect of the dead load is by minimising the cross-sectional area of a beam which is made of a material with high tensile strength. Steel I-girders are a prime example of this method, which retain the necessary flange and webs while removing the excess mass that increases the dead load.

When concrete beams are cast on-site using traditional methods, the supporting formwork and props are removed and the beam must support its own weight. There’s a similar result in precast beams, in which the concrete is cast in a mould and cured off-site. Tensioning the reinforcing within a concrete beam can increase the tensile strength of the beam, which improves its load supporting capacity. However, precast beams can also experience fractural cracking during transport.

 

The InQuik bridge system is a completely different way of constructing reinforced concrete bridges. When InQuik deck panels are cast on-site, the internal reinforcing connects to and supports the permanent steel formwork while the concrete sets and cures. This simple construction method has a number of profound structural consequences: 1) The whole dead load of the bridge deck is fully supported by the reinforcing through the formwork, and not by the concrete itself. 2) The concrete sets and cures in a completely neutral state, so the dead load of the bridge deck doesn’t create tension forces on the bottom of the concrete in the beams. 3) The weight of the concrete (both when wet and set) applies forces on the supporting reinforcing trusses, which put the bottom chords of the truss into tension. All of these factors mean that the concrete in the bridge deck will only be subjected to a distortive force when under a live load (traffic).

The InQuik technology was introduced to the market in 2017 as the next evolutionary step in concrete bridge construction. While we are learning a lot about the constructability and practical advantages of the system, we have only scratched the surface of the structural advantages that this methodology provides.