Holdfast Training Services Trial Blog

For my Phase 3 attachment I have moved to the Engineering Consultant BG&E and am working on a large road infrastructure project in Sydney. BG&E are designing the 13 bridges for the John Holland Seymour Whyte joint venture and I am leading on the pedestrian and cycle bridge over the canal to the north of the airport.

Spanning 75m it will be a steel concrete composite bridge with varying depth. The design interfaces with a few other design aspects and these have resulted in some very tight restrictions on the structure. Four key ones are:

Deck height

The deck is limited by the slope of the path from the underpass of the planned adjacent bridge to the centre of the bridge and the sight distance over the span. This slope is set to the maximum that is allowed by the disability act in Australia. This limits the max height at the centre of the span.

Bridge soffit

The underside of the bridge is limited by flood modelling. I assumed this would be set at 500 mm above the 1/100-year flood event as is stipulated in the contract. However, there is also a clause that the project cannot increase the predicted level of flooding at a substation 1.5km upstream during the Probable Maximum Flood (PMF).

The PMF is used to define the limit of flood prone land and has an annual exceedance probability of once in every 10,000 to 10,000,000 years. The issue is that the underside of one of the upstream bridges has already been set at the limit that causes an afflux at the substation, this means that the model is now very sensitive to reducing the height of my bridge.

The frustrating bit is that the substation will be flooded by 2 m during the PMF. The contract clause means this can’t increase to 2.01 m. I’m hoping sense prevails and this can be adjusted.

Alignment

This bridge was moved after tender submission and now the alignment is constrained by land ownership boundaries. When combined with the approach path radius it limits the angle at which the bridge crosses the canal. This pushes up the span to 75m.

Piers

Why no piers? This is because the canal is so heavily polluted form years of industrial use. Transport for New South Wales would rather pay for the bridges to span the canal, rather than deal with the risk of disturbing this pollution.

Impact

Because of all these factors the bridge has ended up with a span to depth ratio of 26 and about 130 tonnes of steel which has to be in one lift. My main concern is bridge dynamics as pedestrians are far more sensitive to bridge movements. Guess there could be a blog post in it…