What’s In The Bridge Engineering Report That Caused The Crisis


Today’s rush of activity around the state and future of the Auckland Harbour Bridge follows the delivery of an engineering report which warns the agency:

“The bridge is a major steel structure so the current “level of service” will be reduced when either the bridge load capacity is exceeded and can’t be further increased, or when the rate of fatigue cracking is occuring at a rate faster than repairs can be completed.”

bridge waterThe NZ Transport Agency commissioned the report from Beca Infrastructure on the effect of future traffic growth on the bridge.

The report concludes that the present strengthening of the extension bridges due for completion in a year’s time is the maximum practicable for those structures “without changing their structural form.”

So is the bridge safe?

NZTA insists it is.
The report says that with respect to fatigure, a study carried out in 2002-2004 indicated that provided the high performance Bolidt surfacing on the extension decks are maintained in good condition, there is a “low risk of fatigue becoming the limiting factor in the next 40 years.”

But it adds that that study didn’t consider the closure of lane 1N to heavy vehicles in 2007 nor the effect of the proposed 2009 Vehicle Dimensions and Mass Rule Amendments.
The proposed amendment provides for legal axle weight increased of about 10% for most trucks, A 10% increase in most axle weights could result in a “significant shortening of deck fatigue lives.
Similarly, the closure of lane 1N has diverted heavy vehicles to the most critical lane for fatigue – lane 2N, thus shortening its fatigue life.” The report says these effects need a further study and one will be carried out “in the near future.”

The northbound extension bridge peak traffic loading is heavier than the soundbound. The highest traffic loading is between 4am and 6am when the percentage of heavier vehicles is highest in the traffic lanes. The most critical component is the central part of the main 244m bridge span “which is governed by the differential temperature “ as well as the higher daytime heavy traffic loading, says the report.

Solution: Spray the bridge with water?

It seems the best long term hope the report can offer is finding a way to cool down the steel structure and the rather bizarre suggestion of spraying water on it.
The report says a significant factor for the capacity is differential temperature effects together with daytime peak traffic loads. Beca suggests controlling the temperatures by say, spraying water on the bridge, might extend the life of the present bridge but this is largely untried.
If it was possible to reduce these by a “waterspray system” on the northbound extension bridge deck, then a “substantial increase” in the time to reach load capacity could be achieved. But the report admits the feasibility of this system is yet to be established.

The main truss bridge members have recently been strengthened, including provision to carry Transpower cables to be installed in 2011 and the additional cables to be installed about 2030. If it was possible to transfer load from the main spans of the extension bridge to the truss bridge and further strengthen the truss bridge, then additional load capacity in the extension bridges “could be achieved.”

But the report warns that such a scheme would “limit the future available load capacity and the time to reach that capacity for the truss bridge.”
Walking / cycling options

On the issue, dear to some of our hearts, of a walkway or cycleway on the bridge, the report’s findings say that without any management measures in place, adding a 2.4m walkway /cycleway to the northbound extension bridge would reduce the accessed time to reach load capacity by about 10 to years.

But are  the southbound clips ons still an option for waking or cycling?

NZTA, in its commentary on Beca’s report, says:

“The NZTA board decided last year that it would not fund walking and cycling facilities in the southbound box girder. The NZTA has indicated it would consider a new proposal from supporters of cycling and walking facilities on the bridge , providing they are able to obtain funding for this facility.”





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