Documents reveal Auckland Harbour Bridge wobbles when crowds walk over it

Documents from Waka Kotahi have revealed that the Auckland Harbour Bridge has a tendency to wobble when large crowds of people walk over it, reports Newshub.

According to a memo released on 20 May 2022, the northbound and southbound extension lanes on either side of the bridge have natural frequencies of lateral vibration that are close to that of people walking.

If a group of just 250 people walk on the outer lanes, it can cause the structure to vibrate in what is known as ‘synchronous lateral excitation’.

It was found during previous marches that the gap between the original bridge and clip on lanes could open by about 50mm, posing a hazard of ‘serious crushing injuries’.

Even though the transport agency has known about this for some time now, it still had plans to run walking events on the bridge earlier this year.

However, the document states that Auckland Marathon events could go ahead because crowd densities are managed and running/jogging doesn’t cause the bridge to wobble. It also mentions that crowd numbers should be limited to 250 people per span for any other future events.

The good news is that Waka Kotahi says it doesn’t anticipate structural failure if people were to walk across the bridge during events but it may cause the box girder deck to ‘bang’ against the truss deck, possibly causing some local damage.

Newshub states that engineering consultancy Beca told Waka Kotahi that there are multiple ways the problem could be fixed. One solution is to bolt plastic tanks to the underside of the bridge that could be filled with water prior to a walking event in order to interrupt the frequency. The other is to use dampers.

Neither of the proposed solutions to fix the wobbling Auckland Harbour Bridge have been implemented though.