Fremantle Traffic Bridge could collapse with a glancing blow from a vessel the size of the Rottnest ferry, a new engineering consultants' report says.
The report by Arup in May for the Public Transport Authority and Main Roads was obtained by _The West Australian _ through Freedom of Information laws.
It says frequent impacts in the past 40 years have damaged the bridge but so far there has been no major structural failure.
However, the bridge was "at continued risk of a more sufficient impact".
Based on the assumption a new bridge would be built in the next 10 to 15 years, Arup recommended building "glancing protection frames" to shield the old bridge from boat impacts in the meantime.
A Main Roads spokesman confirmed a reinforced steel fender system would be built next year "to mitigate impacts of glancing blows from large marine craft and to give added protection in the event of a head-on impact".
New in-river navigational aids would also be installed.
In a briefing note to new Transport Minister Dean Nalder in March, Main Roads said $2 million had been allocated to repair the bridge's fendering system in 2013-14 and 2014-15.
"However, it is inevitable that funds in excess of this will be required to undertake the works currently being designed," it said.
"It is anticipated funds in the order of $15 million will be required."
Three years ago, _The West Australian _ revealed a previously secret engineering report from 2004 that found the bridge was approaching the end of its serviceable life.
It said the risk of collapse and multiple fatalities if a vessel hit it was unacceptably high.
The report, commissioned by Main Roads, described the risk of doing nothing to replace or upgrade the predominately timber bridge as "intolerable".
It said the risk of the bridge collapsing from being hit by a vessel was 66 times above accepted standards.
Mr Nalder would not say when a new bridge would be built or how much would be spent to make it safe.
"The Fremantle Traffic Bridge is being maintained in a safe and serviceable condition," he said.
"Main Roads expects that with programmed maintenance, the life of the bridge can be extended 10 to 15 years, so that will provide us with a buffer to look at the designs for a replacement bridge."
Fremantle MP Simone McGurk said the Barnett Government had done everything it could to hide the risks associated with the traffic bridge, along with its failures to address the risks effectively.
"Labor allocated $80 million towards a new bridge but the Liberal Government stripped that allocation and has since done a series of expensive patch-up jobs," she said.
"It's an outrageously haphazard approach to a major piece of traffic infrastructure."