Poor maintenance and the use of untreated timber contributed to the fatal collapse of a bridge built by the Army, according to a report it tried to keep secret.

Retired Army engineer Colonel George Butcher said he did not believe untreated timber should have been used on structural parts of a bridge which was "obviously intended" to be a semi-permanent or permanent structure.

"The decision to use untreated timber in the original bridge cannot be supported."

He said timber support transoms rotted more quickly because they were built from separate beams joined together without any flashing, which allowed water to penetrate and "significantly" reduced their life.

Mr Butcher also said King Country couple Keith and Margaret Berryman did not recognise the need for a regular inspection and maintenance programme for the bridge, which led on to their farm.

Beekeeper Ken Richards was killed when the rotting bridge collapsed in 1994, sending his utility plunging 30m into a ravine.

Mr Butcher's report, which is actually a series of written answers rather than one document, was evidence to an Army Court of Inquiry held that year.

Both it and the inquiry's findings - which omit some of Mr Butcher's critical evidence - were withheld from a 1997 coroner's inquest into Mr Richards' death.

Instead, the Army told the coroner the bridge did not fail because of design or construction errors and that it was "fit for its intended use" when handed over to the Berrymans.

However, Mr Butcher's evidence, which the High Court has ruled can be published, has criticised the design and building of the bridge, and timbers used.

He said the failure of a timber transom caused three bays of the timber-deck suspension bridge to collapse.

The transoms are the principal support beams which hang off wire ropes strung under the deck, and which carry the weight of the bridge. They were built with untreated timber, Oregon or Douglas fir, which the Berrymans bought but which the Army agreed to use.

Mr Butcher said that timber had a very short life when constantly exposed to the weather and was usually used under cover.

The reduced strength of the transoms which broke "was in my opinion entirely due to decay of the untreated timber".

"In my opinion, Oregon timber should not have been used for the main structural members of the deck structure of the bridge when it was intended to be semi-permanent or permanent in nature."

He said there were several technical errors and errors of judgment in the design calculations which "by good fortune were self-cancelling".

The allowable stresses for bending used in the design were too high for the timber grades and species used, and higher than permitted under design codes at the time.

Mr Butcher said the design procedure for the timber deck structure was "unsatisfactory". It was fortuitous the Berrymans had throughout the life of the bridge kept running planks they installed on it as a precaution well nailed down.

"I can only conclude that the design of the deck structure could not be considered as adequate for the bridge's intended use."

The coroner's file includes an indemnity signed by the Berrymans agreeing to take on responsibility for the bridge. The Army engineer who designed the bridge, and whose name is suppressed, agreed it was a standard form which all landowners would have been required to sign.

Credibility gap

What the Army told the coroner in 1997:

* In a causative sense, there was nothing in the entire construction of this bridge that contributed to the accident.
* What Colonel Butcher told the Army in 1994:
* The decision to use untreated timber in the original bridge cannot be supported.