Grafton Bridge Design Fault

Press, Volume CIX, Issue 32073, 22 August 1969, Page 15

Grafton Bridge Design Fault

(N.Z. Press Association) AUCKLAND, Aug. 21. A design fault has been revealed as the cause of cracking problems on the $230,000 Grafton Road bridge which was designed by the Ministry of Works.

The cause of the trouble has been shown to be an insufficient allowance at certain points for contraction of parts of the bridge. The total cost of investigations and repairs isestimatefl at $20,000, and remedial work should be completed by mid-December. Another source of embarrassment to the Ministry of Works, the $2.5m Newmarket motorway viaduct, should be open to heavy traffic again about the same time.

The heavy traffic retriction, a ban on vehicles weighing more than two tons, was placed on the structure last November to enable $280,000 of repairs to be made after the discovery of cracks in some joints in the viaduct As with the viaduct high tensile steel bolts will be used as the major method of fixing the Grafton Road bridge. Deep Cracking The District Commissioner of works in Auckland, (Mr F. F. Abey) said today that on only two of the four .outer beams of the bridge had the cracking shown right through from the outside to the inside. The other . two showed cracking on the outside only. Cracking on other beams had been insignificant “Allowance must be made

in all bridges for the members to slide as changes in length occur due to shrinkage of the concrete, both due to the setting of the concrete and the effects of the prestressing,” he said.

Allowance was also made for changes in length due to expansion and contraction as a result of temperature changes. Severe Skew

. In the Grafton Road bridge shrinkages and temperature movements which occurred were not parallel to the line of the bridge, because the abutments were on a severe skew to the line of the bridge.

and because of a low length to width ratio. “Insufficient allowance was made for freedom of movement across the abutment; and when shrinkage and contraction took place binding occurred,” Mr Abey said. The binding caused excessive movement elsewhere, which led to the cracking. The restraints on movement at the abutment had now been removed, Mr Abey said.

He said he was not sure that there had generally been enough allowance made by engineers in the past for this type of effect