HANGAR ROOF COLLAPSE

Press, Volume LXXV, Issue 22883, 2 December 1939, Page 14

HANGAR ROOF COLLAPSE

HAREWOOD MISHAP, INVESTIGATED ERROR IN RIGGING SAFETY OF DESIGN PROVED An error in the rigging has been found to have caused the collapse of the steelwork of the hangar at Harewood this week. An official investigation of the mishap was made yesterday at Harewood by the Minister for Public Works (the Hon. R. Semple), the Engineer-in-Chief (Mr John Wood), and the District Engineer (Mr F. Langbein). “The mishap was due to a slignt error in the adjustment of the guy ropes,” said Mr Semple. "It was bad luck for the steel sub-contractors (Cable and Company, of Wellington), who took a risk in the construction in their desire to help the Government by expediting the building when the permanent end bracing girders were not available. Only a very small percentage of the steel —not more than 10 per cent.—will be unusable. The whole of the steel will be taken back to the contractor’s works at Wellington for reconditioning and straightening, and only six or eight feet which is badly buckled in the middle of each joist and has lost stability, will be useless.” There was nothing wrong with the design, said Mr Semple. The hangar was identical with one which had been completed at the Hobsonville Base for housing the trans-Tasman fly-ing-boats, and a hundred built in Australia. The collapse was due to no fault in the design, which was thoroughly tested and proved to be safe, and it was proposed to go ahead with the second hangar at Harewood immediately, taking greater care that the permanent bracing was put into position in the early stages. Collapsed Like Concertina The riggers employed were all experienced men—some had been 20 years at the work —yet the frank admission was made by the sub-contrac-tors that an error had been made. With the end braces not being on the job, they pushed ahead with the erection to help the Government. Two guy ropes gave way, and with the extra strain, one of the arched ribs was pulled out of plumb. “And when one goes, the whole frame collapses like' a concertina," said Mr Semple. "The rest of the ribs were pulled off balance and the roof trusses gradually settled down in the middle. A statement that some damage was caused to the concrete foundations is entirely erroneous; they are not affected in any way.” Although it was a costly experiment —the damage was estimated to be £looo—the misfortune had provided a wonderful test of the strength of the welding, approved and tested by the department’s inspectors, said the Minister. There was no breaking of any of the welds, although the steel had bent. Twenty tons of steel will have tobe repaired. It is expected that the reconditioned steel will be returned to Harewood within one month. . Official Explanation “At the time that these hangars were called for by the Royal New Zealand Air Force, the Public Works Department,” says an official statement, “did not have a suitable design available, and tenders were called from engineering firms Interested in certain proprietary constructions. The contract was awarded to B. Moore and Co. (Christchurch); their consulting engineers, who prepared the plans, were Campbell and Morrison; and Cable and Company, Wellington, were sub-contractors for the supply and fabrication of the steelwork. “The construction consists of rolled steel joists bent in arched form and tied together against spreading. Each element of the construction, therefore, is in the form of a bow and bowstring, and in fact is sometimes termed ‘bow-string arch construction.’ The ends of the arches rest upon steel supports, which are buttressed and connected to heavy foundations to resist the side thrust from wind action, on the completed building. In common with many types of roof construction, these elements are very flexible until they are properly braced together into the completed roof structure. “Inspection of the steelwork showed that the steel erectors had not erected the end arch ribs complete with their bracing, which would have stiffened and made safe the remainder of the arch ribs during the erection process,but apparently had fabricated the intermediate unbraced arch ribs first and had erected 12 of these in line relying for stability against collapse on the support given to the end rib by two Oregon strut§ about 40-45 ft long. The arch rib was held against these props by four wire rope ties which did not, however, pull directly against the props; in fact two of them were very far removed from the props so that the pull exerted by the ropes tended.to bend and distort the arch ribs. “Immediately before the collapse an attempt was being-made to line up the steelwork by adjusting the pulls in the wire guys and the action of tightening these ropes by the contractor for the steel work, gives adequate explanation for the collapse; and as all the arch ribs were connected one to the other with steel spacing angles, the collapse of one rib meant the collapse of all the ribs.”