Reconstructing Darwin after “Tracy” and resolving the issues of fatigue failure and debris attack
In January 1975 I had been posted back to Brisbane as Head of Discipline Structures which used to be Supervising Structural Engineer and it all tied in as if I had planned the move. At that time Cyclone Tracy hit Darwin and demolished the area. This was a disaster of the first order and the Department was fully involved in what had to be done.
From 11th to 15th March I attended a seminar at New Port, Sydney on Welded Steel Fitments and Structures by Blodgett, an expert in the field. Then, on 26th March I was sent to Darwin returning on the 2nd May, a period of 27 days. Officers from each region were seconded to work up there and assist where needed as well as obtaining first-hand knowledge of the havoc caused by Cyclone Tracy. Harry Berg was a Structural Engineer III in Moresby before I went to PNG and had transferred to Darwin on a permanent transfer. His house was partially demolished and inhabitable so he and his family were transferred to Adelaide for some time. His car remained in Darwin, undamaged and he gave us permission to use it, so we had transport at the week-end. It was two months since the cyclone had struck and by the time we arrived most of the debris had been cleared away or stacked to be moved and the landscape was just a desolate outlook of houses partly, or wholly demolished with floors only on high stumps, trees stripped of branches or blown out, power poles down and water mains closed off. Very little was spared and there had to be whole new thinking about buildings in cyclone prone areas.
The traditional approach was to build to support a given load, a dead load i.e., self weight or an applied live load. Tracy showed there were definite deficiencies in this approach. Tracey produced estimated wind gusts of approximately 65 to 70 metres per second, considerably above the design wind speed and it must be remembered the force applied is proportional to the square of the velocity. The eye of Tracy also moved very slowly across the town area so the buildings were battered by a larger force for a longer time. While houses were sealed they had a good chance of surviving with minimum damage but once the roof is damaged, or external walls or windows broken the internal wind pressure changes dramatically. The force on a wall of a building is the resultant of the forces acting on both sides of the wall. Openings in a windward wall permits the internal pressure in the building to increase and produce outward forces on the various surfaces resulting in decreased forces on windward wall and increased forces on leeward wall. The pressure coefficients are defined in the Australian Standard as 1170 for the whole building and its components.
Tracy clearly illustrated a phenomena not properly appreciated before. This was fatigue failure of corrugated iron roof sheeting at the fixing points, due to sustained fluttering of the sheeting under the wind action. Roofing in cyclonic conditions is subject to long periods of repeated loading. For CGI room sheeting fixed with Buildex type 17 hexagon head self-drilling, self-tapping fasteners tests show, that under dynamic loading, sheeting failures can occur at loads only a fraction of the static load required to cause failure, as low as 15% of the static load under 800 cycles of loading. This phenomena is termed low cycle fatigue and at 50% of the static load failure occurred at 127 cycles. It was found that roof sheeting vibrates at approximately 3HZ and at this frequency it takes an hour to reach 10,000 cycles. The Darwin Reconstruction Commission Darwin Building Manual requires sheeting and fittings to be tested to ensure no failure occurs under 10,000 cycles of working load from zero to maximum followed by a static load of 1.8 times the working load. One solution to this problem was the use of cyclone washers fitting the sheet profile. This stopped the tearing of the sheet at the highly stressed area and was accepted as a “deemed to comply” solution.
Debris attack was another problem; to estimate the size and velocity of appropriate design missiles. Some work had been done in U.S. on debris attack in tornadoes. Texas Technical University, for in residence shelters, recommended a piece of wood 100mm x 50mm x 3500mm long weighing 7kg. travelling at 45 m/sec. The Darwin Reconstruction Commission Building Manual adopted two criteria. For design of shutters, 4 kg., cross-section 100 x 50 mm. Striking on end at 20m/sec. For the design of refuge shelters a timber plank of 8 kg. Mass, having a cross section 100 x 50mm striking on end at 30m/sec. Note: 4 kg. @ 20 ,/sec is equivalent to dropping 4 kg. From height 20m. 8 kg @ 30m/sec is equivalent to dropping 8kg. From a height of 45m.
Two other effects of Tracy were storm surge and rainfall. It was estimated the storm surge for Tracy was approximately one metre. If this had occurred at the time of the highest astronomical tide a maximum water lever of approximately 11 metres Town Datum would have occurred and cause flooding to a depth of three metres in some lower housing areas along the coastal fringe. At least Darwin was spared this with Tracy at work while the tide was low and the tidal range is around six metres. High rainfall is also associated with cyclones and can cause flooding. Here the biggest problem was water damage to salvageable articles exposed after the wind had done its damage. The Darwin Reconstruction Commission Building Manual became the standard for acceptance and it’s deemed to comply requirements made it possible to accept or reject materials covered for the rebuilding. It was some time before the Australian Standards covered the lessons learnt. From all experiences in the reconstruction of Darwin, the building construction industry learnt the importance of the ABC of construction, anchorage, bracing and continuity.
We had barracks type accommodation in Darwin within walking distance to the CDW office which had suffered minor damage only. This was the centre of activity and so much was in progress; I have forgotten the details. With temporary accommodation, e.g., caravans, transportable barracks etc., the problem was to anchor them down. A series of tests were conducted drilling or auguring holes in the ground and grouting a reinforcing steel bar full length which could be secured to the transportable chassis by welding or bolting. There were tests noting hole diameter, soil type, depth of hole and rod diameter with pull out load. Applying a suitable factor of safety, the anchor point could be determined with a degree of certainty instead of large mass concrete footings. We were there to do a job, in fact, any job delegated from records, design, or supervision.
Several of us took advantage of Harry Berg’s car and took a couple of trips down the road on Sundays, visiting Howard Springs, Humpy Doo, Rum Jungle and as far down as the Adelaide River. It was fascinating country and it was surprising to see glider clubs having outings just off the road on what used to be emergency landing strips. A vehicle would two a glider along the strip until it was air-borne or, better still, a light aircraft would tow the glider to a required height before the glider cast off the tow-line. Another time I visited the Yacht Club at Fanny Bay and at the far end of the beach there was a fibreglass yacht, half filled with sand and a hole in its side, with ‘Dove’ on its transome. It was about 25 foot long and was one of three used in making the film ‘The Dove’ about Robert Lee Graham’s circumnavigation of the world, the youngest solo sailor to do it. At least he passed through the cyclone area without incident.