PROGRESSIVE COLLAPSE triggered by a timber box beam failure is thought to have caused the cave-in of a German ice rink killing 15 in early January.
Design drawings obtained by NCE suggest that failure of a single box beam could have brought down the whole structure because the roof lacked alternative load paths.
The Bad Reichenhall ice rink roof collapsed at 4pm on 2 January under the weight of 300mm of snow.
Leading UK engineered timber fabricator Gordon Cowley told NCE that timber box beams of the type that appear to have failed at Bad Reichenhall are prone to 'compression failures masquerading as tensile failures.
'What happens is that the top glulam chord is hit by dampness and starts to compress. This massively increases tensile stresses in the bottom glulam chord, which eventually fails close to midspan.
'If there isn't enough load sharing potential in the design then rapid progressive collapse is likely even if only one beam fails.' Leading German engineered timber expert Professor Heinz Bruninghoff said that timber roof failures were rarely catastrophic and almost never involved loss of life (see box).
'The most common reasons for collapse are severe design failures or long term durability problems caused by excessive moisture.
'Damp can cause timber decay or breakdown of old style urea formaldehyde adhesives, which are not recommended for prolonged low temperature use.' The 75m by 48m Bad Reichenhall roof was built in the early 1970s using the then locally popular Kastentrager structural system. The main span was 40m, with two end cantilevers of 4m each, supported off concrete columns.
Main beams were ultra slim timber box girders at 7.5m centres, 2,870mm deep but approximately only 300mm wide.
Top and bottom chords were 200mm deep glulam sections; webs were 65mm thick 'two-ply' laminations of spruce boards glued together with their grains running at an angle of 15º to each other. Urea formaldehyde adhesives were used.
A combination of purlins and K-shaped glulam cross beams provided lateral restraint against buckling (see diagram).
Four cross beams - which may have also supported a false ceiling - were spaced at 8.6m centres along the gaps between the beams. Between these were 120mm by 460mm purlins. Double diagonal tie rods were provided in some bays to withstand wind loading.