Lessons from gale for farm buildings

Press, Volume CXV, Issue 33928, 22 August 1975, Page 7

Lessons from gale for farm buildings

About 80 per cent of the damage caused to farm buildings by the recent north-westerly gale was the result of weak connections, according to an estimate of Mr G. C. Pinnell, an assistant engineer of the Ministry of Agriculture and Fisheries at Lincoln.

These weak connections Included insufficient leadhead nails, purlins, rafters, or trusses not securely tied down, concrete footings too small, or poles in the case of pole-frame buildings being too shallow in the ground.

Failures in braces also caused buildings to sway sideways.

Mr Pinnell says that it is important to remember that a building fails at its weakest part. If these details were adequate some other part might have failed, but for a few dollars in expense and a little extra time in making connections. many of the failures could have been prevented.

Although the storm affected a large region, only pockets within the region were struck with extreme winds, Mr Pinnell noted.

A very high wind speed was, for instance, recorded at Cust in North Canterbury, where it gusted to 116 miles an hour. This would have produced loads twice those currently used in design. Meteorological Service records, he said, showed there were higher maximum wind speeds in coastal parts of Canterbury than in inland areas. The worst of the damage caused by this storm was near the coast. in a storm, the roof of a building acted like an aeroplane wing, and like an aeroplane it wanted to take off. This uplift was caused largely by suction on the upper surface of the corrugated iron. Where buildings were open into the wind the wind also exerted pressure on the iron from below. The net effect caused many a disaster. Pressure and suction forces also acted at right angles

to the wall-cladding, causing side-sway in a building. Referring to the siting of buildings, Mr Pinnell said that in Canterbury the most severe winds blew from the north-west and sometimes from the southerly quarter. Seldom were

there severe storms from other directions. However, building openings required

a sunny aspect, so it was suggested that openings be sited to face from east to north to minimise the possibility of wind attack. Where buildings were adjacent to shelter belts there was a danger of damage from falling trees, and where shelter belts were located behind buildings they might cause local

accelerations of wind and result in increased loadings on the buildings. Mr Pinnell said that many frames had failed in glasshouses during the

gale. If these had been stronger, not only could the frames have been saved, but also the majority of the glass, and hopefully also, the crop, by reglazing before the first frost or other adverse climatic effect. One portable grain silo was reported to have blown a mile and a half in the storm. Mr Pinnell said that the best advice that could be given about these seemed to be to tow them into a sheltered area when they were emptied. On the merits of timber compared with steel structures, Mr Pinnell said that both types could be made sufficiently strong to resist design wind speeds, but when fastening timber a more conscious effort was required to resist uplift loads produced by the wind.

As a result of the storm Mr Pinnell has a few recommendations to make

when farmers are rebuilding. He says that lead head nails should be spaced every second corrugation and galvanised and spiral shanked nails are better products. Special care, he emphasises, needs to be taken along the edges of roofs as the wind attack is most severe in these places.

The maximum purlin spacing for 0.45 mm (26gauge) corrugated iron is 0.9 m (3ft). A rough but useful rule for purlins over 3m span is — the minimum depth of purlin is 40mm per metre of span. Thus, use a 150 mm by 50mm (6in by 2in), purlin for a 3.7 m (12ft) span, or a 200 mm by 50mm (Bin by 2in) purlin or a 4.5 m (15ft) span. The purlin depths selected are the next sizes up from the depths of 148 mm and 180 mm, as calculated from the rule. For altitudes above 300 m the purlins should, in most cases, be larger than those above to support the heavy snow loads. Discussing tie-downs for purlins, Mr Pinnell said that there were a number of methods. These included the two Bmm (5/16in) diameter bolts at each end; or 4mm (No 8) galvanised wire wrapped over the purlin and securely stapled with 50mm barbed staples; or proprietory fasteners such as Z-nails (two at each end) or Trip-L-Grips (one at each end). Turning to' truss tiedowns, he said uplift loads were very high because of the large roof area but it was impossible to make any general recommendation because of the big range of trusses used. Bolts or proprietory timber fasteners, such as nail plates, circular-tooth plate connectors, or framing anchors, were the most, reliable. Each joint needed to be designed. Concrete footings needed to be quite big to hold the building down. For example, for an open-front building with 4.5 m wide by 6m deep bays (15ft by 20ft) each column required a block of concrete 600 mm by 600 mm by 1000 mm (2ft by 2ft by 3ft). The amount of concrete could be reduced if a reinforced spread footing buried in the ground was used.