Snow drift implications due to building extension

Part of a roof collapsed in a factory and warehouse facility shortly after a storm event. The affected building contains factory and warehouse facilities. 

To the side of this building, a modern industrial unit was built some 20 years later. This unit’s footprint is some 2,000m² and also rectangular. The side wall of this unit is approximately 2.5m above the level of the affected factory/warehouse roof, which created the conditions needed for snow drift to form on the roof of the affected building.

The storm resulted in a severe weather warning for the area with snow /ice and wind gusts up to 50 knots which is comparable to a Force 10 storm. Relatively large amounts of snow were deposited over several days resulting in drifts of up to 3m in depth.

These were on the side of the building that abutted the extension (where the collapse occurred) and on the opposite side concentrated near the centre of the building (where a partial collapse of the roof occurred). The immediate cause of this incident was the snow loading.

The projecting side wall from the extension allowed a significant depth of snow drift to form for the first time since the warehouse’s construction which increased the loading on the affected trusses by at least a factor of two. 

The tendency for snow drift to form on a roof in front of a projecting wall is referenced in the following guidance documents:

1. Wind microclimate around buildings (DG 520), Building Research Establishment, 2011
2. Roof loads due to local drifting of snow, Building Research Establishment, 1999
3. The Institution of Structural Engineers (IStructE) Technical Guidance Note (Level 1, No.5) provides helpful guidance on the derivation of snow and snow drift loads

The collapse affected three bays of roof structure, amounting to a length of around 60m. The forces involved in the collapse were sufficient to drag the adjoining parts of the building, including the side wall, downwards and inwards towards the collapse zone. This completely buckled of the structural steel perimeter column located in the centre of the collapse zone.

The affected roof is formed from a series of ‘bow string trusses’, spaced 2.5m apart and spanning a relatively large distance of 15m. Several of the trusses were severely damaged and there was secondary damage to other parts of the building.

The design of the roof trusses incorporated tie rods placed in an ‘interrupted’ alignment. This also increased the loading on the affected trusses by a factor of two. Furthermore, there were variations in the position of the tie rods at the end of the struts which increased the loading on the affected trusses. Variations in quality of workmanship used to form the ends of the struts and the holes that receive the tie rods also increased the loading on the affected trusses.