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CROSS Safety Report

Wind loads during construction

Report ID: 133 Published: 1 January 2009 Region: CROSS-UK

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Overview

A reporter raises concerns about wind load reduction factors in BS6399-2.

Key Learning Outcomes

For civil and structural design engineers:

  • It is important to assess the risk from winds on structures when wind is the dominant load, particularly on partially completed structures or a relatively light structure with a large surface area such as a sheeted scaffold

  • If you take a reduction in wind speed bear in mind that if your structure is sensitive to wind loads, a minor error in wind speed could lead to failure

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Another report is also about wind loading. During construction the structure and associated temporary works are subjected to wind loading.  This load case is typically short term and BS 6399 Part 2 allows for a reduction in wind speed where the load is of a temporary nature.  What is not made clear in the code, but is in the supporting documentation, is that this reduction is based on the expectation that in times of high wind load the structure will not be in use, and therefore the consequence of failure is not severe.

When considering the stability of a partially erected structure or sheeted scaffold this expectation is, in the view of the reporter, not valid as the consequences of failure from wind could potentially be severe. Therefore, unless it can be justified and agreed, the reporter’s firm does not permit a reduction in wind load due to the load case being short term. Their designers do however take advantage of seasonal reductions where appropriate.

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This report is about structures that are incomplete rather than about structures that are temporary. Temporary structures are only in place in a specific location for a relatively short period and advice on wind loads, as well as other aspects, is given in the 2007 publication from the Advisory Group on Temporary Structures (AGOTS).

It is important to assess the risk from winds on structures when wind is the dominant load on, say, a partially completed structure or a relatively light structure with a large surface area such as a sheeted scaffold. Small increases in wind speed, particularly when considering low wind speed, can result in much higher loadings since the force is proportional to the speed squared.

In the partially completed state there may not be the stability, dead load, or robustness that will exist on completion. Designers taking a reduction in wind speed should bear in mind that if their structure is sensitive to wind loads, a minor error in wind speed could lead to failure. The overall lesson is that designers should be wary in reducing wind loads and they need to weigh up the overall uncertainties, the effects of a minor change in wind speed assumptions, and the consequences should there be a sudden gust, or a storm arise.

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