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CROSS Safety Report

Use of proprietary element beyond stated values

Report ID: 667 Published: 1 April 2018 Region: CROSS-UK

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Overview

After the construction of a 6m high lighting frame, the contractor noticed that the frame was both twisting and deflecting excessively under wind loading.

Key Learning Outcomes

For civil and structural design engineers:

  • Careful consideration needs to be given to dynamic effects of wind loads acting on framed structures as the torsional capacity of the frame may become critical

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On an industrial site, the on-site electrical engineers required the construction of a 6m high lighting frame. For unknown reasons, the engineers specified the use of proprietary elements for the design and asked the civil engineers working in the same organisation to check a sketch done by them. The design was done statically (axial, bending moment, deflection and shear), and met the requirements of the Eurocodes.

However, there were issues with the design as the length of the members was well outside the published load tables for the elements. Accordingly, the load bearing capacities of the frame had to be calculated using the published properties and codes of practice. After the construction, the contractor noticed that the frame was both twisting and deflecting excessively under wind loading. It was discovered that the torsional capacity of the frame was critical due to the dynamic effect of the wind.

The original civil design team quickly proposed a solution of lowering the height of the lighting frame and installing some raking members from supports to arrest the movement. After a discussion, the civil team concluded that they should have questioned the design more thoroughly and gone with an off the shelf product (e.g. a lamp post or a universal beam or column).

After a discussion, the civil team concluded that they should have questioned the design more thoroughly

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