Accommodating building deflection

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Wall ties should be incorporated within 300mm above and below the support.It is important to select the correct support system to ensure that building tolerances can be accommodated. Ancon brackets have a slot at the back to provide vertical adjustment. Longitudinal adjustment is provided by an Ancon cast-in channel in concrete structures, or horizontally slotted holes in steel framed structures.The maximum size of a masonry panel should be restricted to limit the effects of differential movement.This is particularly important if clay brickwork is used with concrete blockwork and a concrete frame.We therefore build a telescopic head arrangement which allows the deflection to take place without putting any load on the wall below.The problem is the deflection head is a relatively complicated, and hence expensive detail to construct.This means that concrete always cracks; the number, size and location of the cracks is controlled by the amount of steel reinforcement embedded in the concrete and by the number and location of expansion joints and control joints. This process continues for a period of time that is estimated to be from 5 to 10 years following construction.

It will also cause balconies to deflect towards the outside edge.The question often arises as to whether it is really required at all; I know of numerous cases of clients asking the drywall contractor whether they need to be used, and if so how big a deflection needs to be accommodated.The answer to whether or not they are needed lies with the structural engineer.In the absence of more specific criteria, criteria for structures with brittle finishes (as found in code documents for years) is frequently used.This simplistic criteria puts a limit of the span divided by 360 on the incremental deflection due to live (or transient) load only and a limit of the span divided by 240 on deflection under total load.

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