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In the design of butt weld strength, it is generally assumed that its strength is at least equivalent to the parent metal. To enhance proper welding operation, the gap between two metals to be welded should not be too small, otherwise the root would be inadequately fused during welding and the butt weld strength would be reduced. On the other hand, the gap should be not set too large because the weld metal would simply pass through it. The function of the gap between adjoining root faces is to increase the depth of penetration down to the root of the weld.
However, it is not always possible to have access to both sides of the butt weld. Hence, the use of backing plates or rings can enhance the quality of welding from one side only. By inserting a backing plate inside the steel member, the correct alignment could be maintained and certain amount of tolerance on longitudinal fit can be permitted