Q. Butt weld – from one side vs from both sides?
In the design of butt weld strength, it's normally presumed that its strength is at least equivalent to parent metal. To improve proper welding operation, gap between two metals to be welded must not be too small, or else the root will be inadequately fused at the time of welding and the butt weld strength will be decreased. Alternatively the gap must be not set too large since the weld metal would simply pass through it. Function of the gap amid adjoining root faces is to increase the depth of penetration down to the root of weld.
Though it's not always possible to have access to both sides of the butt weld. Therefore the use of backing plates or rings can improve the quality of welding from one side only. By inserting a backing plate inside the steel member, correct alignment could be maintained and specific amount of tolerance on longitudinal fit can be permitted.