In many structures stresses are easily relieved, but with large members with long thick dissimilar welded plates or with numerous stiff and redundant members, stresses do not disappear until substantial relative movement has taken place. Even if it only takes 1mm of stretch to reduce tensile stresses to 0, this 1mm can be a crack through lots of the member, even the whole way through it; or a complete failure of flange plate to flange weld over a long length.