structure having reduced structural integrity in a defined region again applied to the channel shaped member having a base wall 21 opposing side walls 22 23 referring to fig4 it will be seen that the side walls have been interrupted for example by cutting out a portion of the side walls so that for a portion 30 of the extent of the side plate no side walls are present within this region the base wall is weakened as shown by deforming the base wall in a direction opposite to the extent of the side walls to provide a shape 31 in other words each side wall is interrupted for example by a slit in a portion corresponding to the region of weakness in the base wall it will be clear to one skilled in the art that other methods can be used for reducing the structural integrity of the side plate in a predetermined region thereof for example perforating the region with holes 52 fig1 fig5 and 6 show modifications of the embodiment of fig4 in which not only are portions of the side walls interrupted but also one or more slits 50 are cut in the base wall the slits not extending for the full extent of the base wall a remainder of the base wall.