Stiffness of restraint for steel struts with elastic end supports

The force induced in a restraint for steel struts is given by BS 5950, Part 1, 1990 as 1% of the applied axial load, and for struts forming the compression flange of lattice girders as 2.5%. These values are used in the ultimate strength design of restraints. The code also states that these restraints should have sufficient stiffness but it does not quantify this requirement. The available literature deals with the determination of restraint forces and their required stiffness for struts with pinned or fixed ends. In practice, it is rare to have such ideal end conditions, but somewhere in between. This is due to the fact that connections in structural steelwork possess resistance to moment ranging from nominal to full rigidity depending on the momentrotation characteristics of the joint under consideration. This paper deals with the design of restraints considering strength as well as stiffness through a mathematical model based on the elastic behaviour of a strut elastically supported at its ends and restrained at a point located along its length by an elastic restraint. The results obtained show that, for a given induced restraint force, the restraint stiffness requirement decreases with the increase of rotational stiffness at the ends of the strut. Although this, to some extent, is expected from a qualitative point of view, it does deserve to be better known by designers, and the present work attempts to quantify the effect of the various parameters involved so that the results can be used in the relevant design cases.