The effect of composite action on the behaviour of space structures

 A.I. El-Sheikh

During the past few decades, the space (3-D) structures consisting of double-layer grids have been widely accepted as large span structures. In spite of the advantages of space structures, sudden and progressive collapses have been experienced, and it has been found that the major problem facing these structures is the brittle type of failure caused by buckling of critical compression chord members.

Besides the well-known double-layer trusses (with nearly frictionless pinned member-joint connections), the study includes a consideration of the relatively new Cubic Space Frame system (CSF) (with rigid member-joint connections).

The present study investigates the possibility of using a concrete slab as an upper chord to increase the strength and improve the behaviour of space structures. By reducing the buckling problems of the top chords, the overall behaviour can be made to depend on the ductile characteristics of the tension members.

A series of full-scale tests has been carried out on three double-layer square-on-square space trusses including two non-composite and one composite truss. It was found that failure of the non-composite trusses was catastrophic, and their ultimate strengths were less than predicted. Overstrengthening the compression members of the non-composite trusses, beside being uneconomic, could not prevent the catastrophic collapse. On the other hand, failure of the composite truss was gradual with adequate warning against collapse as the behaviour was dependent on the tension properties of the bottom members.

A series of tests was also carried out on CSF nodes and splices in order to develop a numerical model for the prediction of their strength and flexural stiffness.

In the analytical part of the study, the finite element method was considered as a possible approach for the analysis of space structures. A new nonlinear program based on the F.E.M. has been developed, verified and used. The program was designed to analyse combinations of concrete slabs and steel beams. A nonlinear beam element with central and/or end springs was developed to model truss and frame members. This element is general and capable of modelling any combination of central or end fixity conditions. Geometric nonlinearities have been considered as well as material nonlinearities. Also, the program includes a nonlinear rectangular plate element to model the concrete slab. This element takes into account the effect of concrete cracking and crushing and steel yielding on the element stiffness.

This program has been verified against the present experimental work and against the experimental and numerical results of other researchers, and is therefore considered to be a reliable means to predict the ultimate strength and behaviour of composite and non-composite space structures.

From the work carried out, it was seen that composite action was effective in increasing the strength and improving the reliability of space structures. The concrete slab successfully prevented buckling of the compression members, and therefore resulted in an overall ductile behaviour.