Effect of composite action on the dynamic behaviour of space structures
The application of composite action ushered a new era in the use of double-layer spaceframes as efficient floor systems in addition to their competitiveness as roof coveringstructural systems. Earlier research on space frames demonstrated large improvementsin their static behaviour caused by the introduction of composite action. Theseimprovements included an increase in ductility to avoid progressive collapse, a largeincrease in load-carrying capacity and a considerable reduction in materialconsumption.In this work, the effect of introducing composite action in changing the dynamiccharacteristics of space frames, in particular the natural frequencies and damping ratioswas presented. The study was expanded to determine the effect of composite action inchanging the response to dynamic excitations. The measured responses included thelateral displacements and changes in the internal member force distribution undershaking table vibrations.Three aluminium space frame models of the square on square (SOS) configuration weremanufactured. The first model was non-composite, while composite action was appliedto the other two models with a top aluminium deck and a timber deck, respectively.Two common cases of support conditions were used in connecting the models to theloading frame, which was the platform of the shaking table.Initial displacement method (snap test) was used to determine the frequency of vibrationand the damping ratio of test models in the vertical and horizontal directions usinglogarithmic decrement method. All models were then exposed to shaking tablevibrations to determine the changes in dynamic responses between different models.These tests were repeated for the three models after the successive removal of panelsfrom one direction to identify the changes to their characteristics and behaviour withdifferent aspect ratios.The second part of the study was carried out numerically by using the finite elementpackage ABAQUS. It started by selecting a valid finite element model from nineproposed models using experimental test results on physical structures. A parametricstudy was conducted using the validated finite element model to expand the study toinclude two common space frame configurations; the square on large square (SOLS)and square on diagonal (SOD), and two other cases of support configurations, namely,fully edge-supported and supports at corners and middle edges of models.Based on the work done in this study, it can be concluded that composite actionchanged the dynamic characteristics of space frames, which was clear in the increase oftheir vibration frequencies in all directions as a result of the increase in stiffness.Furthermore, the increase in stiffness resulted in a general reduction in the dampingratio of space frames covered with aluminium deck, while the high friction with topjoints and the nature of timber as a good energy absorbent material resulted in a variableeffect on the damping ratio associated with the increase in aspect ratio.The effect of composite action was clear in reducing the lateral displacement ofcomposite models by more than 50% compared to the non-composite case. Moreover,composite action resulted in changing the distribution of internal forces in diagonal andlower chord members such that forces became more concentrated at corners and edgesparallel to the direction of vibrations in both cases of corner and edge-supportedmodels.
Space truss, Composite action, Double-layer grid, Dynamic analysis
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