Effects of roofing members on free vibration characteristics of cylindrical lattice shell roofs

Abstract

Spatial structures are used as evacuation facilities and aid stations when a large earthquake occurs. In order to ensure sufficient seismic performances, it is important to grasp free vibration characteristics. Therefore, there are studies on eigenmode shapes, natural periods and damping factors. The damping factors in particular have been investigated in actual behavior since it is difficult to evaluate them theoretically. In addition, there are studies on free vibration characteristics of existing structures and effects of roofing members on damping factors of spatial structures as buildings have not only structural elements but also nonstructural elements. However, the effects of differences of rigidity of roofing members and coefficients of friction between roofing and lattice members on free vibration characteristics have not been sufficiently investigated. In this paper, the effects of differences of ratios of flexural rigidity of roofing members to that of lattice members and coefficients of friction between roofing and lattice members on free vibration characteristics and seismic response behavior of cylindrical lattice shell roofs under horizontal motions are investigated by shaking table tests. The roof structure for experimental model is the cylindrical lattice shell roof with span for gable direction Lx is 750mm. The half open angle 9 is 30deg.. The shell roof is made of cold rolled steel plates (SPCC) 0.8mm thick. The materials of roofing members are ETFE film, aluminum plate with 0.1 and 0.2 mm thick. The responses of structure are measured with the accelerometers, the motion capture system (VENUS3D-1.7MW made by Nobby Tech) and the strain gauges. From the experimental results, it is concluded as follows. 1) Regardless of existence and kinds of roofing members, the eigenmodes with same shapes appear and their natural periods become shorter with an increase in the flexural rigidity ratio rD of roofing members to lattice members. 2) As roofing members are integrally deformed with lattice members on vibration modes with long wavelength, the added flexural rigidity by roofing members is larger. On the other hand, as the lattice members are possible to be deformed by roofing members running in clearance between holes of roofing members and bolts during vibration on those with short wavelength, the added flexural rigidity by roofing members is smaller. 3) Vertical response acceleration magnification factors in the higher order modes decrease by installing roofing members. On the other hand, there are eigenmodes whose amplification factors increase by installing roofing members in the lower order modes. As a result, amplification factors in the lower order modes for structure with roofing members become larger. 4) The effect of flexural rigidity ratio rD on the damping factors is small. However, damping factors increase by installing roofing members which have larger internal loss. On the other hand, focusing on the eigenmodes with smaller rotation of hinges on boundary, the damping factors increase by installing roofing members. 5) The effects of coefficients of friction between roofing members and lattice members on response amplification factors and damping factors are small. On the other hand, the effects of those on natural periods are small for the modes with short wavelength. These effects are large for the modes with long wavelength and consequently the natural periods become shorter with a decrease in the coefficient of friction. 6) In seismic response behavior, the responses of models with roofing members are smaller than those of Rs model without roofing member since amplification factors of higher order modes are smaller. On the other hand, distribution behavior of response displacements of each model are different as amplification factors in each mode differ for each model.