Dynamic behaviour of long span cantilever steel-concrete composite floor

Abstract

Vibration and deflection are two main parameters that always govern the constructability of long span cantilever slab. This paper present the dynamic behavior of a 12.5m long span cantilever steel-concrete composite floor of an actual new proposed construction project. STAAD PRO software was used to analyze the structure subjected to both static and the dynamic loading. From the preliminary analysis using static loading, it was found that the original proposed structural configuration does not pass the deflection limit and is not constructable due to requirement for too big steel section not readily available in market. Consequently, modification to shorten the cantilever length to 6m is introduced and finally makes the structure possible to be build using a ready size of steel beams that are available in Malaysian market. In the detail dynamic analysis, excitation of dynamic loadings similar to human activity at a few random locations is applied to produced various mode shape. Results from the dynamic analysis gives acceleration on adjacent panels. The acceleration vs time graph is then used to calculate the critical natural frequency of the adjacent panels. This value of natural frequency then used to determine the range of recommended peak acceleration using the graph introduced by AISC Design Guide No. 11. It is found that the natural frequencies of the adjacent floor are in the range of 4 – 7 Hz, which is considered a low frequency floors. With the combination of low acceleration and low natural frequencies, it makes the modified floor which the new length is 6m still not comfortable to be used. Therefore, recommendation to thicken the concrete slab is proposed to increase the natural frequency of the floor, so that a comfortable construction is obtained.