Statics: Smaller Internal Forces

A floor structure experiencing excessive vibration

Fig. 9-15: A floor structure experiencing excessive vibration then stiffened using profiled post-tensioned cables (Courtesy of Prof. Jida Zhao, China Academy of Building Research, China)

A floor in a factory on which machines were operated on a daily basis experienced severe vibrations causing significant discomfort for workers. It was found that resonance occurred when the machines operated. The solution to the problem was to avoid the resonance by increasing the stiffness of the floor or its natural frequency.

It was not feasible to stiffen the floor by positioning additional column supports. However, the resonance problem was solved in a simple and economical manner by the Institute of Building Structures, China Academy of Building Research, Beijing, through stiffening the floor using external post-tensioned tendons as shown in Fig. 9-15.

Due to the profile of the tendons and the post-tension forces applied, additional upward forces or elastic supports are provided at the points where the steel bars react against the concrete beams which support the floor. This reduces the internal forces (bending moments) in the beams, making the floor system stiffer with increased natural frequencies. With the natural frequencies avoiding the operating frequency of the machine, resonance did not occur and the response of the floor was significantly reduced. The static behaviour of the structure is demonstrated in Section 9.3.2.