Semi-Active Control of Post-tensioned Steel Frames

After the Northridge 1994 earthquake it was found that failures in steel frame buildings were mainly concentrated in beam-column connections. This prompted researchers to work on improvements, mainly focussed on increasing the rotational ductility capacity in connections. Most of these improvements however did not eliminate the residual deformations in the connections. To overcome this weakness, researchers introduced post-tensioned steel connections, composed of post-tensioned steel strands and energy dissipating devices. In this research, a single-element model of post-tensioned connection was developed and incorporated in a new computer program for non-linear dynamic frame analysis, which was then used to investigate the effects of the level of post-tensioning forces on seismic behaviour of frame buildings. When used in moment resisting frames, post-tensioned connections reduce residual displacements and prevent development of plastic hinges in the beams. The initial stiffness of post-tensioned frames is also similar to conventional moment resisting frames but their energy dissipation capacity is lower. The performance of the structure is sensitive to the level of post-tensioning forces, and in some cases the use of post-tensioned connections leads to increased displacements. The aim of this research was to investigate semi-active control of the post-tensioning forces as an approach for improving the seismic behaviour of multi-storey steel frame buildings. Three control approaches were proposed to improve the dynamic behaviour of post-tensioned frames: (i) energy dissipation approach which aims to increase the energy dissipation capacity of the frame, (ii) stiffness control approach which aims to change the frequency of the frame by softening or stiffening to avoid excitation by major frequency components of the earthquake and (iii) deformation regulation approach which aims to improve the distribution of deformations along the height of the frame. The three control approaches showed different results. Increasing energy dissipation in the connections is not an efficient approach for reducing the frame response, especially when large displacements occur in the early stages of loading. The stiffness control approach showed good performance, reducing both floor displacements and force demand on the elements. The deformation regulation approach also improved the response, providing more uniform inter-storey drift distribution. In general, the research presented here shows that semi-active control can be used to improve the seismic performance of post-tensioned steel frames.