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PROCEEDINGS OF THE ELEVENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Computational Analysis of Precast Concrete Frames with Post-Tensioned Tendons
O. Oddbjornsson1, N.A. Alexander1, C.A. Taylor1 and R. Sigbjornsson2
1Department of Civil Engineering, University of Bristol, United Kingdom
O. Oddbjornsson, N.A. Alexander, C.A. Taylor, R. Sigbjornsson, "Computational Analysis of Precast Concrete Frames with Post-Tensioned Tendons", in B.H.V. Topping, (Editor), "Proceedings of the Eleventh International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 167, 2007. doi:10.4203/ccp.86.167
Keywords: nonlinear elastic, self centring, post tensioned, precast concrete, distinct element code, jointed system.
The design philosophy used in earthquake resistant engineering today has succeeded in reducing the number of casualties during earthquakes. However the way safety is achieved results in large economical losses during large seismic events. This drawback to current design philosophy has led to a growing interest in damage resistant buildings. This class of structure exhibits nonlinear elastic behaviour, thus limited damage or residual displacement is induced by the seismic event. Typical examples of these nonlinear elastic structures are precast concrete frames with post-tensioned tendons connecting structural elements . This type of connection between structural elements allows for large deformations without serious damage to the structural elements.
The joint mechanics of both isolated joints and joints within frame assembly are explored under quasi-static and dynamic excitations. Joint/frame assembly stiffness-rotation/lateral deformation relationship is determined by conducting a quasi-static push over analysis. Tendon tension, joint sliding, contact area and element flexure are monitored during the quasi-static push over analysis to increase understanding of the mechanics during joint opening. The nonlinear dynamics of the frame assembly are investigated by sinusoidal base excitation at variable frequency and amplitude. The system features during the dynamic loading are monitored and analysed by utilizing conventional mathematical tools from the nonlinear dynamics field are used to identify and classify the dynamics of the system.
The analysis of the joint mechanics reveals that the joints exhibit softening nonlinear stiffness characteristics, having the largest stiffness when joint rotations are small then softening as joint rotations increase. During the quasi-static analysis no joint sliding is observed as the design assumptions assume but during dynamic analysis joint sliding is encountered. Joint sliding during dynamic loading has some influence on the over all structural response, but everything suggest that this influence is secondary compared with the overall structural behaviour. Although the influence of the joint sliding is subtle it introduces phenomenon that are very interesting from nonlinear dynamics point of view. Phenomenon's like quasi-periodicity where the structure reaches steady state vibrating at its forcing frequency plus one or more incommensurate fundamental frequencies. The joint sliding also keeps the structure from reaching steady state solution when the structure is forced at a frequency close to the jump from high amplitude response to low amplitude response introducing another extended period to the structural response.
For the test frame researched in this paper everything suggest that the design assumptions made when designing the frame to with stand seismic excitation hold. This does still not imply that these assumptions hold in general for all structures of this class. Since the system is nonlinear it is necessary to determine whether the design assumptions hold for larger structures as well, this will be attempted in future publications.
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