|Session 9.1 - Precast Concrete Structures|
For the last four years an international committee of Commission 7 : Seismic Design of the International Federation of Structural Concrete (fib) has been preparing a state-of-the-art report on the seismic design of precast concrete buildings. The report includes aspects of both precast reinforced and prestressed concrete. The committee has had contributions from 32 members, including six members from New Zealand (R. Park, D. Bull, L. McSaveney, A.O’Leary, N. Priestley and J. Restrepo). The other contributions have been from members in Canada, Chile, Indonesia, Italy, Japan, Mexico and the United States. The co-convenors of the committee are R. Park (New Zealand) and F. Watanabe (Japan). The report is due for completion in early 2003.
The main sections of the report are: state of the practice in various countries; advantages and disadvantages of incorporating precast reinforced and prestressed concrete in construction; lessons learned from previous earthquakes; precast construction concepts; design approaches; primary lateral load resisting systems (moment resisting frames and structural walls including dual systems); diaphragms of precast concrete floor units; modelling and analytical methods; gravity load resisting systems; foundations and miscellaneous.
The object of the report is to present existing practice, to recommend good practice, and to discuss current developments.
The paper presents a summary of the contents of the report.
R.B. Fleischman and K.T. Farrow
Floor systems are intended to provide diaphragm action between elements of the lateral-load resisting system during seismic response. Recently, significant design issues have been raised related to precast concrete diaphragms in long floor span structures. These issues include the possibility of larger than expected forces, joints in critical locations that may not possess sufficient strength or ductility, and the potential for large drift demand on the gravity system due to excessive diaphragm flexibility.
An analytical research program has examined the behavior of floor diaphragms in long floor span precast concrete structures, focusing on parking structures. A two stage approach was taken in the research: Nonlinear static (pushover) analyses of topped and pretopped floor diaphragms were performed to determine the effect of structural dimension and construction details on the service level stiffness and ultimate capacity of the diaphragms; and, diaphragm demands were established for structures with diaphragms of varying strength and flexibility through nonlinear transient dynamic analysis.
Design recommendations are proposed based on a comparison of the diaphragm capacities to the diaphragm demands. These recommendations include provisions for the appropriate design strength, allowable flexibility, and ductility requirements for precast concrete floor diaphragms in long span construction, including parking structures.
Keywords: earthquake design, precast concrete, flexible diaphragms, parking structures
L. Tuleasca, A. Cuciureanu and J.M. Ingham
An experimental investigation to evaluate the response of a monolithic precast joint, with cast-in-place steel fibre reinforced concrete (SFRC), when subjected to earthquake-type lateral loading is described in this paper. The use of SFRC in monolithic precast joints subjected to high magnitudes of static and dynamic actions is desirable due to its isotropic nature. Design variables that control the response of a SFRC joint subjected to lateral loading were evaluated, with special attention given to the influence of these parameters on the cast-in-place joint between the double-tee beams and the column. Response of the test specimen was also examined in terms of beam plastic hinge spreading.
It was found that this joint, when using SFRC, possessed greater strength and deformation capacity than when constructed of ordinary concrete, despite incorporating a smaller quantity of conventional reinforcement. The joint developed only superficial damage at later stages of testing, corresponding to the development of full beam plastic hinging.
S. Sritharan and S. Vernu
Precast hybrid frames with dry jointed connections have been implemented in seismic design of buildings in the United States. The use of unbonded post-tensioning tendons and mild steel reinforcement debonded over a short distance at the connections between precast beams and columns provide several advantages for this framing concept. However, the strain-incompatibility that exists between concrete and steel at the connection makes the analysis and design of the hybrid frames more complex than the monolithic frame systems. Consequently, the available analysis and design techniques are based on several simplified assumptions. This paper provides an improved version of the equivalent monolithic concept to analyze the hybrid frame at the connection and system levels. By comparing analytical values with experimental data, it is shown that the improved analysis method provides satisfactory prediction of the moment-rotation response, neutral axis depth and elongation of the post-tensioning steel. By reversing the analysis method, an alternative design concept for hybrid frame connections is also presented.
Keywords: Precast, Hybrid frame, Unbonded, Post-tensioning, Seismic, Analysis, PRESSS