CREEP AND SHRINKAGE EFFECTS ON SEISMIC RESPONSE OF PRESTRESSED CONCRETE STRUCTURES

Abstract

Prestressed concrete sections are economical compared to reinforced concrete and hence, have a wide use in bridges, nuclear containment structures, buildings and other structures. Reliability of these structures mainly depends on the performance of the prestressing force over a design period of these structures. During a major earthquake, these prestressed concrete structures will be subjected to severe ground motions. The actual behaviour of prestressed concrete structures subjected to dynamic loading is not as yet well established. Their behaviour is quite involved, being influenced by several interacting factors viz creep and shrinkage, and rather distant from the elastic-plastic model, which is commonly taken as reference for reinforced concrete or steel structures. In general, time-dependent deformations of concrete may severely affect the serviceability, durability and stability of the structure. The creep deformations may be several times of the initial deformation and may exceed the serviceability limit state. Creep and shrinkage causes shortening of the concrete of prestressed concrete structures and thereby reduces the stress in the prestressing steel and thus, results in a decrease in prestressing force. Consequently, the stresses in the concrete, due to prestressing force, also undergo changes. Seismic studies of prestressed concrete structures considering creep and shrinkage effect are non existent in the literature. So in the present study, efforts have been made to study the seismic behaviour of prestressed concrete structures considering creep and shrinkage effects. Prediction of the response of prestressed concrete structures requires the three-dimensional structural idealization and true modelling of nonlinear behaviour of concrete, reinforcing steel and prestressing tendons. Concrete is highly nonlinear material and its nonlinear response is mainly due to progressive cracking and nonlinear deformations. Behaviour of concrete is found to be different under uniaxial and multiaxial loading conditions. Triaxial tests performed on concrete shows that concrete is pressure sensitive material and under high hydrostatic compression, concrete flows like metals in a limited manner. The dynamic behavior of concrete is significantly different from static behavior. The compressive, tensile and flexural strengths of concrete depend upon the rate of loading. An approach based on elasto-viscoplasticity theory is adopted here to take into account the loading rate effects. A generalized three-dimensional failure criterion for concrete is implemented into the finite element codes. The proposed model takes into account the pressure dependent behavior of concrete in three-dimensional loading situations. The strain-rate dependent behaviour of materials is taken into account. The precise modelling of reinforcements and prestressing tendons in prestressed concrete structures is important for determining its response under static as well as dynamic loading conditions. Depending upon the prestressing force and the profile of the prestressing tendon, a line load is exerted on the surrounded concrete which is generally in the opposite directions of external applied forces. Adiscrete formulation of the prestressing tendon, lying inside the 3-dimensional solid element, is used to model the tendon. This formulation accurately predicts the line load exerted on the surrounding concrete. Stiffness of the prestressing tendons isadded in the stiffness ofthe concrete sections to get the overall stiffness. Various researchers have studied the creep and shrinkage effects on concrete and several analytical models have been developed to predict them. In this study, the results predicted by these analytical models are compared with the experimental results. Based on this study, an analytical model is selected for prediction of creep and shrinkage strain in concrete. The modelling ofsteel reinforcements and prestressing tendons plays an important role in determining the realistic behaviour. The stiffness of reinforcing steel and prestressing tendons is included in the evaluation of the overall stiffness of the structure. Reinforcing steel and prestressing tendons also exhibit nonlinear behaviour upon loading. Nonlinear constitutive relationships for steel reinforcements and prestressing tendons are also modelled in the finite element code. All these effects have been considered to determine the response of the prestressed concrete structures subjected to static as well as dynamic loads. Based on the above, a procedure and computer software has been developed for the three-dimensional nonlinear finite element analysis ofprestressed concrete structures understatic as well as dynamic loading conditions. Validation of the software has been done by analyzing two experimental and one analytical study of other researchers. The results of these three validation studies, obtained by the software, have been compared with the results of other researchers and found to be in a good agreement. Using the developed algorithm and software, New ITO bridge - a prestressed concrete single cell box girder bridge constructed at ITO, New Delhi has been analysed. This bridge was constructed about 9years ago and during the construction, instruments were embedded in it to monitor its long-term performance. Results ofthe analytical study are compared with the observed results and found to be in close agreement. The bridge has also been analysed for several cases of earthquake loading, applied at various ages of concrete ranging up to 20 years after the construction of the bridge, to find the change in the bridge behaviour due to effects of creep and shrinkage.