24-07-2014, 04:10 PM
Push Over Analysis of Unstiffened Steel Plate Shear Wall
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Abstract
Steel Plate Shear Wall (SPSW) is made from thin steel plate which in turn are framed by the beams and columns of structural system. In recent year they are proved to be very effective and economical for resisting lateral loads such as earthquake and wind loads.The present work targets to study the behaviour of single frame SPSW with different plate thicknesses, rigid and non-rigid frame members, pinned and moment resisting joints. For this purpose, pushover analysis is performed using ANSYS. Push-over curves and distribution of stresses in the plate is demonstrated. A Comparative study for different stiffness of shear wall to resist the lateral loads is carried out. Ultimate load carrying capacity of the plate is determined for different model. It is found that the effect of stiffness of the beams and columns on the ultimate load carrying capacity of SPSW may be ignored until the members are strong enough to resist the force applied on them before tearing failure of plate. The ultimate load carrying capacity increases linearly with increasing the thickness of steel plate. For moment resisting frame with no shear plate, the initial stiffness is low, but it is increased by many folds when steel plate is also used to resist the lateral load.
INTRODUCTION
steel plate shear wall (SPSW) is a lateral load resisting system consisting of vertical steel plate infill connected to the surrounding beams and columns and installed in one or more bays along the full height of the structure to form a cantilever wall. SPSW subjected to cyclic inelastic deformations exhibit high initial stiffness, behave in a very ductile manner, and dissipate significant amounts of energy. These characteristics make them suitable to resist and dissipate seismic loading. SPSWs can be used not only for the design of new buildings but also for the retrofit of existing constructions. Beam-to-column connections in SPSWs may in principle be either of the simple type or moment-resistant. Prior to key research performed in the 1980‟s, the design limit state for SPSW was considered to be out-of-plane buckling of the infill panel. To prevent buckling, engineers designed steel walls with heavily stiffened infill plates that were not economically competitive with reinforced concrete shear walls. However, several experimental and analytical studies using both quasi-static and dynamic loading showed that the post-buckling strength of thin SPSW can be substantial. The web plates in steel plate shear walls are categorized according to their ability to resist buckling. The web plates can be sufficiently stiffened to preclude buckling and allow the full shear strength of the web to be reached. These are known as “stiffened” web plates. While stiffening increases the effectiveness of a web plate, it is typically not as economical as the use of the “unstiffened” web plate in which buckling of the web plate is expected. The vertical steel plate connected to the columns and beams is referred to as the web plate. The columns in SPSW are referred to as vertical boundary elements (VBE) and the beams are referred to as horizontal boundary elements (HBE). In typical designs the webs of steel plate shear walls are unstiffened and slender. The webs are therefore capable of resisting large tension forces, but little or no compression. As lateral loads are imposed on the system, shear stresses develop in the web until the principal compression stresses (oriented at a 45° angle to the shear stress) exceed the compression strength of the plate. At this point, the web plate buckles and forms diagonal fold lines.
MODELLING
Three sets of SPSW specified by RB, NRB, and M are modelled in ANSYS. The length and height of all the models are 1500 mm and 1000 mm respectively. In the first set the supporting beams and columns were made perfectly rigid and connections between them were pinned connection. This set consisted of five models having plate thicknesses 1mm, 2mm, 4mm, 6mm and 8 mm and are denoted as RB1, RB2, RB4, RB6 and RB8 respectively as shown in Fig. 1.
RESULTS AND DISCUSSIONS
The load displacement curve for model RB1, RB2, RB4, RB6, RB8 is presented in Fig. 5. The push-over analysis of SPSW with rigid frame-members and pinned connections showed that their ultimate lateral load carrying capacities are proportional to the thickness of the plates. Initial stiffness is varying linearly with the thickness of plate. The ultimate lateral load carrying capacities of SPSW with pinned beam-column connections increase linearly with increase in the thickness of the steel plate.
CONCLUSIONS
The steel plate wall has a large initial stiffness and load-carrying capacity. SPSW with relatively larger aspect ratio exhibits the greater load-carrying capacity and deformation capacity than the one with smaller aspect ratio. Both strength and stiffness ratios are approximately the same as the ratio of their aspect ratios. In the shear-dominated walls, most infill steel plates yield early across the wall height, and at a large plastic deformation, plastic hinges are developed at the frame members by the moment frame action. In the flexure-dominated walls, the columns in the first storey yield and a plastic hinge are developed at the wall base. SPSW systems are usually more flexible in comparison to concrete shear walls, primarily due to their flexural flexibility. Therefore, when using SPSW in tall buildings, the engineer must provide additional flexural stiffness. Large composite concrete infill steel pipe columns can be used at all corners of the core wall toimprove the system‟s flexural stiffness as well as its overturning capacity. Load deflection curve of SPSW is linear for smaller values of displacement but it attains a constant value, known as the ultimate lateral load carrying capacity of the SPSW with increase in displacement. If the beams and columns do not behave like perfectly rigid members, then more strain is developed in the middle region of the plate and decreases gradually towards the two corners in the perpendicular direction to the tension field. The ultimate lateral load carrying capacities of SPSW with pinned beam-column connections increase linearly with increase in the thickness of the steel plate. The initial stiffness of SPSW with pinned beam-column connections increase linearly with increase in the thickness of the steel plate. For SPSW with pinned connections, the effect of stiffness of the frame members on the ultimate load carrying capacity of SPSW can be ignored, until the members are strong enough to resist the force applied on them by the plate and do not fail before the tearing of plate. But it increases linearly with increase in the thickness of steel plate used.