3.2 Structural Engineering Innovations
Integrated Modelling of the Seismic Response of a Multi-Storey Framed Structure Supported on Pile Foundations
L.M. Wotherspoon & M.J. Pender
ABSTRACT: For some time we have been espousing the integrated design of structure-foundation systems. The purpose of this paper is to demonstrate the practical implementation of this in evaluating the response of a multi-storey framed structure supported on pile foundations. We use the features available in SAP 2000 and Ruaumoko to model the nonlinear soil-pile response and also the opening and closing of gaps between the pile shaft and the soil when the piles are embedded in cohesive soils. Our objective in using SAP2000 is to demonstrate how the features in a commercially available software suite are capable of contributing to realistic modelling of both the above ground structure and the pile foundation beneath the ground. Selected results from analysis presented in this paper indicate the importance of looking at the structure and foundation together as a single entity. If the results from fixed base analysis are used in design, the distribution of actions throughout the structure could be significantly different to the actions developed using integrated models.
Effect of Construction Quality Variability on Seismic Fragility of Reinforced Concrete Building
P. Rajeev & S. Tesfamariam
ABSTRACT: This paper highlights developing a probabilistic seismic demand model (PSDM) for reinforced concrete (RC) frames. A six-storey three-bay moment resisting RC frame is designed to a 1984 Canadian building design code. The RC frame is further modified to investigate variability of construction quality (CQ) on the PSDM. Three levels of CQ are considered, poor, average, and good. Forty five ground motion records were used to study the ground motion variability. The numerical model of the frame was developed in OpenSees and nonlinear dynamic analyses were performed, and the maximum interstorey drift is obtained as a response parameter for all simulations. The PSDM parameters are calculated using cloud analysis for all combination of construction quality. The variation in the PSDM parameters is studied. Finally, the effects of CQ on the seismic fragilities are discussed.
The Continuous Column Concept - Development and Use
Gregory A. MacRae
ABSTRACT: This paper describes the continuous column concept. This concept recognizes that all continuous columns and walls in a multi-storey building provide stiffness. This stiffness discourages the formation of a soft-storey mechanism. The concept is more powerful, and it is more general, than the “capacity design” approaches that have been advocated in the past. This paper describes the background to the development of this methodology, quantification of the amount of drift concentration for specified column stiffness, and the importance of the continuous column concept in reducing the tendency of the structure to develop significant drifts in one direction due to P-delta effects. Finally, some applications of the concept are described.
Low Cost Lightweight Buckling Restrained Braces for Low Rise Buildings
A.S. Jones
ABSTRACT: Buckling restrained braces are becoming an increasing attractive alternative to conventional concentric bracing in seismic loading, due to their ability to have equal compression and tension capacities in the inelastic range. This paper investigates a lightweight, low cost buckling restrained brace for low rise building in developed and developing countries. The brace composes of a round steel bar core as the primary yielding element surrounded by low density expandable polyurethane foam, providing full lateral restraint against buckling in compression, thus allowing the inelastic action in compression. These are encased in a bamboo culm which acts elastically to develop the required lateral restraint. The ends of the bars in these tests were rotationally fixed at both ends to reduce induced moment and were loaded axially. The brace successfully sustained tension forces above yield capacity, deforming only 20% of that expected. The compression capacity was significantly lower than calculated with no test brace reaching the steel member compression capacity. Failure in compression was by splitting of the bamboo laterally through localised holes; however the bamboo retained previous shape and position after failure successfully self-centring the brace. The test was therefore partially successful, however further planned tests on smaller bars and larger diameter bamboo may yet develop the required compression restraint.
Lateral-Resisting Systems Capable of Multiple Seismic Performances
T. Trombetti, S. Silvestri, G. Gasparini & I. Ricci
ABSTRACT: This paper aims at presenting an innovative approach for an optimised/full-controlled seismic design of structures which combines recent contributions in the field of earthquake engineering and overcomes the traditional design approach leading to the identification of the characteristics of the lateral-resisting system capable of satisfying multiple seismic performance objectives. In this respect, it is fundamental the total conceptual separation between the structural systems resisting to vertical and horizontal loads. With reference to both (1) a braced pendular frame structure and (2) a shear-type frame system coupled with a lateral-resisting element (such as a reinforced concrete core or a bracing system), the approach here presented identifies the characteristics (strength, stiffness, ductility, energy-absorption) of the system resisting to horizontal loads which enables to satisfy prescribed seismic performance objectives. This is achieved through the identification of an objectives curve, in the Force-Displacement diagram, of the mechanical characteristics of the structure. The lateral-resisting system is obtained by means of (1) special braces in the case of the braced pendular frame structure and (2) special connection elements in the case of the shear-type system coupled with a lateral-resisting element.
Experimental Investigation on the In-Plane Behaviour of Non-Ductile RC Walls
A.S. Gebreyohaness, G.C. Clifton & J.W. Butterworth
ABSTRACT: Past earthquakes have demonstrated that buildings with non-ductile RC components as their primary lateral load resisting system pose a significant seismic risk. Assessment and retrofit of such buildings entails a careful evaluation of the as-built performance of the lateral load resisting components. A series of quasi-static cyclic tests on reconstructed RC wall specimens of an existing building are being undertaken at the University of Auckland, to determine the seismic performance of non-ductile walls in the framework of a research project addressing the seismic assessment and retrofit of existing buildings in New Zealand Wall thickness, aspect ratio, level of axial load, and the effects of splices and boundary reinforcement are amongst the important parameters being investigated.
The test setup and loading regime and details of the first two experimental tests are presented herein. The lateral load capacity of the lightly reinforced non-ductile RC walls investigated is found to be dictated by the flexural strength at their base. However, due to the plain round bars used, the walls didn’t develop distributed flexural cracks but rather exhibited a predominantly rocking response about a single crack located at the foundation-wall interface. In addition, they had Ltd ductility capacity, which was dependent on the level of axial load. The current NZSEE guideline underestimated the contribution of concrete to the shear strength of the walls and predicted the wrong mode of failure for one of the walls.