Seismic assessment of reinforced concrete walls in Australia

© 2017 Dr. Ryan HoultSome non-ductile reinforced concrete walls in buildings were observed to perform poorly in the 2011 Christchurch earthquake, with most of the lives lost from the event caused by the collapse of buildings that relied on these structural elements for lateral support. Reinforced concrete (RC) walls are widely used throughout the Australian building stock as the primary lateral support elements. It is possible that some of these structural elements would perform poorly in a very rare earthquake due to the low standard of detailing that is currently required in Australia, as well as the low earthquake return period that the Building Code of Australia stipulates for their design. The aim of this research has been to assess the seismic performance of reinforced concrete structural walls, both rectangular and C-shaped, in Australia, a region of low-to-moderate seismicity. The current Australian Standard for Earthquake Actions, AS 1170.4:2007, stipulates earthquake hazard values that are based on a seismic hazard map that is over two decades old. A probabilistic seismic hazard analysis was conducted for most of the capital cities in Australia using the AUS5 model to provide a more accurate prediction of seismic hazard in Australia. The results indicate that for some cities, such as Melbourne, the response spectrum is expected to be higher for large return periods in comparison to the design spectra derived using AS 1170.4:2007. Furthermore, a site response study was conducted using equivalent linear analyses to investigate the amplification of the soil response as classified in AS 1170.4:2007 using a range of ground motions that would be expected in Australia. The primary conclusions from the study showed that there can be a large dependency of the soil amplification on the intensity of the earthquake ground motions for the softer soil classes. Moreover, the low intensity ground motions resulted in a higher spectral shape factor for soil class Be and Ce in comparison to factors derived from the current AS 1170.4:2007. An investigation was undertaken to find the displacement capacity of rectangular lightly reinforced and unconfined walls using a finite element modelling (FEM) program, with emphasis on finding the equivalent plastic hinge length. A Secondary Cracking Model (SCM) was formulated, which is a simple, mathematical model that has the potential to predict if a RC wall has a sufficient longitudinal reinforcement ratio to enable “secondary cracking” to occur. The SCM has been validated by comparison with results from the FEM analyses. Equivalent plastic hinge length equations were derived for the rectangular walls that were observed to form secondary cracking and a single, primary crack, and this can be used to predict the displacement capacity of these walls. This estimate of the displacement capacity assumes that the inelastic rotation that occurs over the inelastic region at the base of the wall can be modelled using an equivalent plastic hinge length over which the curvature is assumed to be a constant value. These estimates of the equivalent plastic hinge length are more appropriate for RC structural walls commonly found in Australia due to the parameters used in deriving them (e.g. mechanical properties of steel, longitudinal reinforcement ratio). Moreover, some expressions for the equivalent plastic hinge length that have derived by previous researchers were found to be inappropriate for the walls analysed in this research; these were particularly inaccurate for walls that do not have sufficient longitudinal reinforcement to force secondary cracks to form. The new expressions provide better estimates of the displacement capacity of lightly reinforced and unconfined walls when compared with recent experimental observations. One of the most widely used and popular cross-sections used in structural design of RC walls is the C-shaped section. There is a paucity of information available on the inelastic behaviour of such elements, and virtually no experimental data exists on non-rectangular concrete walls with inferior details commonly found in regions of low-to-moderate seismicity. An extensive number of nonlinear pushover analyses have been conducted based on FEM to investigate the seismic behaviour of C-shaped walls with detailing commonly found in Australia. Based on the FEM results, the SCM, that has been developed for rectangular walls, was found to be able to predict the potential of a single-crack forming in the walls. The direction of loading and mode of bending was found to be particularly important for the seismic performance of these walls. A non-ductile failure was observed for the majority of the walls investigated due to crushing of the unconfined concrete at the ends of the flanges in the governing direction of loading. Further analyses were conducted in the FEM program but with confined boundary ends to emphasise the importance of such structural detailing in allowing some plastic behaviour to be achieved for the governing direction of loading. The equivalent plastic hinge lengths derived from the extensive number of FEM analyses correlated poorly in comparison to the estimates from a number of expressions that exist in the literature, including a recently developed equation specifically for C-shaped walls. Therefore, equivalent plastic hinge lengths were derived from these results and for each direction of loading. A program has been written in MATLAB to derive vulnerability functions for low-rise, mid-rise and high-rise buildings in Australia that use structural walls as their lateral force-resisting system. The city of Melbourne was used as a template for conducting the analyses, and a dataset of thousands of buildings obtained from the National Exposure Information System (NEXIS) and Census of Land Use and Employment (CLUE) databases was included in the assessment. The displacement capacity of each of the buildings was estimated using a moment-curvature analysis followed by a plastic hinge analysis. A range of artificial earthquakes from GENQKE and real earthquakes from the PEER ground motion database on “weathered bedrock” conditions were obtained. These ground motions were subsequently used in equivalent linear analyses using the program SHAKE2000 to find the site response at the surface of different soil columns from shear wave velocity profiles taken predominantly from sites around Melbourne. The National Regolith Site Classification Map was used to estimate the soil conditions underlying each of the building sites. The acceleration and displacement response spectra resulting from these ground motions were used to represent the seismic demand for different site conditions in the capacity spectrum method and to ultimately estimate the vulnerability of the buildings. Thus, vulnerability functions were derived from the results

RC U-shaped walls subjected to in-plane, diagonal, and torsional loading: New experimental findings

Although reinforced concrete U-shaped walls are popular in construction practice internationally, there is a paucity of experimental research investigating the seismic performance of such salient elements. The present paper summarizes an experimental campaign on two slender U-shaped reinforced concrete walls detailed with a single-layer of reinforcement. State-of-the-art instrumentation was used to capture the three-dimensional displacement field of the wall surfaces using digital image correlation techniques. Experimental findings are presented, including strain profiles, equivalent plastic hinge lengths, longitudinal strains at the base, cracking distributions and widths, and out-of-plane deformations. Approximately half of the yielding zone length was found to be equal to the equivalent plastic hinge lengths, which were found to decrease as a function of drift. The longitudinal strains at the base of these walls showed some shear lag effects when subjected to in-plane or diagonal loading. For most directions of loading, the largest crack widths were found to be associated with flexural-shear or shear cracks. When subjected to a pure torque, the vertical strain distribution at the base of the wall correlated with the theoretical distribution for an open section governed by warping torsion. The out-of plane deformations were primarily concentrated within a small region towards the ends of the flanges prior to the local buckling failures that were observed experimentally

Residual displacements of flexure-governed RC walls detailed with conventional steel and shape memory alloy rebars

Recent seismic events have shown that permanent damage and deformations of buildings prevent the structure from being serviceable, imposing high costs associated with repairs or demolition. The yielding and inelasticity of the steel rebars in the boundary ends of modern designed reinforced concrete walls are generally the source of residual displacements for reinforced concrete buildings. This paper investigates the residual displacement of reinforced concrete walls detailed with either conventional steel or shape memory alloys in the boundary ends of the wall. The force-displacement response of a large dataset of reinforced concrete flexurally-governed walls is analysed to derive the residual displacement as a function of inplane displacement (or drift). The existing very few experimental results on reinforced concrete walls detailed with shape memory alloys are also examined. On average, walls detailed with conventional steel are found to attain residual displacement less than the permissible limit for drifts up to 1.5%. The shape memory alloy walls are generally shown to perform better, with an estimate of the permissible limit being reached at approximately 2.0% drift. However, some design deficiencies from two of three wall specimens detailed with shape memory alloys resulted in poor performance. Thus, more experimental testing is needed on reinforced concrete walls detailed with shape memory alloys to increase confidence in using these materials in practice

Seismic performance of slender RC U-shaped walls with a single-layer of reinforcement

Reinforced concrete walls are commonly used to resist the lateral loading induced by wind and earthquake actions. While most walls include two vertical reinforcement layers, some regions of the world construct slender, non-rectangular concrete walls with a single vertical layer of reinforcement. The seismic performance of such elements is largely unknown given the paucity of experimental research. This paper presents the results of two slender reinforced concrete U-shaped walls tested at the Earthquake Engineering and Structural Dynamics Laboratory (EESD Lab), École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. Both wall specimens, designed similar to construction practice in Colombia, were tested using quasi-static cyclic loading to observe if out-of-plane instability would develop when deformations were limited to prevent the flange boundary ends crushing. Initial failure of both wall specimens corresponded with local out-of-plane buckling in the boundary ends of the flanges occurring on load reversal. The buckling lengths were approximately 700–800 mm, which corresponded to 44–50 bar diameters. The crack patterns were observed to be steepest in the web of the walls, demonstrating the increased shear demand in comparison to that of a rectangular wall. Both wall specimens reached ultimate drifts larger than 2.5–3.0% before global failure occurring in the web-flange intersection due to crushing. A small twist was subjected to one of the walls when centered and loaded diagonally, which showed that the decay in torsional stiffness is proportional to the decay in translational stiffness

Residual displacements of reinforced concrete walls detailed with conventional steel and shape memory alloy rebars

Modern reinforced concrete design codes can generally achieve the primary performance level of no collapse in the event of a rare to very rare earthquake. However, recent seismic events have shown that permanent damage and deformations of buildings prevent the structure from being serviceable, imposing high costs associated with repairs or demolition. Shape memory alloys have the ability to recover large strains upon removal of stress. Thus, replacing conventional steel with superelastic alloy rebars in the boundary ends of reinforced concrete walls has the potential to reduce residual seismic displacements for these types of buildings. This research paper investigates the lateral residual displacement of reinforced concrete walls detailed with conventional steel and shape memory alloy bars as a function of the in-plane drift. Namely, the force-displacement hysteresis of a large dataset of experimental walls with conventional steel are used to study the residual displacement as a function of several key design parameters. A state-of-the-art finite element modelling program is then used to investigate the residual displacements of walls detailed with shape memory alloy bars, and a parametric study is undertaken to investigate the influence of residual displacements of these types of walls. Most of the walls reinforced with shape memory alloys achieved residual displacements less than the permissible limit at large drift levels. The axial load was found to help suppress the residual displacements of walls with increasing drift. The curvatures were found to be distributed over a limited height at the base that was equivalent to the length of the shape memory alloy bar used. Plastic hinge analysis expressions are adapted to estimate the operational displacement of reinforced concrete walls with shape memory alloys

Core versus Surface Sensors for Reinforced Concrete Structures: A Comparison of Fiber-Optic Strain Sensing to Conventional Instrumentation

High-resolution distributed reinforcement strain measurements can provide invaluable information for developing and evaluating numerical and analytical models of reinforced concrete structures. A recent testing campaign conducted at UCLouvain in Belgium used fiber-optic sensors embedded along several longitudinal steel rebars of three reinforced concrete U-shaped walls. The resulting experimental dataset provides an opportunity to evaluate and compare, for different types of loading, the strain measurements obtained with the fiber-optic sensors in the confined core of the structural member against more conventional and state-of-the-practice sensors that monitor surface displacements and deformations. This work highlights the need to average strain measurements from digital image correlation techniques in order to obtain coherent results with the strains measured from fiber optics, and investigates proposals to achieve this relevant goal for research and engineering practices. The longitudinal strains measured by the fiber optics also provide additional detailed information on the behavior of these wall units compared to the more conventional instrumentation, such as strain penetration into the foundation and head of the wall units, which are studied in detail

The plastic hinge length of planar and non-planar RC walls

When using a plastic hinge analysis, the displacement capacity of a reinforced concrete wall is highly dependent on the assumed value for the plastic hinge length (Lp). Most of the expressions that are available in the literature have been derived specifically for beams, columns, bridge piers, or planar (rectangular) walls with an applicability to a small range of design values. This paper introduces an expression for the Lp derived from recent research using an extensive database of experimental and numerical results of both planar and non-planar walls. The design expression presented for Lp is found to provide more conservative estimates in comparison to that currently used in some building codes of 0.5 times the wall length. The assessment expression for Lp can provide more reasonable estimates, which will in turn provide more accurate estimates of the displacement capacity of the wall

A Comparison of Facial Muscle Activation for Vocalists and Instrumentalists

The purpose of this study was to compare the muscle activation of singers and instrumentalists while performing simple vocal exercises. Volunteer participants (N = 28) were undergraduate music majors and minors, with an equal number being vocalists and instrumentalists. Participants performed five vowel sounds (ah, eh, ee, oh, oo), while electromyography of the zygomaticus and masseter muscles was sampled at 1,000 Hz. A statistically significant multivariate analysis of variance effect was obtained and follow-up analyses of variance showed instrumentalists had more masseter muscle activation than vocalists when performing “eh” and “ee.” Instrumentalists also had more zygomaticus muscle activation than vocalists when performing the “eh” vowel, but vocalists had more zygomaticus muscle activation when performing the “ah” vowel