Journal of Structural Engineering

Buckling of reinforcement is one of the possible phenomena that limit the deformation capacity of reinforced concrete members under reversed cyclic loading. Previous experimental research suggests that occurrence of buckling is linked to displacement history, a parameter that is not explicitly accounted for in the available expressions for ultimate drift or curvature ductility capacities. This problem is explored in the present paper, by following through analytical expressions that relate the critical buckling strain as defined by the hysteretic stress-strain model of the reinforcement and the imposed cyclic history in terms of displacement. The analytical expressions thus derived are evaluated parametrically in order to establish behavioral trends. It is shown that when controlled by bar buckling, deformation capacity cannot be defined uniquely as it varies with the path of applied load. A primary conclusion of the research is that any quantifiable indices of deformation capacity referred to in the framework of displacement-based design using deterministic approaches need be adjusted to represent conservative lower bounds rather than approximations to the actual values of nominal failure.

The pseudodynamic testing of two reinforced concrete bridge columns with a reduced scale of 2/5 was performed to obtain their seismic responses to near-fault ground motions. An identical column was also cyclically tested under constant axial load plus biaxial bending to obtain the basic properties of these bridge columns. Biaxial hysteretic loops from both tests show significant stiffness and strength degradations and pinching effect. This attested to the difficulty in developing a mathematical model to realistically capture the load-displacement relation for a reinforced concrete column. The distinct characteristics of round corners and negative stiffness were found in biaxial hysteretic loops obtained from pseudodynamic tests. The scatter plots of pseudodynamic test results also show that the orientations of the resultant flexural moments do not bias in any direction. Thus, this aspect might be considered in the seismic design of a column. Since pseudodynamic results are believed to be more accurate than numerical solutions they can be treated as reference solutions in developing a finite element model. Meanwhile, the hysteretic response obtained from cyclic loading test may be used to match a mathematical model to mimic the very complicated load-displacement relation for analysis. The biaxial hysteretic loops show during unloading more stiffness degradation and pinching than the uniaxial hysteretic loops for both cyclic loading and pseudodynamic tests. This behavior indicates that damage caused in one direction weakens the seismic resistance in the other direction.

An experimental and numerical investigation carried out on RC wide beam-column joints when subjected to seismic loads is reported within this paper. The experimental study was conducted by subjecting three full-scale wide exterior beam-column specimens to simulated seismic loads at the Nanyang Technological University of Singapore. Simulations of earthquake loadings were applied on to the specimens via quasistatic load reversals. Seismic performance analysis of the joints were based on the results obtained from the experiment in terms of their general behavior, hysteresis response, and the strain profiles of the longitudinal reinforcement within the specimens. These experimental results were then used to validate a three-dimensional (3D) nonlinear finite-element (FE) model. The behavior of the joints under the influence of critical influencing factors like column axial load, transverse beam, and beam bar anchorage ratio were also analyzed through the parametric studies carried out.

Low-rise buildings are typically designed with shear walls and horizontal diaphragms to resist lateral load during windstorms and earthquakes. Although seismic performance and lateral load resistance behavior are generally well-understood for light-frame wood construction, this is not true for log construction. Research is needed to characterize log shear wall behavior subject to racking loads, and to develop recommendations for designing log walls to resist lateral loads from seismic or wind events. Monotonic and reverse-cyclic tests were conducted on log shear walls with 1:1, 2:1, and 4:1 aspect ratios. These walls showed higher resistance than conventional light-frame shear walls and similar hysteretic behavior to concrete and masonry shear walls. Log walls withstood large in-plane displacements without significant load reduction using lateral bracing typical of that used for light-frame wall testing; however, the test procedure would be improved by providing full-wall support to resist out-of-plane displacements of log shear walls. Design recommendations were developed based on the experiments reported herein.

This paper describes tests on a novel composite floor system constructed by connecting prefabricated concrete slabs to timber joists. Seven types of shear connectors have been developed and tested: lag screws, either alone or combined with a notch cut from each timber joist; metal plates embedded in the concrete slab and either nailed or glued to the joists; dowels embedded in the concrete and glued to the timber; and toothed metal plates embedded in the concrete and pressed into the timber. Four-point bending tests to failure were performed on five, full-scale, 4.8 m long specimens connected with lag screws or metal plates nailed to the timber. Values of deflection and relative slip between the concrete slab and the timber obtained in these tests showed high correspondence with values obtained from a uniaxial finite element model developed for nonlinear analyses of composite beams. The model was also used to perform a numerical analysis to failure of composite beams with the other four connection systems that were developed but not tested on full-scale specimens. The outcomes of the experimental tests and numerical analyses show that the newly developed system can provide good structural performance, especially if connections with coach screws and notches in the timber are used. The economic advantages of prefabrication and the possibility to disassemble the structure and reuse the timber beams and concrete panels at the end of the service life make the proposed floor system very promising.

The experimental seismic responses of a full-scale two-story light-frame wood townhouse building, designed to modern U.S. engineered seismic design requirements, were compared against the predictions of a new software package entitled seismic analysis package for woodframe structures (SAPWood) developed recently within the NEESWood Project. The main objective of this paper was to verify the accuracy of the predictions from the SAPWood model, which incorporates shear deformations of shear walls as well as cumulative floor displacements caused by the out-of-plane rotations of the floor and ceiling diaphragms. A comparison was conducted on interstory drifts and shear wall deformations for various structural configurations (construction phases) of the test building and excitation levels. Good agreement was found between the numerical predictions and test results for the four different construction phases. The SAPWood model was shown to be a promising numerical tool for predicting the seismic response of light-frame wood structures.

Design models for local buckling of fire-exposed aluminum sections are currently lacking. Based on analyses with validated finite-element models, this paper investigates local buckling of extruded sections with stress-strain relationships representative for fire-exposed aluminum alloys. Due to the fact that these stress-strain relationships are more curved than at ambient temperature, existing design models developed for ambient temperature cannot be used for fire design. This paper presents a new design model for local buckling under fire conditions. The study concludes that the local buckling resistance decreases less fast than the plastic capacity at increasing temperature. This is mainly due to the fact that the ratio between the modulus of elasticity and the 0.2% proof stress increases with increasing temperature for structural aluminum alloys.

This paper presents a procedure for calculating the effective equivalent structural properties (stiffness and damping) of hysteretic systems from the time histories of their dynamic response. The method is based on the finding that the effective dynamic damping is proportional to the product of the structural natural frequency and the delay between the overall restoring force and the structural displacement. The delay is calculated from the time-shifted cross correlation between the structural force and displacement; and the same procedure also allows us to compute the instantaneous period of the response. It is then shown how, by computing the standard deviations of the force and displacement signals, it is possible to compute both the equivalent damping and the linearized stiffness. By implementing this procedure sequentially through moving windows, it is possible to monitor the nonlinear evolution in time. Some proofs are given for some easily tractable mathematical cases such as for linear simple harmonic and seismic responses, conjecturing that the method can be equally applied for transient forcing functions of nonlinear systems.

A nondestructive damage detection approach based on measured structural responses is presented and verified by controlled laboratory experiments and by tests on real structures. In the experiments, defined magnitudes of damages are assigned to reinforced concrete beams, and the dynamic responses of the undamaged and damaged specimens are measured. The measured frequencies of the specimens and their sensitivity to change in mechanical characteristics, called sensitivity factors, are used to predict the location and the magnitude of the damage. The damage locations associated with the investigated test series are predicted with high accuracy by the proposed identification approach. This approach has been applied to real structures, including an existing bridge in Switzerland. In order to provide successful damage detection in cases where the damage does not affect or only marginal affects dynamic structural characteristics, the approach has been extended to handle simultaneous multiple structural response characteristics, such as frequencies, and modal and static displacements. The identification approach satisfies ease-of-use requirements in existing reliability assessment methods for deteriorating structures. The main purposes of this paper are to (1) present the proposed identification algorithm; (2) discuss the adjustment of the stochastic algorithm inputs, such as cross-sectional stiffness, geometrical, and mechanical properties, by monitored structural responses; and (3) present the interaction between the developed identification algorithm and the stratified Monte Carlo randomization technique, thus allowing the application of the algorithm to large engineering structures. Additionally, it is shown that the developed code is suitable for incorporation into commercial software packages.

Wind tunnel tests of open-frame, low-rise buildings were carried out to determine the drag (base shear) and bracing loads in the direction normal to the frames (parallel to the ridge). In total, 18 configurations were examined in an open country terrain at a scale of 1:100. The worst wind angles for all configurations are between 0°–40° with 20°–30° typically yielding slightly higher loads, 0° being parallel to the ridge. The largest load coefficients are observed for the smallest frame buildings, consistent with observations for enclosed buildings, which is due to three-dimensional (edge) effects. The solidity ratio has a clear effect on the load coefficients with higher coefficients for lower solidity, similar to the behavior observed on lattice frames or trussed towers. However, when these coefficients are multiplied by the solidity ratio, so that they can be directly compared to enclosed building coefficients, it is clear that the total load increases monotonically with solid area. Bracing was observed to take up to 75% of the total drag load. An empirical model was developed for obtaining design loads.

Current design wind loads for buildings and other structures are based upon model tests in low-speed boundary-layer wind tunnels that generate straight-line winds. Winds resulting from tornadoes that can occur during extreme weather events such as thunderstorms or hurricanes differ greatly from conventionally conceived atmospheric boundary-layer winds. This paper presents transient wind loads on a one-story, gable-roofed building in a laboratory-simulated tornado and compares them with the provisions of building standards. Tornadoes were simulated in smooth, open terrain with vortex core diameters from roughly five to twelve times the plan dimension of the building model (0.46 to 1.06 m). A 1:100 scale model of a building with dimensions of $9.1\text{m}\times 9.1\text{m}\times 6.6\text{m}$ and gable roof angle of 35° was used for this study. Comparisons of peak loads measured in this study showed that tornado-like vortices can generate load coefficients greater than those prescribed by ASCE 7-05 for straight-line wind over open terrain. For lateral forces, these coefficients were found to be as much as 50% larger than the standard provisions. Vertical uplift coefficients were found to be two to three times the provisions most likely due to the effect of a large negative static pressure generated at the core of the vortex.

We present a novel methodology for the estimation of the aerodynamic damping that affects the along-wind response of tall buildings. The estimation uses (1) measurements of the external pressures on the windward and leeward faces under wind normal to a building face; (2) a simplified model of the relation between those pressures and the longitudinal fluctuating velocities with respect to the building; and (3) time-domain estimates of the along-wind building response that preserve the phase relationships between pressures and response. The methodology accounts explicitly for the shapes of the building’s modes of vibration, the natural frequencies of vibration, and the mechanical damping ratios, and takes advantage of the recent development of efficient time-domain algorithms for calculating dynamic structural response to wind. The methodology is applied to an isolated tall building in suburban terrain. The results of the calculations show that, for the example being considered, the aerodynamic damping (1) is positive, that is, it has a favorable effect; (2) is small relative to the mechanical damping; (3) is for practical purposes independent of the shape of the fundamental mode of vibration; and (4) is unaffected by higher vibration modes.