Journal of Structural Engineering

A comprehensive study has been conducted to investigate the long-term behavior and strength of partially encased composite columns made with thin-walled welded I sections stiffened with transverse links. The paper presents the strain measurement history and load deformation response of five $300\times 300\mathrm{mm}$ and two $450\times 450\mathrm{mm}$ stub-column specimens. Four specimens were loaded for 150 days, following a typical construction sequence, and five columns were tested up to failure. Long-term axial deformations due to shrinkage and creep of the concrete were recorded and are compared to prediction models. The tests showed that the relatively higher stresses in the steel shape due to the sequence of loading and shrinkage and creep of the concrete had no significant effect on the failure mode and ultimate capacity for this type of column. The transverse stresses in the web of the steel shape were lower in specimens subjected to long-term loading. The axial stresses in the transverse links at peak load, caused by the transverse expansion of the concrete, were found not to depend on the loading sequence.

A conventional longitudinal branch plate-to-HSS member connection tends to cause excessive distortion or plastification of the HSS connecting face. In an effort to reduce the inherent flexibility of longitudinal branch plate connections, stiffening plates or structural tees are sometimes welded to the HSS connecting face. This paper presents the results of an experimental test program and a corresponding numerical study, using the finite element method, with the aim of determining the minimum required “effectively rigid” stiffening plate thickness. An empirical formula is developed to predict the required stiffening plate thickness, satisfying both a serviceability and an ultimate limit state condition, for use in a proposed connection design procedure.

Flatbar stiffeners are attractive in stiffened plates from a fabrication point of view. Flatbars are susceptible to torsional buckling and collapse. Current approaches to assess torsional buckling strength are uncertain. For instance, formulations for torsional buckling of stiffeners in steel panels are still debated. Compared to steel structures, the strength of aluminum structures is influenced not only by the residual stresses and initial imperfections, but also reductions in mechanical properties in heat-affected zones (HAZ). In the present paper, an experimental and numerical investigation of the torsional buckling strength of longitudinal stiffeners in aluminum (aluminum alloy 5083-H116 and 6082-T6) panels subjected to axial compression is conducted. The experimental results are presented and compared with numerical predictions by using finite element method code ABAQUS, considering the influence of the HAZ and actual initial deflections. The effect of the HAZ on the torsional buckling strength of the stiffened panels is discussed. It is found that the torsional buckling strength of the stiffened panels is affected by the HAZ and the reduction of ultimate strength is up to 34% when the HAZ is accounted for in numerical prediction for AA6082-T6. The load eccentricity also affects the ultimate strength significantly. The ultimate capacity is not found to be sensitive to welding residual stresses in aluminum stiffened panel.

An experimental investigation into the column strength of cold-formed stainless steel rectangular hollow sections is described in this paper. The columns tested displayed four cross-section sizes and were compressed between fixed ends. The tests were performed over a range of column lengths in order to obtain a column curve for each series of tests. The failure modes of the columns involved local buckling, overall flexural buckling, and combined local and overall buckling. The test strengths are compared with the design strengths predicted using the American, Australian/New Zealand, and European specifications for cold-formed stainless steel structures. The design strengths were calculated using the material properties obtained from both tensile coupon and stub column tests. It is concluded that the design strengths predicted by the three specifications are generally conservative. Reliability analysis of the design rules in the three specifications is performed. It is shown that the design strengths predicted by the Australian/New Zealand Standard are generally more reliable than the design strengths predicted by the American and European specifications.

Masonry infill panels in framed structures have been long known to affect strength, stiffness, and ductility of the composite structure. In seismic areas, ignoring the composite action is not always on the safe side, since the interaction between the panel and the frame under lateral loads dramatically changes the stiffness and the dynamic characteristics of the composite structure, and hence, its response to seismic loads. This study presents a simple method of estimating the stiffness and the lateral load capacity of concrete masonry-infilled steel frames (CMISFs) failing in corner crushing mode, as well as the internal forces in the steel frame members. In this method, each masonry panel is replaced by three struts with force-deformation characteristics based on the orthotropic behavior of the masonry infill. A simplified steel frame model is also presented based on the documented modes of failure of the CMISF. The method can be easily computerized and included in nonlinear analysis and design of three-dimensional CMISF structures.

Fatigue sensitive details in aging steel girders is one of the common problems that structural engineers are facing today. The design characteristics of steel members can be enhanced significantly by epoxy bonding carbon fiber reinforced polymers (CFRP) laminates to the critically stressed tension areas. This paper presents the results of a study on the retrofitting of notched steel beams with CFRP patches for medium cycle fatigue loading $(R=0.1)\text{.}$ A total of 21 specimens made of $S127\times 4.5$ A36 steel beams were prepared and tested. Unretrofitted beams were also tested as control specimens. The steel beams were tested under four point bending with the loading rate of between 5 and 10 Hz. Different constant stress ranges between 69 and 379 MPa were considered. The length and thickness of the patch were kept the same for all the retrofitted specimens. In addition to the number of cycles to failure, changes in the stiffness and crack initiation and growth were monitored during each experiment. The results showed that the CFRP patch not only tends to extend the fatigue life of a detail more than three times, but also decreases the crack growth rate significantly.

A comprehensive experimental investigation on the dynamic characteristic and seismic response of adjacent buildings linked by fluid dampers is carried out using a 4 m×4 m seismic simulator. Two building models are built as one five-story steel frame 5 m high and the other six-story steel frame 6 m high. The fluid dampers connecting the two building models are of linear force–velocity property. Two types of ground motions are applied to the adjacent buildings. The control performance of fluid dampers is first assessed through the comparison of the dynamic characteristic and seismic response of the adjacent buildings linked by fluid dampers with those linked by rigid rods or without any connections. The effects of the number, location, and linking pattern of fluid dampers and the effects of ground motion on control performance are then investigated. The first natural frequency ratio of the two building models is finally changed and its effect on control performance is evaluated. The experimental results show that the installation of fluid dampers can significantly reduce the seismic responses of both buildings. The number, location, and linking pattern of fluid damper, the frequency ratio of adjacent buildings, and the type of ground motion should be considered properly to achieve good control performance.

This paper discusses the rationality of the design force and damping reduction factors adopted by a few seismic design provisions for buildings with and without added passive energy dissipation systems. The issue will first be pointed out that the damping reduction factors adopted by those provisions are derived from the effects of viscous damping on displacement responses, but are used to reduce the design force of buildings. Statistical results from 1053 ground motions recorded in the U.S. show that it may lead to unconservative results, especially for systems with damping ratios greater than 10% and periods longer than 0.15 s. Furthermore, although there is no doubt that the additions of extra damping to a structure will always reduce the displacement responses, many documents argue the effect of added damping to reduce the force responses of the buildings. Therefore, this paper also addresses the effects of viscous damping on the inertial force and elastic restoring force in order to use the damping reduction factors correctly. Results of this study suggest that if the damping of structures comes from the hysteretic response of the building, the design force of the structures should be the inertial force and the damping reduction factors should be derived from the acceleration responses. Otherwise, if the additional damping of structures comes from the added energy dissipation devices, the design force should be the restoring force and the damping reduction factors should be derived from the displacement responses.

A finite-element formulation that captures both the St. Venant and warping torsional effects of open sections is developed. The formulation is applicable to prismatic thin-walled open sections commonly used in steel construction and is able to represent general torsional and warping end restraints. The element has two end nodes, each having two degrees of freedoms, and featuring continuity of the first derivative of the angle of twist (C1 continuity). A family of hyperbolic interpolation functions is adopted in the derivation in lieu of conventional polynomial displacement interpolation schemes. The stiffness matrix and the associated energy equivalent load vector are demonstrated to yield nodal generalized displacements and forces in exact agreement with classical solutions for any externally applied twisting moment distribution. Consequently, the formulation allows the adoption of coarser subdivisions than those needed in other finite-element formulations while preserving the exactness of the results. The formulation is simple and provides a basis to assess the quality of results of other approximate finite-element solutions. The implications of the exactness of the formulation are discussed and illustrated through examples.

An efficient and accurate algorithm is developed to evaluate the reliability of a steel frame and reinforced concrete shear wall structural system subjected to static loading. In a companion paper, the algorithm is extended to consider dynamic loading, including seismic loading. The concept integrates the finite-element method and the first-order reliability method, leading to a stochastic finite-element-based approach. In the deterministic finite-element representation, the steel frame is represented by beam-column elements and the shear walls are represented by plate elements. The stiffness matrix for the combined system is then developed. The deterministic finite-element algorithm is verified using a commercially available computer program. The deterministic algorithm is then extended to consider the uncertainty in the random variables. The reliability of a steel frame with and without the presence of reinforced concrete shear walls is evaluated for the strength and serviceability performance functions. The results are verified using Monte Carlo simulations. The algorithm quantitatively confirms the beneficial effect of shear walls, particularly when the steel frame is weak in satisfying the serviceability requirement of lateral deflection. The algorithm can be used to estimate the reliability of any complicated structural system consisting of different structural elements and materials when subjected to static loading. The procedure will be useful in the performance-based design guidelines under development by the profession.

A hybrid reliability evaluation approach is proposed, consisting of the response surface method, the finite-element method, the first-order reliability method, and the linear iterative scheme when the load is applied dynamically to a complicated structural system. The method is especially applicable for realistic structural systems consisting of different types of structural elements and materials. The method was specifically developed to evaluate the reliability of a steel frame and reinforced concrete shear wall structural system subjected to seismic loading. The unique feature of this algorithm is that the earthquake loading can be applied in the time domain, enabling a realistic representation of the seismic loading conditions. The steel members are represented by beam-column elements and the shear walls are represented by plate elements. The deterministic algorithm, discussed in detail in the companion paper, is then extended to consider the uncertainty in the random variables. The reliability of a frame without and with RC shear walls is evaluated for the serviceability performance functions. The results are verified using Monte Carlo simulation. The method estimates the probability of failure very accurately, and is very efficient compared to simulation. The algorithm can be used to estimate the reliability of complicated structural systems consisting of different types of structural elements and materials under dynamic including seismic loading applied in time domain.

This paper presents a new confinement model based on strain compatibility and transverse force equilibrium. This new approach is capable of predicting the effectiveness of transverse reinforcement, which is key in modeling the behavior of high-strength concrete confined with high-yield-strength steel. The model is validated on test results from more than 200 circular and square large-scale columns tested under slow and fast concentric loading. In addition, results from about 50 square and circular large-scale columns tested under constant axial load and reversed cyclic bending were also used in the assessment of the model adaptability to seismic-type loading. All the predictions are in very good agreement with the experimental results. The model is especially effective in assessing the effectiveness of high-yield-strength steel.

In this paper, results from fire resistance experiments on five types of reinforced concrete columns are presented. The variables considered in the study include concrete strength [normal-strength concrete (NSC) and high-strength concrete (HSC)], aggregate type (siliceous and carbonate aggregate), and fiber reinforcement (steel and polypropylene fibers). Data from the study is used to determine the structural behavior of HSC columns at elevated temperatures. A comparison is made of the fire resistance performance of HSC columns with that of NSC and fiber-reinforced HSC columns. The factors that influence the thermal and structural behavior of HSC concrete columns under fire conditions are discussed. The results show that the fire resistance of a NSC column is higher than that of a HSC column. Also, the addition of polypropylene fibers and the use of carbonate aggregate improve fire resistance.

The work presented here investigates the possibility of using changes in the nonlinear vibration characteristics to detect damage in reinforced concrete elements. The nonlinear vibration characteristics were studied by conducting impact excitation vibration tests on reinforced concrete beams. The nonlinearities were detected by examining the changes in the fundamental frequency during the decay of an impulse response with time (and hence, with amplitude of vibration). The tests demonstrated that there is a change in nonlinear vibration behavior with damage. The change is greatest at low levels of damage. After the beam has been loaded to 27% of the failure load in three-point bending there is a reversal in the trend and a slight reduction in nonlinearity with further damage. These results indicate that the method is unlikely to be useful in detecting high levels of damage in reinforced concrete bridges, however, it is capable of detecting low levels of damage not visible to the naked eye, and so has potential for assessment of special structures where very high levels of structural integrity are required.