Journal of Structural Engineering

In previous studies, a finite-difference procedure was developed to analyze the dynamic response of simply supported normal reinforced concrete (NRC) slabs under blast loads. Ultra high performance concrete (UHPC) is a relatively new material with high strength and high deformation capacity in comparison with conventional normal strength concrete. Therefore, the finite-difference procedure for analysis of conventional reinforced concrete members against blast loads needs to be significantly adapted and extended to accommodate UHPC. In this paper, an advanced moment-rotation analysis model, employed to simulate the behavior of the plastic hinge of an UHPC member, is incorporated into the finite-difference procedure for the dynamic response analysis of reinforced UHPC slabs under blast loads. The accuracy of the finite-difference analysis model that utilized the moment-rotation analysis technique was validated using results from blast tests conducted on UHPC slabs. The validated finite-difference model was then used to generate pressure impulse (PI) curves. Parametric studies were then conducted to investigate the effects of various sectional and member properties on PI curves. Based on the simulated results, two equations were derived that can be used to normalize a PI curve. Further numerical testing of the normalization equations for UHPC members was then undertaken. The generated normalized PI curve, accompanied by the derived normalization equations, can be used for the purposes of general UHPC blast design.

The performance of RC frame members under earthquake loading may differ significantly from that of the frame. Even if the story drifts are not significant, a collapse mechanism may be observed because of extensive deformations in the beams, columns, or joints. Therefore, the seismic performance of each member should be investigated independently from that of the frame. In this experimental research, the seismic performance of one exterior and one interior beam-to-column connection region of a one-half–scale 3-story, three-bay RC frame is studied. The RC moment resisting frame building is designed following the code recommendations and earthquake excitations with different intensities applied sequentially to the frame by pseudodynamic testing method. At the end of the tests, under high seismic loads, the strength of members was exceeded, and a collapse mechanism formed. The joint shear distortions were significant for even the basic safety earthquake level, when soft soil conditions are present. In this experimental study, both the individual member responses and the performance of the frame are examined for earthquake loadings with different probabilities of exceedance. The interaction between the beam and joint responses, the effect of connection performance on the seismic behavior of the frame, and the influence of different soil conditions are also investigated.

Available capacity models for the assessment of seismic capacity of RC columns are mainly based on experimental tests under axial load and uniaxial bending. Furthermore, experimental tests have been mainly performed on columns representative of new constructions. To analyze the effects of biaxial bending on the seismic performances of existing RC columns with design characteristics nonconforming to present day seismic codes and practices, this paper presents the results of a first stage of tests on RC columns under both axial load and biaxial bending, which effectively simulate the seismic actions. In particular, this paper presents results of four tests on full-scale square columns reinforced with plain bars subjected to constant axial load and biaxial cyclic actions. Two different cyclic displacement paths were investigated: horizontal displacements with an inclination angle of 45 or 30°, with respect to the cross-section principal axes. The influence of biaxial bending actions on the global behavior of existing RC columns in terms of stiffness, strength, deformation capacity, and the effects of different horizontal displacements orientation on their performances are herein investigated and discussed; the comparison with experimental outcomes provided by uniaxial bending tests on companion specimens are also presented. Experimental findings showed that biaxial bending actions affect columns rotational capacity more than strength. The rotational capacity reduction because of biaxial bending actions, even if more significant than the flexural capacity reduction, is not currently taken into account in the available seismic guidelines related to existing RC buildings.

Over the last few decades, a significant effort has been made to achieve accurate evaluation of the resistance of RC elements subjected to pure shear or combined internal forces including shear. Regarding the latter, continuum models characterized by simplified stress fields have recently been applied for the evaluation of the ultimate capacity interaction diagram of rectangular RC cross sections undergoing combined axial force, bending moment, and shear force. This paper illustrates the natural progress of these studies and describes an analytical tool for the calculation of the ultimate strength of RC columns with a circular cross section. The proposed method is based on the application of the static theorem of limit analysis and takes into account both truss and arch effects. To ascertain the accuracy and reliability of the method, the relations developed are applied with reference to a large number of tests reported in the literature, and a comparison between the theoretical and experimental results is drawn. Finally, the predictions of the proposed method are compared with those of other simplified methods presented in the literature.

The study presents the results of an experimental program aimed at investigating the flexural behavior of externally plated RC beams. A total of 13 full-scale rectangular RC beams were tested. The effects on the behavior of the plate thickness, the anchorage of the plate to the beam through anchor bolts or side plates (collars), and the use of perforated plates instead of solid ones were investigated. The experiments indicated that the beam ductility increases as the plate thickness decreases, and the anchorage of the plate to the beam through bolts or collars proves to be an efficient method for preventing the premature plate peeling failure and achieving sufficient ductility in beams strengthened with thick solid plates. Anchor bolts had adverse effects on the ductilities of the beams with thin plates, and the use of perforated steel plates was found to be an efficient method for increasing the ductility of a strengthened beam. The beams repaired with perforated plates anchored to the beam with collars had load-carrying capacities close to those of the undamaged beams.

A new model is proposed by an assumption of quartic polynomial function to simulate the relative slip of steel bar and surrounding concrete in RC tensile member. In the new model, the postcracking behavior with a tension-stiffening effect is considered. Through the extension of the new model, the relative slip of the perfectly bonded interface of RC bending member can also be determined, based on which an interpolating function can be introduced to study the relative slips under different bending moments for corroded RC bending members. Finally, two experimental results of corroded RC beams under four-point bending are used to verify the new models.

The prediction of creep deformations of concrete and reinforced concrete structures has been a field of extensive research. Several different creep models have been developed. Most of the models were developed for constant concrete stresses; thus, in the case of varying stresses, a specific time integration is necessary. Currently, when modeling concrete creep, the engineering focus is on the application of sophisticated time-integration methods rather than choosing more appropriate creep models or considering the scatter of input parameters. For this reason, this paper presents a method to quantify the uncertainties of creep prediction originating from the selection of creep models, from the time-integration methods, and from input parameter uncertainty. By adapting variance-based global sensitivity analysis, a methodology is developed to quantify the influence of different sources of uncertainty. By application of the developed method, it is determined that selecting more appropriate creep models is of higher importance than choosing sophisticated time-integration methods.

In North America, the design of reinforced masonry (RM) shear walls as a seismic force-resisting system (SFRS) is very conservative in terms of quantifying the wall’s shear strength and ductility capacity. In the current study, eight full-scale squat RM walls were tested at McMaster University under quasi-static reversed cyclic loading to quantify their shear strength, idealized displacement ductility, drift-damage relationships, lateral stiffness degradation, and hysteretic damping. The test walls demonstrated shear-strength capacities up to 200% of those predicted by the Canadian Standards Association’s (CSA) masonry design code. In addition, the results demonstrated that the Canadian shear strength expression was, on average, 32 and 34% more conservative than the Masonry Standards Joint Committee (MSJC) and the New Zealand Standards (NZS) design codes for masonry, respectively. Moreover, the simplified modified compression field theory (SMCFT), adopted directly from the Canadian concrete design code, gave the most accurate predictions of the tested walls’ shear strength compared with masonry code expressions. Using bilinear idealization, the wall displacement ductility levels were found to range from 3.8 to 9.0 with corresponding equivalent viscous damping ratios of 8.7–19.2%, respectively. Design-oriented expressions were developed to relate the lateral stiffness degradation to ductility and drift levels. The wall drift-damage relationships were presented in fragility curve format for two predefined damage states. In general, the study attempts to shed some light on the behavior of this understudied SFRS (i.e., RM squat walls) to facilitate its adoption in the next generation of performance-based seismic design codes in North America.

An extensive statistical analysis is conducted on experimental data to achieve a formula for the strength and stiffness of masonry- infilled frames having central openings. For this, most of the available experimental data were collected and categorized based on their confining frames and opening types. The reliability of existing empirical relations was investigated, in which a reduction factor was suggested that shows the ratio of strength or stiffness of perforated infill to a similar solid one. The study shows that the relation recommended by the literature is the most accurate, among others, to estimate the lateral strength and stiffness of perforated infilled frames. Modified formulas derived from trend analysis of collected experimental data were proposed to determine the mechanical properties of perforated infilled frames. It is also shown that the reduction factor of the ultimate strength of infilled frames caused by the presence of openings depends highly on the material of the confining frames (steel or concrete), but the reduction factor of stiffness is not affected by the frame type. Therefore, different equations are proposed for the strength and stiffness of infills with openings.

This paper presents a novel solution procedure for inverse reliability problems with implicit response functions without requiring the derivatives of the response functions with respect to uncertain variables. This can be used to determine the unknown design parameters such that the prescribed reliability indexes are attained in the presence of mixed uncertain (both random and fuzzy) variables. The proposed computational procedure involves three steps: (1) probability of failure calculation using high-dimensional model representation (HDMR) for the limit state/performance function approximation, transformation technique to obtain the contribution of the fuzzy variables to the convolution integral, and fast-Fourier transform for solving the convolution integral; (2) reliability index update; and (3) most probable point update. The limit state/performance function approximation is obtained by linear and quadratic approximations of the first-order HDMR component functions at most probable point. This is a versatile method that can solve even highly nonlinear problems or the problems with multiple parameters. The methodology developed is applicable for inverse reliability analysis involving any number of fuzzy variables and random variables with any kind of distribution. The accuracy and efficiency of the proposed method is demonstrated through four examples involving explicit/implicit performance functions.

The contribution of higher vibration modes is important to the wind-induced response of long-span roofs, which makes it difficult to estimate the wind-induced response and equivalent static wind loads accurately. The authors have studied this topic for a number of years, and some research results are presented. The Ritz-proper orthogonal decomposition method for choosing dominant modes of the wind-induced response is proposed. Eigenvectors of fluctuating wind pressure obtained from the proper orthogonal decomposition are regarded as the spatial distribution of the load to generate Ritz vectors. The background and resonant response are redefined on the basis of the modal acceleration method, and a method is proposed to calculate the correlation coefficient between the background and the resonant responses. Target responses are expressed as the linear combination of basis vectors, which are the dominant eigenmodes of wind pressure and structural vibration modes representing the background and the resonant responses, respectively. The combination factors of these basis vectors are determined with the least-squares approximation method. As a case study, these methods are applied in a single-layer reticular shell.

Transmission tower collapses caused by strong winds or severe freezing rains frequently occur in many parts around the world. To study the mechanical behavior and failure mechanism of tower structures under such environmental loads, three pairs of tower subassemblages, including one single-panel pair and two double-panel pairs, were designed and statically tested at Tongji University. The difference between the two specimens in each pair was that one had an additional diaphragm as a method of retrofitting while the other did not. One pair of double-panel specimens was tested to simulate and capture the failure pattern of transmission towers subjected to wind load. Correspondingly, two pairs were used to investigate the relevant response of structures under the combination of vertical ice and wind loads. A comparative study on strains at critical sections and local and global deformations as well as failure modes of the tower subassemblages was carried out. The favorable effects of adding the diaphragm, in terms of improving the structural performance of the tower subassemblages, were emphasized in the study. The test results showed that the complex interaction between the main legs and the diagonal bracing members significantly affect the overall stability performance of the towers and their behavior at the ultimate limit state. The study demonstrated that the diaphragms comprehensively improved the mechanical behavior and increased the load-carrying capacity of the transmission tower structures. For specimens subjected to simulated wind load only, the bearing capacity increased by 22.1% after adding horizontal diaphragms. On the other hand, for double- and single-panel specimens subjected to simulated ice and wind loads, the corresponding increases were 17.6 and 18.3%, respectively. The implications of the test results and their relevant practical applications in the wind- and ice-resistant design of transmission towers are summarized and discussed in this paper.

This paper presents the seismic performance of buildings with a hybrid bracing system in which buckling-restrained braces (BRBs) are used at the lower stories with the higher level of ductility demands, and conventional buckling-type braces consisting of steel hollow structural sections (HSS) are used in the upper stories with relatively smaller ductility demands to minimize the probability of their fracture under reversed cyclic displacements. This type of hybrid braced frame (HBF) could prove economical, especially for retrofitted buildings in seismically active regions. A series of nonlinear time-history analyses was conducted to investigate the seismic performance of 3- and 6-story buildings with the hybrid bracing system. The main parameters studied are the seismic design parameters, maximum interstory drift ratios, fracture response of the HSS, and the optimal deployment of the BRBs. The seismic performance of the HBFs was compared with conventional concentrically braced frames and buckling-restrained braced frames (BRBFs) designed according to the current code provisions. Results showed that using BRBs at only a few lower levels of the HBFs gives similar performance to that of BRBFs. The incident of brace fractures in conventional HSS can be delayed or completely eliminated by using the proposed hybrid bracing systems.

A new self-centering beam-to-column connection is proposed. The connection uses posttensioned high-strength steel bars to provide self-centering capability and carefully designed energy-dissipation (ED) elements that consist of steel cylindrical pins with an hourglass shape. The proposed ED elements have superior ED and fracture capacity, and are placed between the upper and the bottom flanges of the beam such that they do not interfere with the composite slab. A simplified performance-based procedure was used to design the proposed connection. The connection performance was experimentally validated under quasi-static cyclic loading. The specimens were imposed to drift levels beyond the expected design ones to identify all possible failure modes. The experimental results show that the proposed connection eliminates residual drifts and beam damage for drifts lower than or equal to 6%. A simplified analytical procedure using plastic analysis and simple mechanics was found to accurately predict the connection behavior. Repeated tests on a connection specimen were conducted, along with replacing damaged ED elements. These tests showed that the proposed ED elements can be easily replaced without welding or bolting, and hence the proposed connection can be repaired with minimal disturbance to building use or occupation in the aftermath of a major earthquake.

Flexural tests on full-scale I-shaped beams, built up from high-strength steels (HSB800 and HSA800) with a nominal tensile strength of 800 MPa, were carried out to study the effect of flange slenderness on flexural strength and rotation capacity. The primary objective was to investigate the appropriateness of extrapolating current stability criteria (originally developed for ordinary steel) to high-strength steel. For comparison purposes, specimens with ordinary steel (SM490) were also tested and showed sufficient flexural strength and rotation capacity in accordance with the AISC specification. The performance of high-strength steel specimens was also very satisfactory from the strength, but not from the rotation capacity, perspective. The inferior rotation capacity of high-strength steel beams was shown to be directly attributable to the absence of a distinct yield plateau and the high yield ratio of the material. When a higher rotation capacity is required as in plastic design, the testing clearly showed that high-strength steel beams were vulnerable to brittle fracture when full-height transverse stiffeners were welded to the tension flange in the plastic hinge region. Residual stress measurements reconfirmed that the magnitude of the residual stress is almost independent of the yield stress of the base metal.

This paper deals with the ultimate strength and design of fixed-ended lipped channel columns experiencing local-distortional buckling mode interaction. First, the paper reports the results of an experimental investigation involving a set of 26 columns with several cross-section dimensions and yield stresses that were tested to determine their failure loads and also to provide experimental evidence of the occurrence of local-distortional mode interaction. These results consist of the column geometries, material properties, initial geometric imperfections, nonlinear equilibrium paths, and ultimate strength values. Then, after comparing the experimental column ultimate loads with the estimates provided by the current direct strength method (DSM) design curves against local and distortional failures, which clearly show that they lead to inaccurate and often very unsafe ultimate strength estimates, the paper presents and assesses the quality of DSM-based design procedures based on approaches providing nominal strengths against local-distortional and distortional-local interactive failures. Next, an in-depth comparison is made between all the experimental ultimate strength results available in the literature and their estimates provided by the preceding DSM design procedures. Finally, the paper closes with design considerations and recommendations, motivated by the conclusions drawn from this investigation.

The determination of the state of internal stress is important to define the rigidity of a tensegrity structure and its stability. Several methods can be used; some are based on direct measurements of the forces in the elements, but are not easily transferable to a real structure. The authors opt for indirect measurement techniques, which seem more appropriate for implementation on-site. One can consider the vibratory analysis of the elements, the vibratory analysis of the whole structure, or the analysis of the structure’s behavior under static loading. Here, the node displacement fields of a tensegrity structure in different states of self-stress under several strategies of static loadings is studied by comparing the measurement obtained by a tachometer with simulations. The aim of this work is to show the feasibility of a displacement field to identify the state of self-stress by this analysis. It is shown that under certain conditions, plans can be made to replace the direct measurement of the forces by indirect analysis.

Current seismic design codes provide empirical equations for the calculation of the fundamental period of a framed structure primarily as a function of height, and do not consider the effect of beam-column joint stiffness. In the literature, far too little attention has been paid to incorporate this effect in approximate formulas to calculate by hand the natural periods of semirigid steel frames, which is the aim of this paper. The proposed formulas have been developed after modifications of existing formulas in the literature for rigid-jointed plane steel frames. Then, the accuracy of these approximate formulas was verified by a finite element analysis using ABAQUS software. Finally, a parametric study was conducted to highlight the effect of semirigid connections on the natural periods of vibration of plane steel frames.

The strength of thin-walled stainless steel columns has been investigated extensively over the last few years. In European standards, the concept of section classification for determining the cross section capacity is used. In this system, for Class 4 cross sections, the effective width method (EWM) must be used to account for the effect of local buckling. Because of the complexity and limitations of this method, other methods have been developed, such as the direct strength method (DSM) for cold-formed thin-walled profiles and the continuous strength method (CSM), initially established for members made of nonlinear metallic materials. In the CSM, to take advantage of strain hardening, a deformation-based design approach using a continuous relationship between the cross-sectional slenderness and the cross-sectional deformation capacity is used. To a large extent, the CSM yields accurate predictions, especially in the low slenderness range where the current DSM design procedures for members submitted to pure compression tend to produce conservative predictions for materials with pronounced strain hardening such as stainless steel alloys. The present paper presents an extension of the traditional DSM, which provides accurate design strength predictions in the low slenderness range for stainless steel thin-walled section columns failing by distortional, local, and overall buckling. It contains practical information concerning the reference experimental data and draws conclusions about the justification of the proposed analytical formula. The paper is divided into three main parts: the description of the database, the establishment of the design model, and a reliability analysis of the method.