Journal of Structural Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

In the construction of reinforced concrete structures, bent reinforcing bars (rebars) are manually straightened to connect wall-slab joints in tall buildings, retaining wall-slab joints, and temporary openings, among other uses. Plastic deformation is unavoidable in the cold bending and straightening of rebars, and the work-hardening effect, the aging-hardening effect, and the Bauschinger effect weaken the rebars. Mechanical splices offer an alternative to the bending and straightening of rebars. In this study, tensile tests were conducted on cold-bent and straightened rebars to identify the resulting changes in yield strength, modulus of elasticity, tensile strength, and ductility. Cyclic loading tests were subsequently conducted on wall-slab joints to compare the structural performance of joints with mechanical splices with that of joints constructed with cold-straightened rebars. The test results indicate that the joints with mechanical splices exhibited typical flexural behavior and were equivalent in capacity to monolithic joints. In contrast, the joints with cold-straightened rebars developed a greater amount of bond cracks in the surface and spalling of the side cover concrete of the slab; these joints consequently displayed less strength and deformation capacity and lost stiffness more rapidly than did the joints with straight rebars. This phenomenon was apparent for joints with larger diameters and for higher-grade rebars. Therefore, joints with cold-straightened rebars should be used with caution in design and construction.

The results of a series of finite-element (FE) simulations and experimental studies are used to develop two prying models that predict the failure strength of thick-flange built-up T-stub connections with complete joint penetration (CJP) and fillet welds. A parametric study based on identifying the major geometric and force-related parameters that vary within current acceptable steel fabrication and design practices is used to select the cases for analysis. The strength prying models predict the capacity of thick-flange built-up T-stub connections with CJP and fillet welds for the failure limit state of the partial yielding tension flange followed by bolt fracture. The accuracy of the developed strength models are verified with FE results of built-up T-stubs designed for full strength compared with existing models reported in the literature. Based on the results of this study, a refined design procedure that takes into account the partial yielding in thick-flange T-stubs is proposed.

Twenty-seven wall-splice specimens reinforced with number (No.) 15 (0.6 in.) deformed bars were tested to evaluate the effect of transverse bar spacing on the tensile resistance of the lap-spliced reinforcement. All specimens were $21/2$ blocks wide and 13 courses tall and were constructed in running bond with all cells fully grouted. In all cases, the lapped bars were located within a single block cell and were either spaced 0, 25, or 50 mm (0, 1, or 2 in.) apart. It was determined that the calculated tensile resistance of the reinforcement was greater for bars that were in contact and furthermore, is insensitive to the magnitude of the transverse spacing provided in the case of noncontact lap splices. It would appear that both the American and Canadian provisions are appropriate for both contact and noncontact lapped bars given the resulting levels of conservatism.

This paper presents a new predictive formula for the shear capacity evaluation of reinforced concrete members subjected to combined axial, bending, and shear forces. The formula is based on a tensorial approach to concrete shear resistance that is very similar to the one used in the shear-enhanced fiber beam element. The total shear resistance is broken down into the contribution of concrete and contribution of shear reinforcement. The concrete contribution to the shear resistance is calculated using a normal-shear stress failure envelope. Normal (longitudinal) stresses are calculated from axial and bending forces acting on the concrete member. In the formulation, a number of simplifications are made to keep the formula as simple as possible but still sufficiently accurate. The resulting formulation, although capable of accounting for all of the major variables that influence the shear strength, including size effect, remains particularly simple and with a compact notation. The predictions of the proposed formula are compared with those used by the American Concrete Institute (ACI), the European Committee for Standardization (CEN) Eurocode, and the International Federation for Structural Concrete (fib) model code, and its accuracy is checked against a vast experimental database available in the literature. Results and comparisons are very encouraging and confirm the soundness of the underlying mechanical model. The capability of this model to provide a unified approach for reinforced and unreinforced members opens up the possibility to extend the application of the proposed formula to engineered cementitious composites, such as fiber-reinforced concrete.

There is a need to develop a reinforced masonry (RM) seismic performance database (SPD) of experimental results in order to facilitate adoption of RM seismic force-resisting systems (SFRS) in the next generation of performance-based seismic design (PBSD) codes. As a contribution to this SPD, and within a larger ongoing research program, this paper reports on recent shake table tests performed on lightly reinforced masonry shear walls. The test walls covered a range of design parameters to facilitate benchmarking, further performance investigation, and calibration of numerical models as well as future development of fragility curves within the context of PBSD. The design parameters of the walls were selected to meet the requirements of the lightly reinforced conventional construction category. This particular RM wall category is not permitted in moderate and high seismic zones in the Canada based on Canadian masonry design code. Within the context of PBSD, the wall seismic performances, in terms of damage levels observed during the shake table tests, were generally acceptable considering the levels of earthquake record intensities to which the walls were subjected. The walls did not experience any damage during low amplitude tests, corresponding to frequent minor earthquake events, incurred repairable damage under higher level tests, and maintained their integrity under the highest level record selected to mimic the seismic hazard levels of the highest seismicity zones in Canada. The tests also demonstrated the capability of lightly reinforced masonry shear walls to adequately dissipate energy through nonlinear flexural response. The test results are expected to contribute to the growing SPD of RM SFRS components, and to facilitate a better understanding of the behavior and performance of lightly reinforced masonry walls under realistic seismic loads.

A series of tests on the concrete-filled tubular circular hollow section (CHS) connections is presented in this paper. Four types (T, Y, K, and KT) of 12 connections were tested. The tests covered two different chord wall slenderness ratios (D/t) of 60 and 75. The test specimens were first welded by steel tubes to form connections, and then self-compact concrete was filled in the main chord in full length. The material properties of steel and concrete used in the test specimens were measured. Each connection was tested to failure with the branch member loaded in axial tension. Punching shear failure of chord member was the main failure mode observed during the tests. The test results were also compared with current AISC design predictions, which are only for circular hollow section steel tubular connections. The design strengths predicted by the current AISC standard are shown to be quite conservative for the test specimens in this study. Therefore, the contribution of concrete should be considered in the design because it has a significant effect on the ultimate bearing capacity of concrete-filled CHS tubular connections.

The slew of high profile engineering calamities in the past decade has demonstrated the disastrous consequence of progressive collapse. However, the low probability of such events actually occurring means it is uneconomical to spend extreme resources to design every building against progressive collapse. A more feasible proposition would be to consider alternative fall-back parameters such as secondary load carrying mechanisms that can help to reduce the severity of the collapse, should it actually occur. However, to date, very limited studies have been carried out to quantify the effectiveness of such secondary load carrying mechanisms in resisting progressive collapse, especially membrane actions developed in RC slabs. Therefore, a series of 6 one-quarter scaled specimens were tested and the failure modes, load-displacement relationships, load redistribution responses, and strain gauge results are presented herein. The contribution of each mechanism on the load-carrying capacity is discussed. A series of analyses are also carried out to better quantify the findings made in the study.

Since being pioneered in France in 1986, prestressed concrete girder bridges with corrugated steel webs have emerged as one of the most promising forms of steel-concrete composite bridges. Considerable theoretical and experimental studies on this type of bridge have been carried out since the 1970s. This paper presents a comprehensive review of these studies, covering relevant publications written in English, Chinese, and Japanese. The review is organized into topics covering the major characteristics of structural behavior, including the flexural, shear, and torsional behavior of this type of bridge, shear buckling of corrugated steel webs, dynamic and seismic behavior, etc. Some suggestions are put forward for future research on this promising form of steel-concrete composite bridge.

Hollow structural sections (HSS) are currently underutilized in seismic moment frame systems as possible alternatives to typical wide-flange members. To explore the potential use of HSS moment connections in seismic moment frame systems and evaluate connection detailing requirements, experimental tests of welded unreinforced and reinforced HSS-to-HSS connections are conducted under increasing cyclic rotations to failure. The behavior of the unreinforced connections is limited by their inability to isolate inelasticity to the beam member and panel zone region leading to eventual fracture at lower than anticipated drift levels. However, through plate and external diaphragm plate reinforced HSS-to-HSS moment connections are able to move the location of inelasticity into the HSS beam member resulting in desired beam plastic hinging. Although the unreinforced connections have limited ductility, the reinforced connection detailing leads to stable degradation of the moment capacity at increasing drift levels with fracture initiating only at very large rotations attributable to eventual onset of local buckling in the plastic hinge region. The through and diaphragm plates are found to have an important effect in ensuring that the sources of inelastic rotation are limited to the beam and panel zone region. Additionally, the use of reinforcement improves the connection stiffness. Overall, the study shows that fully welded HSS-to-HSS seismic moment connections are viable provided proper detailing of the connection is performed.

In existing wood strength prediction models for parallel to grain failure in timber connections using dowel-type fasteners, different methods consider the minimum, maximum, or summation of the tensile and shear capacities of the failed wood block planes. It is postulated that these methods are not appropriate since the stiffness of the adjacent wood loading the tensile and shear planes differs, and this leads to uneven load distribution among the resisting planes. A closed-form analytical method to determine the load-carrying capacity of wood under parallel-to-grain loading in small-dowel-type connections in timber products is thus proposed. For the wood strength, the stiffness of the adjacent loading volumes and strength of the failure planes subjected to nonuniform shear and tension stresses are considered. The effective wood thickness for the brittle failure mode is derived and related to the elastic deformation of the fastener. A mixed failure mode is also defined (a mixture of brittle and ductile) and depends on the governing ductile failure mode of the fastener. To help the designer, an algorithm is presented that allows the designer to calculate the resistances associated with predictions of the different possible brittle, ductile, and mixed failure modes. The proposed stiffness-based model has already been verified in brittle and mixed failure modes of timber rivet connections. In the research reported in this paper, an extended application is proposed for other small-dowel-type fasteners such as nails and screws. Results of nailed joint tests on laminated veneer lumber (LVL) and the test data available from the literature on glulam confirm the validity of this new method, and show that it can be used as a design provision for wood load-carrying capacity prediction of small-dowel-type timber connections.

High performance fiber reinforced cementitious composite (HPFRCC) materials exhibit strain hardening behavior under tensile loading. This strain hardening response occurs after the first cracking of the material. In this paper, experimental and parametric studies were conducted to assess the influence of the compressive strength, loading type, and tension reinforcement ratio ($\rho$) on the ultimate deformation characteristics of reinforced HPFRCC beams. The analytical and numerical results for simply supported beams with different amounts of tension reinforcement ratios under three different loading conditions are presented and compared with each other and with the available experimental data. The plastic hinge rotation capacity increases as the loading condition changes from the concentrated load in the middle to the uniform load, and it reaches a maximum in the case of two-point loading. The effect of loading type on the plastic hinge rotation capacity of the reinforced beams with a high amount of $\rho$ is not as significant as that of lightly reinforced beams. Based on the analytical results, new simple equations as a function of the tension reinforcement ratio and the loading type are proposed. Analytical results indicate that the proposed equations can be used with sufficient accuracy to calculate the plastic hinge rotation capacity of reinforced HPFRCC beams.

The use of empirically calibrated moment-rotation models that account for strength and stiffness deterioration of steel frame members is paramount in evaluating the performance of steel structures prone to collapse under seismic loading. These deterioration models are typically used as zero-length springs in a concentrated plasticity formulation; however, a calibration procedure is required when they are used to represent the moment-curvature $(M-\chi )$ behavior in distributed plasticity formulations because the resulting moment-rotation $(M-\theta )$ response depends on the element integration method. A plastic hinge integration method for using deterioration models in force-based elements is developed and validated using flexural stiffness modifications parameters to recover the exact solution for linear problems while ensuring objective softening response. To guarantee accurate results in both the linear and nonlinear range of response, the flexural stiffness modification parameters are computed at the beginning of the analysis as a function of the user-specified plastic hinge length. With this approach, moment-rotation models that account for strength and stiffness deterioration can be applied in conjunction with force-based plastic hinge beam-column elements to support collapse prediction without increased modeling complexity.

This paper investigates the structural performance of steel-concrete-steel (SCS) sandwich beams interconnected by channel connectors with large interval. Nine simply supported SCS beams with shear span/depth ratio between 1.0 and 5.5 were tested under static load. The contribution of the channel connectors was also studied. All the beams failed because of either tension plate failure in midspan (flexural failure) or tension plate failure in shear span (vertical shear failure). As shear span decreased, failure tended to be governed by shear, whereas for larger shear span, failure was governed by bending. The channel connectors were confirmed to directly affect the vertical shear resistance and ductile performance after critical diagonal cracking. On the basis of the observed failure mode, a mechanical model was developed to predict the ultimate shear resistance of SCS deep beams after critical diagonal cracking. The accuracy of this lower-bound approach was ascertained by comparing with the test results.

This paper describes the influence of steel fiber-reinforcement on the design of cost-optimized, prestressed concrete, precast road bridges, with a double U-shaped crosssection and isostatic spans. A memetic algorithm with variable-depth neighborhood search is applied to the economic cost of these structures at different stages of manufacturing, transportation, and construction. The problem involved 41 discrete design variables for the geometry of the beam and the slab, materials in the two elements, active and passive reinforcement, as well as residual flexural tensile strength corresponding to the fibers. The use of fibers decreases the mean weight of the beam by 1.72% and reduces the number of strands an average of 3.59%, but it increases the passive reinforcement by 8.71% on average, respectively. Finally, despite the higher cost of the fibers, their use is economically feasible since the average relative difference in cost is less than 0.19%.

Some existing qualification/validation data for alternatives to code-compliant light-frame cold-formed steel (CFS) shear wall construction were developed before requirements for test specimens with $1:1$ aspect ratios and the inclusion of a vertical panel joint. These requirements were introduced on the notion/assumption that walls with an included panel joint and $1:1$ aspect ratio provide a lower bound estimate of wall performance (strength, displacement, and toughness). In this technical note, data from three independent experimental test programs were analyzed to shed light on panel joint and aspect ratio effects on cold-formed steel frame shear wall performance. The analysis showed that the performance of walls without a panel joint and with aspect ratios as high as $2\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.:1$ provide reasonable lower bound measures of performance.

Although straw-bale construction originated in the midwestern United States in the late 1800s, the engineering of plastered straw-bale walls to resist in-plane lateral loads is a modern development. Relevant experimental studies are reviewed and allowable shears are developed for use in design for wind and seismic loading. Clay, soil-cement, lime, lime-cement, and cement plasters having various thicknesses and reinforced with welded steel wire mesh, woven steel wire mesh, or polypropylene mesh are considered. Experimental values are adjusted on the basis of moment-curvature analysis results to account for lower-bound model code material strengths, plaster thicknesses, and mesh reinforcement. A conventional model for shear strength is used to provide a preliminary assessment of true shear strength. The resulting design values and associated detailing requirements are summarized in a format suitable for implementation in model code provisions for straw-bale construction.

Because three-pinned circular steel arches are statically determinate, it is commonly thought that linear analysis is sufficiently accurate for predicting their in-plane elastic behavior and buckling. This paper shows that the linear analysis underestimates the in-plane structural response of three-pinned circular steel arches to the external load, but overestimates their elastic in-plane buckling load. To predict the in-plane structural behavior and buckling load of three-pinned circular steel arches correctly, nonlinear analysis is required.