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.

The solution of impact problems requires advanced computational techniques to overcome the difficulties associated with large short-duration loads. Such cases typically use the explicit time integration method because it provides a stable solution for problems such as the analysis of structures subjected to shock and impact loads. However, most explicit-based finite elements were developed for continuum models such as membrane and solid elements, which renders the problem computationally expensive. On the other hand, the development of fiber-based beam finite elements allows for the simulation of the global structural behavior with very few degrees of freedom, while accounting for the detailed material nonlinearity along the element length. However, explicit-based fiber beam elements have not been properly formulated, in particular for the case of the emerging force-based beam element. This paper develops two fiber plane beam elements that consider an explicit time integration scheme for the solution of the dynamic equation of motion. The first element uses a displacement-based formulation, whereas the second element uses a force-based formulation. For the latter case, a new algorithm that eliminates the need for iterations at the element level is proposed. The developed elements require the use of a lumped mass matrix and a small time increment to ensure numerical stability. No iterations or convergence checks are required, which renders the problem numerically efficient. The developed explicit fiber beam-column models, particularly the force-based element, represent a simple yet powerful tool for simulating the nonlinear complex effect of impact loads on structures accurately while using very few finite elements. The traditional implicit method of analysis typically fails to provide numerically stable behavior for such short-duration problems. Two correlation studies are presented to highlight the efficiency of the developed elements in modeling impact problems in which the material models consider the strain rate effect. These examples confirm the accuracy and efficiency of the presented elements.

This paper presents the results of an investigation of the decrease in residual axial force in corroded high-strength bolts in friction joints for steel bridge girders. The results of a finite-element analysis revealed that the root-mean-squared value of the thickness reduction near the washer was a useful parameter for evaluating the residual axial force for the corroded bolts. Furthermore, the effect of the corroded bolt’s shape on the residual axial force could be evaluated based on the thickness reduction near the washer. To verify the validity of this approach, measurements were carried out on corroded bolts cut from a steel bridge girder. The results indicated that the proposed method can estimate the residual axial force to an accuracy of $\pm 15\%$.

Experimental results are presented for a new method of reinforcing concrete columns with galvanized steel equal-angle (GSEA) sections. For the same cross-sectional area, a GSEA section has a higher second moment of area than a conventional steel bar, which leads to a higher bending stiffness of the GSEA reinforced concrete member. In addition, the area of confined concrete is higher in GSEA reinforced concrete members than in steel bar reinforced members, which results in higher strength and ductility. The experimental program involved testing of 20 square, high-strength concrete (HSC) specimens under concentric axial load, eccentric axial load, and four-point loading. The specimens were reinforced longitudinally with either four N12 (12-mm-diameter deformed steel) bars or four GSEA sections and transversely with R10 (10-mm-diameter plain steel) bars. The specimens were 800 mm high with a $210\times 210\mathrm{mm}$ square cross section. Fifteen specimens were tested under either a concentric or eccentric axial load. The remaining five specimens were tested under four-point loading. Effects of the type of longitudinal reinforcement, spacing of transverse reinforcement, and loading conditions on the behavior of HSC specimens were investigated and discussed. Experimental results showed that, in general, specimens reinforced with GSEA sections had higher load-carrying capacities than the specimens reinforced with steel bars. In addition, the postpeak load-deformation behavior was observed to be more pronounced in specimens reinforced with GSEA sections than in specimens reinforced with steel bars.

Low-ductility steel concentrically braced frames (CBFs) are widely used in buildings as a lateral resistance system in seismic regions around the world due to their high stiffness and strength. Steel CBF buildings have been observed to have reserve capacity after brace fracture in past earthquakes. However, it is difficult for most practicing engineers to evaluate the contribution of the reserve system to the collapse prevention performance of CBF buildings because of its complex inelastic behavior. This study proposes a method to define the reserve system of multistory low-ductility CBF buildings for earthquake loads by performing a static pushover analysis. The multistory CBF building is converted to an equivalent single-degree-of-freedom (SDOF) CBF system with induced reserve parameters. A simple mathematical model describing the ductility response spectra for the reserve system in moderate seismic regions is established from the results of an extensive parametric study employing the SDOF mode to represent multistory CBF buildings. The collapse prevention capacity of the reserve system for a multistory CBF building can be evaluated by comparing the period of the multistory CBF system with a threshold value. The proposed approach is validated by application to buildings in two case studies. The results demonstrate that the reserve capacity of a multistory CBF plays an important role in collapse prevention. This simplified procedure is intended for a rapid evaluation on the multistory CBF system using an approximate seismic response.

Tornadoes are considered a low-probability high-consequence event that can cause significant damage to community infrastructure, resulting in injuries and fatalities and ultimately creating long-term socioeconomic losses. Community resilience requires not only that the performance level of individual facilities be modeled and understood, but also their synthesis in space and time. Fragilities are conditional statistical distributions that provide the probability of exceeding analyst-defined performance levels as a function of hazard (or loading) intensity. Fragilities are becoming a core component in community resilience studies and enable the analyst to model performance of individual components or a cluster of the infrastructure, thereby supporting risk-informed decision making at the community level. In this paper, tornado fragilities for a portfolio of nonresidential buildings are developed. These fragilities, combined with several existing tornado building fragilities from the literature, are proposed to represent a first comprehensive minimum size portfolio of tornado building fragilities needed to model a community. For illustration, they are then used in the Centerville virtual community to perform community-level building damage assessment. This minimal-level portfolio of building fragilities lays the foundation for post-tornado recovery and resilience studies of a community, which eventually requires inclusion of all physical and nonphysical infrastructure.

A restoring force model is proposed that is applicable for predicting the hysteretic behaviors of the pre-pressed spring self-centering energy dissipation (PS-SCED) braces. Two Bouc-Wen models were used to portray the behaviors of the self-centering and energy dissipation mechanisms, and a linear elastic model was used to simulate the behaviors of the connecting ends. A genetic algorithm (GA) was used to identify the dominant parameters of this model on the basis of experimental data. Comparisons with experimental results for three prototype braces demonstrated that the proposed model could effectively portray the hysteretic behavior of the PS-SCED bracing system by selecting reasonable dominant parameters. Parametric analysis of the proposed model was performed, and a dimensionless function was defined to compare the influence of each parameter on the hysteretic behaviors of the bracing system. The results indicated that the energy dissipation parameter and the parameters that determined the stiffness of the loading and unloading phases could cause significant increases in the error function, whereas the other parameters could be set to constant values to simplify the model of the PS-SCED braces with the same configurations.

This paper presents an experimental study on dowelled connections in cross-laminated timber (CLT) and laminated veneer lumber (LVL) using ϕ20-mm mild steel dowels and internal steel plates. Connections designed to fail in brittle row shear and group tear-out were tested under monotonic loading to assess the validity of analytical models from literature and code provisions. Connections designed to provide nonlinearity before failure and thus produce ductility were tested under both monotonic and cyclic loading to study the influence of cyclic loading on ductility and the possibility of mode cross-over. It was found that cross layers in CLT improve ductility. Furthermore, mode cross-over from ductile response to brittle failure was observed in both CLT and LVL connections. Nevertheless, a good amount of ductility was achieved in all layouts (except the LVL connections designed for group tear-out failure) before cross-over to brittle failure occurred.

Steel plate reinforced concrete (SPRC) composite shear walls, which are composed of steel columns embedded in boundary elements and an embedded steel plate in the wall web, have been used in super-high-rise buildings. When subjected to rare earthquake loads, combined tension-bending-shear actions are often generated in the shear walls of super-high-rise buildings because of the increasing demand for a greater height–width ratio. Based on quasistatic tests on seven SPRC shear walls under tension-bending loads, the seismic behavior of SPRC shear walls with various steel-content ratios and axial tension ratios was investigated. The failure mode, strength and displacement capacity, stiffness degradation, shear deformation, damping coefficient, strain, and cracking of each test specimen are presented in detail. The failure mode is divided into tension-bending failure, anchorage failure, and torsional buckling failure according to the test results. The strength, stiffness, and ductility of the SPRC shear walls were significantly reduced with an increase in the axial tension ratio. The fiber beam-column finite-element (FE) model was simulated using MSC.MARC software. A comparison showed that the FE model predicted the load-displacement relationship, stiffness degradation, and ultimate capacity with a reasonable level of accuracy. Based on the test results, a design method is proposed for predicting the ultimate strength of the SPRC shear walls under tension-bending combined loads, and recommendations for improved anchorage design are proposed.

Large-scale tests on concrete-filled double-skin tubes, composed of pairs of concentric cold-formed rectangular hollow section members with the annulus between them filled with grout, are performed under air-blast loading. The recorded data from the field tests, along with further laboratory experimentation, are used to validate numerical models developed for single-degree-of-freedom and explicit finite-element analysis. The influence of several key variables on the response of concrete-filled double-skin tubes is ascertained by means of a parametric study. Overall, the ultimate moment capacity is found to have a stronger influence on the response than do mass, stiffness, hollowness ratio, or confinement factor. The results of this investigation are used to provide design guidance for the use of these members in protective design applications.

Wind tunnel tests were carried out in an open terrain to measure the wind pressure on six cylindrical roofs with the rise-span ratios of 1/4, 1/6, and 1/8 and length-span ratios of 1 and 2. Using the measured wind pressures on the roof, a parametric study using numerical simulation with finite-element (FE) models was conducted to investigate the effects of the structural span, rise-span ratio, length-span ratio, roof distributed mass, and mean wind velocity on the wind-induced response and equivalent static wind loads (ESWLs) of 432 single-layer cylindrical steel shells with supports along four edges. The results demonstrated that the wind directions of 0 and 45° were unfavorable in the view of the wind-induced response; the first several vibration modes made the most important contribution to the wind-induced response; and the background response was larger than the resonant response for most cases. In the balance of precision and simplicity of the parametric analysis for convenient application, the universal ESWLs realizing the equivalence of multiple peak response were expressed by two dominating vibration modes simplified as sine functions. With this method and parametric analysis, the pressure coefficients of the ESWLs were proposed as a function of reduced frequency for engineering applications.

Priestley’s oscillatory process theory is widely employed to model nonstationary excitation. However, the underlying coherence function of this theory can only be time invariant. To take into account the time-varying coherence function, the Sigma-oscillatory process theory and Wold-Cramer decomposition model have been introduced in literature. Due to its straightforwardness, the Wold-Cramer decomposition model is used in this study. Based on the Wold-Cramer decomposition model of nonstationary wind excitation, the alongwind response analysis framework of tall buildings to nonstationary winds is presented. The time-varying coherence function models of two downburst events are developed using the measured downburst data. The influence of time-varying coherence of nonstationary winds on the alongwind tall building response is investigated. Numerical examples show that the time-varying coherence function may lead to larger structural response compared with the corresponding time-invariant coherence function model. Considering the large variation of nonstationary downburst winds, the observation drawn from this study should be further evaluated using more measurement wind data. It can be expected that the time-varying coherence function may play an important role in better modeling the nonstationary winds and their load effects on structures.

A new structural system, called steel frame with cold-formed steel shear infill walls (SFIW), is developed by using cold-formed steel shear walls with steel sheathing plates on both sides as infill walls in steel frame. The purpose is to investigate the ultimate strength of SFIW by a theoretical analysis and to study the cyclic behavior of SFIW by tests. Cyclic loading tests were conducted to characterize the seismic behavior and failure mechanism of the proposed system, and to identify the shear resistance contribution ratio between the infill wall and the steel frame. The results showed that SFIW exhibited relatively high ultimate strength, good ductility, and substantial energy absorption capacity. Furthermore, plastic analysis of this structural system was performed to investigate the ultimate strength of SFIW. The calculated values of the ultimate strengths by the proposed analytical theories were compared with the tested values and reasonable agreement was observed.

In dry-stacked concrete masonry construction, units are laid without mortar, increasing the speed and reducing the cost of masonry installation. Despite these benefits, mortarless construction has not gained widespread acceptance as a viable alternative to traditional bonded masonry. This is largely attributed to the fact that the effect of surface roughness characteristics on the mechanical behavior of dry-stack masonry is not yet fully understood. To address this knowledge gap, the authors develop experimentally validated, predictive models that explicitly take the bed surface topography into account while representing the localized, nonlinear behavior at dry joints. With these validated models, a parametric analysis is completed to derive relationships between unit variables (e.g., material properties, surface roughness, and grout strength) and the performance of mortarless prisms under axial compressive loads. The derived relationships contribute to the knowledge base in terms of the behavior of dry-stacked masonry construction.

For reinforced concrete (RC) slabs without shear reinforcement, shear and punching can be the governing failure mode at the ultimate limit state if subjected to large concentrated loads. Shear and punching of RC slabs without shear reinforcement has been a challenging problem in assessment based on current standards. To examine a previously developed enhanced analysis approach, this study was conducted by applying a multilevel assessment strategy to a 55-year old RC bridge deck slab subjected to concentrated loads near the main girder in a field failure test. This strategy clearly provides the engineering community a framework for using successively improved structural analysis methods for enhanced assessment in a straightforward manner. The differences between analysis methods at different levels of assessment were discussed regarding one-way shear and punching shear behavior of the slab. The influences of parameters, such as boundary conditions, location of concentrated loads, and shear force distribution, were investigated.

This paper studies the seismic response of an innovative hybrid coupled shear wall (HCW) system, obtained through the connection of a reinforced concrete (RC) wall to two steel side columns by means of steel links. This structural solution is conceived to exploit both the stiffness of the RC wall, required to limit building damage under low-intensity earthquakes, and the ductility of the steel links, necessary to dissipate energy under medium-intensity and high-intensity earthquakes. The seismic performance of the proposed HCW system is evaluated through multirecord nonlinear dynamic analysis of a set of case studies designed on the basis of a specific ductile design procedure. The adopted finite-element models are illustrated and validated with experimental tests and more complex three-dimensional numerical models. A selection of results is presented and discussed to highlight the potential of the proposed innovative HCW systems and the possibility to develop a ductile mechanism in which plastic deformations are mainly attained in the steel links, with minor damages in the RC wall.

There is an opportunity to revisit and generalize classical theories for concrete cracking in light of increased interest in the use of nonconventional reinforcing materials and material efficiency. Fracture-based models to describe concrete cracking have potential but a limitation has been that many variables and different phenomena have to be incorporated to produce realistic material models. In this paper, an integrated fracture-based model (IFBM) is developed to predict the behavior of lightly reinforced concrete beams. The proposed model is a closed-form solution that integrates different local phenomena to more precisely describe the onset of cracking, crack propagation and crack rotation. The IFBM incorporates postcracking tensile stresses in the concrete, the bond-slip behavior between the reinforcement and concrete, and compression softening in the concrete compressive zone. The model can predict parameters such as the crack length development and crack mouth opening displacement in Mode I lightly reinforced concrete flexural specimens subjected to three-point bending. The predictions show a fairly good agreement with experimental results for small-scale reinforced concrete beams with low reinforcement ratios (0.15–0.5%). The ability of the IFBM to identify specific failure modes and to capture the crack propagation and crack rotation stages of behavior in lightly reinforced concrete beams are particular advantages. Such an approach provides a powerful tool to study the problem of minimum reinforcement requirements.

A timber composite beam consists of a cross-laminated timber (CLT) panel attached to a girder such as a laminated veneer lumber (LVL) beam. Under positive bending moment, part of the CLT panel acts as the flange of the LVL girder and resists compression. When the spacing between LVL girders becomes large, simple beam theory is not applicable because the compressive stresses in the flange vary with the distance from the LVL girder web, and the flange area over the web is more highly stressed than the extremities. This phenomenon is termed shear lag. For the design of steel-concrete composite sections, the effective flange width concept has been introduced into national and international design specifications. Despite the large number of studies regarding steel and concrete composite structures, comparative, comprehensive research has not been conducted on timber composite structures. In this study, a numerical model is developed and experimentally validated for analyzing different configurations of timber composite beams. Based on a parametric study, a formula is proposed for determining the effective flange width of timber composite beams.

A recent alkali silica reaction (ASR)/delayed ettringite formation (DEF) expansion model was modified to account for the load-induced precracks within a structure. Factors such as the orientation of the exposure face, the reinforcement details, and the location of the strain measurements were also considered. The model was implemented for environmentally exposed large-scale reinforced concrete specimens. The model provided a satisfactory simulation of the expansion results, with most results within the extremities of the measured data. The model successfully simulated the expansion strains inside and outside the development length zone. The expansion induced by ASR/DEF deterioration mechanisms in large structural concrete members could be effectively modeled using the minimalist semiempirical formulation.

A method to accurately calculate seismic shear forces in post-tensioned hybrid (PH) precast concrete walls under risk-targeted maximum considered earthquake (${\mathrm{MCE}}_{\mathrm{R}}$) level ground motions is essential to avoid unexpected damage or possibly failure. Although extensive research on seismic performance evaluation of PH precast concrete walls has been carried out previously, the postelastic amplification of shear forces due to higher-mode effects, relative to the design shear forces computed using the equivalent lateral force (ELF) procedure, has been addressed only in a limited manner. In the present study, first the equations to calculate shear forces in PH precast concrete walls, post-tensioned rocking (PR) precast concrete walls, and reinforced concrete (RC) shear walls are examined and their limitations discussed. Then an extensive numerical study of PH precast concrete walls designed using the equivalent lateral force procedure is carried out. The applicability of currently available equations to calculate seismic base shear forces in PH precast concrete walls are evaluated by comparing them with the results of nonlinear response history analysis (NLRHA) using a suite of 40 ground motions. Next, the shear force distribution over the wall height in PH precast concrete walls is also investigated. Regarding the base shear force, it is found that the existing methods developed for PR precast concrete walls and RC shear walls can produce reasonable estimates of the base shear. In the case of the wall shear force along the height of the wall, existing methods for RC shear walls are found to provide good estimates. Based on the results obtained, (1) an equation to calculate the peak base shear force and (2) a multilinear shear force design envelope to calculate wall shear forces along the height of the PH precast concrete walls are proposed under ${\mathrm{MCE}}_{\mathrm{R}}$-level.

Reinforced concrete (RC) flat-plate structures are vulnerable to punching shear failure at their slab-column connections, potentially leading to a catastrophic progressive collapse. In practice, the slab-column connection above an interior column, removed due to abnormal loads, may be subjected to a concentrated downward force because of the absence of the supporting column and further being pushed as a result of different live load intensities on individual stories. This force is different from the full design load that the column withstands in normal situations and, combined with the gravity load acting on the slab, may cause punching shear failure at the interior slab-column connection. This will further trigger failure propagation to the surrounding slab-column connections. This paper presents the experimental tests performed on two identical large-scale $2\times 2$-bay RC flat-plate specimens under an interior column removal scenario. A 5-kPa uniformly distributed load was applied first to the slab followed by an incremental concentrated force imposed on the slab-column connection above the removed interior column. The complete collapse-resistant behavior and load redistribution pattern of the specimens were investigated and are reported herein. Results show that more than 90% of the applied concentrated force is solely distributed to the four nearest adjacent columns. Three load-carrying mechanism phases, in the form of flexural, tensile membrane, and a combination of one-way catenary and dowel actions can be distinguished in resisting the applied concentrated load.

A comprehensive wind tunnel experimental study for $4:12$-sloped hip roofs of L- and T-shaped low buildings was carried out in a simulated open terrain exposure to examine wind load characteristics and assess the applicability of wind provisions specified by the American wind load standard for such geometries. Results show that considering roof shape effects, hip roofs with rectangular or complex plans perform differently from rectangular gable roofs and incur smaller local and area-averaged suction. For L- and T-shaped hip roofs, distinctive pressure distributions occur, particularly along roof eaves near building re-entrant corners, where considerable suction appears for the wind blowing toward these building re-entrant corners. Furthermore, the building length-to-width aspect ratio effects are in most cases moderate. Generally, the wind load provisions of the American wind load standard are adequate for L- and T-shaped hip roofs, except for the edge zone along ridge and hip with large areas, for which measured values exceed the code-specified wind pressure coefficients. Finally, it was found more appropriate to utilize the entire L- or T-shaped roof dimensions, as opposed to the rectangular section only, to determine the least horizontal dimension of such nonrectangular hip roofs for the definition of roof zones.

An experimental program investigating the potential of fiber-reinforced polymers (FRPs) as a strengthening option for glulam beams subjected to simulated blast loads was undertaken. A total of four different retrofit configurations were investigated along with a fifth alternative for restoring previously damaged beams. Increases in resistance and maximum deflection in the range of 1.35–1.66 and 1.3–1.62, respectively, were obtained when FRP tension laminates with and without confinement were used. Partial-length and full-length confinement prevented premature debonding and significantly altered the failure mode from simple tension failure to a combination of brash tension and compression failure while limiting the damage to a small region. The results also showed that the addition of FRP contributed to an increase of 1.17 in the tensile failure strain relative to the unretrofitted beams. A material predictive model that accounts for strain-rate effects was developed using experimental stress–strain relationships determined from coupon tests. The proposed model compared well with resistance curves obtained from full-scale testing of glulam beam elements subjected to simulated blast loading.

A method is proposed to indirectly determine values of the transfer length in cracked RC based on the observed load-deformation responses of RC tension members and their crack configurations under increasing load. The transfer length is characterized on the basis of results of several tension stiffening studies in the literature, including recent tests by the authors. In the present study, the transfer length is revealed to be strongly load-dependent, accounting for the gradual deterioration of the tension stiffening effect with increasing load. A solution is identified for the classical problem governing the relationship between the average crack spacing and the transfer length, and it is demonstrated to yield accurate estimates of average crack spacing for a wide range of RC tension and flexural members. Based on empirical crack spacing distributions of real RC members, a statistical approach is adopted to characterize the minimum and maximum spacing of cracks.

The feasibility and cost-effectiveness of various retrofit techniques to improve the seismic performance of an existing 35-story steel building are examined. Three types of supplemental energy dissipation devices are used in conjunction with basic retrofit measures to achieve the collapse prevention performance objective of current standards. Devices considered include fluid viscous dampers (FVDs), viscous wall dampers, and buckling restrained braces. The placement of the devices was kept the same in all three cases, considering overall architectural, programmatic, and constructability issues. The mechanical characteristics of the devices were selected using a simplified approach to achieve the same overall effective damping ratios and story drifts consistent with the targeted collapse prevention performance objective. The results of nonlinear dynamic analyses indicate that the FVD scheme was the most efficient for this structure in achieving the targeted performance goal and provided the most cost-effective means of improving the structural behavior and reducing economic losses for Level 2 basic safety earthquake hazard events. Future research needs related to the use of supplemental energy dissipation devices in existing buildings are also discussed.

In the present study, a semipassive resettable stiffness damper using motion amplification in the resetting mechanism is presented for seismic protection of short-period structures. Validation of the resetting mechanism with amplification was achieved through damper hystereses obtained from laboratory testing of a small-scale prototype. Numerical simulations were performed for a 3-story scaled building model subject to three benchmark earthquake ground motions. The results showed that increasing amplification led to more overall energy dissipated by the damper and a better structure response. However, it was shown that increasing amplification also resulted in more partial resets of the damper, and a reduction in the damper efficiency. Using a programmable valve in place of an on-off valve to reduce partial resets resulted in a poorer damper performance, which was attributed to the programmable valve being in the open position for a longer period of time during the earthquakes. It was concluded that motion amplification is an effective means of enhancing the energy dissipation of the damper in the presence of small piston displacements.

Laboratory tests have shown that current standardized design rules lead to overly optimistic results for calculated ultimate loads of plated structures, especially for internal elements such as box beams. Additionally, the validity of applying equations to compute local buckling resistance might be restricted to steel grades for which the yield strength is less than 500–700 MPa. Both the literature survey and recent experimental buckling tests using ultrahigh-strength steel (UHSS) show that the current design equations can be extended to at least a nominal yield strength of 1,000 MPa. Additionally, a new simplified equation for the reduction factor $\rho$ is proposed and a statistical calculation is presented to compare current and proposed calculation methods.