Journal of Structural Engineering

Significant progress has been made in the preceding two decades in the area of seismic engineering. Design codes are very quickly migrating from prescriptive procedures intended to preserve life safety to reliability-based design with less prescription intended to quantify risk associated with designs. Therefore, all stakeholders are given the opportunity to speak a common language (probability and risk) leading to structural designs that not only reliably preserve life safety after rare ground motions, but minimize damage after more frequent ground motions and thereby minimize life-cycle costs. Probabilistic performance-based design is in between traditional prescriptive design methods and full reliability-based design methodologies. The present paper provides an overview of a state-of-the-art model-code performance-based design methodology and casts this design procedure into multiple-objective optimization problems for single-story and multistory structural steel frameworks with fully and partially restrained connections. A methodology for applying an evolutionary (genetic) algorithm with radial fitness and balanced fitness functions is discussed in detail. A companion paper provides applications of the automated design algorithm to single-story frames and multistory frames with a variety of connection characteristics and beam-to-column moment capacity ratios.

Design codes are migrating from prescriptive procedures intended to preserve life safety to reliability-based design. All stakeholders are given the opportunity to speak the common language of risk and structural designs can be developed to not only reliably preserve life safety after rare ground motions, but minimize damage after more frequent ground motions (minimize life-cycle costs). A companion paper presents a methodology for using an evolutionary (genetic) algorithm with radial fitness and balanced fitness functions to generate solutions to optimal design problems formulated within a probabilistic performance-based design framework. The present paper outlines application of the automated algorithm to design steel frames with fully restrained and a variety of partially restrained connections. Comparison of optimal designs resulting from application of the algorithm proposed with those found in the literature is made. Detailed discussion of algorithm performance and response of the resulting optimized designs during pushover and time-history analysis is provided.

Due to variations in concrete properties, environmental conditions, and other factors, the rate of corrosion of reinforcing steel can be highly variable within a given structural component. By placing multiple corrosion rate sensors throughout a reinforced concrete (RC) bridge deck, this variability can be monitored and corrected for use in a reliability model. There is a limit, however, on the number of sensors that can be feasibly placed in a structure, due to economic and constructability constraints. The constraints on the design variables of a permanent structural health monitoring (SHM) sensor network can be used to formulate a multiobjective optimization problem. This investigation describes the formulation of this problem for a RC bridge deck to be outfitted with corrosion rate sensors. The total cost of sensor system installation and the maximum lifetime dispersion of corrosion current density serve as the two objective functions to be minimized. The design variables are the spacing between adjacent sensors and the unit cost of each sensor. A set of optimal (Pareto) solutions are found for various assumptions using a multiobjective goal seeking algorithm in conjunction with Bayesian updating and interpolation techniques. The set of optimal combinations of sensor spacing and unit cost provide the best tradeoff between total SHM system cost and performance for a given set of constraints.

This paper presents novel analytical solutions to describe the behavior of composite steel–concrete beams with partial interaction at elevated temperatures. The analytical model is derived by means of the principle of virtual work and, based on its strong form, solutions are derived in closed form for the cases of a simply supported beam and of a propped cantilever subjected to a generic regime of temperature. The materials are assumed to behave in a linear fashion, but their elastic moduli are modified to account for the degradation which the materials exhibit at elevated temperatures. The accuracy of the proposed solutions is tested against the results obtained by means of a finite-element method, as no directly applicable solutions appear to be available in the literature with which to validate the formulation. A refined 16 degrees of freedom finite-element is selected for this purpose, and its derivation and modification are outlined briefly. Applications are proposed to illustrate the ease of use of the analytical solutions to gain a better insight into the fundamental structural response, and to provide a convenient design tool. For structural design, the use of a simplified integration method to account for the thermal effects and for the material degradation has been considered in order to circumvent the complex calculation of section properties that can arise when the degraded elastic moduli and induced thermal gradients vary across the cross section. This simplification is valid in general, but it leads to slight underestimations of the structural deformation state at elevated temperatures for high levels of shear connection. A second application of the theory is given, in which the stress state due to thermal effects is superimposed with that due to external sustained loading. The load level to attain first yield of the member is shown to depend significantly on the stiffness of the shear connection.

A nonlinear finite-element analysis (FEA) model was developed in this paper based on the elastoplastic finite-element theory to analyze the load versus deformation relation of steel beam to concrete-filled steel tubular column connections. Six tests on steel beam to concrete-filled steel tubular (CFST) column connections using external ring after exposure to the ISO-834 standard fire were used to verify the theoretical model. The test parameters included the column cross-sectional type, the fire duration time, the level of axial load in the column, and the beam-column strength ratio. Each test specimen consisted of a CFST column and two steel beam segments in cruciform arrangement to represent an interior joint in a building. Three of the six composite connection specimens had circular cross sections and three had square cross sections. Five of the test specimens were simultaneously exposed to the standard ISO-834 fire condition. After they had cooled down to room temperature, each was tested under a constant axial load and a cyclically increasing flexural load. This paper presents an analysis of the experimental results to validate the FEA model and to evaluate the influences of different testing parameters on various characteristics of the beam-column connection performance. Comparisons between the predicted results and the experimental results indicate that the FEA model can predict the $P–\underset{̣}{\Delta }$ relations of steel beam to CFST column connections and the column lateral load resistance after fire with reasonable accuracy. Finally, the FEA model was used to make a parametric study of the influence of the various factors on the postfire behaviors of steel beam to CFST column connections.

This paper presents the experimental work on 18 steel-plate girders loaded predominantly in shear. The test girders were grouped into five series, two of which were hot-rolled steel universal column sections, and the other three were fabricated sections. One specimen from each series was tested under ambient temperature condition, and the other three at isothermal conditions at 400, 550, and $700°\mathrm{C}$ . The tests were to investigate the shear buckling behavior, the diagonal web tensile field action, and the development of plastic hinge mechanism in the flange plates of the girder panels. The experimental results were then compared with analytical and numerical finite-element predictions. The reported test results are believed to be the first of their kind in the field of structural fire resistance of steel members. The knowledge gained from both experimental and numerical works will hopefully shed some light on the column and beam web panel shear behavior in a steel beam-to-column connection.

A constitutive strain-based confinement model is developed for circular concrete columns confined with external fiber-reinforced polymer (FRP) composite jackets. The model includes the properties of the concrete microstructure through a series of relationships developed from experimental data and the fundamental stress-strain behavior of concrete. The secant concrete modulus was used in the model and expressed as a function of the secant modulus softening rate. In addition, the ultimate radial strain was related to the FRP composite effectiveness. The distinguishing feature of the model is that it includes the effects of FRP confinement on the concrete microstructure by evaluating the internal concrete damage using the axial, radial, and volumetric strains. The FRP-confined concrete model predicts the axial stress at any level of axial strain accurately for any normalized effective confinement stiffness of the FRP composite jacket as verified in comparisons with experiments.

Reinforced concrete slabs typically have relatively small tensile reinforcement ratios and are generally regarded as very ductile structural elements. This is not the case for slabs reinforced with low-ductility welded wire fabric. Such slabs fail with little warning by fracture of the reinforcement in a brittle and catastrophic manner and, as a consequence, many of the assumptions implicit in structural design are not applicable. Slabs containing low-ductility reinforcement lack robustness and cannot absorb the energy required to resist significant impact or blast loading. This paper outlines the importance of ductility in concrete structures and describes the observed failure mode and ductility of reinforced concrete slabs containing both normal and low-ductility reinforcement. For slabs reinforced with low-ductility welded wire fabric, failure is characterized by relatively little plastic deformation prior to collapse, with small rotational capacities of the critical regions and, consequently, little moment redistribution.

In reinforced concrete shear walls subjected to seismic loads the flexural, web shear and sliding shear mechanisms are activated. The measurements obtained from the tests of 11 wall specimens with aspect ratio 1.0 and 1.5 are used in order to set up analytical models that predict the deformations associated with each mechanism and the corresponding strength. The quantity and arrangement of the reinforcement of the specimens were chosen to ensure a flexural type of failure. It was observed during the tests that the web shear deformation increased until maximum strength was achieved. After the point of maximum strength, web shear deformations decreased, while sliding shear deformations along the base main flexural crack increased. Together with the previously noted web shear and sliding shear deformations, flexural deformations were also measured. The relationships proposed for the description of the strength and the deformation characteristics of these mechanisms derived from appropriately modified existing models along with new ones. The analytically calculated quantities are compared with the experimental measurements and a good match is noted. It is concluded that in shear walls with low aspect ratio sliding shear deformations appear at the base plastic hinge, even in the case where the flexural behavior initially prevails the response. Neglecting of such shear deformations results in underestimating shear forces at the base of other vertical structural elements.

The difficulties of modeling the vertical flexure of mechanically fastened wooden ship hulls as that of box beams, a mainstay of naval architecture when applied to iron and steel ships, can largely be overcome by factoring the incomplete composite action of timber components in terms of a reduced shear modulus, an increased shear lag, and a reduced sectional area in tension (owing to butt joints). Sample computations on a large wooden hull indicate that its deflection can be limited to about twice that of a completely composite hull if stiff fasteners (drift pins) are used at a much greater density than is typical of traditional construction. The lengths of timber pieces become severely limiting only if they are below $1∕5\mathrm{th}$ of hull length. The methodology has broad application to the preliminary design of many-piece timber box beam structures in general.

The use of special concentrically braced frames has increased since the 1994 Northridge and 1995 Hyogoken–Nanbu Earthquakes. However, past performance suggests limited ductility and energy dissipation in braced frame systems due to buckling of conventional braces. In order to address this limitation, three- and six-story concentrically braced frames with superelastic shape memory alloy (SMA) braces are studied to evaluate their seismic performance in comparison to traditional systems. SMAs are unique metallic alloys that have the ability to undergo large deformations while reverting back to their original undeformed shape providing recentering capabilities to the braced frame. Detailed analytical models of the frames with SMA braces are developed and two suites of ground motions are used to evaluate the structures with respect to interstory drift and residual drift. The results suggest that the SMA braces are effective in limiting interstory drifts and residual drifts during an earthquake, in part, due to the recentering nature of superelastic SMAs.

A detailed comparison of the recent full-scale tests conducted at Mississippi State University (MSU) using simulated wind loads on a portion of a standing seam metal roof and model-scale wind tunnel tests carried out at the University of Western Ontario of an aeroelastic “failure” model of the same roof system is presented. In spite of the significantly different approaches, the results obtained were remarkably consistent. The tests suggest that, at the roof corner, the E1592 uniform pressure test contains conservatism of about 50% for the roof system tested by both approaches; and up to about 80% for the other roof systems tested only at MSU. This conservatism arises if the roof system is required to withstand the code-recommended pressure applied as uniform pressure in the E1592 test, without accounting for the reality of the dynamic spatially varying properties of the wind-induced pressures. In general, the tests also suggest that influence surface approaches together with experimentally measured pressure distributions provide an excellent way of determining the effective loads on various components.

A mathematical model is proposed and its analytical solution derived for the analysis of the geometrically and materially linear two-layer beams with different material and geometric characteristics of an individual layer. The model takes into account the effect of the transverse shear deformation on displacements in each layer. The analytical study is carried out to evaluate the influence of the transverse shear deformation on the static and kinematic quantities. We study a simply supported two-layer planar beam subjected to the uniformly distributed load. Parametric studies have been performed to investigate the influence of shear by varying material and geometric parameters, such as interlayer slip modulus $\left(K\right)$ , flexural-to-shear moduli ratios $(E∕G)$ and span-to-depth ratios $(L∕h)$ . The comparison of the results for vertical deflections shows that shear deformations are more important for high slip modulus, for “short” beams with small $L∕h$ ratios, and beams with high $E∕G$ ratios. In these cases, the effect of the shear deformations becomes significant and has to be addressed in design. It also becomes apparent that models, which consider the partial interaction between the layers, should be employed if beams have very flexible connections.

Single point, self-drilling fasteners are commonly used in light-gauge steel constructions including domestic, agricultural, and industrial structures. Fasteners installed in practice often suffer from a variety of in situ conditions that can affect the capacity of the fastener, such as construction tolerances creating combined shear and bending in the fastener (rather than pure shear), overtorqueing the fastener which can cause stripping of the thread or shearing off the head, nonperpendicular insertion of the fastener and accidental impact on the structure creating dynamic loads on the fastener. This note describes a series of 120 single lap connection tests of single screw fasteners under a variety of the above-mentioned conditions, performed on fasteners of different gauge inserted into different thicknesses and grades of steel. A design equation is proposed to modify the nominal shear strength to account for these conditions.

The tensile capacity of concrete is usually neglected when calculating the strength of a reinforced concrete beam or slab, even though concrete continues to carry tensile stress between the cracks due to the transfer of forces from the tensile reinforcement to the concrete through bond. This contribution of the tensile concrete is known as tension stiffening and it affects the member’s stiffness after cracking and hence the deflection of the member and the width of the cracks under service loads. For lightly reinforced members, such as floor slabs, the flexural stiffness of a fully cracked section is many times smaller than that of an uncracked section, and tension stiffening contributes greatly to the postcracking stiffness. In this paper, the approaches to account for tension stiffening in the ACI, European, and British codes are evaluated critically and predictions are compared with experimental observations. Finally, recommendations are included for modeling tension stiffening in the design of reinforced concrete floor slabs for deflection control.