Journal of Structural Engineering

Steel storage racks, made of cold-formed steel, are used extensively in industry for storing goods. Two main racking systems prevail, referred to as selective racks and drive-in racks. International racking design codes deal mainly with selective racks, but limited design guidelines are available for drive-in racks. Drive-in racks require minimum floor space by storing pallets one after the other with no space between them. The forklift truck drives into the rack to store the pallets on the first-in, last-out principle. To allow forklift passage, drive-in racks can be braced only at the back (spine bracing) and at the top (plan bracing) in the down-aisle direction, resulting in a complex slender structure with poorly understood three-dimensional (3D) behavior and increased risk of collapse. Tests on drive-in rack systems to accurately capture their 3D behavior have not previously been available in the literature. This paper presents experimental results from full-scale tests conducted on a complete drive-in rack system. Experimental investigations of the load transfer and relative stiffness under various horizontal loading conditions are presented. Experiments have been performed on loaded and unloaded racks.

Steel drive-in racks are susceptible to structural failure from accidental impact by operating forklift trucks. Under impact, the upright bends and the supported pallets may drop through the rack to cause structural collapse if the bay opening exceeds the pallet bearing width. This drop-through limit state has not been considered in existing rack design standards. This paper proposes a simple equation to calculate the equivalent static impact force based on recent tests and nonlinear dynamic analysis of drive-in steel racks. An impact load factor is developed on the basis of a structural reliability assessment, taking into account the uncertain nature of the impact force, structural resistance, and models used in structural analysis. In design practice, the bay opening is determined from factored impact loads and is not to exceed specified limits.

The seismic performance of low-rise frames with energy dissipative semirigid connections is evaluated under far and near field artificial ground motion records. Four different connection capacities are employed in the frames and are characterized by two different moment-hardening ratios and two different hysteresis models. Nonlinear time-history and Fourier analyses are utilized to assess the seismic performance of 24 semirigid and 2 rigid frames. In the nonlinear time-history analyses, nine far field and nine near field artificial ground motion records generated from three different attenuation relations are used. Finally, the seismic performance of the rigid frames and the semirigid frames are compared, and the conditions under which the semirigid frames show better seismic performance than rigid frames are investigated. The results show that 25 of 26 sample frames satisfy all acceptance criteria and exhibit reliable seismic performance.

This work aims to enhance the energy-dissipation capacity of classical rectangular T-stubs by proposing an hourglass shape for the T-stub flange according to the approach usually adopted for added damping and stiffness (ADAS) devices. A new type of axial damper is developed. First, a mechanical model of the device is set up and a finite-element model is carried out in ABAQUS code. The accuracy of both models is verified through comparison with experimental results. Next, on the basis of cyclic tests, the improvement of the energy-dissipation capacity of classical T-stubs provided by the proposed approach is quantified, and the low-cycle fatigue curves are determined with reference to the case of both T-stubs on rigid support and of coupled T-stubs. The results of the work also represent a useful tool for designing a dissipative double split tee connection.

As part of a major study on the seismic response of bridge systems with conventional and advanced details, a large-scale model of a 4-span bridge incorporating several innovative plastic hinges was recently tested on shake tables at the University of Nevada, Reno. The bridge model included six columns, each pair of which utilized a different unconventional detail at the bottom plastic hinges: shape memory alloys (SMAs), engineered cementitious composites (ECCs), elastomeric pads embedded into columns, and posttensioning tendons. The bridge model was subjected to two horizontal components of simulated earthquake records of the 1994 Northridge earthquake in California. More than 340 channels of data were collected. Test results showed the effectiveness of the innovative materials in reducing damage and permanent displacements. The damage was minimal in plastic hinges with SMA/ECC and those with built-in elastomeric pads. Conventional reinforced concrete plastic hinges were severely damaged because of spalling of concrete and rupture of longitudinal and transverse reinforcement.

This paper is motivated from the wider need in system identification studies to identify and interpret the eigenvalues of seismically isolated bridges from field measurements. The paper examines the transverse eigenvalues of multispan bridges which are isolated in both transverse and longitudinal directions at all supports including all center piers and end abutments. The paper shows that regardless of the value of the longitudinal isolation period of the deck, the length of the bridge, and the number of spans, the first transverse (isolation) period is always longer than the longitudinal isolation period of the deck. This result cannot be captured with the limiting idealization of a beam on continuously distributed springs (beam on a Winkler foundation) which yields the opposite result of the first transverse period always being shorter than the longitudinal isolation period. This fundamental difference between the response of a flexural beam supported on distinct, equally spaced springs and that of a beam supported on continuously distributed springs has not received the attention it deserves in the literature of structural mechanics-dynamics. Finally, the paper shows that the first normalized transverse eigenperiod of any finite-span isolated deck follows a single master curve and the solutions from all configurations are self-similar and are not dependent on the longitudinal isolation period or on whether the deck is isolated on elastomeric or spherical-sliding bearings.

The factors that significantly influence the three-dimensional analysis of arch dams are identified: the semiunbounded size of the reservoir and foundation-rock domains, dam-water interaction, wave absorption at the reservoir boundary, water compressibility, dam–foundation rock interaction, and spatial variations in ground motion at the dam-rock interface. Through a series of example analyses of actual dams, it is demonstrated that (1) by neglecting water compressibility, the stresses may be significantly underestimated for some dams or overestimated for others; (2) by neglecting foundation-rock mass and damping, the stresses may be overestimated by a factor of 2 to 3; and (3) spatial variations in ground motion, typically ignored in dam engineering practice, can have profound influence on the earthquake-induced stresses in the dam. This influence obviously depends on the degree to which ground motion varies spatially along the dam-rock interface. For the same dam, this influence would differ from one earthquake to the next, depending on the location and depth of the causative fault relative to the dam site. How to select damping values for the dam concrete and foundation rock is also addressed.

Elastomeric and lead-rubber bearings are two commonly used types of seismic isolation devices. During seismic events, some of the bearings in an isolation system will be subjected to large axial compressive loads, caused by gravity plus overturning forces, accompanied by simultaneous large lateral displacements. However, the critical load capacity of elastomeric bearings has been shown to reduce with increasing lateral displacement. The design of isolation systems composed of these types of bearings therefore requires that stability at the maximum displacement be demonstrated. The current procedure to assess the stability uses a ratio of areas, referred to as the overlapping area method, to determine the critical load capacity at a given lateral displacement that must be greater than a combination of axial forces imposed on the bearing. Although the overlapping area method provides a simple means of calculating the critical load at a given lateral displacement, it lacks a rigorous theoretical basis and has not been experimentally verified for bearings with shape factors representative of those used for seismic isolation (i.e., 10–30) or for lead-rubber bearings. Experimental testing and detailed nonlinear finite element analysis were employed to investigate the critical load capacities of an elastomeric bearing and a lead-rubber bearing with shape factors of 10 and 12, respectively, at large lateral displacements. The results of these investigations showed the lead core has a negligible effect on the critical load over a range of lateral displacements corresponding to 150–280% shear strain in comparison with the elastomeric bearing. The overlapping area method is shown to conservatively estimate the critical load capacity of this bearing in comparison with the experimental results.

This paper provides an experimental validation on the analytical modeling and design of unbonded posttensioned precast concrete coupling beams. Previous test results from a series of floor-level coupling beam subassembly specimens are compared with the predicted results from a fiber-element analytical model. The test results are also used to validate an idealized coupling beam end moment versus chord rotation relationship that was developed as a design tool following basic principles of equilibrium, compatibility, and constitutive relationships. Based on the experimental and analytical comparisons, the paper presents a seismic design approach to achieve the desired lateral strength, stiffness, ductility, and energy dissipation capacity in unbonded posttensioned precast coupling beams.

Experimental research was carried out regarding the flexural behavior of deep beams cast with plain and fibrous concrete with hooked steel fibers, and subjected to monotonic vertical loads. Four deep fiber-reinforced concrete beams were cast. Two of them were made of plain concrete with main and web steel reinforcements (RC), and two were made of hooked steel fiber-reinforced concrete (SFRC) with main steel reinforcements. The experimental results show the brittle behavior of reinforced deep RC members characterized by crushing of concrete struts and fracture of web steel bars. SFRC deep beams exhibit higher strength and, above all, ductility with respect to RC members due to the bridging actions of fibers across main and secondary cracks. From the analytical point of view, a softened strut-and-tie macromodel able to reproduce the flexural behavior (in terms of multilinear load-deflection curves) of deep beams in plain and fibrous concrete and with the presence of horizontal steel bars, including softening of compressed struts and yielding of steel bars, is presented. The model is able to take into account the tensile behavior of main bars embedded in the surrounding concrete and the softening of the compressed strut, the arrangement and percentage of the steel bars, and the percentage and aspect ratio of steel fibers. The experimental results obtained here and those available in the literature are compared with the results obtained through the proposed model and with the other models given in the literature, showing good agreement. Finally, a validation of the proposed model is made numerically by using a nonlinear finite-element program (ATENA-2D) able to analyze the flexural behavior of deep members.

Precast spandrel beams are often used on the perimeter of precast buildings to support the precast floor units. These elements are typically not considered part of the lateral force resisting system. However, the presence of the spandrel beams in the floor system may modify the strength, stiffness, and deformation capacity of the precast floor diaphragm. The nature of this response is highly dependent on the characteristics of the details connecting the spandrel to the precast floor system. These details are often welded connections used primarily for erection stability and designed without diaphragm action in mind. With emerging design methodologies for precast diaphragms requiring better-defined performance, the impact of the spandrel beams must be accounted for. Accordingly, analytical research is presented here that examines the effect of spandrel-beam-connecting details on the global characteristics and local demands of a flexure-controlled precast floor diaphragm. Design recommendations are provided.

Despite much experience that low-rise wood buildings are vulnerable to damage by extreme wind events, few such structures have been tested in fullsize to understand how they respond to wind loads as a whole system. This paper presents a study that measured internal force flows throughout the framing of a typical North American single-story structure with platform construction. Applied forces were concentrated horizontal loads normal to walls and patches of gravity loads on the sloped roof. Two series of load cells were embedded into the system, between the roof trusses and supporting walls and between the floor platform and the foundation. It was observed that even localized external forces have effects that propagate through the entire system. For instance, horizontal loads applied near eave level or to the roof were reacted at the top of the foundation around the entire wall perimeter of the building footprint, both parallel and transverse to the applied loading. For vertical loads, measurements showed that systemic effects dominate the response. Observed three-dimensional behavior stems from the relatively stiff interconnection of roof, wall, and floor platform substructures.

Bearing or compressive-strength capacity perpendicular to the grain of timber beams is a troublesome issue. Not only do many different load cases occur in practice that are not covered by structural timber design codes, but also these codes provide only a basic provision and vary throughout continents. Code design rules require the standardized compressive or bearing strength to be determined by test standards. An assessment of the results of standard test methods of the European Union, North America, and Australia/New Zealand shows incompatibility. It is demonstrated how previously incompatible results can be made compatible by using a physical model and some calibration tests. The proposed model offers a consistent and simple way to bridge the differences between both test standards and structural design codes.

This paper presents an optimization approach to the design of simply supported, post-tensioned, prestressed concrete I-girder bridges. The objective is to minimize the total cost of the structure, considering cost of materials, fabrication, and installation. For a particular girder span and bridge width, the design variables considered for cost minimization of the bridge system are girder spacing, various cross-sectional dimensions of the girder, number of strands per tendon, number of tendons, tendon layout and configuration, slab thickness, slab rebar, and shear rebar for the girder. Explicit constraints on the design variables are developed on the basis of geometric requirements, practical conditions for construction, and code restrictions. Implicit constraints for design are formulated as per the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications. The optimization problem is characterized by having a combination of continuous, discrete, and integer sets of design variables and multiple local minima. An optimization algorithm, evolutionary operation (EVOP), is used that is capable of locating directly with high probability the global minimum without requiring information on gradient or subgradient of the objective function. The present optimization approach is used for a real-life bridge project, leading to a feasible and acceptable design resulting in around 35% savings in cost per square meter of the deck area. Computational time required for optimization of the present problem is only a few seconds. Because constant design parameters have influence on the optimum design, this cost minimization procedure is performed for a range of such parameters.

This paper deals with the accurate evaluation of complete three-dimensional (3D) stress fields in beam structures with compact and bridge-like sections. A refined beam finite-element (FE) formulation is employed, which permits any-order expansions for the three displacement components over the section domain by means of the Carrera Unified Formulation (CUF). Classical (Euler-Bernoulli and Timoshenko) beam theories are considered as particular cases. Comparisons with 3D solid FE analyses are provided. End effects caused by the boundary conditions are investigated. Bending and torsional loadings are considered. The proposed formulation has shown its capability of leading to quasi-3D stress fields over the beam domain. Higher-order beam theories are necessary for the case of bridge-like sections. Various theories are also compared in terms of shear correction factors on the basis of definitions found in the open literature. It has been confirmed that different theories could lead to very different values of shear correction factors, the accuracy of which is subordinate to a great extent to the section geometries and loading conditions. However, an accurate evaluation of shear correction factors is obtained by means of the present higher-order theories.