Journal of Structural Engineering

The load-bearing behavior of a symmetric spindle-shaped Tensairity girder with 5-m span and thin chords is studied experimentally, numerically, and analytically. The influence of the air pressure on the load-deflection behavior is investigated for homogeneous distributed load, asymmetric distributed load, and central local load. An m-shaped deflection with two maxima at about one- and three-quarter of the span was obtained for homogeneous distributed loads whose distribution is not linearly dependent on the applied load. The slope of the load-deflection curve as well as the maximal load increases with increasing air pressure, demonstrating the stabilizing role of the inflated hull. An analytical model based on two beams coupled by an elastic foundation with air pressure-dependent properties is presented for the homogeneous distributed load case, and simple predictions for the average displacement and the maximal load are given. The model reveals the subtle interplay between the chords and the inflated hull leading to the peculiar displacement distribution of the system. Finite-element analysis shows the limiting influence of the low fabric shear modulus on the stiffness and load-bearing capacity of the Tensairity girder for local and asymmetric distributed load. The investigated spindle-shaped Tensairity girder is optimal for homogeneous distributed loads, where a live load to dead load ratio of more than 50 has been achieved.

The analysis of a cable with a small bending stiffness is a problem encountered in many engineering applications such as the fatigue assessment of stay cables, the modeling of pipeline laying operation, or the determination of bending stresses in drillpipe assemblies. Because this phenomenon is modeled by a singularly perturbed equation, standard numerical techniques fail to solve these problems efficiently. As an alternative, provided the complexity of the analytical developments does not preclude their application, these problems may be tackled with appealing analytical procedures such as matching asymptotics or multiple scales. Otherwise, advanced numerical simulations combining patching asymptotics within a numerical framework are the only possible approach for problems where the governing equations are too complex. Patching asymptotics also feature a number of merits such as the possibility of using a boundary layer with a finite extent. Aiming at a better understanding of this latter technique, the purpose of this paper was to determine the solution of a cable with a small bending stiffness. Interesting details about patchability conditions and about how to restore higher derivative continuity are included. The accuracy of the patching asymptotics approach is also compared with that of matched asymptotics.

It is known that the effectiveness of tuned mass dampers (TMDs) decreases as the input duration shortens. As a result, their use is commonly discouraged against short-duration, pulse-like ground motions, such as those occurring in near-field (NF) zones in the presence of forward directivity or fling-step effects. Yet a systematic assessment of such control impairment is still missing. In this paper, a recent analytical model of ground motion pulses is applied to the design and evaluation of TMDs against impulsive earthquakes. Based on this model, first, a new optimization method is introduced as an alternative to the classical $H_{\text{∞}}$ approach. Then, the two strategies are tested on single- and multi- degrees-of-freedom linear structures subject both to analytical pulses and to a large set of NF records possessing pulse-like features. The resulting statistical evaluation, expressed by percentile response spectra, shows the pros and cons of a pulse-oriented TMD design and improves the general understanding of TMD performance under impulsive ground motions.

Steel concentrically braced frames (CBFs) are used widely as a seismic lateral force–resisting system. Although modern CBFs designed for high seismic regions have considerable ductility, CBFs in moderate seismic regions are expected to have limited ductility, even when designed using modern provisions. In addition, older CBFs in high seismic regions also are expected to have limited ductility. In these low-ductility systems, reserve capacity (i.e., secondary strength and stiffness) plays an important role in seismic collapse prevention. Thus, quantifying the impact of reserve capacity on earthquake response for low-ductility systems is critical. This paper presents research that used single-degree-of-freedom systems to represent low-ductility CBFs, where brace fracture causes a sudden loss of strength and stiffness. Postbrace fracture stability was studied by considering variations in reserve system strength and stiffness parameters. Performance was evaluated by considering local ductility demands on the reserve system and global drift demands. For the cases considered, the ductility capacity of the reserve system was typically a more critical constraint than global drift capacity. Reserve capacity is demonstrated to appreciably influence seismic collapse behavior, whereas primary system strength has a small influence.

A procedure for conducting reliability analysis of RC beams subjected to a fire load is presented. This involves identifying relevant load combinations, specifying critical load and resistance random variables, and establishing a high-temperature performance model for beam capacity. Based on the procedure, an initial reliability analysis is conducted using currently available data. Significant load random variables are taken to be dead load, sustained live load, and fire temperature. Resistance is in terms of moment capacity, with random variables taken as steel yield strength, concrete compressive strength, placement of reinforcement, beam width, and thermal diffusivity. A semiempirical model is used to estimate beam moment capacity as a function of fire exposure time, which is calibrated to experimental data available in the literature. The effect of various beam parameters was considered, including cover, beam width, aggregate type, compressive strength, dead to live load ratio, reinforcement ratio, support conditions, mean fire temperature, and other parameters. Using the suggested procedure, reliability was estimated from zero to 4 h of fire exposure using Monte Carlo simulation. It was found that reliability decreased nonlinearly as a function of time, whereas the most significant parameters were concrete cover; span/depth ratio when axial restraints are present, mean fire temperature, and support conditions.

It is not uncommon that cover cracking, spalling, and delamination occur in many corroding RC structures. Previous research has mainly been concerned with corrosion levels leading to cover cracking along the main reinforcement, whereas corrosion of stirrups is often overlooked. Corrosion phenomena, including stirrup corrosion, were studied in an experimental investigation presented in this paper. High levels of corrosion were reached, up to 20% of the main bars and 34% of the stirrups legs. The occurrence of crack initiation, propagation, and cover delamination were examined. The specimens had the shape of a beam end and were corroded with an accelerated method; an imposed current was used, taking care to keep the current density as low as practically possible for the duration of the laboratory testing. The effects of this process were compared with those of natural corrosion using models from the literature. The location of the bar, middle and corner placement, the amount of transverse reinforcement, and the corrosion level of longitudinal reinforcement and of transverse reinforcement were studied. The results concerning the concrete cracking in the experimental campaign are presented. The crack patterns and widths were analyzed, showing differences between specimens with or without stirrups and whether stirrups were corroding. Finally, the effect of corrosion was simulated as the expansion of corrosion products in a finite-element (FE) model, and the results, mainly the crack pattern and width, were compared with the test results. The conclusions addressed the importance of taking into consideration both high corrosion levels and corrosion of stirrups for the assessment of deteriorated structures.

Six RC beam-column subassemblages, consisting of two single-bay beams, one middle joint, and two end column stubs, were quasi-statically tested under a middle column removal scenario. The tests were aimed at investigating whether there are alternate load paths that can mitigate progressive collapse. With adequate axial restraints, both compressive arch action (CAA) and catenary action could be mobilized, significantly increasing the structural resistance beyond the beam flexural capacity. The effects of the top and bottom reinforcement ratios at the joint interfaces and beam span-to-depth ratio on structural behavior were studied. The results show that CAA is more beneficial to subassemblages with a short span-to-depth ratio and a low reinforcement ratio, whereas catenary action is more favorable to subassemblages with a large span-to-depth ratio and a high reinforcement ratio, particularly the top reinforcement ratio. As the last defense mechanism to prevent structural collapse, the development of catenary action is highlighted. The onset of catenary action corresponds to the transition of beam axial force from compression to tension, typically occurring at a central deflection around one beam depth in the tests if no shear failure precedes catenary action. At the catenary action stage, prior to fracture of the bottom bars, structural resistance is contributed by both beam axial tension from longitudinal reinforcement and shear force because of dowel action. If the contribution from rising axial tension exceeds the loss as a result of declining shear force, the structural resistance will still keep on increasing until the fracture of the top bars. Finally, the authors suggest a deformation criterion to determine the catenary action of RC subassemblages; i.e., when the deflection at the middle joint attains 10% of the total beam span length, catenary action capacity is reached. The conservatism of this criterion for design purposes is also discussed.

Tornadoes represent a unique natural hazard because of the very low probability of occurrence, short warning times (on the order of only a few minutes), and the intense and destructive forces imposed on engineered and nonengineered buildings. The very low-probability/very high-consequence nature of a tornado strike makes designing for survival and reducing damage under typical financial constraints a substantial challenge. On April 27, 2011, an enhanced Fujita (EF) 4 (EF4) tornado devastated an almost 10-km (5.9-mi) long, 0.8-km-wide (1/2-mi-wide) path, through the city of Tuscaloosa, Alabama, and continued on the ground for 130 km (80 mi). This paper presents the design concept that resulted following a week-long data reconnaissance deployment throughout the city of Tuscaloosa by the authors. The dual-objective philosophy proposed herein is intended to focus on both building damage and loss reduction in low-to-moderate tornado wind speeds and building occupant life safety in more damaging wind-speed events such as EF4 and EF5 tornadoes. The philosophy articulates a design methodology that is the basis upon which structural engineering was formed—namely, provide life safety and control damage—but the new philosophy is focused at separate tornado intensity levels.

Analytical and associated numerical investigations of the fluctuating internal pressures induced through a dominant opening in real buildings with leaky and flexible envelopes are undertaken. The damping effect of these factors both separately and in combination are quantified using RMS internal pressure coefficients and equivalent damping ratios for a range of envelope flexibilities and background porosities for the case of the Texas Technical University test building and a large-span industrial building. Simulated ratios of the RMS internal pressures and the peak spectral response of internal pressure for leaky and flexible buildings to that of rigid nonporous envelopes are presented in nondimensional format for a range of building volumes, opening areas, and porosity ratios. Additionally, nondimensional curves of the RMS internal to external pressure ratios for real flexible and leaky envelopes are presented along with experimental data reported in the literature in a form suitable for design purposes.

A large number of wind pressure experiments have been performed to investigate the influence of surrounding buildings on wind pressures and forces on a cubical low-rise building. The primary purpose of the present work was to understand the effects of surrounding buildings and to examine methods for estimating wind loads. The considered parameters were area density and upstream distance under a turbulent boundary layer representing a suburban area. Results show that wind pressures decrease with increasing area density because of the surrounding buildings, but the variations of mean and fluctuating components within one area density are relatively small. The local wind pressure coefficients increase significantly, and this is originated from the rapid decrease in the local velocity pressure at the model position. Last, after discussing the design approach of a target low-rise building in a large group, a methodology for estimating wind loads on a target low-rise building was examined by introducing the (zoning) interference factor.

ASCE 7 does not provide design wind loads for roof-mounted solar panels. This paper discusses the use of the wind tunnel test method, called Method 3 in ASCE 7-05, which was originally intended for obtaining design wind loads for individual buildings. Because roof-mounted solar arrays are generally mounted in many configurations on many buildings of many different shapes, additional requirements are necessary to use Method 3 in this situation. The paper describes these additional requirements.

A Rayleigh quotient is developed to calculate estimates of the fundamental frequencies of tall slender buildings. The quotient provides simple analytical frequency estimates that agree well with results reported using a more detailed beam model and finite-element analyses. It is also shown that the fundamental frequency varies inversely with the height $H$ of the building being modeled, and that the additional stiffening provided by one model of an outrigger appears not to appreciably alter the fundamental frequency.

In this paper, a set of simplified design equations for an unbonded posttensioned (PT) precast split hybrid shear wall system is developed. Such a system was proposed by the Precast Seismic Structural Systems (PRESSS) and can be used as a bearing wall or a special reinforced concrete wall for building frames, as defined in ASCE/SEI 7, for eliminating residual drift after seismic events. The panels are anchored to the foundation with unbonded PT tendons located at the panel center; adjacent panels are connected with uniformly distributed energy dissipating coupling devices. An iterative design procedure to calculate the required area of the PT reinforcement and the total yield force of all shear connectors in one vertical joint was proposed by PRESSS. This iterative procedure is exact, but tedious and lengthy. A simpler nondimensional chart-based design procedure was proposed by the writers. This procedure is efficient, but has limitations. A noniterative procedure adopted by ACI ITG-5.2, although simple, does not yield the optimum combination of the shear connectors force and PT force. The set of simplified design equations proposed in this study overcomes the shortcomings of these three procedures. It is simple, direct (noniterative), general, and yields the optimum design in fewer number of steps. The proposed design equations predicted values are accurate and are normally within ± 2% of the exact optimum solution.