Journal of Structural Engineering

Sparse fatigue test data was available for gusset stiffener details on through plate socket connections when the AASHTO 2001 specification for signal, sign, and luminaire structures was developed. An effort was carried out to investigate the effect of base plate thickness on various sizes and geometries of gusset stiffener details for through plate socket connections, based on previous research study results at Valmont, the University of Minnesota, and the University of Texas at Austin. Fatigue test results revealed an improved fatigue resistance for socket connections compared to stiffened details. Parametric studies using the finite-element method revealed behavior that tends to contradict current AASHTO specifications. Local stresses were nearly constant after the stiffener reached a $102\mathrm{mm}$ $\left(4\mathrm{in.}\right)$ vertical length. Further, local stresses appear highest for short stiffeners that are currently recognized as Category C details. The study also demonstrated that fatigue investigations and experiments are essential to validate current fatigue categories for such structures.

The deformation, ultimate load capacity, and fracture behavior of full-scale welded K-joints fabricated from cold-formed rectangular hollow sections have been studied both numerically and experimentally. An extensive experimental program of welded K-joints tested to failure at temperatures between 23 and $-60°\mathrm{C}$ revealed that the primary failure mode was ductile tearing of the chord flange at the toe of the tension brace-to-chord weld. For some joints tested at $-40$ and $-60°\mathrm{C}$ , initial ductile tearing led to brittle fracture. Finite-element based $J$ -integral assessment of an advancing crack for one K-joint geometry was combined with $J–\Delta a$ material curves measured at $-40°\mathrm{C}$ . Assessment revealed that crack advance, once initiated, would be expected to continue at a nearly constant load. The assumed shape and size of the initial crack in the finite-element model were considered to be conservative and the estimated maximum load capacity and joint ductility were conservatively predicted with respect to the experimental results. This procedure was found to be promising for assessing the ultimate ductility of tubular joints at low temperatures.

This paper presents research on unstiffened elements made from high strength G550 steel and subjected to uniform compression. The elements are $1\mathrm{mm}$ or less in thickness and prone to local buckling. Current design rules for these steels specify additional reduction factors to compensate for their reduced strength, which has previously been attributed to the lack of strain-hardening. The paper presents tests and advanced finite-elements analyses of unstiffened elements in compression and identifies appropriate reduction factors for this type of element. The analytical studies demonstrate that the effect of gradual yielding may be the main reason for the relatively low strength of G550 steels in the intermediate slenderness range, rather than the lack of strain hardening. Tests are performed on short single-angle stub columns and on double-angle stub columns. A comprehensive series of material tests of tension and compression coupons is also presented. The test strengths are compared with current American and Australian specifications for cold-formed steel structures, and design recommendations specific to G550 steels are made suggesting that a reduction factor of 0.9 is appropriate for unstiffened plate elements.

This paper presents the results of two series of experimental and analytical studies on the structural behavior of an innovative moment-resisting connection to concrete-filled circular hollow sections using blind bolts. The first part of the paper presents a large-scale T-stub connection with full instrumentation tested to failure in tension, and the second part presents a series of pullout tests to explore the anchorage behavior of cogged bars within concrete-filled steel tubes. The favorable strength and stiffness characteristics of this type of connection showed that it could be an alternative to conventional welded moment-resisting connections. A 3D finite-element (FE) model was developed to simulate the behavior of blind-bolted T-stub connections when subject to tension. The FE model took into account material nonlinearities, geometrical discontinuities, and complex contact interactions among the bolt heads, tube wall, end plates, and nuts. The FE results were found to be in good quantitative agreement with the actual connection behavior. A stiffness model composed of various nonlinear springs was also developed to predict the pullout behavior of cogged bars within concrete-filled steel tubes. The cogged extensions to the blind bolts were very effective in relieving the stress concentration on the thin tube wall. The tensile load can be shared between membrane action of the tube wall and anchorage of the cogged bars. Thus, by using the blind-bolted connections with cogged extensions to connect the beam flanges to the column, moment transfer between the beam and column can be effectively achieved within a structural frame of composite construction.

The paper addresses the problem of describing the anisotropic damage process and the dissipative behavior of masonry structures under static incremental and dynamic loads. A homogenized continuum model, based on simplified micromechanical hypotheses, is presented. The plane stress is considered. The finite-element method is adopted as a framework for numerical implementation. Masonry is considered as a composite material made up of blocks, mortar bed joints, and mortar head joints. Mortar bed joints are schematized as interfaces characterized by cohesion, tensile strength and friction, whereas mortar head joints are considered as geometrical discontinuities. Internal symmetries of the material leads to distinguishing two couples of emisymmetric bed joints, characterized by equal state variables. The computation of the displacement jumps in these two couples of joints is sufficient to evaluate the displacement jumps of all the joints contained in the assumed unit cell. Constitutive equations consider the nonlinear stress–strain relation in terms of mean stresses and mean strains. The latter are produced by an elastic strain contribution and by different inelastic strain contributions depending on the damage in mortar joints and in blocks. The damage processes are described by means of an energetic approach. The hysteretic behavior is described by considering a Coulomb-type friction law on the mortar bed joints. The model is implemented in a general purpose finite-element code (ANSYS). A simple example of a cyclic load history is presented in order to demonstrate the effectiveness of the model.

In this paper simple empirical models are suggested to evaluate the reduction in bond strength as a function of reinforcement corrosion in reinforced concrete (RC). These models are proposed by considering a wide range of published experimental studies related to the bond strength degradation due to reinforcement corrosion. The models are then validated based on the referred experimental studies. It has been found that the proposed models provide estimates of the reduction in bond strength for corroded reinforcement that are in reasonably good agreement with the experimentally observed values. The predictions of the proposed models have also been found to be in line with those of other published data on empirical predictions. An attempt has also been made to evaluate the flexural strength of RC beams with corroded reinforcement failing in bond. It has been found that the analytical predictions for the flexural strength of corroded RC beams based on the suggested bond degradation models are in line with those of the experimentally observed and empirically predicted values of the referred experimental study. The present study concerning the progressive loss of bond strength due to reinforcement corrosion could be very useful for predicting the service life of structures serving in an aggressive environment.

A conjugate gradient search method is coupled with a general model for the ultimate strength of a rectangular reinforced concrete (RC) cross section. Reinforcement required to provide reliable resistance identically equal to required strengths (due to factored load effects) is determined. The total reinforcement is determined optimally, either to obtain a global minimum or to obtain a minimum subject to various constraints, such as the use of equal reinforcement on opposite faces or the use of equal reinforcement on all faces. The model is useful for the design of beam, column, and wall sections that are subjected to uniaxial or biaxial bending. Solutions are obtained directly, and make use of computing power contained within spreadsheet programs that are widely available. Avoiding the somewhat tedious hand calculations and approximations required in conventional iterative design approaches avoids errors and potentially unsafe designs, while the use of optimum reinforcement quantities advances the aim of improving the sustainability of RC construction.

Performance-based seismic design (PBSD) of wood frame structures has been only preliminarily investigated by a handful of researchers around the world and is currently under development in the United States as well as other countries worldwide. This paper presents the basic concept of applying system identification logic as one alternative for PBSD of wood frame structures. In PBSD of wood frame structures, one prescribes combinations of hazard level and corresponding performance, and “designs” the system such that these performance metrics are met. For wood frame buildings this procedure fits the definition of a system identification problem, i.e., the input and output are known and the nonlinear system between them is sought. In most cases the shearwall locations, i.e., geometry, are known, and only the wall design is sought. In this paper, the concept is presented and a procedure to apply it within a PBSD framework is proposed. The validity of the procedure is confirmed through two illustrative examples, namely the design for a two-story wood shearwall and a two-story wood frame building. An existing ten-parameter hysteresis model commonly used for seismic analysis of wood frame structures was used in both examples to represent the nonlinear system to be identified. The “design” is then checked by applying a suite of ordinary ground motion records and comparing the multirecord incremental dynamic analysis of the peak deformation response to the target (performance) curve. The approach was found to work well for the example structures and shows promise for extension to more complicated structures and performance measures.

Load tests of the effect of fiber-reinforced polymer (FRP) reinforcing rods (Z-spikes) on the flexural stiffness of deteriorated railroad crossties were conducted. Reinforcement was provided by installing $13\mathrm{mm}$ $\left(0.5\mathrm{in.}\right)$ diameter polyglass polyester resin fiberglass rods bonded in place using epoxy-resin. The Z-spikes were inserted into the deteriorated railroad crossties perpendicular to the primary bending axis to provide horizontal shear reinforcement and improve flexural stiffness. They were installed in pairs starting at each end of the crosstie specimen and moving inward. A total of ten rows were sequentially inserted into each crosstie specimen with load tests performed after each successive row installation. Results from load tests show an average increase in effective flexural stiffness (EI) of 58% with all ten rows of shear spikes inserted into the crossties.

Vibration of lightweight engineered timber floors (ETFs) is a problem occasionally reported by occupants of dwellings constructed with such floors. Vibration of timber floors has been studied extensively in the past resulting in the development of design criteria to prevent dynamic serviceability problems. Nevertheless, unsatisfactory serviceability continues to be reported. The present experimental investigation has examined several lightweight ETF configurations and found that most satisfy existing design criteria intended to preclude bodily oscillations under foot traffic. However, these same floors failed to display satisfactory performance with respect to vibrational disturbances such as drumminess and shaking. Common variations in design such as changes in fasteners or the inclusion of blocking have little effect on vibrational disturbances. Instead, experimental evidence suggests that minimum levels of structural damping may be required to control such phenomena.

The performance of a damper in controlling large-amplitude vibrations of a stay cable in a bridge is influenced by essential parameters such as the amount of sag and flexural rigidity of the cable, and the stiffness of the damper support. In this paper, accurate asymptotic formulas are analytically derived for the modal damping ratio of a general cable. The formulation includes the parameters mentioned above and is appropriate for a damper having a finite support stiffness. For a viscous damper, the influential parameters are explicitly incorporated as reduction and modification factors in the modal damping formula, which significantly simplifies the design procedure for stay cables. The analytical results are also extended to high-damping rubber dampers which have recently been encountered in practice. Finally, important formulas relevant to the design of both types of dampers are tabulated.

The seismic performance of a composite moment resisting frame (MRF) comprised of concrete filled tube (CFT) columns and wide flange beams was investigated experimentally. The four-story composite MRF test structure was designed using performance-based design concepts. The performance objectives include achieving: (1) the operational performance level under the frequent occurrence earthquake; (2) the life safety performance level under the design basis earthquake; and (3) the collapse prevention performance level under the maximum considered earthquake. The hybrid pseudodynamic test method was used to subject the test structure to these various seismic input levels. $P\text{-}\Delta$ effects associated with the gravity frames in the prototype building were included analytically in the tests. Results from the tests indicated that the structural performance under the simulated seismic loading was consistent with the expected performance for all three earthquake levels, indicating that effective seismic performance of composite MRFs with CFT columns can be achieved.

Results of experimental and analytical investigations of the seismic response of fully restrained steel moment connections between deep W-shape beams and deep and square built-up box columns are presented. Prototype beam-column connections were selected from the steel moment-resisting frames in a building built before the 1994 Northridge earthquake. Two full-size, single sided, pre-Northridge box column connection assemblage specimens were fabricated using the prototype connection data. Pretest analyses showed that the responses of box column connections can be substantially different from W-shape column connections. Both specimens were tested using a standard connection prequalification test protocol and each failed by brittle fracture in the complete joint penetration (CJP) welds of the beam flanges at connection drift angles less than $0.8\%\mathrm{rad}$ . The pre-Northridge beam flange CJP welds are likely to fail in a brittle manner regardless of the type of column cross section. ABAQUS finite-element models of the specimens were validated using the test results. The locations of the cracks in the beam flange CJP welds coincide with the region of high axial stresses, which varies depending on the column type, presence of continuity plates, and thickness of box-column plates.

Active control of confined masonry walls with nonlinear hysteretic response has been studied through numerical simulation in this paper. The objective has been to reduce the response to below the failure level when the walls have been subjected to earthquakes. An active tendon control mechanism, comprised of prestressed tendons and an actuator, has been used. A nonlinear control algorithm has also been developed based on Wilson’s- $\theta$ instantaneous linear optimal control method and applied. To achieve the best results, weights in the performance index corresponding to displacement, velocity, acceleration, and control force have been optimized by distributed genetic algorithms. An illustrative example has been included where, for designing and testing the controller, a series of white noise ground accelerations of different amplitudes as well as a number of strong earthquakes, such as Chi Chi in 1999 and Northridge in 1994, have been used. Some practical aspects such as actuator-wall interaction, actuator saturation, sensor saturation, and time delay have not been considered in this phase of the study. The designed controllers have been clearly successful in all the tests, even for earthquakes of very high intensity.

A semiactive control strategy for the vibration control of structures using a stiffness varying tuned mass damper (TMD) with application of bang-bang control force to TMD is proposed. The control strategy involves a combination of controlling the response of structure using bang-bang control with a control force applied to the TMD, and the use of the principle of minimizing the maximum value of the frequency response function (FRF) for the coupled structure-variable stiffness TMD system. The optimal tuning parameter and damping ratio for the TMD are obtained based on this principle. When the control force is applied to the TMD, the TMD is simultaneously retuned (by varying the stiffness) in real time to achieve the minimax principle for the desired level of response reduction. When the active control is ineffective or is turned off, the damper system acts as a passive TMD. The performance of the control strategy is examined under harmonic excitations and is compared to previous results on bang-bang control and minimax shaping of the FRF when the TMD is not retuned in real time. In order to examine the robustness of the controller, a wind excitation obtained from the Davenport spectrum is applied to a structure with low natural frequency. The control strategy is shown to achieve substantial response reduction with relatively low control force requirements.

A nonlinear model-based system identification method is proposed, formulated, implemented, and applied to a three-span continuous base-isolated bridge. Transverse and rotational rigid-body motions of the bridge superstructure are formulated into two degrees-of-freedom dynamic governing equations. To model hysteretic behavior of lead–rubber bearings, the Menegotto–Pinto model is used. The proposed system identification procedures consist of two phases to address ill-conditioning issues. The uncertainty of parameters is considered, which results in probability distributions instead of single values for identified parameters. The proposed system identification is adopted in the comparison of experimental data sets. Hypothesis testing is used to determine the closeness of identification results. The proposed method is applied to quick-release field experiments on the bridge to investigate aging and temperature dependent effects in lead–rubber bearings.

Light rail systems are an important transportation mode in urban centers. An advanced rail transit system has been developed that utilizes linear induction motors (LIM) for propulsion and braking of the vehicles. Due to the tight motor-to-rail air gap tolerances required for the efficient operation of the LIM system, the trackwork requires special consideration. An innovative crosstie trackwork system has been developed to address competing requirements for high stiffness to ensure efficient operation of the LIM, and low track stiffness for acceptable ride quality. The crosstie incorporates a hollow structural steel section welded at each end to formed steel base plates which are supported by elastomeric pads. The crosstie supports steel running rails. This paper describes a finite-element model developed to predict the response of a crosstie to loads that simulate the passage of a steel wheeled transit vehicle. The model uses tetrahedral elements for the hollow structural steel section and the base plates. The elastomeric pads are modeled using elastic foundation elements. The model predicts stresses and deflections in the crosstie. An experimental program that was carried out on the crosstie to investigate its fatigue performance and to obtain data to verify the finite-element model is also described. The model conservatively predicts crosstie deflections and stresses. The hot spot stress approach is used to provide a conservative fatigue life prediction of the crosstie.

This paper studies the effects of random properties on the strength and reliability of two steel frames. Monte Carlo simulation is used to analyze the behavior of a low-rise industrial frame and a grain storage bin. Both structures exhibit significant second-order effects with their strength limited by loss of frame stability. Both frames have been previously studied for the development of the direct analysis provisions of the 2005 AISC Specification for Structural Steel Buildings. The analyses consider randomness in yield strength, elastic modulus, residual stresses, and out-of-plumb ( $P$ –Delta) and out-of-straightness ( $P$ –delta) imperfections. The effects of spatial correlation of these properties are also studied. The simulation results are used to estimate probability distributions of frame strength and reliability, or probability of failure. The results provide probabilistic relationships between material and geometric properties of steel frames and the overall strength of the frames. Randomness in elastic modulus and residual stresses are shown to have a significant impact on strength and reliability of the frames by increasing lateral deflections and thereby second-order forces. The results confirm the importance of including geometric imperfections for proper assessment of frame stability.