Journal of Structural Engineering

The paper analyzes the reliability consequences of the fact that the current design codes for concrete structure contain covert (or hidden) understrength (or capacity reduction) factors. This prevents distinguishing between different combinations of separate risks due to the statistical scatter of material properties, the error of the design formula, and the degree of brittleness of failure mode, and also makes any prediction of structural reliability (or survival probability) impossible. The covert formula error factor is implied by the fact that the design formula was calibrated to pass not through the mean but through the fringe (or periphery, margin) of the supporting experimental data. The covert material randomness factor is the ratio of the reduced concrete strength required for design to the mean of the strength tests. As a remedy, the covert understrength factor of design formula should be made overt, its coefficient of variation (based on the supporting test data) should be specified, and the type of probability distribution (e.g., Gaussian or Weibull) indicated (which then also implies the probability cutoff). Alternatively, the code could give the mean formula, specify its coefficient of variation and type of distribution, and either prescribe the probability cutoff or overtly declare the understrength factor. The mean of strength tests required for quality control should be figured out from the required design strength on the basis of a specified probability cutoff and the coefficient of variation of these tests. Furthermore, it is proposed that the currently used empirical understrength factor, which accounts mainly for the risks of structural brittleness (or lack of ductility), should be based on the expected maximum kinetic energy that could be imparted to the structure. The reliability integral taking into account the randomness of both the load and structural resistance is generalized for the case of multiple (statistically independent) understrength factors. Finally, it is pointed out that the currently assumed proportionality of the tensile and shear strengths to the square root of compressive strength of concrete is realistic only for the mean, but grossly underestimates the scatter of tensile and shear strengths.

This first part of two companion papers deals with the numerical modeling of timber–concrete composite beams (TCCs) under long-term loading. All phenomena affecting the long-term behavior of timber, concrete, and the connection system, such as creep, mechanosorptive creep, shrinkage/swelling, and temperature variations, are fully considered. The structural problem is solved through a uniaxial finite element model with flexible connection and a step-by-step numerical procedure over time. The important role played by the environmental thermohygrometric variations on TCCs is highlighted through some analyses. The proposed numerical procedure is validated on two long-term experimental tests in outdoor conditions. Despite some uncertainties in environmental conditions and material properties, a good fit between experimental and numerical results is obtained. A parametric analysis is performed in the second part, showing the contribution of different rheological phenomena and thermohygrometric variations on beam deflection and connection slip. Based on results carried out, a simplified approach for long-term evaluation of TCCs is then proposed.

This second part of two companion papers investigates the contribution of different rheological phenomena and thermohygrometric variations on long-term behavior of timber–concrete composite beams (TCCs) in outdoor conditions. The numerical algorithm presented and validated against two experimental tests in the first part is employed with this aim. Such a model fully considers all rheological phenomena and, therefore, leads to rigorous solutions. Effects on the beam response include the creep and mechanosorptive creep of both timber and connection, along with concrete creep and shrinkage, and may markedly increase the elastic deflection due to live load. The inelastic strains due to yearly and daily variations of environmental conditions (temperature and relative humidity) produce an important fluctuation of the deflection. A simplified method, which is suitable for practical design of TCCs under long-term loading, is at last proposed. The effects of load, concrete shrinkage, and inelastic strains due to environmental variations are evaluated one by one using approximate formulas and are then superimposed. Creep and mechanosorptive creep are taken into account by adopting modified elastic moduli. The reliability of the proposed method is checked by way of some comparisons with numerical results. The applicability for the case of TCCs in heated indoor conditions is also discussed.

A numerical model for the study of prestressed members with bonded and unbonded tendons has been implemented, considering the hybrid type finite element formulation for planar frames. With such an approach, accurate curvature distributions are obtained, allowing a good computational performance and an adequate unbonded tendon stresses evaluation. The computer program also includes geometric nonlinearity, composite construction, and time-dependent effects. Linear Maxwell chain models are used for both concrete and prestressed steel time-dependent behavior. This paper presents a general description of the numerical model proposed, including the structural idealization approach, a hybrid type finite element formulation review, the material constitutive models, and the main numerical procedures. A brief review of the behavior of unbonded prestressed members is also presented. Numerical and experimental results are compared, showing the good performance of the model for the analysis of unbonded prestressed elements. Unbonded tendon stresses and cracking pattern results are included, showing the capability of the numerical model. The model is proposed as an efficient analytical tool for further research on unbonded prestressed members.

When subject to combinations of internal pressure and axial compression, experience shows that the often thin cylindrical shell walls of tanks and silos are prone to a local form of buckling failure that is essentially axisymmetric and is strongly influenced by the nature of the end or intermediate support conditions. This paper will outline the basis of an analytical solution to the inherently nonlinear elastic–plastic buckling into axisymmetric modes, in which end constraints and arbitrary radial pressure loading are modeled as essentially loading imperfections on an otherwise linear, classical, critical buckling formulation. Closed form analytical expressions for the imperfect behavior, including geometric imperfections, are then used to predict either first surface or full section plasticity, allowing buckling failure to be summarized in a form that is closely analogous to the Ayrton–Perry formula for the buckling of columns. It is suggested that a suitably simplified form of this general approach to axisymmetric buckling, when combined with similarly simple methods for the buckling into nonaxisymmetric modes, could provide an important alternative to the current procedures for the buckling design of thin-walled, cylindrical, tanks and silos.

The minimum weight criterion, which has been widely adopted in the literature for optimal design of steel structural systems, is inadequate to fully reflect the initial monetary investment due to lack of consideration of additional construction expenses resulting from varied degree of design complexity such as different member sections and splice/connection types. In this paper, design optimization of seismic steel moment-resisting frames involves simultaneous consideration of two competing objective functions: the steel material weight and an approximate measure of design complexity in terms of the number of different standard steel member section types. The code-compliant seismic structural design follows the equivalent lateral force procedure of the 2000 National Earthquake Hazards Reduction Program seismic provisions in conjunction with American Institute of Steel Construction load resistance factor design seismic steel design criteria. A genetic algorithm is used for the posed biobjective structural optimization problem to produce a set of alternative designs establishing optimized trade-off between the two merit objectives. Numerical examples show that a minimum weight design with a balanced degree of design complexity is expected to achieve an initial investment economy with more accuracy.

A new methodology for calculating equivalent linear parameters for use in predicting the earthquake response of structures is presented. The methodology is applied to single-degree-of-freedom inelastic structural models subjected to a suite of earthquake acceleration time histories. Error distributions over a two-dimensional parameter space of effective period and effective damping are analyzed through a statistical approach with an optimization criterion most applicable to structural design. Four hysteretic models are analyzed: bilinear, stiffness degrading, and two pinching models. Evaluation of the equivalent linear parameters is performed by two separate approaches. The first compares the maximum response of the equivalent linear system to the maximum response of the nonlinear system. The second compares the performance point displacement prediction calculated by the capacity spectrum method using the equivalent linear parameters developed in this paper to the maximum response of the nonlinear system. Results are compared to analysis using the existing equivalent linear parameters as set forth in Applied Technology Council-40 (ATC-40). The error analysis reveals that the ATC-40 equations are significantly unconservative at lower values of ductility and conservative at the higher values of ductility. The new equivalent linear parameters lead to a substantial improvement for both error measures over the entire ductility range.

An event-driven distributed control system for conducting continuous seismic response simulations using geographically distributed hybrid models is presented. Hybrid models, comprising appropriately scaled experimental and numerical substructures in networked geographically distributed laboratories or computers, can be used to realistically and cost-effectively evaluate the performance of complex structures at large scales. The principal advantage of the proposed event-driven control system is its ability to mitigate the adverse effects of random completion times of communication, computation, actuation and measurement tasks during a hybrid simulation on the stability, accuracy and reliability of the simulation results. The efficiency of the proposed continuous extrapolation/interpolation hybrid simulation method is demonstrated by examining the earthquake response of a two-story shear building model that comprises two experimental substructures and a numerical integrator connected through the internet. An evaluation of results from these hybrid simulations suggests that distributed hybrid simulation conducted using the proposed procedure provides accurate and reliable results.

Brittle fractures have been observed in beam-to-column connections of welded steel moment frames following several recent earthquakes. To help understand the effects of connection fractures on seismic safety and response, a series of shaking table experiments and numerical simulations were carried out on a one-third scale, two-story, one-bay moment-resisting steel frame. Simplified mechanical connections were utilized in the experiments to create different types and distributions of fracture behavior. These investigations demonstrate that connection fracture may not always have deleterious consequences, especially when few fractures occur. On the other hand, the experimental and numerical investigations clearly demonstrate that severe strength loss due to the combination of numerous fractures, undesirable postfracture hysteretic behavior, and large excitation can have adverse consequences including collapse.

In this paper, a control design process incorporating robustness criteria is developed specifically for wind-excited high-rise buildings and its feasibility is experimentally verified by implementing on a four degree-of-freedom scaled (1:300) model of a high-rise building through wind tunnel tests in along-wind and across-wind directions. For feasibility toward mature application, considerations in the control design process are made as practical and thorough as possible. The main features of the proposed design process include a suitable identification scheme that can realistically model wind loads and the possible interaction between control devices and structure, as well as a systematic way of incorporating robustness criteria in the controller for reducing tracking error, rejecting noise, attenuating disturbance, and maintaining stability in the presence of system uncertainty. To account for the complexity of wind load in system identification, wind spectrum is obtained through the high-frequency force-balance tests, and the corresponding state space wind model is constructed. The nominal system with the robustness criteria employed is transformed into a generalized $H_{\infty }$ control problem that can be further converted into a set of linear matrix inequalities (LMIs). Consequently, strictly proper output feedback $H_{\infty }$ controllers are thus determined by a simpler solution procedure in searching the solution of LMIs. As observed from experimental results, the performances of proposed $H_{\infty }$ controllers are quite remarkable and comparable to those of classical linear quadratic Gaussian (LQG) controllers. Additionally, the proposed controllers are numerically demonstrated to be more robust than classical LQG controllers under the existence of system uncertainty. This successful implementation shows that the design process and robust controllers proposed are feasible for wind-excited high-rise buildings and the resulting control performance is promising.

A new dynamic time-delay fuzzy wavelet neural network model is presented for nonparametric identification of structures using the nonlinear autoregressive moving average with exogenous inputs approach. The model is based on the integration of four different computing concepts: dynamic time delay neural network, wavelet, fuzzy logic, and the reconstructed state space concept from the chaos theory. Noise in the signals is removed using the discrete wavelet packet transform method. In order to preserve the dynamics of time series, the reconstructed state space concept from the chaos theory is employed to construct the input vector. In addition to denoising, wavelets are employed in combination with two soft computing techniques, neural networks and fuzzy logic, to create a new pattern recognition model to capture the characteristics of the time series sensor data accurately and efficiently. The model balances the global and local influences of the training data and incorporates the imprecision existing in the sensor data effectively. Experimental results on a five-story steel frame are employed to validate the computational model and demonstrate its accuracy and efficiency.

Current engineering practice evaluates the seismic performance of bridges based on nonlinear dynamic response analyses. Although various models exist for envelope curves of concrete confined by transverse reinforcement, only a few simple models represent the hysteretic behavior of the confined concrete; therefore, development of an accurate stress–strain model of unloading and reloading paths for confined concrete is urgently needed. In this paper, an unloading and reloading stress–strain model for concrete confined by transverse reinforcement is presented, which is based on the results from a series of compressive loading tests of reinforced concrete column specimens. The model focuses on the effect of repeated unloading/reloading cycles and partial loading excursions, and provides equations that take into account the number of unloading/reloading cycles to predict essential parameters, such as plastic strain and unloaded stress after repeated loading cycles. The proposed model provides good agreement with the test results.

For a reliable and efficient design of fiber reinforced polymer (FRP) wrapping reinforcement, an accurate stress–strain prediction of reinforced concrete (RC) columns confined with FRP composites is essential. The stress–strain characteristics of FRP-confined concrete have been extensively studied, testing small-scale cylindrical specimens. In this paper, the stress–strain response of axially loaded large-scale columns confined with FRP wrapping reinforcement is investigated. The effective circumferential FRP failure strain and the effect of increasing confining action on the stress–strain behavior of RC columns confined with FRP are examined. Different models for the prediction of the stress–strain behavior of FRP-confined concrete have been reviewed. The existing stress–strain models are applied to predict the stress–strain response of the experimental results presented in this study. A comparison between the experimental results and those predicted by the existing models are presented. Few of the available models that were derived based on small-size cylinders were found to accurately predict the stress–strain response of large-scale columns.

An experimental investigation of cold-formed stainless steel hollow sections subjected to web crippling is presented in this paper. Tests were conducted on square and rectangular hollow sections of austenitic stainless steel type 304. Tensile and compression coupon tests were performed to obtain the longitudinal tension and transverse compression material properties. The web crippling tests were conducted under two loading conditions for end-two-flange and interior-two-flange specified in the current American Specification and Australian/New Zealand Standard for cold-formed stainless steel structures. The test strengths obtained from this study and the test strengths of stainless steel lipped channel sections under end-one-flange and interior-one-flange loading conditions conducted by other researchers are compared with the design strengths obtained using the current American Specification, Australian/New Zealand Standard and European Code for cold-formed stainless steel structures. In addition, the test strengths are also compared with the design strengths obtained using the unified web crippling equation as specified in the North American Specification for cold-formed carbon steel structural members. A unified web crippling equation for cold-formed stainless steel sections with single web is proposed in this paper.

In this study, full-scale tests were conducted on unblocked shear walls with various sheathing details and reference blocked shear walls. A total of eight $4.88\mathrm{m}\times 4.88\mathrm{m}$ shear walls were tested. Monotonic and reversed cyclic displacement schedules were used as loading protocols. Load–displacement response of each test specimen was recorded, from which the initial stiffness, yield point, ultimate load and displacement, and energy dissipation were calculated. The test results were analyzed to study the influence of different configurations on unblocked shear wall strengths. Relative load-carrying capacities between unblocked and blocked shear walls were determined. The results show that the provisions currently in CSA O86-01 for 2.44 m tall shear walls may be conservatively applied to the four wall configurations tested in this study. Data from this study are suitable for verifying structural analysis models that may be used to further refine the adjustment factors.

This study addresses the development and structural testing of a hybrid sheathing panel designed to improve the lateral resistance of light wood-frame shear walls. The panel consists of thin outer sheets of oriented-strand board (OSB) bonded to strips of fiber-reinforced polymer (FRP) that are sandwiched between the OSB panels at the edges. The FRP-reinforced panel material selection and fabrication are summarized. Monotonic and cyclic tests were conducted to assess the performance of single-connector sheathing-to-framing connections using both OSB and FRP-reinforced OSB. Twenty-four full-scale shear walls were tested under monotonic and cyclic racking loads to determine the benefits of FRP-reinforced OSB to wall lateral load resistance. For a 102 mm perimeter nail spacing, FRP-reinforced OSB-sheathed walls exhibited increases in peak capacity and energy dissipation under cyclic loading of 27 and 73%, respectively, compared to walls sheathed with conventional OSB. Based on these experimental results, FRP-reinforced OSB panels appear to have significant potential for increasing the energy dissipation capacity and lateral load resistance of wood-frame structures subjected to extreme lateral loads. However, their full utilization will require higher strengths for critical shear wall components such as tension ties and framing interconnections, as well as improvements in sheathing-to-framing fastener performance.

A formula for the design of the nonconstrained flagpole is presented. This formula provides a direct solution that is close to the result of the Uniform Building Code (UBC). The formula is simple and can be used as an alternative or as a complement to check results using the UBC’s formula.