Journal of Structural Engineering

In recent years, significant advances have been accomplished in the fields of modeling, analysis, and design of deteriorating civil engineering systems, and novel approaches to life-cycle assessment, maintenance planning, and optimal design of structural systems have been proposed. Despite these advances, life-cycle concepts are far from being explicitly addressed in design and assessment codes and effectively implemented into practice. There is, therefore, a need to promote further research in the field of life-cycle performance of structural systems under uncertainty and to fill the gap between theory and practice by incorporating life-cycle concepts in structural design and assessment codes. An effort is currently ongoing within the Structural Engineering Institute (SEI)/ASCE to meet this need. This paper is part of this effort and is aimed at presenting a review of the main principles, concepts, methods, and strategies for life-cycle assessment and design of deteriorating structural systems under uncertainty. General criteria for deterioration modeling are first presented, with emphasis on the effects of corrosion and fatigue in steel structures and chloride-induced corrosion in concrete structures. The time-variant structural performance is then investigated with reference to a set of probabilistic performance indicators, and the structural lifetime associated with a reliability target is formulated. The role of inspection and monitoring, the effects of maintenance and repair interventions, and the definition of cost-effective maintenance strategies are discussed. The concepts of life-cycle performance assessment and maintenance planning are used to formulate the life-cycle reliability-based design problem in an optimization context.

The implementation of reliability methods for designing new structures and assessing the safety and evaluating the performance of existing structures and infrastructure systems has gained widespread acceptance. Consequently, reliability-based design specifications in the form of load and resistance factor design (LRFD) methods have dominated the development of current codes and standards. This paper reviews the reliability-based performance criteria used to calibrate design and evaluation codes and standards for assessing the strength, serviceability, and fatigue resistance of structural components. The review shows that large differences exist in the target reliability levels adopted for evaluating the strength of various types of structural members and materials. These differences result from many factors, including (1) intended structure design and service life; (2) expected member modes of failure (e.g., ductile or brittle); (3) importance of the individual member to overall system integrity (secondary member, column, or connection); (4) experiences with previous designs; (5) material and construction costs; (6) structure type and occupancy; and (7) risk tolerance of the engineering community and the public within a code’s jurisdiction. For other than seismic hazards, current specifications remain primarily focused on the evaluation of individual structural members and components, although recently proposed performance-based design (PBD) procedures apply varying target member reliability levels that depend on structure categories, modes of failure, and required levels of structural performance. The implementation of reliability-based durability criteria in design standards is still a subject of research owing to difficulties encountered in modeling material degradation mechanisms and their interactions and in the collection and mapping of long-term site-specific data on degrading agents. Because of large epistemic uncertainties, the evaluation of the fatigue safety of structural components in engineering practice still relies on conservative basic models of damage accumulation using $S\text{-}N$ curves or basic fracture mechanics crack growth models. Overall, reliability-calibrated structural standards are producing designs that offer a good balance between safety and cost. The future implementation of risk-based methods will further enhance the ability to meet structure-specific performance requirements set by owners and users.

Establishing consistent criteria for assessing the performance of structural systems and infrastructure networks is a critical component of communities’ efforts to optimize investment decisions for the upkeep and renewal of the built environment. Although member-level performance and reliability assessment procedures are currently well-established, it is widely recognized that a member-oriented approach does not necessarily lead to an efficient utilization of limited resources when making decisions related to the management of existing deteriorating structures or lifeline systems, especially those that may be exposed to extreme events. For this reason, researchers have renewed their interests in developing system-level assessment methods as a basis to modern structural and infrastructure performance evaluation and design processes. Specifically, system-level performance metrics and characteristics such as reliability, redundancy, robustness, resilience, and risk continue to be refined. The objective of this paper is to extend the content of the accompanying paper on reliability-based performance indicators for structural members by reviewing proposals for the development and implementation of performance-based criteria for structural systems and infrastructure networks. The paper reviews established concepts of reliability design along with emerging ideas of performance-based and resilience-based design that are especially relevant for assessing and managing system-level risk. The paper also studies structural redundancy and robustness concepts as well as network-level performance metrics along with ranking approaches. Insights from these analyses reveal the need for transitioning structural and infrastructure design processes from a traditional component-level reliability-based approach, to one that seeks uniform levels of risk across scales (from structural systems to interconnected infrastructure networks across communities). Implementation examples are drawn from experiences with buildings, bridges, offshore oil and gas platforms, and a variety of infrastructure systems. The paper also reflects on promising avenues for pursuing practical and calibrated system-level performance indicators that support life cycle performance, safety, reliability, and risk of structural and infrastructure systems as integral parts of resilient communities.

Infrastructure management plays an important role in sustainable development and is essential for the evaluation of the socioeconomic feasibility of existing projects and the development of new ones. This paper presents a review of key aspects involved in the maintenance and operation of infrastructure under uncertainty. It discusses the main conceptual and theoretical principles and guides the reader through different aspects of the problem by offering a large set of state-of-the-art references. Although the paper does not provide a comprehensive description of all maintenance methods and policies available in the literature, it critically reviews the main approaches. The concepts presented and discussed in this paper provide the basis to build models that can be used for making better decisions for maintaining and operating infrastructure systems.

The development of infrastructure systems that are sustainable and resilient is a challenging task that involves a broad range of performance indicators over the system lifecycle that affect system functionality and recovery. Sustainability indicators address economic, social, and environmental performance metrics and resilient indicators address strength, functionality, and recovery-time metrics following a hazard event. Sustainable systems consider environmental impact and conservation of nonrenewable resources over the life of the system. Resilient systems consider performance levels relative to potential damage levels and recovery times from events. Both concepts address adequate system performance and lifecycle costs, but put a different emphasis on other indicators. Numerous sources of uncertainties associated with the lifecycle performance of infrastructure systems require the use of a risk-informed decision-making approach to properly account for uncertainties and to identify cost-effective strategies to manage risk. A framework for risk-informed decision making for the lifecycle performance of infrastructure facilities that includes consideration of sustainability and resilience is presented. Separate examples are provided for the same highway bridge deck system to illustrate sustainable and resilient performance objectives with the design and rehabilitation of highway bridge decks. The sustainability assessment considers the effect of corrosion degradation mechanisms on lifecycle costs, environmental impact (${\mathrm{CO}}_2$ and waste), and social impacts (accidents and user time) while maintaining service life and structural safety. The resilience assessment considers the effect of seismic hazard events on structural damage levels and recovery time while maintaining system functionality and structural safety.

A large number of experimental curvatures in plastic hinges of concrete members are used alongside analytical moment-curvature relations to backestimate strains in the bars, the extreme concrete fibers of a section, or its confined core at the ultimate conditions of concrete sections in flexure with axial loads. Strain limits derived from these ultimate strains can support fiber models for prismatic members or models with finite-length plastic hinges at the ends. The measurements come from circular or rectangular columns (some tested diagonally), walls, or beams. The ultimate strains derived for steel bars and concrete are not local material properties (especially for cyclic loading): they depend on the geometric features of the section as a whole and of the immediate vicinity of the most critical point in the section. The size effects are clear: (1) concrete ultimate strains increase in a small compression zone, (2) the monotonic ultimate strain of tension bars increases with decreasing number of bars in the tension zone, (3) the ultimate strain of steel in cyclic tests increases with the increasing number of bars in the compression zone, and (4) the ultimate strain of confined concrete is larger at a section corner in biaxial bending than along the full side of a rectangular compression zone in uniaxial flexure, while at the perimeter of a circular section, it is in between these two extremes. The cyclic ultimate strain of steel in tension increases as the bar diameter–to–stirrup spacing ratio increases, thanks to the delay of bar buckling in previous compression half-cycles. The ultimate strains derived apply both as mean values in a plastic hinge and at the end section of prismatic members. Compared to experimental ultimate curvatures, those computed from the proposed ultimate strains do not have bias and exhibit much less scatter than those obtained from arbitrary ultimate strains specified in some codes. These code predictions are, in general, unsafe.

This study examines the behavior of high-mast lighting pole (HMLP) foundations through the use of nonlinear finite-element modeling. HMLPs utilize nuts and threaded rods to clamp the light pole structure’s base plate to the foundation. Inspections by the Alaska Department of Transportation and Public Facilities (AKDOT&PF) have revealed widespread loosening of the nuts used on HMLPs over time. The threaded rods used are F1554 Grade 55, which are heat treated from mild steel rods, $F_y=248\mathrm{MPa}$ (36 ksi) to gain additional yield strength. This study briefly highlights the difference in postyield behavior in mild steel and high-strength steel, and proposes a probable mechanism behind the clamp-load loss experienced by Alaska’s HMLPs. Because the HMLPs are much taller than traditional lighting poles, they experience larger external wind loading. This wind load is potentially stressing threaded rods into the postyield range. A previous study shows that the current tightening procedure is likely not undertightening nuts, which could be a major culprit behind loosening. As a result of this and previous research, it is believed that the nuts aren’t loosening in the traditional sense—losing clamp load by rotating on the threads. Instead, the threaded rods undergo significant postyield strain during external loading. This plastic deformation is large enough in magnitude to cause the bolted joint interface to lose clamp load when the external force is removed. One factor that exacerbates this effect in some pole configurations is prying action, which is not accounted for in the design. The clamp loss mechanism is examined by applying wind loads to three pretensioned HMLP configurations using finite-element analysis software (ABAQUS). The analysis uses 3-D solid elements, contact surfaces that allow for separation, bonding surfaces where contact surfaces are not required, boundary conditions that approximate reality, displacement controlled pretension, and external wind loading represented by coupled moments.

This paper deals with the effects of geometric imperfections on flexural inelastic buckling resistance of axially loaded columns with equally spaced lateral braces and presents a simplified way of forming the critical geometric imperfection for finite-element analyses, which leads to rational results for buckling resistances. First, laterally braced columns with geometric imperfections formed by the traditional single half sine wave or the eigenbuckling modes were analyzed by FEM to illustrate the effect of different imperfection shapes. Second, geometric imperfections of columns acquired from measurements in laboratory tests were adopted, and the statistical characteristic and probability distribution of initial deflections along the column length were obtained; then, extensive numerical results from finite-element analyses were obtained through random imperfection simulations. Last, the comparison was made between statistical results based on the random imperfections and those based on eigenbuckling modes to propose a critical geometric imperfection shape as a linear superposition of a limited number of scaled eigenbuckling modes, making it possible to obtain a rational buckling resistance result by one-time analysis, which is safe but not too conservative. The commonly used lowest eigenbuckling mode or equivalent sine shape for initial geometric imperfections do not provide a rational prediction for the inelastic buckling resistance of laterally braced columns. The critical geometric imperfection shape presented in this paper can be used in advanced analysis for inelastic buckling resistance of braced columns.

Concrete-filled steel tubes (CFSTs) frequently are a more efficient and economical structural solution then conventional reinforced concrete and structural steel. For the same size member, CFSTs offer increased strength and stiffness as well as accelerated construction. CFSTs may be used for bridge columns that will yield under earthquake loading, but this requires robust connections capable of sustaining large cyclic loads. To facilitate accelerated bridge construction, precast cap beams and girders are commonly used. Although CFST column-to-foundation connections have been studied, CFST column-to-precast cap beam connections have not. This is the focus of this research study. In contrast to cast-in-place (CIP) components, precast pier cap connections require additional design considerations including construction methodology and limits on geometry. To investigate this connection an integrated analytical and experimental research program was performed. Initially, continuum finite-element methods were used to conduct a parametric study; the results from that study determined the values of the study parameters and selection of the experimental specimens. Three different connection types were studied: (1) an embedded ring CFST, (2) a welded dowel, and (3) an interior reinforced concrete connection. Large-scale, pseudostatic tests were conducted. Results from the experimental investigation showed that the three connection types provide excellent deformability under reversed-cyclic loading and minimized damage in the cap beam.

This paper attempts to enhance (1) the flexibility-corotational beam formulation for simulating highly geometrically nonlinear problems, and (2) the arc length method of integration to develop a robust and economical solution scheme for random quasi-static loading input. The existing flexibility-corotational formulation of a three dimensional beam is improved by (1) updating the rotation matrices by using the quaternions, and (2) proposing novel methods for tracking the displacement compatibility. Further, the cylindrical arc length method is modified by (1) introducing capabilities for handling random quasi-static loading, and (2) proposing automatic adjustment of the arc length. By using these developments several classical geometric nonlinearity problems are solved and path independency for buckling and postbuckling path with the random quasi-static loading is verified. The proposed variation in the arc length solution scheme is also shown to be better than the implementation of the arc length method in the state of the art commercial software.

Mobility analysis plays a key role in form finding and design of novel kinematically indeterminate structures. For large-scale or complex structures, it demands considerable computations and analyses, and, thus, efficient method is of great interest. Because many structures could be viewed as the product of two or three subgraphs, such structures are called regular structures and usually hold certain symmetries. Combining graph theory with group representation theory, this paper proposes an improved symmetry method for the mobility of kinematically indeterminate pin-jointed structures. The concepts of graph products are described and utilized, to simplify the conventional symmetry-extended mobility rule. Based on the definitions of the Cartesian product, the direct product, and the strong Cartesian product, the authors establish the representations of nodes and members for the graph products, respectively. The proposed method focuses on the simple subgraphs, which generate the entire structure, and computes the matrix representations of the nodes and members under each symmetry operation. Therefore, symmetry analysis of the entire structure is transformed into independent evaluations on the subgraphs. Mobility of symmetric structures with a large number of nodes and members is studied, and the static and kinematic indeterminacy of the structures is evaluated using the proposed method.

Concentrically braced frames (CBFs) have been used as seismic force resisting systems in steel structures for many decades. CBFs designed prior to about 1988 were not capacity designed for the expected brace strength, and many are therefore prone to undesirable failure modes when subjected to large earthquakes. The consequences of other factors, such as weld toughness, gusset plate clearance to permit brace end rotation, and local slenderness of the brace, were also not fully understood, so older CBFs may also exhibit premature failure relative to modern, special CBFs (SCBFs). Thus, these older CBFs are expected to exhibit low inelastic deformation capacity and are termed nonductile CBFs (NCBFs) here. An infrastructure review showed that many NCBFs are deficient by SCBF standards and an experimental program was undertaken to explore the impact of brace and connection deficiencies in particular. Brace local slenderness and brace-to-gusset plate weld deficiencies were found to be highly detrimental to drift capacity. Beyond these concerns, welds joining the gusset plate to the frame were found to be vulnerable to fracture even if they had sufficient strength based upon the yield capacity of the brace. Bolted connections were found to be generally more robust, especially if bolt bearing and fracture resistances were similar. The findings presented here will aid engineers in determining need and type of retrofit for NCBFs.

Probabilistic seismic risk analysis (SRA) is often used for accurately quantifying seismic risk of critical engineering structures. It combines the probabilistic model of the behavior of structural response given a ground motion parameter (GMP) (e.g., seismic fragility model) and the probabilistic seismic hazard analysis (PSHA) for the GMP in a mathematically rigorous manner. In current engineering practice, SRA is performed based on a single GMP. For structures whose responses can be better predicted using multiple GMPs, a vector-valued SRA (VSRA) gives more accurate estimates of risk. However, due to the extensive computational efforts required for the multiple-dimensional cases, VSRA has not been widely applied in engineering practice. This paper presents a simplified approach to VSRA, which can substantially improve computational efficiency without losing accuracy. A new seismic hazard deaggregation procedure is proposed to determine a set of controlling earthquakes in terms of magnitude, source-site distance, and occurrence rate for the site of interest. VSRA is then performed based on the controlling earthquakes rather than all possible earthquake occurrences. Two numerical examples are presented to validate the effectiveness and accuracy of the simplified approach. Factors affecting the approximations in the simplified approach are discussed.

Most codes for the design of fiber-reinforced concrete (FRC) structures are based on experience gained over the years with steel fibers. Recent codes include the possibility of applying the same considerations for FRC structures with plastic fiber. However, the consequences of assuming identical design considerations regardless of the type of fiber is scarcely known in terms of the structural behavior of full-scale elements. The main goal of this paper is to assess the influence of the type of fiber on the performance of full-scale concrete slabs, emphasizing the consequences of using a common design approach. For that, a comparative experimental study was conducted to expose differences regarding crack patterns and load-deflection behavior. Then, finite-element simulations were performed using the constitutive equations from a commonly used model code. The results indicate distinct levels of overestimation of the structural behavior measured experimentally, confirming that specific design considerations are required depending on the type of fiber used. Based on the findings, correction factors are proposed for the design of FRC slabs with each fiber.

This paper studies the postfire performance of steel-reinforced concrete (SRC) columns subjected to an entire loading and fire phase, including ambient temperature loading, heating and cooling with constant external loads, and postfire loading. A sequentially coupled thermal-stress analysis module was employed in a finite-element analysis (FEA) program to establish temperature field analysis and structural-analysis models of SRC columns subjected to the entire loading and fire phase. To improve the precision of the FEA model, the influence of fire-induced concrete explosive spalling (CES) was considered by using element change technique. Besides the existing fire-resistance test data of SRC columns, to further validate the accuracy of the FEA model in the postfire phase, a set of fire tests on SRC columns, including a fire-resistance test and postfire test subjected to the entire loading and fire phase, were conducted. Compressive failure and bending failure were observed in the test. Comparison between predicted and test results indicates that the accuracy of the FEA model was acceptable, and then the FEA model was extended to simulate a full-scale SRC column in a real project subjected to the entire loading and fire phase. The temperature distribution, load versus axial deformation relations, and load redistribution of the full-scale SRC column during the entire loading and fire phase were investigated. It was found that the postfire peak load of the calculated column can be 26.7% lower than that at ambient temperature. The load carried by shaped steel rises from 20% to the peak value of 93% during the heating and cooling phase, indicating the significant contribution of shaped steel to resist the external loads during fire exposure.

A continuous and smooth hysteretic model for shear-critical reinforced concrete (RC) columns was identified and calibrated based on an efficient stochastic search technique and an experimental database consisting of 30 shear-critical RC columns. The inelastic restoring force of shear-critical columns was described by the Bouc-Wen-Baber-Noori (BWBN) model to produce the requisite strength and stiffness degradation as well as pinching effect. Meanwhile, the backward Euler and Newton-Raphson schemes were adopted to determine the inelastic restoring force by solving the ordinary differential equations. Then a differential evolution (DE) algorithm was utilized to identify the control parameters of the BWBN hysteretic model based on an experimental database consisting of 30 shear-critical rectangular columns under cyclic loading. Finally, prediction equations for the BWBN model parameters were developed in terms of four structural physical parameters. The capability and accuracy of the calibrated hysteretic model were demonstrated by comparison with the available experimental data.

Concrete-filled steel tube (CFT) columns offer significant advantages over columns made of either steel or concrete alone, such as large energy dissipation and increased strength and stiffness. To further improve the seismic performance of these columns, an experimental investigation was conducted into CFT columns using ultrahigh-strength steel. More specifically, seven square and circular specimens made with high-strength and conventional steel were subjected to constant compressive axial load and cyclic flexural load protocols with two and 20 cycles imposed at each drift level. Based on the test results, the influence on the CFT’s cyclic behavior of the high-strength steel, cross-sectional shape, axial load, and number of cycles in lateral loading history was studied. In comparison with the conventional steel specimens, larger elastic deformation, higher strength, and delay of local buckling were observed in the high-strength steel specimens, while compared with the circular specimens, the square specimens sustained larger drift angles without fracture of their steel tubes because of the development and progress of serious local buckling. Furthermore, a simple analytical model based on the concept of the superposed strength method was proposed. The accuracy of this model was confirmed with the experimental results.

Lipped channel beams (LCBs) are commonly used as flexural members such as floor joists and bearers in the construction industry. These thin-walled LCBs are subjected to specific buckling and failure modes, one of them being web crippling. Despite considerable research in this area, some recent studies have shown that the current web crippling design rules are unable to predict the test capacities under end-two-flange (ETF) and interior-two-flange (ITF) load conditions. In many instances, web crippling predictions by the available design standards such as AISI S100, AS/NZS 4600 and Eurocode 3 Part 1-3 are inconsistent, i.e., unconservative in some cases, although they are conservative in other cases. Hence, experimental studies consisting of 36 tests were conducted in this research to assess the web crippling behavior and capacities of high-strength LCBs under two-flange load cases (ETF and ITF). Experimental results were then compared with the predictions from current design rules. Comparison of the ultimate web crippling capacities from tests showed that the design equations are very unconservative for LCB sections under the ETF load case and are conservative for the ITF load case. Hence, improved equations were proposed to determine the web crippling capacities of LCBs based on the experimental results from this study. Current design equations do not provide the direct strength method (DSM) provisions for web crippling. Hence, suitable design rules were also developed under the DSM format using the test results and buckling analyses using finite-element analyses.