Chris P. Pantelides

Performance-based design using structural optimization

A new methodology for seismic design is proposed based on structural optimization with performance-based constraints. Performance-based criteria are introduced for the seismic design of new buildings. These criteria are derived from the National Guidelines for Seismic Rehabilitation of Buildings (Reference [19], Federal Emergency Management Agency (FEMA), ‘NHERP Guidelines for seismic rehabilitation of buildings’, Report Nos 273 and 274, Washington, DC, 1997) for retrofitting existing structures. The proposed design methodology takes into account the non-linear behaviour of the structure. The goal is to incorporate in the design the actual performance levels of the structure, i.e. how much reserve capacity the structure has in an earthquake of a given magnitude. The optimal design of the structure minimizes the structural cost subjected to performance constraints on plastic rotations of beams and columns, as well as behavioural constraints for reinforced concrete frames. Uncertainties in the structural period and in the earthquake excitation are taken into account using convex models. The optimization routine incorporates a non-linear analysis program and the procedure is automated. The proposed methodology leads to a structural design for which the levels of reliability (performance levels) are assumed to be quantifiable. Furthermore, the entire behaviour of the structure well into the non-linear range is investigated in the design process. Copyright

Optimum structural design via convex model superposition

Abstract A new approach is presented for implementing a multidimensional convex model for the optimal design of structures subjected to bounded but uncertain loads. This is a non-probabilistic method for including uncertainty in the design. In previous studies, a two-stage optimization process was used to predict the effect of uncertainties, modeled using convex theory, on the response constraints. Here, a superposition method is used to find the response of structures with load uncertainties. The convex model is implemented on the effect of the uncertain loads, i.e., the displacements and stresses. The advantage of the method is that one optimization process is eliminated, and the constraints do not need to be expressed explicitly as a function of the design variables.

Linear and nonlinear pounding of structural systems

Abstract Structural pounding occurs frequently during strong earthquakes between two buildings or different parts of the same building. Structural pounding can also occur in bridges in the longitudinal direction at the abutments or at expansion hinges, and laterally between narrowly separated superstructures. The dynamic behavior of a damped single-degree-of-freedom (SDF) structural system with onesided pounding during an earthquake is examined. The structural response of the SDF structure with either elastic or inelastic structural behavior is analyzed. The pounding phenomenon is modeled as a Hertz impact force, which represents the behavior of two colliding bodies during a completely elastic impact. Artificial, as well as actual earthquake excitations, and realistic parameters for the pounding model are used in numerical evaluations of the seismic response. The effects of separation distance and inelastic structural behavior on the magnitude of the pounding force are examined. An increase in the damping energy absorption capacity of the pounding structure results in the reduction of the pounding forces. The present model and method of analysis can be used in investigations of pounding between buildings or pounding which occurs in bridges during strong earthquakes.

Convex model for seismic design of structures-I: Analysis

A convex model is used to estimate the maximum response of structural systems subjected to uncertain seismic excitations. The convex model is based on the assumption that the energy of the excitation is bounded . A reduction factor, defined in the modal domain by dividing the results obtained from the convex model by those from the time history of the actual record, is used to calibrate the convex model. An average reduction factor is also defined by averaging a set of excitation-specific reduction factors. The average reduction factor can be used for unknown excitations with an assumed energy bound and certain common earthquake characteristics. These common characteristics can be defined either by a set of previous earthquakes in the region or by regional earthquake spectra. The convex model using the average reduction factor yields acceptable predictions of the maximum response.

Load and resistance convex models for optimum design

This paper is concerned with the optimal design of structures that are affected by uncertainties present in the loads applied to the structure, and by uncertainties affecting the internal resistance of the structural members. The magnitude of the applied loads and the modulus of elasticity of the structural members are assumed to vary within deterministic bounds. These uncertainties are idealized using a nonprobabilistic method, the convex model. The two types of uncertainties are considered simultaneously by employing the Cartesian product of convex sets. Two different convex models are examined to account for the uncertainties: the ellipsoidal convex model, and the uniform bound convex model. The optimum designs of a truss using the two convex models are compared to a worst case scenario optimum design in order to evaluate their performance. It is shown that it is not possible to identify a single worst case scenario that would be able to account for all possible combinations of uncertainties. However, both the ellipsoidal and the uniform bound convex model designs are found to be superior to the worst case scenario design in terms of constraint violations.

Performance-Based Evaluation of Reinforced Concrete Building Exterior Joints for Seismic Excitation

Beam-column joints of nonductile reinforced concrete buildings that were built prior to the current seismic code provisions have been investigated using several performance-based criteria. Four half-scale reinforced concrete exterior joints were tested to investigate their behavior in a shear-critical failure mode. The joints were subjected to quasi-static cyclic loading, and their performance was examined in terms of lateral load capacity, drift ratio, axial load reduction in the column at high drift ratios, joint shear strength, ductility, shear deformation angle of the joint, and residual strength. Two levels of axial compressive column load were investigated to determine how this variable might influence the performance of the joint. Specific performance levels for this type of reinforced concrete joint were established and a comparison was made to current design and rehabilitation standards. A limit states model was established, which could be used for performance evaluation or seismic rehabilitation.

Axial Load Behavior of Concrete Columns Confined with GFRP Spirals

The writers evaluated the confinement that was provided by glass fiber-reinforced polymer (GFRP) spirals in concrete columns under axial load. Given that GFRP spirals are resistant to chloride-induced corrosion, the option of replacing steel spirals with GFRP spirals was explored to determine whether this would reduce the corrosion of the vertical steel bars in hybrid columns. The writers investigated the axial load behavior of 10 spirally reinforced concrete columns. Six of the 254-mm diameter columns were confined with a GFRP spiral and four were confined with a steel spiral. Some of the columns that were confined with a GFRP spiral utilized steel vertical bars (hybrid columns), whereas others utilized GFRP vertical bars (all-GFRP columns). The stress-strain and load-displacement behavior of all columns was studied. Analytical expressions predicted the axial load capacity of the hybrid and all-GFRP-reinforced concrete columns. Axial compression tests of all-steel-reinforced and hybrid specimens subjected to accelerated corrosion were also carried out. The latter exhibited a smaller corrosion rate, similar axial load capacity, and equal or higher ductility relative to steel corroded columns.

Optimal design of dynamically constrained structures

Abstract A modified iterated simulated annealing (MISA) method is proposed for optimal design of structural systems with dynamic constraints. The MISA method uses a random sequence of designs to find the optimal design. In performing the optimization, the MISA method employs two new features: the first is automatic reduction of the search region; and the second is sensitivity analysis of the design variables. The MISA method deviates from traditional annealing algorithms because of these two features and is considered to be a new method. A two-story frame structure with dynamic constraints was used in evaluating the performance of the MISA method. Classical optimal design methods were used to solve the same problem and the results were compared to MISA. The comparisons show that MISA is able to provide the global minimum even when infeasible initial designs are attempted. By contrast, some classical optimization methods either fail to converge or converge to local minima.

CONVEX MODEL FOR SEISMIC DESIGN OF STRUCTURES—II: DESIGN OF CONVENTIONAL AND ACTIVE STRUCTURES

The optimal design of the members of conventional structures or structures equipped with active bracing systems, known as active structures, is presented for uncertain excitations. Three approaches are used for obtaining the optimal structural design: (1) the time-history analysis of an actual earthquake record (AR), (2) the global energy-bound convex model adjusted with an excitation-specific reduction factor (RGEB), and (3) the global energy-bound convex model adjusted with an average reduction factor (ARGEB) for a set of excitations with common characteristics. The optimal structures obtained using the RGEB and ARGEB convex models have different sizes for their conventional members from the designs based on a time-history analysis of the actual earthquake (AR). The optimal design of the structure is carried out using a modified annealing algorithm. The advantage of using convex models to perform the optimization is that they represent a more general excitation than a single earthquake record. In addition, the RGEB and ARGEB convex models require considerably less computational effort since the constraints of the optimization become time-independent. A comparison between optimal designs of structures with conventional members only, and active structures indicates that the latter are more efficient by combining the conventional and active members.

In‐Situ Verification of Rehabilitation and Repair of Reinforced Concrete Bridge Bents under Simulated Seismic Loads

Three in-situ tests were performed on two bents of a reinforced concrete (RC) bridge under quasi-static cyclic loads. The bridge was built in 1963 and did not possess the necessary reinforcement details for ductile performance. The tests included an as-built bent, a bent rehabilitated with carbon fiber reinforced polymer (FRP) composite jackets, and a damaged bent repaired with epoxy injection and carbon FRP composite jackets. Two new concepts of strengthening bridge bents with FRP composites were implemented in this study. The first involves shear strengthening and confinement of a beam cap-column joint through an FRP composite “ankle-wrap.” The second is an FRP composite “U-strap” to improve the anchorage of column longitudinal steel reinforcement extending into the joint. FRP composite jackets were also implemented in the columns and beam cap. An additional rehabilitation measure was that of anchorage of the piles to the pile cap using epoxied high strength steel bars. The performance of the bent in the as-built condition and that of the rehabilitated and repaired bents is described in terms of strength, stiffness, displacement ductility, and energy dissipation.