A. Ghobarah

PERFORMANCE-BASED DESIGN IN EARTHQUAKE ENGINEERING: STATE OF DEVELOPMENT

Abstract The design objectives in current building codes address life safety, control damage in minor and moderate earthquakes, and prevent collapse in a major earthquake. However, the actual reliability of the design in achieving the objectives is not known. There is a general agreement among researchers and professionals that future seismic design needs to be based on achieving stated multiple performance objectives. Future seismic design practice will be based on explicit performance criteria that can be quantified, considering multiple performance and hazard levels. There are several challenges to be addressed before procedures for performance-based design can be widely accepted. The development in performance-based design in seismic engineering will be directed towards the definition of performance objectives, a general design methodology, issues of ground motion modeling, and demand and capacity evaluations.

Response‐based damage assessment of structures

The structures ability to survive an earthquake may be measured in terms of the expected state of damage of the structure after the earthquake. Damage may be quantified by using any of several damage indices defined as functions whose values can be related to particular structural damage states. A number of available response-based damage indices are discussed and critically evaluated for their applicability in seismic damage evaluation. A new rational approach for damage assessment is presented which provides a measure of the physical response characteristics of the structure and is better suited for non-linear structural analysis. A practical method based on the static pushover analysis is proposed to estimate the expected damage to structures when subjected to earthquakes of different intensities. Results of the analysis of ductile and non-ductile reinforced concrete buildings show that the proposed procedure for damage assessment gives a simple, consistent and rational damage indicator for structures.

Shear strengthening of beam-column joints

Shear failure of beam-column joints is identified as the principal cause of collapse of many moment-resisting frame buildings during recent earthquakes. Effective and economical rehabilitation techniques for the upgrade of the joint shear-resistance capacity in existing structures are needed. The objective of this research is to develop effective selective rehabilitation schemes for reinforced concrete beam-column joints using advanced composite materials. Several reinforced concrete beam-column joints were constructed. The joints were designed to simulate nonductile detailing characteristics of pre-seismic code construction. The control specimens showed joint shear failure when subjected to cyclic loading at the beam tip. Different fibre-wrap rehabilitation schemes were applied to the joint panel with the objective of upgrading the shear strength of the joint. The tested rehabilitation techniques were successful in improving the shear resistance of the joint and in eliminating or delaying the shear mode of failure.

Seismic rehabilitation of beam–column joint using GFRP sheets

Abstract Techniques for upgrading reinforced concrete beam–column joints are proposed. The test specimens represent a typical joint that was built in accordance to pre-1970s’ codes. The objective of the rehabilitation is to upgrade the shear strength of these joints and reduce the potential for bond-slip of the bottom bars of the beam. Glass fibre-reinforced polymer (GFRP) sheets are wrapped around the joint to prevent the joint shear failure. GFRP sheets are attached to the bottom beam face to replace the inadequately anchored steel bars. Three beam–column joints are tested; namely, a control specimen and two rehabilitated specimens. The specimens are tested under quasi-static load to failure. The control specimen showed combined brittle joint shear and bond failure modes while the rehabilitated specimens showed a more ductile failure mode. A simple design methodology for the rehabilitation scheme is proposed.

Rehabilitation of a reinforced concrete frame using eccentric steel bracing

Abstract The seismic performance of a low-rise nonductile reinforced concrete (RC) building rehabilitated using eccentric steel bracing is investigated. A three-story office building was analyzed using various ground motion records. The effectiveness of the eccentric steel bracing in rehabilitating the building was examined. The effect of distributing the steel bracing over the height of the RC frame on the seismic performance of the rehabilitated building was studied. The behavior of the nonductile RC frame members is represented by a beam–column element capable of modeling the strength deterioration and the effect of the axial force on the yield moment and the deformation capacities at peak strength of the members. The link behavior is modeled using tri-linear moment and shear force representations. The performance of the building is evaluated in terms of story drifts and damage indices.

Seismic performance of highway bridges

Abstract The seismic response of both isolated and non-isolated highway bridges is investigated with the objective of determining the effect of some of the design parameters on the bridge response. A dynamic bridge model is used to compare the effect of various energy-dissipating concepts when the bridge is subjected to moderate to severe earthquake ground motions. The seismic energy is assumed to be dissipated by the inelastic pier behaviour or by the use of base-isolation devices such as the lead-rubber bearing system. It was found that allowing the pier to deform inelastically requires very high ductility design. Structural damage in the form of permanent deformation is unavoidable. The use of lead plugs in the isolation devices is a very efficient energy dissipation system. Selecting the force required to yield the lead plugs to be 5% of the superstructures weight provides a reasonable balance between reduced shear force transmitted to the pier and increased displacement of the bridge deck. It was also concluded that the shearing force in the pier can be substantially reduced by locating the lead plugs at the abutments only.

SEISMIC REHABILITATION OF BEAM-COLUMN JOINTS USING FRP LAMINATES

An innovative and practical technique for the seismic rehabilitation of beam-column joints using fiber reinforced polymers (FRP) is presented. The procedure is to upgrade the shear capacity of the joint and thus allow the ductile ftexural hinge to form in the beam. An experimental study is conducted in order to evaluate the performance of a full-scale reinforced concrete external beam-column joint from a moment resisting frame designed to earlier code then repaired using the proposed technique. The beam-column joint is tested under cyclic loading applied at the free end of the beam and axial column load. The suggested repair procedure was applied to the tested specimen. The composite laminate system proved to be effective in upgrading the shear capacity of the nonductile beam-column joint. Comparison between the behaviour of the specimen before and after the repair is presented. A design methodology for fibre jacketing to upgrade the shear capacity of existing beam-column joints in reinforced concrete moment resisting frames is proposed.

Nonlinear seismic response of concrete gravity dams with dam-reservoir interaction

Abstract The nonlinear seismic fracture response of concrete gravity dams is conducted when the effect of the dam–reservoir interaction is taken into account. The dam–reservoir interaction is included in the time domain analysis using the staggered solution method. Smeared crack analysis model based on a nonlinear fracture mechanics crack propagation criterion is used to study the cracking and response of the dam. Results of the analysis are compared to the case when the dam–reservoir interaction was represented by added masses. It is found that the nonlinear analysis of concrete gravity dams that includes dam–reservoir interaction gives a crack pattern that is close to the observed damage to the Sefid-rud dam during the 1990 Manjil (Iran) earthquake. The predicted crack pattern is different from that of the case when the dam–reservoir interaction is approximated using the added mass approach. It is concluded that proper modelling of the dam–reservoir interaction is important in the nonlinear response analysis of concrete gravity dams.

Dynamic analysis of reinforced concrete frames including joint shear deformation

Abstract The rehabilitation of existing buildings requires an assessment of their lateral load resisting capacity which may be limited by the strength and ductility capacity of their critical regions. From this assessment, a rehabilitation strategy can be formulated. Lack of adequate confinement and shear reinforcement in the beam–column joints of existing reinforced concrete frames may be the cause of brittle failure during a seismic event. Most of the nonlinear dynamic analysis programs assume infinitely rigid beam–column joints in concrete frames regardless of the reinforcement detail. To properly analyze existing structures, a joint element is proposed and introduced in the nonlinear dynamic analysis. The developed joint element accounts for inelastic shear deformation and bar bond slip. The response of three- and nine-story existing frames with joint elements when subjected to dynamic loading was compared with the response of frames with rigid joint assumption and the response of rehabilitated frames. The results show that the modelling of inelastic shear deformation in joints has a significant effect on the seismic response in terms of drift and damage. The rigid joint assumption was found to be inappropriate when assessing the behaviour of existing nonductile structures.

Rehabilitation of Reinforced Concrete Frame Connections Using Corrugated Steel Jacketing

An experimental investigation was conducted to study the failure mode of existing reinforced concrete beam-column connections designed during the 1970s. The effectiveness of using innovative corrugated steel jackets for enhancing the seismic shear strength and ductility of these types of connections was examined. Four large-scale beam-column connections were tested under cyclic loading. The four connections represent existing frame connection, current code detailed, and two rehabilitated connections. The variables in the test specimens included the amount of joint and column transverse reinforcement and the jacketing of column only or both column and beam. Test results indicated that the shear strength ofjacketed joints can be estimated using an approach that is similar to the current design recommendations for beam-column joints. The corrugated jacket was found to be efficient in the rehabilitation of existing structures not meeting current seismic code requirements. A method is proposed for the design of the corrugated steel jacket to enhance the shear strength and ductility of the beam-column joint.