Riyadh Hindi

A Proposed Damage Model for RC Bridge Columns under Cyclic Loading

This paper defines a damage index based on the predicted hysteretic behavior of a concrete column. The model yields a damage index at a point in the time history for the element, based on the predicted monotonic response from the point in time to failure. The model takes into account the parameters that describe the hysteretic behavior: stiffness degradation, strength deterioration, and ultimate displacement reduction. Therefore, the damage model is accumulative and it combines energy, ductility, and low-cycle fatigue. The model is based on the work needed to fail a reinforced concrete column monotonically after it experiences a cyclic loading. The model modifies the ultimate displacement that the column can achieve, due to low-cycle fatigue in the longitudinal reinforcement using the Coffin-Manson rule in combination with Miners hypothesis. The proposed model is applied to bridge columns tested by others, and compared to existing damage indices. The proposed model gives a realistic prediction of damage throughout the loading cycles for several test specimens investigated.

Influence of Different Confinement Patterns on the Axial Behavior of R/C Columns

This paper studies the axial behavior of reinforced concrete columns using a new confinement technique that the first author has developed. Two opposing spirals (cross spirals) are used to confine reinforced concrete circular columns in order to enhance their strength and ductility or to increase the spiral spacing (pitch) to facilitate the flow of concrete during construction. The influence of the new confinement technique on the axial strength and ductility of reinforced concrete circular columns is experimentally investigated and compared to columns confined with conventional single spiral. Ten small-scale reinforced concrete circular columns confined with different spiral patterns were constructed and tested subjected to concentric axial compressive loading. The axial force, axial displacement and strains in the concrete and confining and longitudinal reinforcement were measured during the testing. This study showed that columns confined with two opposing spirals behaved almost similar to columns confined with single spiral when the same amount of confining reinforcement is used. The study also showed that the new confinement technique could significantly improve the axial strength and ductility of circular columns without hindering the flow of concrete during construction.

Simplified Trilinear Behavior of Diagonally ReinforcedCoupling Beams

This paper presents a simplified procedure to predict the monotonic behavior of diagonally reinforced coupling beams of shearwalls. The procedure yields a trilinear monotonic force-displacement response up to failure. The proposed simplified procedure can be easily used by design engineers. The procedure assumes all load is resisted by diagonal tension and compression. The diagonal compression is carried by the diagonal reinforcement and the concrete core, while the diagonal tension is carried only by the diagonal reinforcement. Either rupture of the diagonal reinforcement or crushing of the concrete core defines failure point of the coupling beam. The proposed trilinear approximation is applied and compared with 13 specimens that were experimentally tested by other investigators. The specimens vary in terms of dimensions, reinforcement, and material properties.

Distortion-Induced Fatigue Cracking in a Seismically Retrofitted Steel Bridge

This study examines the effect of a seismic retrofit on distortion-induced fatigue cracking in a steel girder bridge in the Midwestern United States. The seismic retrofit of the bridge was deemed necessary following a structural review initiated by the Federal Highway Administration (FHWA) in response to the 1989 Loma Prieta Earthquake. Following the review, the bridge was modified using a conventional seismic retrofit strategy to meet the 1995 FHWA standards for survivability and seismic performance. Upon completion of the retrofit, subsequent inspections identified new cracks (at the location of the retrofit) in the webs of longitudinal girders at transverse stiffener locations in an area known as the web gap. This study investigates the influence of the seismic retrofit strategy on crack formation by comparing principal stresses in the web-gap region of the original and retrofitted bridge under dead, live and thermal loading. A three-dimensional linear-elastic finite element analysis of the bridge before and after the retrofit is presented with results obtained from a commercial finite element software package. Comparisons of stresses before and after the retrofit indicate a measurable increase in stress near the web-gap region attributable to thermal and live loads following the retrofit. In addition, several viable repair strategies to limit these web-gap stresses and subsequent distortion-induced fatigue are presented. The results to date clearly demonstrate the importance of considering fatigue-sensitive details in seismic-retrofit strategies.

Effect of Modifying Bearing Fixities on the Seismic Response of Short- to Medium-Length Bridges with Heavy Substructures

The effect of modifying the fixity conditions of the bearings on the seismic response and vulnerability of existing bridges with heavy substructures is studied. For this purpose, nonlinear seismic analysis of a typical bridge with heavy substructures is conducted to assess its seismic response and vulnerabilities. Next, the bearings are modified to obtain four different configurations of bearing fixities over the substructures. Nonlinear seismic analyses of the bridge are conducted to assess its seismic response and vulnerability for each bearing fixity configuration. It is found that changing the fixities of the bearings may be an effective response modification technique to mitigate the effect of seismic forces on vulnerable substructures of the bridge under consideration. The thermal load effects on the bridge with fixed bearings are found to be generally negligible compared to seismic load effects. Thus such a response modification technique may be used for the economical design of new or seismic retrofitting of existing short- to medium-length bridges similar to that considered in this study and subjected to low- to moderate-intensity ground motions.

Prediction of damage in R/C shear panels subjected to reversed cyclic loading

In this paper, the damage prediction of shear-dominated reinforced concrete (RC) elements subjected to reversed cyclic shear is presented using an existing damage model. The damage model is primarily based on the monotonic energy dissipating capacity of structural elements before and after the application of reversed cyclic loading. Therefore, it could be universally applicable to different types of structural members, includeing shear-dominated RC members. The applicability of the damage model to shear-dominated RC members is assessed using the results from reversed cyclic shear load tests conducted earlier on eleven RC panels. First, the monotonic energy dissipating capacities of the panels before and after the application of reversed cyclic loading are estimated and employed in the damage model. Next, a detailed comparison between the analytically predicted damage and the observed damage from the experimental tests of the panels is performed throughout the loading history. Subsequently, the effects of two important parameters, the orientation and the percentage of reinforcement, on the damage of such shear-dominated panels are studied. The research results demonstrated that the analytically predicted damage is in reasonably good agreement with the observed damage throughout the entire loading history. Furthermore, the orientation and percentage of reinforcement is found to have considerable effect on the extent of damage.

Live Load Distribution Factor for Highway Bridges Based on AASHTO-LRFD and Finite Element Analysis

The AASHTO-LRFD (2004) live load distribution factors for highway bridges show significant change compared to the standard AASHTO (1996) that have been used for the last 50 years. This paper presents a comparison between the moments distribution factors of concrete bridges due to live load calculated in accordance with the current AASHTOLRFD (2004) formulas and finite-element analysis. Several three-dimensional linear elastic models were built using the structural analysis program SAP2000 (2004) to obtain the most accurate method to model the bridge superstructure. The bridge deck was modeled as quadrilateral shell elements and the girders as space frame elements. The live load used in the analysis is the vehicular load plus the standard uniform lane load as specified by AASHTO–LRFD (2004). The live load is positioned at the longitudinal location that produces the maximum moments, then the load is moved transversely across the bridge width in order to investigate all possibilities of bridge loading (one, two and three lanes loaded). In this comparison the range of applicability specified by the AASHTO-LRFD (2004) is fully covered in terms of span length, slab thickness, girder spacing and longitudinal stiffness. One parameter is considered at a time while the remaining parameters are fixed. All the AASHTO-PCI girders (Type I to VI) are considered to cover the complete range of longitudinal stiffness specified in the AASHTO–LRFD (2004) specifications. The results of this study are presented in format of graphs and some recommendations for specific bridge geometries are presented.

SEISMIC RETROFITTING OF BRIDGES BY RESPONSE MODIFICATION TECHNIQUES BASED ON ALTERING BEARING FIXITIES

Feasibility of a proposed seismic retrofitting technique for typical bridges in the Central US has been studied. The retrofitting technique is based on modifying the fixity conditions of the bearings for response modification purposes to eliminate the need for costly retrofitting of substructures. For this purpose, a seismically vulnerable bridge, typical of those in the Central US was selected. Detailed seismic analyses of the bridge were then conducted. It was found that its bearings, wing-walls and pier foundations needed to be retrofitted. A conventional retrofitting strategy was developed and the cost of retrofit was estimated. Next, the abutment bearings were fixed longitudinally to modify the response of the bridge so as to alleviate the effect of seismic forces transferred to vulnerable pier foundations. It was observed that the proposed retrofitting technique effectively mitigated the seismic forces transferred to the pier foundations and eliminated the need for their costly retrofitting. Thus, the proposed retrofitting method may be used for economical seismic retrofitting of such bridges in the Central US or in similar regions of low to moderate risk of seismic activity.

Investigation of Distortion-Induced Web-Gap Cracking in a Seismically Retrofitted Steel Bridge: Repair Measures

AbstractThis paper studies distortion-induced fatigue cracks in a 1960s-era welded plate girder bridge that developed cracks in the web-gap region shortly after completion of a comprehensive seismic retrofit. Finite-element analysis (FEA) and field testing were conducted to investigate the cause of cracking and then to recommend appropriate repair measures for the bridge. The field test results validated the high-stress concentrations in the web-gap region of the bridge that led to fatigue cracking after only a limited number of cycles. FEA results indicated that the high-stress concentrations were principally a result of replacing K-type diaphragms with stiffer cross diaphragms as part of the seismic retrofit. Two conventional repair measures in addition to two innovative repair measures were evaluated. The proposed innovative repair measures focused on ease of installation compared with more conventional options and were found to reduce the distortion-induced stresses well below the constant amplitude f...

NONLINEAR BEHAVIOR OF DIAGONALLY REINFORCED COUPLING BEAMS

This paper summarizes a theoretical model to predict the monotonic load -deformation behavior of diagonally reinforced coupling beams. The model assumes all load is resisted by diagonal tension and compression. The diagonal compression is carried by the diagonal reinforcement and the concrete core surrounded by the diagonal bars in that direction, while the diagonal tension is carried only by the diagonal reinforcement. Either rupture of the diagonal reinforcement or crushing of the concrete core defines the end point of the behavior (failure) of the coupling beam. The strain hardening of the reinforcing steel is considered in this study, which is an important source of strength at high deformation. The model is automated in a computer program that can be easily used by design engineers to predict the monotonic load-deformation response of such elements. The model is applied and compared to experimental tests that were done by several investigators (total of 13 specimens). The specimens vary in terms of dimensions, reinforcement and material properties. The model gave good results compared to the test results.