Ahmad Itani

Cyclic Behavior of Shear Links of Various Grades of Plate Steel

Plate steel shear links offer enhanced design freedom, including the ability to use steel grades that are not available in rolled shapes. This paper discusses the cyclic response of five types of shear links designed using steel grades that ranged from 485 to 100 MPa yield strength. The design and subsequent performance was categorized into two groups; conventional design and those designed without stiffeners using low yield point steels. Ductile failure modes and desirable hysteretic behavior that resulted in efficient cyclic energy dissipation were observed for all specimens. The links designed without stiffeners using low yield point steel reached shear deformations exceeding 0.20 rad, outperforming the conventional designs that exhibited approximately 0.12 rad capacity. The enhanced performance was the result of shifting the failure mode by excluding intermediate stiffeners and providing low web compactness. This combination eliminated welding and decreased measured plastic strain demands on the face of the inelastically deforming webs.

Floor Accelerations in Yielding Special Moment Resisting Frame Structures

Severe damage to acceleration sensitive nonstructural components in recent earthquakes has resulted in unprecedented losses. Recent research has been aimed at increasing the understanding of acceleration demands on nonstructural components in buildings. This investigation subjects a set of four special moment resisting frame (SMRF) building models to a suite of 21 far-field ground motions using the incremental dynamic analysis procedure. Full three-dimensional models including floor slabs are used to extract both the horizontal and vertical responses. Floor acceleration response spectra are generated to assess the acceleration demands on elastic nonstructural components. Changes to the current code provisions that include the influence of structural period are proposed. An alternative design approach that directly amplifies the ground acceleration spectrum to achieve the desired floor acceleration spectrum is presented.

Experimental and Analytical Studies of Hospital Piping Assemblies Subjected to Seismic Loading

The seismic characteristics of welded and threaded hospital piping assemblies were investigated with and without seismic restrainers under various intensities of seismic loading using a biaxial shake table. Experimental results showed that the restrainers limited the displacements; however, they did not reduce the acceleration responses. No leakage was detected in the welded assembly up to a drift ratio of 4.3%; however, threaded piping suffered minor leaks at a drift ratio of 2.2% and experienced connection failure at a drift ratio of 4.3%. A simplified computational model was developed and calibrated with experimental data using SAP2000. The effective stiffness of the seismic restrainers was determined to be 10% of full stiffness due to their initial slack. The analyses showed that the dynamic response of the piping system as braced in these experiments with similar boundary conditions was predominantly kinematic with minimal inertial effects.

Analytical Fragility Functions for Horizontally Curved Steel I-Girder Highway Bridges

Horizontally curved bridges were investigated following a statistical evaluation of typical details commonly used in the United States. Both seismically and non-seismically designed bridges are considered where the primary differences are in column confinement, type of bearings and abutment support length. Columns and bearings were found to be the most seismically vulnerable components for both categories. Central angle was identified as an important factor that increases the demand on various components, particularly columns. Furthermore, larger angles lead to increased deformations at the supports which adversely affect the seismic vulnerability. Consistent with the fragility curves that account for the central angle explicitly, a second set of system fragility curves were introduced for cases when central angle is not specified such as the case in the National Bridge Inventory. Comparison of fragility parameters to those suggested by HAZUS-MH highlighted the need for revisions to account for current design practices and central angle.

Finite element investigation of steel built-up shear links subjected to inelastic deformations

Non-linear finite element models accounting for large displacements have been used to investigate the behavior of steel built-up shear links that had previously been tested using large-scale experiments. The links were designed using steel grades with yield points ranging from high to low strengths. The objectives of the numerical analyses were to further investigate the non-linear behavior and to correlate the numerical results with experimental observations. Elasto-plastic as well as cyclic stress-strain material properties were incorporated to study the influence of material behavior on the overall shear link response. Non-linear monotonie analyses of the shear links incorporating the cyclic stress-strain steel properties resulted in similar trends in the response as the backbone curves recorded from the physical experiments. The numerical models of built-up shear links utilizing structural grade steels closely correlated to the experimentally recorded shear strength. Models utilizing low yield point steels overestimated the shear strength, which was caused by the characteristics of cyclic behavior of those steels. The detailed numerical models also allowed for investigation of the plastic strain demands on the different components of the link. It was shown that finite element models combined with appropriate stress-strain relationship may be used with confidence to check the design of shear links of different steel grades and sectional geometries.

Seismic analysis and design of modern steel highway connectors

This paper discusses the seismic analysis and design of modern steel plate girder connectors. Full 3-D finite element analyses were conducted on a two-span two-girder bridge to determine its seismic load path and to establish the seismic performance of the bridge with several cross frame configurations. The results of the study showed that the cross frames play a significant role in the seismic behavior since they transfer the inertia forces to the substructure, a phenomenon not accounted for in the current design bridge codes.

SEISMIC PERFORMANCE OF TWO-COLUMN BENTS--PART II: RETROFIT WITH INFILL WALLS

As part of an ongoing study on seismic behavior of 2-column bents built before the 1970s, the effectiveness of an infill wall as a retrofit technique was evaluated. Experimental work was conducted on 1/4-scale models using the shake tables at the University of Nevada, Reno. Caltrans current construction practice was implemented in the retrofit. The wall had a gap between the top of the wall and the bottom of the cap beam, which modified the lateral load path and caused the wall and columns to partially separate. Deterioration of the structure was concentrated in the cap beam and top of the columns. The overall performance of the infilled bent was significantly improved in terms of the displacement ductility capacity and the lateral load strength. A simple model was developed based on experimental results to determine the force-displacement curves of infilled bent.

SEISMIC PERFORMANCE OF TWO-COLUMN BENTS--PART I: RETROFIT WITH CARBON FIBER-REINFORCED POLYMER FABRICS

A study on the seismic behavior of 2-column bents with drop cap beams built prior to the 1970s was conducted at the University of Nevada, Reno. Identical 1/4-scale models follow typical geometric and reinforcement properties of typical substandard bridges. One specimen tested in the as-built condition, while a second one was retrofitted using carbon fiber-reinforced polymer (CFRP) fibers before testing. The retrofit was designed using current design recommendations of the California Department of Transportation (Caltrans). The specimens were tested under identical conditions using shaketables. The as-built bent had severe shear damage and low displacement ductilities. The CFRP retrofit efficiently modified the failure mode and enhanced the overall performance, lateral load-carrying capacity, and displacement ductility. A simple model was developed for FRP-confined concrete stress-strain curves and was implemented in the pushover analysis. A discussion on the experimental and analytical results and the effectiveness of the CFRP retrofit is presented.

Seismic Retrofit of Octagonal Columns with Pedestal and One-Way Hinge at Base

Recent earthquake experience has shown that the response of retrofitted bridges is generally satisfactory. However, many seismically-deficient bridges remain, notably those that are supported on piers that lack proper detailing to resist strong earthquakes. This article reports on a study of seismic retrofit of bridge columns that are irregular both in the cross section and along the height, with the latter caused by the presence of a pedestal and a one-way hinge connection between the footing and the pedestal. Three identical quarter-scale column specimens were built for shaketable testing. One was tested as-built, the others were enhanced with seismic retrofits. The columns were subjected to the 1994 Northridge Sylmar earthquake in the strong direction until failure. Results showed that lateral steel of the as-built pedestal was highly deficient, resulting in an undesired pedestal failure. The first of the retrofitted columns was tested with a pedestal extension and a glass fiber-reinforced polymer (GFRP) pedestal jacket. The second was tested with the same retrofit as the first but several of the column bars were severed at the base of the column to reduce plastic shear demand. Results showed that the pedestal retrofit was successful in strengthening the pedestal sufficiently to shift the plastic hinging into the column. The severed column bars lowered shear demand and increased the plastic deformation. The authors also used analytical models to determine the shear strength and push-over behavior. The authors suggest that bond slip, shear deformation, and strain rate effects be accounted for in any effort to obtain a reasonable estimate of the push-over response of piers.

Response of a 2-story test-bed structure for the seismic evaluation of nonstructural systems

A full-scale, two-story, two-by-one bay, steel braced-frame was subjected to a number of unidirectional ground motions using three shake tables at the UNR-NEES site. The test-bed frame was designed to study the seismic performance of nonstructural systems including steel-framed gypsum partition walls, suspended ceilings and fire sprinkler systems. The frame can be configured to perform as an elastic or inelastic system to generate large floor accelerations or large inter story drift, respectively. In this study, the dynamic performance of the linear and nonlinear test-beds was comprehensively studied. The seismic performance of nonstructural systems installed in the linear and nonlinear test-beds were assessed during extreme excitations. In addition, the dynamic interactions of the test-bed and installed nonstructural systems are investigated.