Stuart S Chen

Ambient vibration and seismic evaluation of a cantilever truss bridge

Abstract An experimental ambient vibration field investigation with a companion computational modeling study of a cantilever truss bridge is presented. The ambient vibration experiments were conducted on the central suspended span and two adjacent anchor spans of the North Grand Island (NGI) Bridge. Three-dimensional finite element model of the bridge is validated against the experimental results. Results from time history analyses using selected ground motions are used to assess the damage threshold of the bridge. A nonlinear static procedure using the capacity–demand spectrum approach is then employed to evaluate the expected seismic performance of the structure. Results show that sway frame members are especially prone to damage under earthquake-induced deformations. However, overall response of the structure may be considered satisfactory from a life safety standpoint.

BrIM for project delivery and the life-cycle: state of the art

Bridge Information Modeling (BrIM) was introduced to bridge enterprise stakeholders in design, construction, operations, and management. These stakeholders are increasingly realizing that a well thought out leveraging of bridge data for multiple purposes through the entire bridge life cycle is important. This paper surveys the genesis and development of BrIM supported by NSBA, NCHRP, AASHTO, and FHWA. This includes aspects that distinguish it from its close cousin, Building Information Modeling (BIM). Principal questions, issues, and challenges that have been raised by various stakeholders about BrIM are summarized to help clarify the way forward to increased industry acceptance and deployment of BrIM-enabled workflow.

Methodologies for Evaluation of Effective Slab Width

A composite section is made up of a steel girder and concrete slab connected by shear connectors. The shear lag phenomenon usually takes place in such a section and results in underestimation of stresses and strains at the web-to-flange intersections of the girder. With the introduction of the concept of effective slab width, the actual width can be replaced by an appropriate reduced slab width. The classical effective slab width definition does not take into account the strain variation through the slab thickness. More sophisticated definitions are introduced and used with finite element analyses. The method of finite element modeling is discussed, and the model is successfully verified with experimental results. Parametric study is conducted to investigate the effective slab width for both positive and negative moment sections. The effective slab width is computed and compared with the current AASHTO load and resistance factor design (LRFD) specifications. The results demonstrate that full width can be used as the effective slab width in the design and analysis in most cases for the design and analysis of both positive and negative moment sections. The current AASHTO LRFD specifications are found to be conservative for configurations with widely spaced girders, especially in negative moment sections.

ENERGY BASED SYSTEM IDENTIFICATION USING QUICK-RELEASE EXPERIMENTS

This study presents a time-domain system identification method which is used to identify nonlinear hysteretic properties of isolation bearings installed in seismically isolated bridge structures from quick release experiments. The method utilizes a computational model of the bridge and an optimization technique to minimize an objective function which ensures the energy balance within the structural system at any given instant of time. It is shown that in the proposed method that the objective function is an algebraic quantity which does not require the solution of nonlinear second-order differential equations of motion. Hence, computational effort is minimal. INTRODUCTION Full-scale testing of bridge structures has gained interest during the last two decades in order to better understand their overall behavior under in-situ conditions. Dynamic testing methods such as ambient vibration and forced vibration (e.g. quick-release) testing are considered to be the most viable experimental methods. These experiments can be used to develop reliable nonlinear as well as equivalent linear models that are representative of the type of bridges tested. However, this can be achieved if certain parameters that govern the behavior of the structural system are accurately extracted from the experimental recordings. The suitability and accuracy of the technique used to extract these parameters, which is generally referred to as system identification, is highly dependent on the type of structural system. The dynamic response of seismically isolated structures is controlled by the nonlinear hysteretic properties of the isolators. Most of the research involving seismic isolation has been conducted under controlled-laboratory conditions and on the individual isolation bearings only. Several experimental studies have been undertaken to investigate the overall system behavior on scaled model bridges (Kelly, 1985; Tsopelas, 1994). However, it is generally accepted that in-situ conditions may be different and field verification of the entire structural system is needed. Among all the forced vibration experiments conducted on bridges, only a limited number of full- scale experiments were on bridges with nonlinear isolation bearings (Lam, 1990; Kakinuma et al., 1994; Hasegawa et al., 1994; Gilani et al., 1995; Wendichansky et al., 1998). In their study, Wendichansky et al. (1998) conducted numerous ambient vibration and quick-release experiments on two three-span slab-on-girder highway bridges located on Rte 400 in Erie County, New York before and after the replacement of the steel bearings with isolation bearings. The bearings on the Northbound bridge consisted of laminated neoprene bearings. At the abutments of the Southbound bridge, the steel bearings were replaced by laminated lead-rubber bearings and the same type of bearings without the lead core were used over the piers (fig. 1). Experiments were conducted by pulling (transversely) and releasing the superstructure above the bearings at either both piers simultaneously or at one pier only to excite various modes of vibration. Wendichansky et al. (1998) proposed a dual time-frequency domain system identification method which was used to identify the structural dynamic characteristics of the overall structural system. The nonlinear bearing properties were determined using static test data and based on the laboratory

Experimental Study on Ultimate Behavior at Negative Moment Regions of Composite Bridges

This paper presents experimental results of the ultimate behavior of the negative moment region of a quarter-scale full model and a half-scale subassemblage model of a two-span continuous composite bridge of concrete deck slab on steel girder. The two specimens are based on a prototype bridge that has a large girder spacing [3,800 mm (13 ft)]. At the ultimate state, it is shown that a larger portion of the deck is activated to resist tensile stress compared with the effective width specified in the AASHTO load and resistance factor design bridge specifications. Also, a plastic hinge that forms at the internal support has enough rotational capacity (ductility) to enable development of a second plastic hinge within the span. Experimental results show a reasonably good match with accompanying finite element method analyses.