Seismic Assessment of WSDOT Bridges with Prestressed Hollow Core Piles

Reza Shafiei-Tehrany, Mohamed ElGawady, and William F. Cofer
Washington State Transportation Center (TRAC)
Civil and Environmental Engineering; Sloan Hall, Room 101, Washington State University, Pullman, Washington 99164-2910

Washington State Department of Transportation (WSDOT) developed a retrofitting program to address the State’s bridges that do not meet current seismic standards. Of particular interest for WSDOT are bridges with multiple column bents founded on precast/prestressed hollow core concrete piles, since in the Puget Sound region of Washington State, there are 22 major bridges that are founded on precast/prestressed hollow core concrete piles.

Traditional retrofit techniques, such as supplying additional confinement and longitudinal reinforcement through the plastic hinge region, have been shown to be effective in increasing the shear capacity of hollow piles. However, stiffening the region adjacent to the pile-to-pile-cap connection causes the plastic hinge to form near mid-height of the above ground portion of the pile, reducing displacement ductility in the process (Abebaw, 2008). Currently, no effective retrofitting techniques exist to improve the ductility capacity of prestressed hollow-core piles.

This report investigates the seismic performance of a reinforced concrete bridge with prestressed hollow core piles. Both nonlinear static and nonlinear dynamic analyses were carried out. A three-dimensional “spine” model of the bridge was developed using SAP2000, including modeling of the bridge bearings, expansions joints, and soil-structure interaction. The effect of foundation soil flexibility was examined by running analyses on three different soil types and comparing the results.

The dynamic nonlinear response of the bridge was investigated by using three ground motions with different return periods. The nonlinear static response of the bridge was investigated using different variants of capacity spectrum methods. Nonlinear static analysis provided poor results compared to nonlinear dynamic analysis, due to higher mode effects. Results of both nonlinear static and dynamic analyses showed that the piles fail in a brittle fashion under seismic loading. Using results from 3D finite element analysis of the piles and pile-crossbeam connection, a more advanced spine model was created. The pile-crossbeam connection improved the strength of the bridge.

The work presented in this report is part of a larger research project. In phase one of the project (Greenwood, 2008) finite element analyses of the actual I-5 Ravenna piles and pile-tocross-beam connections have been developed to better understand the performance of hollow core piles. In phase two, the results from phase one were implemented in other finite element models to study the seismic vulnerability of the I-5 Ravenna Bridge.

Dapped-End Strengthening of Precast Prestressed Concrete Double Tee Beams with FRP Composites

Antonio Nanni and Pei-Chang Huang
University of Missouri-Rolla

In recent decades, precast prestressed concrete structures have become more and more prevalent in the construction industries. The use of precast concrete in particular has been shown to be technically advantageous, economically competitive and esthetically superior because of the reduction of cross-sectional dimension and consequent weight savings, enlargement of span length, cracking and deflection control, and larger shear force resistance.

The use of precast concrete can improve the quality of the final products, decrease construction time and assist the progress of construction in adverse weather conditions. Unlike a cast-in-place reinforced concrete structure that is by nature monolithic and continuous, a precast concrete structure is composed of individual prefabricated members that are connected by different types of connections. The type of connections used determines the behavior of a precast structure under load.

The concept of dapped-end beams is extensively used in buildings and parking structures as it provides better lateral stability and reduces the floor height. The design of dapped-end connections is an important consideration in a precast concrete structure even though its analysis is complex. The unusual shape of the dapped-end beam develops a severe stress concentration at the re-entrant corner. Furthermore, in addition to the calculated forces from external loads, dapped-ends are also sensitive to horizontal tension forces arising from restraint of shrinkage or creep shortening of members. Therefore, if suitable reinforcement is not provided close to the re-entrant corner, the diagonal tension crack may grow rapidly and failure may occur with little or no warning. On the basis of the above observations, reinforcing schemes and associated methods of design, which combine simplicity of application with economy of fabrication and which provide the margin of safety required by present building codes, have to be investigated.

This thesis reports on the strengthening and performance of dapped ends that were initially constructed without the required steel reinforcement. The research program focused on precast prestressed concrete double tee members with thin stems. One dapped-end of each member was reinforced with mild steel according to the Prestressed Concrete Institute design method and the other end, intentionally deficient, was strengthened with carbon FRP sheets. Two different configurations were tested and compared to attain a better understanding of the dapped-end behavior and the novel upgrading method of concrete reinforcement with externally bonded FRP composites. A 0^o/90^o wrapping technique was used. The failure mode resulted from peeling of the carbon FRP sheet. In order to attain fiber rupture rather than peeling, an end-anchor was added. The system involves cutting a groove into concrete, applying the sheet to the concrete, and anchoring the sheet in the groove. It was demonstrated that the number of plies (stiffness) of FRP reinforcement and the application of anchor increase ultimate capacity and that the failure by fiber rupture is achieved. Algorithms are provided to estimate the capacity of the dapped-end.