Nien-Yin Chang

Three-Dimensional Properties of MSE Bridge Abutments

Conventionally highway bridge abutments are supported on deep foundations. As an embankment approach settles, it results in a differential settlement between the approach pavement and bridge deck, which causes the traveling vehicle to experience an impact force. This phenomenon is called bridge bump. A mechanically stabilized earth bridge abutment (MSE abutment) minimizes the damaging and unpleasant bridge bump by eliminating the differential settlement. This article reports the result of a series of 3-D nonlinear finite element analyses to investigate the performance of MSE bridge abutments supported on a U-shaped MSE Wall (UMSEW). Overall structural integrity, sufficient clearance, smooth transition from pavement to bridge deck and pleasing aesthetic appearance are critical long-term performance requirements of an MSE abutment. Three MSE abutments supporting a bridge with single span length of 48 m (160 ft) and three different footing sizes (2.17 m, 3.05 m, and 3.96 m), respectively were analyzed for the lateral deformation, vertical settlement, earth pressure, inclusion stresses, and bearing pressure underneath the footing. The analysis results show that, when founded on a sound subsoil or rock, the UMSEW effectively supports the MSE bridge abutment.

Anisotropic Properties of MSE Geo-composite

“Composite material” signifies that two or more materials are combined on a macroscopic scale to form a material exhibiting the quality superior to its constituents (Jones, 1975) by capturing the strength of each constituent. Combining the strength of Ottawa sand (compression) and geosynthetic (tension), this geo-composite is strong in both tension and compression. Finite element analyses are performed on cubic samples of this geo-composite to evaluate its equivalent transversely isotropic properties, which are then written as functions of the isotropic linear elastic properties of sand and geosynthetics, and geosynthetic spacing, etc using regression analyses.

Definition of Yield Zones on Concrete Barrier Structures under a Transverse Impact Load

The American Association of State Highway & Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications, the nationally-accepted specifications for bridge designs, stipulate the design method for concrete barriers on concrete bridge decks. This paper observes stress distributions on the concrete barrier and its adjacent structure when they are under a transverse impact load. Three research models are established for this paper. An area of potential maximum tensile or compressive stress on the concrete barrier, which is called a “yield zone”, can be considered the location of failure under the extreme transverse impact load. The results of this research show that the shapes and locations of yield zone are affected by the support structures and their rigidity. This paper also discusses the limitations of the analysis method used by the AASHTO Specifications.

MULTI-DIRECTIONAL SEISMIC RESPONSES OF HYBRID TEE WALLS

Seismic response analyses were performed on 10- and 20-m hybrid cantilever retaining walls (HTW) with RC cantilever stem and mechanically stabilized earth (MSE). Geogrid is used as the reinforcement and is either attached or detached to the RC wall. The analysis uses the Imperial Valley Earthquake ground motion and its scaled motions to examine the effect of severity of earthquake-induced ground shaking on HTW seismic responses. Wall height, ground motion intensity, multi-directional shaking, and inclusion connection conditions were found to significantly affect the wall displacement and rotation, inclusion stresses, earth pressures, and bearing pressure of HTW. The earth pressure with multi-directional shaking differs from the AASHTOs Mononobe-Okabe (M-O) prediction.

COLORADO TYPE 7 AND 10 RAILS UNDER HIGH TEST LEVEL LOADS

Colorado impact rail designs follow the AASHTO Standard Specification for bridge deck overhangs, whereas it uses a 44.5 kN equivalent service load. In the AASHTO LRFD 2000 Code, the impact load was changed to an ultimate static load of TLI (60 kN) to TL6 (780 kN) without providing design details and the Colorado Department of Transportation (CDOT) began its research to establish design details. Colorado railings in bridge approach usually sit on mechanically stabilized earth (MSE) walls and the impact load transfer from rails to walls becomes critical to rail stability and MSE wall design. Nonlinear quasi-static and impact load finite element analysis were performed to evaluate the stability and impact transfer efficiency of Colorado Type 7 and Type 10 rails with concrete and steel barriers, respectively under TL4 (240 kN) and TL5a (516 kN) impact loads. The finite element modeling demonstrated that the current design was effective at resisting impact loads if the rails were long and continuous. However, the modeling indicated that the system might not perform adequately if the impact occurred close to the end of the rail. Further research is needed to simulate true dynamic loading and to validate the results with actual crash tests.

HYBRID T WALLS UNDER SEISMIC LOADS

An innovation wall system with a reinforced concrete cantilever wall (or T wall) with mechanically stabilized backfill is proposed. The wall system, briefed as hybrid T wall (or HTW) retains the positive and removes the negative aspects of the performances of both reinforced concrete walls and MSE wall systems. HTW and its optimal dimensions were obtained inductively using the nonlinear finite element analyses through the seismic responses of six different wall types and varied HTW dimensions. Accelerograms of none strong earthquakes of magnitudes between 6 though 8 Richter scale were used in the seismic response analyses of HTW. The strength of correlation between the seismic performances of HTW and earthquake strong motion parameters was investigated. The HTW performances were then expressed as the function of ground motion parameters with strong correlation through regression analyses for predicting the performance of HTW with known ground motion parameters and wall height. A comprehensive study is required to improve the wall performance prediction models.