Bridge Abutments over MSE Walls
Very often gravity wall bridge abutments are constructed on top of MSE walls. While some simplified methods exist in AASHTO etc., the design engineer must account for all the interactions between the supported abutment and the supporting MSE wall (Figure 1).
This article examines the MSE wall with the stiffness method as proposed by AASHTO LRFD 2019.
Traditionally, the vertical load due to the abutment is taken as constant and distributed to the wall with a two-way distribution approach.
The base shear and other loads are also transposed with an inverse triangle distribution that is formed from an equivalent active wedge.
Figure 2 presents the impact stresses calculated from the base shear and base traction with DeepEX 2023.
A critical component is how the vertical stress interaction is handled beyond the gravity wall. If the extra surcharge is handled with a distribution approach as assumed in Fig. 2, then there are no additional interaction effects.
However, if that component is handled as a surcharge with the theory of elasticity, then the additional impact loads can quickly become significant.
Figure 3 presents the same analysis using theory of elasticity for both the vertical and base shear stresses from the gravity wall but treating the back soil load as elastic. Figure 4 presents a similar model, but the back load is treated with the one-way distribution. Figure 5 presents a wall without the interaction effects.
Between the examined interaction conditions, the case of including the soil behind the gravity wall with the theory of elasticity produces the greatest maximum forces on the reinforcements (Fig. 3).
The cases with the AASHTO distribution approach and theory of elasticity produced very similar reactions, with the AASHTO approach giving slightly greater force on lower reinforcements.
The elasticity approach though produces greater reactions on upper reinforcements vs. the AASHTO approach (Figures 2 and 4).
The main lesson from this exercise is that one needs to examine the interaction effects with different approaches when possible.
Last, given the uncertainties with simplified methods it is wiser if one designs all reinforcements structurally to consider the greatest required force.
Figure 1: MSE wall supporting gravity wall abutment
Figure 2: MSE wall impact stresses with AASHTO approach
Figure 3: MSE wall impact stresses with theory of elasticity including back soil load
Figure 4: MSE wall impact stresses with theory of elasticity, back soil Load with 1-way distribution
Figure 5: MSE wall model without gravity wall