ABSTRACT
Load ratings following the AASHTO LRFR procedure often result in insufficient ratings for reinforced concrete arch bridges, particularly when thermal forces are considered. The main reason is that the effect of stiffness reduction due to concrete cracking is often ignored because it is complicated to quantify and not comprehensively addressed in the specifications. Typical analyses are based on the original member properties and indeterminacy of the structure is also ignored. Using P-M-φ (Axial force-Moment-Rotation) relationship, bending stiffness in reinforced concrete members can be calculated under combined axial force and bending moment. Evaluation of reinforced concrete arch bridges considering the reduced stiffness due to cracking of concrete will provide more accurate results and it reflects the actual structural behavior.
Reinforced concrete arch bridges contain high structural indeterminacy. Therefore, bending stiffness affects distribution of the moments and axial forces in arches. Also, insufficient rating based on P-M interaction strength curve at one particular location in an arch rib certainly does not cause failure of the rib. The forces and bending moments in the arch will actually be redistributed when the capacity is reached at a cross section. Furthermore, development of the P-M curve implies that part of the section is in tension and is cracked. This results in a reduction of stiffness at the locations that the capacity has been reached. This consequently results in a redistribution of forces, particularly bending moments, in the arch member. To accurately determine the strength of an arch bridge, iterations of analyses taking into account redistribution of internal forces due to reduction of stiffness will be needed.
Accurate evaluation of reinforced concrete arch bridges therefore cannot be purely based on strength at a few controlling locations. Instead, it should be based on performance of the entire structure considering effective stiffness and force redistribution in the arch members. The final failure mechanism will need to be developed to determine the actual load carrying capacity of the bridge.
