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# R value, and abutment stiffness for a precast 2 span simple bridge

## R value, and abutment stiffness for a precast 2 span simple bridge

(OP)
Hello,
For a 2 span simple precast girder bridge, I must find the column seismic forces and abutment bearing movement, in both with or without abutment stiffness options. The bridge concrete deck rests on 10 girders and the rubber supports are given as expansive at abutments, and fixed at bridge midspan, where each of the two spans separately rest on a single pier cap which rests on a single column.

I have a couple questions:

I have found the forces and displacements without considering any abutment stiffness, in my finite element computer model.

1-For the abutment stiffness option do I model it as two springs in series, such as for example one very weak spring to account for the gap between girder end and abutment and one for the abutment itself a very stiff spring? Is that how it should be done? and I have 10 girders supporting the concrete slab. I modeled all springs for the rubber support as equal linear springs on 3 dimensions. but what about the springs when modeling for abutment stiffness? how do I model them? and do I model them all the same or from the middle towards the ends they vary? so it should be like, during earthquake, the bridge will first cover the gap between the abutment wall and and face of girder, and then hit the abutment, and then what, displace it by an amount equal to hitting force / abutment spring constant ? (we are not considering the soil at the back of abutment in our problem here)

2-After I find the maximum moment and force in my column, in the very end, I must divide this by R value (which is given as 1.5)? I mean that value comes into play after I find everything and then modifies the maximum forces I have by simply dividing with it?

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