effective span length

huangyhg

effective span length
i am doing an analysis on a pier cap. see attachment. the pier cap is setting on 4 concrete columns. the bridge is a 4 girder system. each girder sets over a column. in order to do some work on the bearings, i must jack up the bridge and support it on the pier cap. the problem is that when i put the loads on the cap from the jacking it puts a significant amount of flexure into the cap. more than the cap can handle. the distance between the columns/girders is about 7'. can i reduce the span length from 7' to something less, say 6' or so. this is being review by the dot so if i do reduce the span, what reference is available?
try calculating the reinforcing using strut and tie methods.   
you don't have flexure, you have shear.  the pier cap is probably about 36-48" deep, and you are jacking in between.  this is a deep beam.  aci has a pretty good method for analyzing deep beam action.  this one isn't as simple as a free body diaphragm to analyze the loads.
doughantholz
the cap is 24" deep. but i did see that it meets the criteria for deep beams.
where in aci is the method for analyzing deep beam action. i just seen it call out references.  
deep beam = strut and tie = see appendix a of aci 318.  for this situation, i would also look at crsi pile cap design procedures.   
what is your jacking loads and what is the cap dimensions?  what reinforcement exists in the top layer and what in the bottom layer?  i think the loads are probably within the strength limit of the cap using fixed end model.  strut and tie method is also preferred for the disturbed regions, (need to know the tie size/spacing and confinment).
jacking load is 150kip or dist load w = 37.5k/ft. cap is 24"x24". the are no plans that can be found for this structure, but on the ends we got the contractor to chip off enough to field verify that there are only 2 - #8 bars in the bottom layer running longitudinally. top layer unknown. and tie stirrups are unknown. reinf fy = 40ksi. f'c=3ksi. i really am not concerned with shear because center of distrubuted load is approximatly with in d from center of the support.