holes in concrete slabs

huangyhg

holes in concrete slabs
i believe this has been kind of posted before but i would like to ask a question to expand the topic.  what type of conservative analysis would you do with a 6'-0" diameter hole in the top of a box culvert that had an 8'-0" span?  the hole was present due to a precast manhole sitting on top of the culvert.  the problem stemmed from having to apply a live load and obvious dead load of the manhole. i believe that the analysis that was done was too conservative and therefore penalized the design by complicating the process.  the analysis in a nutshell was to support 1/2 of the load by a beam that was spanning approx. 8' and provide bars for cracking.  would appreciate any type of assistance.
if i understand you correctly, i would have done nearly the same thing.  my possible exception would to use two beams either side of the 6' diameter hole spanning the 8' length.  unless the hole in the top slab was somewhere near an end wall.  in which case, pending how close the end of the inlet was to the endwall, i would assume part of the load to carry to each of the walls and a beam.  the beam, i would try to keep within the thickness of the top slab.
i have seen this done many times and have done it myself once or twice.  my reasoning for using the additional beams is fairly simple: a simplified analysis here can produce a satisfactory result with the addtional redundancy.  steel is cheap, especially in light of a long culvert where concrete and excavation are far more costly.  and now, without some rigorous analysis you have a design you won't be digging up later on.
qshake,
i probably should have been more specific, but i did look at 1/2 the load on 2 beams, one on either side of the hole. i guess i was wondering if the 1/2 load to each side of the "beam" was too conservative.  thanks for the reply.
i think its conservative, but unless you do a more sophisticated analysis (finite elements) you really don't have any other choice.
i would think that the primary effect of such a large hole on a "slab" (or what's left of it) is a shear effect.  i would be concerned that the slab around the hole, or the beams you provide, would mitigate the high shears that would build up, especially where the edge of the opening gets close to the supporting walls holding up the slab.
make sure that the thickness of the slab can handle the shear.  how you distribute the shear around the hole is what's difficult.  i'm not sure, without seeing the layout, how the shear would flow.  with beams around four sides of a circle, you probably have it covered.