whl. loads on exist. slab on grade

huangyhg

whl. loads on exist. slab on grade
i am checking an exist. 6" slab on grade (f'c=3000 psi)that was designed in the late 1960's. it was supposed to be designed for 10k fork lift wheel loads, however when i use the aci-360 code and the pca wheel load charts i am coming up w/ a 7-8 inch required slab.  i have also tried the wri and coe charts.  the forklifts have been used for the past 35 years and have not been a major problem.  i need to justify a 15k wheel load is okay.  
any suggestions on why my check of the exist. slab won't work for the original design loads??  are there other methods to use other than the charts mentioned?

if the slab is on exceptionally "good" soil, the thickness of the concrete (up to a point) is of reduced concern. perhaps you get a geotech to investigate the properties of the soil under the slab and avise on what they find.
there is the possibility whilst it was originally designed for 10k, perhaps this load was never realized during its lifetime (and hence there is the perception that the current design is appropriate).  theres also the possibility that this load was exceeded by a substantial margin.  
i'm not sure how much luck you will have getting a geotech to agree to the fact that the entire subgrade has exceptional bearing capacity/stiffness.  my experience with geotechs is that they are very conservative.
you may want to consider testing the slab in discrete locations with a 15k point load distributed over an approximate tire contact area - i know this option is really a pain, but better to have a controlled failure.  afterall, if you cannot justify why the existing design works for 10k, how can you justify it for 50% more?
drw75
one of the situations most often encountered by today's graduates is checking concrete work done in the 1960s and 1970s when allowable stress design was the only method in use.
nowadays, we use ulitmate strength design and when checking work from those bygone days cannot match nor verify what was done.  in many cases, it is easy to go back through an old concrete book or even aci 318-83 and find asd examples.  however, in your case you should check that the tables and graphs in use were based on asd or usd and then work from there.
on the assumption that this might be the problem, look up some old codes and check your situation.
good luck.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.