Point Load on One-Way Slab
Point Load on One-Way Slab
(OP)
I have a floor system with steel beams 6 feet on center and a 12 inch concrete slab on top of them. The steel beems span about 20 feet so the slab acts like a one-way slab. In addition to live and dead loads, during construction I am going to have some pretty large point loads from small mobile cranes that will be driving around to lift equipment into place. How do I treat a point load on a one-way slab system? Everything I can find in the codes and books always assumes that there is a uniform load. But I ahve point loads. What I want to do, and my calculation checker is not agreeing with, is take my point load and distribute it over a width of 4 times the slab thickness. This is what they do in the ACI 318 (13.2.4, 8.10.2) for composite beams, on each side of the web they extend the effective area of flange 4 times the slab thickness of either side. I don't have a composite beam, but I was hoping to use this rationale to justify my effective width. Basically I want to slice my one-way slab into widths 4 feet wide for analysis of point loads. Obviously I will still consider my dead and live loads over the whole 4 foot width. In addition to your opinion any information backing your position would be helpful, so my checker and I can come to a concensus. Thanks for the help! I am just out of college and starting to understand how much I did not learn there.






RE: Point Load on One-Way Slab
AASHTO has a slab distribution formula for concentrated loads placed on bridge decks. Bridge engineers distribute concentrated loads on one-way slabs all the time. They use the AASHTO formula as follows:
The concentrated load is spread over a distance E
E = (4 + 0.06 x S) but not more than 7 feet (or, I would add, the center to center spacing of the posts).
S = span, ft.
Taking a small width of slab, such as 12" or 24" seems irrational as you know the slab will essentially deflect as a whole. And, per Hooke's law, where there's deflection, there's stress. Thus, you know the slab will tend to work as a unit over a wider width than one or two feet.
You should also check punching shear through the slab per ACI 11.12.
RE: Point Load on One-Way Slab
While shown for concrete fill on composite deck, I think the methodology is easily adaptable to formed, cast-in-place concrete slabs.
RE: Point Load on One-Way Slab
1) finite element model
2) effective width method
3) Westergaard method (Today's AASTHO spec is based upon this) It was originally written in 1930 for the Bureau of Public Roads. The original report is more useful for building engineers than the formulas in the AASTHO spec. which have been specifically rewritten to be used with HS20-44 loadings. I suggest getting a copy of the original H. M. Westergaard report thru your local library. The title is "Computations of Stresses in Bridge Slabs Due to Wheel Loads", Public Roads, 11, No. 1, March 1930.
You will need to analyze for the cranes located parallel and perpendicular to the beams. Include impact for the cranes moving across the slab. Do the cranes have outriggers? If so, those could be even larger than the wheel loads.
You need to check bottom and top main steel and distribution steel. Also check beam and punching shear in the slab.
RE: Point Load on One-Way Slab
RE: Point Load on One-Way Slab
RE: Point Load on One-Way Slab
If you use the effective width method, I would suggest that the effective width for one wheel load does not exceed
0.5xE + 0.5x(the space between wheels).
RE: Point Load on One-Way Slab
Try calculating the maximum resisting moment of the slab with the #9 bars over a 12" width. Then look at the worst moment generated by the point load based on your span condition. See how far you have to span the load out and see if it is reasonable. At 12" thick on a 6' span with #9 bars, this was paobably designed to support four feet of gold at Fort Knox.
Mike McCann
MMC Engineering
RE: Point Load on One-Way Slab
Thanks for the help, in the end I am going to use the Westergaard equation that seems to be referenced in several documents of We=0.58*s+2c. Where s is span, c is diamter of applied point load. This comes from Westergaards original analysis in his 1930 publication called "Computation of stresses in bridge slabs due to wheel loads". One of the more experienced engineers in office happened to have access to it. It was a report written for the department of agriculture. The best part is that my checker agrees! Thanks again for all the help!
RE: Point Load on One-Way Slab