Two-way slab design for point loads

[cmbyrd77]

I wanted to see if anyone has any good literature on two-way slab design for point loads. I know that there is AASHTO for deck design using one axle or point loads, but these point loads a are smaller. Every example I've ever seen on two-way slab design is for uniformly distributed loads. Any help would be greatly appreciated.

 

[theonlynamenottaken]

There are quite a few threads on this site discussing this. There are even a few I started and contributed to a few years back. You should search for those, but basically your choices are performing a yield line analysis (try MacGregor & Wight's, "Reinforced Concrete - Mechanics and Design") and obtaining design shears and moments from a FEM analysis. For the latter you should seek out Wood, R.H., “The Reinforcement of Slabs in Accordance with a Pre-Determined Field of Moments" for the original theoretical treatment. For a more modern interpretation you can check this out:

A Finite Element Approach to Reinforced Concrete Slab Design in GT STRUDL

 

[TDIengineer]

Apply equivalent uniform loads for your point loads to analyze the overall slab bending.

Check punching shear for the local point loads. Concrete is so good at redistributing, I would model as uniform.

If it is point loads at midspan, then find the equivalent uniform load to match the same moments.

Anyway, I didn't read the literature but I would think you could apply some logic and come up with a reasonable approach.

 

[UcfSE]

Use the equivalent frame method. It is simpler than yield lines or doing full-blown fea. Basically look at the slab as a "beam" that makes a moment-resisting frame with the columns.

 

[rapt]

It would depend how big the point loads are compared to the remainder of the loading.

For relatively small point loads, especially if they are applied in the middle strip area, then equivalent frame or other simplified methods with distribution of effects into column and middle strips are ok. If the point loads are in the column strip area, they should not be distributed normally as their effect should be contained within the column strip and not distributed to the middle strip.

If they are relatively large point loads as in a transfer slab, then FE analysis to determine the distribution and reinforce accordingly for the concentrations in different areas of the slab.

 

[msquared48]

Conservatively speaking, I don't see that this would be much different than analyzing a two way steel beam system.

Just figure in the appropriate slab width in each direction based on the width of the point load, add in the other uniform loads over the same width strips, get the moments and reactions and design the reinforcing steel. No worries.

Mike McCann
MMC Engineering

 

[theonlynamenottaken]

It all depends doesn't it? Two-way slab systems are almost always continuous and monolithic, so the accuracy of any approximate method truly is relative to several variables, including the magnitude of point loads and the column layout. With a rectangular layout of column bays and relatively small point loads then you can simplify this problem to a certain extent. However, oversimplifying could lead to failing to recognize non-uniform punching shears and the effects of sudden stiffness (reinforcing) changes in a slab.

I humbly disagree that the Equivalent Frame Analysis method is always easier than FEA. With modern software it can be easier to make a reliable FEA plate model than it is to do discrete frame analyses with the Equivalent Frame Analysis method and marry your results together, especially if your office uses solid modeling as opposed to 2D drafting as we do. Approximate methods were formulated to 'approximate' what a reliable FEA model would provide for you internally... At the end of the day it depends on the exact problem parameters and what tools you have at your disposal.

 

[gobsmacked]

Nobody has yet suggested Hillerborg's strip method. But I do.