Concentrated Loads on One/Two-Way Slabs? 1

[theonlynamenottaken]

Hey fellow structure lovers,

I frequently am posed with the problem of concentrated loads on one-way and two-way slabs. Some of these loads are vertical point loads and some are concentrated moments (through a baseplate). I have no problem performing local area calculations like punching shear. What I'm wrestling with is the bending/torsion calcs (i.e. effective widths).

Aside from plate FEA does anyone know of any references or texts that address this subject? I've been told that Westergaard found that the effective width in a one-way slab with a concentrated load = 2/3*span... but I'd like to have that in writing. Others have suggested guessing or assuming an angle for a triangularly increasing effective width extending from the point of load towards supports.

Help!

 

[theonlynamenottaken]

Searching the forum I found "dbuzz"'s post from 2003 that references Australian Standard AS 3600-2001

b_ef = b_load + 2.4*a*(1.0 – a/L_n)

where:
- ‘b_ef’ is the effective width
- ‘b_load’ is the load width
- ‘a’ is the perpendicular distance from the nearer support to the section under consideration
- ‘L_n’ is the length of clear span

As ‘b_load’ approaches 0 and ‘a’ approaches L_n/2, ‘b_ef’ approaches 60% of L_n.

Any others?

 

[UcfSE]

What do you think of looking at it as though it were upside down with the slab bearing on the point load, similar to footing design? Then you would have "flange" on either side of the point load similar to a T-beam. This is just a suggestion, I've no idea if it's a reasonable assumption just off the top of my head without reviewing code and assumptions, etc.

 

[jike]

Westergaard, a professor at the University of Illinois, issued a comprehensive report in March of 1930 to the Bureau of Public Roads "Computation of Stresses in Bridge Slabs due to Wheel Loads". I believe Westergaard's report is the basis for the modern day AASTHO requirements. The AASTHO spec., unfortunately, has taken Westergaard's recommendations and converted them into engineering data that works only for truck loads and bridges. Building engineers must work with varying wheel bases, axle lengths and tire contact areas. You must go back to Westegaard's original report to find useful information. You should be able to order this report thru your local Public Library.

This report has charts that give coefficients both for principal and distribution reinforcements for one way slabs subjected to concentrated loads. It has charts to handle overlapping stresses, when wheel loads are parallel or perpendicular to the span. It even takes into account the contact area of the concentrated loads.

Reinforced Concrete Fundamentals by Phil Ferguson has a Chapter On Distribution of Concentrated Loads.

I hope this helps!

 

[apsix]

The British concrete code BS8110 has the same formula for effective width as the Australian code, as given above.