Concentrated Load on Supported One Way Slab 1

[BoydZander]

I have a need to determine a rational effective width for a supported reinforced concrete slab supporting a concentrated load.

I'm aware of AASTHO having a method and formula but I'm not familiar with AASTHO code nor do I have access to one. I have heard said that it is based on Westergaard's "Computation of Stresses in Bridge Slabs due to Wheel Loads".

Various threads have placed the effective with at about 60% of the clear span. The threads have mentioned Australian and British codes being in close agreement.

My challenge is I need a reference as well as a document to educate myself, a checker, an approver and a client's independent reviewer. If I could get my hand on the Westergaard report, perhaps I would have met my needs.

So, question is, does anyone have a copy of Westergaard's report available, or other reasonable reference, for effective slab width for support of point loads?

Regards,

Boyd

 

[pwht1]

The Australian code (AS3600 - 2009) says for a load not near an unsupported edge (ie cantilever) the effective width is the "load width + 2.4a x (1-a/Ln)"

a = "perp dist from the nearest support to section under consideration".

Sorry no Westergaard report.

 

[Lion06]

I have an old document that deals with this. I'll post when I log in to my work computer.

 

[BoydZander]

Thanks for the prompt response. It is much appreciated.

StructuralEIT, I would be very much interested in your document.

The Australian Code formula is just what I would need, but I can only reference governing codes. I can however, reference published literature and the Westergaard report is regarded by the codes.

It acually is in agreement with the Australian Code at midspan fom what I understand - basically effective slab width = about 60% of the span length.

Westergaard states Beff = 0.58S + 2c, S= Span, c = width of load bearing.

Westergaard is an old document published in 1930. Knowing modern codes essentially agree with it will help bringing the checker to accept it's use.

Another document that would prove to be useful in referencing for this is

Effective Width of Concrete Bridge Slabs Supporting Concentrated Loads, E. F. Kelly, Public Roads,, Vol. 7, NO. 1, March 1926. This is what Westergaard references.

Thanks Again guys.

 

[Lion06]

Check this paper out and let me know if it helps.

 

[BoydZander]

StructuralEIT,

Thanks, I appreciate your digging this item up. It helps. I would like to know where this formula came from, looks like a professor obtained it fom some text or study.

The great thing about your reference is it also provides direction on a line load across the slab too which is a common challenge.

The formula for the point load is 4/3x + d with x being placement in the span and d with bearing width of the load. It basically becomes 0.67L, L being the span length, which compares to the 0.60L specified by Australian Standard AS 3600-2001.

I'm personally comfortable using 0.58L to 0.67L for point load at midspan. I'm still stuck without a reference though.

It seems strange to me why ACI 318 or other is silent on this, as far as I can tell, when it should be a common load that needs consideration.

Regards,

Boyd

 

[weab]

I treat this as a T span with web of 0 thickness. I use this to determine effective width. I believe that this is reasonable.

 

[KootK]

@pwht1: does the Aussie code provide any guidance when the load is positioned on a cantilever?

 

[pwht1]

Yep, the effective width shall be the lesser of the equation above or half that value + the distance from the centre of the load to the cantilever edge.

 

[apsix]

Actually AS3600 doesn't give guidance for loads on a cantilever. pwht1's answer is for concentrated loads adjacent to unsupported edges parallel to the span direction of 1-way slabs.

 

[pwht1]

Apsix, I've never used that clause but I'd always interpreted that section as applying to cantilevers. I had to re-read it but you're correct. Sorry kootk, ignore my previous post.

 

[StructuresJohn]

The British code BS8110 gives
effective width = 0.6L + (width of load)
when load is in centre of a span and L is span


When the load is not central the following formula is used:
2.4x(1-x/L) + (width of load)
where x is distance from centre of point load to centre of support and L is span.

Note that when the point load is near the edge the effective width is reduced.

John

 

[AVak]

The "Composite Deck Design Handbook" published by the Steel Deck Institute has formulas for this situation. I am not sure if it qualifies as a "Code," but I use it for the design of composite steel deck and concrete floors quite frequently. Its use for reinforced concrete may not be appropriate, however.

Adam Vakiener