Slab-on-grade Design Excel Spread Sheet - Live Load (Uniformly Distributed Load) Case 2

[Baek]

Hi,

Below uniformly distributed load tab of slab-on-grade spreadsheet does stress check for negative bending occuring in the aisle due to the stationary live load on both sides of the aisle.

Does anyone know why it compare wLL and wLL(allow)?
wLL is bearinhg pressure and wLL(allow) is from modulus of rupture meaning for tension.

Attaching TM / PCA references (which are mentined in the spread sheet), assumptions of the spread sheet (which is the first tab of the excel file) and the original excel spread sheet file which is freely availalbe on many websites including Link .

 

[Huevo]

If you don't know the answer to this question, you should not be performing this design.

 

[Baek]

Thanks Huevo.

 

[Baek]

If anyone would like to suggest other thoughts, please do.

 

[BridgeSmith]

I'm with Huevo. If you don't know the answer, you're in over your head, and I for one will not encourage the irresponsible use of design tools.

 

[kissymoose]

I guess I'm confused by your question as well Baek. It just looks like your comparing your slab strength to the design loads. It's just a unity check. The allowable live load is based on the strength of the unreinforced slab which is limited by its tensile strength (modulus of rupture). I can't help much in terms of the equations that get you there, but I think Huevo made his comment because it's just comparing design loads to allowable loads.

 

[Ron247]

Baek, the concern about use of the spreadsheet if you are not familiar with how the design is done manually is very valid and sound advice to take. The question you posed is fairly easy in comparison to the Subgrade Modulus value and Factor of Safety value decisions. I have always found these to be the hardest values to accurately decide (i.e. take the most experience). If you do not understand a unity check, then there is concern about the values used elsewhere unless these were determined by a more experienced individual. The wLL versus wLl(allow) is like buying a cup of coffee. The Starbucks cost $5,055 and I only have $2,426, so I cannot buy it. If you are not familiar with "unity check" it is (I need)/(I have) <=1

 

[jayrod12]

I have to ask, what are you putting on a slab that weighs 250 kPa (5ksf)? That's a lot of load

 

[BridgeSmith]

"...what are you putting on a slab that weighs 250 kPa (5ksf)? That's a lot of load"

I assumed it was to design slabs for equipment or truck wheel loads. The standard patch size for a truck wheel load is 10" x 20", so the 5ksf would correspond to a 7 kip wheel load, which is not that large.

 

[jayrod12]

Agree, except even for those type of wheel loads, it's a different design method. This method here is for uniform loading.

 

[Baek]

Thanks for all of your valued feedbacks, however I’m not quite sure if anyone is on the same page as me.

Why wLL for tensile resistance is compared with simply bearing (compressive) stress?

Just a faire advice, it may interest you reading through the attached references.

 

[mmarlow]

I think I understand what you're asking.

So you're saying that the wLL input is for bearing pressure check?

And that wLL(allowed) is checking the bending of the slab?

It appears to me that this slab doesnt check bearing pressure directly.

Your input is the load induced on the slab. Not the bearing capacity. The wLL(allowed) is the max distributed load that the slab can take when considering bending of the slab.

Nowhere in the excel sheet are you entering ,directly, allowable bearing preasure. You're simply checking a beam. I think you're having more confusion with the inputs, hopefully not the concepts.


-MMARLOW EIT

 

[Baek]

Thanks.

I might have been lazy on answering questions and keeping the thread going.

Huevo and Hotrod. Agreed. That’s why it’s such a benefit finding this out.

Kissymoose and Ron
I don’t think you wanna do concrete bearing unity check with modulus of rupture. It’s not compressive strength fck that usually dealt with. Modulus of rupture is the unit resistance in tension per area for uncracked concrete which we usually ignore in flexural resistance of reinforced concrete because members crack at their early stage of loading and after rebar takes the tension to couple compressive resistance. And for this case the tensile resistance from the modulus of rupture should exert horizontally as the uniformly distributed load induces negative moment in the aisle by pressure applied on both adjacents of it until it cracks on the surface.

Jayrod and Hotrod.
The spreadsheet is not for truck wheel loads. It’s for the aisle of the ware house or truck parking area where both sides of it are charged for maintenance equipment, forklifts or stacks of pallets. 250kpa is for a shovel on its chain links given which is significantly loaded than usual cases mentioned above with the shovel truck weight is about 300tonnes and the area of the each chain link is 1m x 6m.

Thanks Mmarlow. You were very close to what is questioned. What’s your opinion on the « max distributed load that the slab can take when considering bending of the slab » that you mentioned is being checked with « uniformly distributed load » ?

Thanks all for your time. If you are not following me, just save your time. Have a good discussions on what interest you.