Designing for Wind Load on a Slab-On-Ground Foundation &/OR Slab with Haunch Foundation

[oengineer]

I am working on designing a foundation for a single story building. The building is made of CMU load bearing walls. My goal is to use a Slab-on-Ground Foundation system OR a Slab with Haunch Foundation System.

For a Slab with Haunch Foundation System, I am able to calculate the size of the Haunch based on the Dead, Live, & Wind Loads acting on the building.

For the Slab-On-Ground Foundation system, (using the Wire Reinforcement Institute - Design of Slab-On-Ground Foundation _Design Guide) I do not see how to consider the effects of wind acting on the building being transferred to the foundation. Is there something that I am not seeing in the Design Guide? Should wind loads not be considered for Slab-On-Ground Foundation systems?

Should wind loads not be considered for a Slab with Haunch Foundation System supporting a Single Story CMU wall building? The wind load is effecting the overturning & width of the haunch.

Suggestions/comments are appreciated.

 

[oengineer]

The link contains the Wire Reinforcement Institute - Design of Slab-On-Ground Foundation_ Design Guide that I am referring to:

https://files.engineering.com/getfi...file=Design_of_Slab-on-Ground_Foundations.pdf

 

[r13]

As the title suggests, the WRI Tech Note is the correct reference for the design of slab-on-ground, for which the wind load does not have direct effect on. But what is the foundation the perimeter walls to sit on? It, whether a turn down from the SOG, or a strip foundation, is the structural element to transfer the wall loads to the subgrade below, and suffers the most from the wind load. I don't understand what is the thing you don't understand about. Maybe I didn't read your post properly/correctly.

 

[oengineer]

But what is the foundation the perimeter walls to sit on? It, whether a turn down from the SOG, or a strip foundation, is the structural element to transfer the wall loads to the subgrade below, and suffers the most from the wind load. I don't understand what is the thing you don't understand about. Maybe I didn't read your post properly/correctly.

The exterior CMU wall shall sit on top of the perimeter grade beam/haunch.

The WRI Tech Note does not provide any information on how to consider the wind load acting on the slab-on-ground foundation. It's as if the wind load is not to effect the SOG foundation system.

 

[r13]

It affects the grade beam, and the underlaying soil, on which the grade beam is to be designed. The slab may feel increased pressure at the edge, if it is cast integrally with the grade beam (forms a L shaped beam). But, it may feel nothing, if a joint is provided at the interface. Again, WRI is focusing on the slab, not the entire foundation system. You need to check foundation textbooks to find design examples.

 

[oengineer]

Would a "Stiffened Beam" foundation system be considered as a "Slab-On-Ground Foundation"?

Or would a "Slab with Haunch Foundation" be also considered as a "Stiffened Beam" foundation system?

The geotechnical report says that this building "may be supported on shallow spread footings or a stiffened-beam-and-slab foundation".

 

[r13]

 

[oengineer]

The image below shows the plan view for the CMU Load Bearing Building. As you can see there are no interior walls inside the building, only exterior load bearing walls.

For the foundation design, the issue I am having is possibly justifying the use of an interior grade beam in the slab.

For CASE 1 (see image below), I am designing the Building foundation system as a "Slab with Haunch Foundation" system. The problem I am having with this design is the Soil bearing & overturning of the individual grade beam. My concern is that the overturning of the grade beam will create a hinge in the slab. The grade beam size in this situation is 24" deep x 18" wide.

Since treating the Building foundations as a "Slab with Haunch Foundation" system has an issue, I tried a different approach.

The second approach is CASE 2. For CASE 2, I am trying to design the Building foundations as a "Slab-On-Ground Foundation" system. For this I am trying to use the Wire Reinforcement Institute - Design of Slab-On-Ground Foundation_ Design Guide. My issues with this approach is trying to apply the wind load acting on the grade beam and the overall foundation system. The WRI Tech Note says that the grade beams should be spaced at a minimum of 15'-0" C-C. For this reason, it would appear that an interior grade beam is required for my building foundation layout. My goal is to justify the use of an interior grade beam, even though we do not have any interior walls in the building.

This is my reason for trying to get clarification on which system would really be considered a "Stiffened Beam" foundation system? Would it be CASE 1 or CASE 2?

 

[r13]

You don't have interior bearing wall, why beam there?

 

[oengineer]

You don't have interior bearing wall, why beam there?

Well, this is based on the Wire Reinforcement Institute - Design of Slab-On-Ground Foundation_ Design Guide that says the following:

If the foundation system is to be considered as a "Slab-On-Ground Foundation" system, then wouldn't the 15 ft spacing apply? If so, wouldn't an interior grade beam be required?

Which system would be considered a "Stiffened Beam" foundation system between "Slab-On-Ground Foundation" or "Slab with Haunch Foundation"?

 

[r13]

Please drop WTI and look for PCA guide for design of slab on ground. What is your application, heavy industry warehouse, light manufacturing/commercial building, or residential? The grid type slab system is rarely seen these days, since it is labor intensive. Below is SOG with interior bearing wall.

 

[oengineer]

Please drop WTI and look for PCA guide for design of slab on ground.

Do you happen to know the exact name of the PCA document so I can obtain it?

What is your application, heavy industry warehouse, light manufacturing/commercial building, or residential?

It's a control building for a WWTP.

Below is SOG with interior bearing wall.

Is the SOG the same as a "Stiffened Beam" foundation system?

 

[r13]

Link, Link

You can also look into ACI360. SOG - Slab on Grade/Ground.

 

[bridgebuster]

I was thumbing through the WRI manual and it brought to mind a webinar about SOG foundations with multiple grade beams. However, as I recall, it was for a situation where the structure would be on an expansive clay. A few years back, I designed a SOG foundation for a decent size electrical generator room and we didn't use grade beams. I'll dig through my files to see what I can find.

 

[r13]

Thinking the headaches on construction (excavation, compaction, forming..) makes grid type foundation very unfavorable, if you can simply make a thicker mat foundation. I am not sure its benefits over expansive soil though.

 

[oengineer]

I was thumbing through the WRI manual and it brought to mind a webinar about SOG foundations with multiple grade beams. However, as I recall, it was for a situation where the structure would be on an expansive clay. A few years back, I designed a SOG foundation for a decent size electrical generator room and we didn't use grade beams. I'll dig through my files to see what I can find.

Please let me know what you find. Thank you.

 

[bridgebuster]

Attached are the calculations we did for the generator building slab.

 

[bridgebuster]

Attached is a DoD manual for SOG design. They show a thickened slab along the perimeter. I was looking at the ASCE webinar slides and it was for SOG on expansive soil. If you don't have this condition you could go with a thickened slab at the perimeter or isolate your slab the way r13 drew it.

The slides are copyrighted; I don't think I can post them.

 

[r13]

oeengineer,

The slab on grade foundation is composed of two parts - foundation and slab. In general, the slab carries the loads imposed on it directly, especially in the form of concentrate load; and the foundation (thickened edges, grade beams) carries the reactions from the building above due to DL, LL, and lateral loads. In most instances, the slab can be isolated from the perimeter foundation element through joints; but occasionally, it will be connected to the perimeter foundation for better distribution of loads, and control of differential settlement between the two elements. The grid system - rigid grade beams (interior and exterior) cast integrally with the slab is used for heavy floor loads with weak soil support. In this case, once soil pressure is determined, you can then design it as two way suspend slab system, with columns and beams as support, and the soil pressure as floor load. Hope this helps.

 

[hoshang]

Attached are the calculations we did for the generator building slab.

https://files.engineering.com/getfile.aspx?folder=869cc912-00bd-41ce-9f64-e

Wind parallel to x-x direction (k)= 1681.276
How did this come?
I tried it this way:
windward & leeward wall
p(max)=9.74-(-7.893)=17.633
[14-(1*0.56)]*12.9458=173.991552
17.633*173.991552= 3067.993036
windward & leeward wall @ corner
p(max)=13.266-(-10.244)=23.51
1*0.56*12.9458=7.249648
23.51*7.249648= 170.4392245
--------------
sum= 3238.432261

Am I missing something?