## Is subgrade modulus still used by structural engineers?

## Is subgrade modulus still used by structural engineers?

(OP)

Hello,

I'm a geotech engineer. From my side of the ground surface, the subgrade modulus, k, also known as modulus of subgrade reaction, is not used anymore for designing foundations. It is not really a property of the soil and depends on so many factors that getting an accurate value requires some amount of work. What's worse, it's not constant throughout the bottom of a footing so that giving a single value isn't exactly correct. Foundations are now just designed using shear failure and settlement analyses. Even mat foundations can be designed without subgrade modulus. Just to be clear, I know subgrade modulus is still used for pavement and sometimes slab-on-grade designs.

However, I've got a structural engineer client who is asking for the subgrade modulus to design a footing. I've already told them the allowable bearing capacity he has to work with but they are insistent on getting a subgrade modulus value. It's making me wonder if structural engineering in general has not caught up with this and is still using it in their designs. Do structural engineering software require a k value to work? Any thoughts on this?

I'm a geotech engineer. From my side of the ground surface, the subgrade modulus, k, also known as modulus of subgrade reaction, is not used anymore for designing foundations. It is not really a property of the soil and depends on so many factors that getting an accurate value requires some amount of work. What's worse, it's not constant throughout the bottom of a footing so that giving a single value isn't exactly correct. Foundations are now just designed using shear failure and settlement analyses. Even mat foundations can be designed without subgrade modulus. Just to be clear, I know subgrade modulus is still used for pavement and sometimes slab-on-grade designs.

However, I've got a structural engineer client who is asking for the subgrade modulus to design a footing. I've already told them the allowable bearing capacity he has to work with but they are insistent on getting a subgrade modulus value. It's making me wonder if structural engineering in general has not caught up with this and is still using it in their designs. Do structural engineering software require a k value to work? Any thoughts on this?

## RE: Is subgrade modulus still used by structural engineers?

The only exception I could think of is if you are designing your column base for partial fixity and allowing some rotation of the base (some where between a fixed and pinned end).

Other than that, its used mostly for slab on grade or raft design.

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

-----*****-----

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

In such cases the subgrade stiffness will determine the slab internal forces and the reinforcement. If we assume uniform reactions under the entire base, that will probably be an conservative approach for member design, but not for verifying geotechnical bearing pressure.

Interested in alternative ways to doing this other than subgrade modulus, as it's not the first time I've heard about the issues with this approach.

## RE: Is subgrade modulus still used by structural engineers?

I'm not sure how you could reasonably design a large raft without it. The results are quite sensitive to it on softer soils. Once you get above 200psi/in or so with typical building footings, it becomes less important.

For isolated pad and strip footings, its not typically used.

## RE: Is subgrade modulus still used by structural engineers?

I design a lot of water retaining concrete structures with exterior and interior walls/columns etc, the subgrade modulus is essential to design the mat slab that forms the floor of the basin.

But yeah for a typical small building with spread footings/continuous wall footings I would never look at it.

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

That's exactly what I did. I'm glad to see I'm not the only one.

@milkshakelake

Yes, you need loading and also footing dimensions. I suppose in a pinch you could use various loads and give a range. I think the idea is that the geotech gives the structural a bearing capacity. Using that and the column loads, the structural sizes the mat foundation and passes it back to the geotech. The geotech uses that loading and size to get the subgrade modulus throughout the matt foundation and gives it to the structural (I provided a contour map). Then the structural can use that in their own FEM to see how the mat foundation will bend and how to reinforce it. If resizing is necessary then the it goes back to the geotech and so on. That's the iterative process that EireChch was talking about.

I think I was a bit surprised that there isn't something other than the subgrade modulus used by structural FEM software to model the bending of a mat foundation.

## RE: Is subgrade modulus still used by structural engineers?

I'm not entirely sure about the FEM concept. Hooke's law states that F=Kx. So you get the F from the engineer (or assume something), get the settlement based on geotechnical equations and soil layers, and then you can calculate K. Why would you need FEM for that? It seems like an equation you apply over a 2D space.

## RE: Is subgrade modulus still used by structural engineers?

## RE: Is subgrade modulus still used by structural engineers?

The process should be iterative but usually the geotechnical engineer is under their own contract with the developer and scope of service doesn't cover much if anything beyond the initial soils report. That leaves us in a position to usually need to do some form of enveloping of the spring stiffness. ACI 336.2R-88 offers us some guidance on variation of the spring constant throughout the plan of the mat to account for local stiffening at the edges.

## RE: Is subgrade modulus still used by structural engineers?

Pretty much all of the structural software uses a winkler model"I think that is the key. Software companies have not developed a more accurate model from a geotech point of view. It could be there isn't a better theory. I don't know. But this makes sense. Even if we all agree the subgrade modulus isn't ideal, it is what we need for advanced modeling.

## RE: Is subgrade modulus still used by structural engineers?

In the best case scenario we'd have been allowed to open a dialog with the Geotechnical engineer and work to get compatible settlements between their analysis and our structural software results.

## RE: Is subgrade modulus still used by structural engineers?

Variable Modulus of Subgrade Reaction - Part 1

Variable Modulus of Subgrade Reaction - Part 2

Subgrade Modulus – Revisited

## RE: Is subgrade modulus still used by structural engineers?