Strip Foundation Design

Part 2: Reinforced Concrete Verification

Hi friends, 👋👋

As promised last week, are we continuing with part 2 of the strip foundation design guide in this email.

Last week we showed how to verify that the soil doesn’t fail/collapse due to loading.

Today, we’ll have a closer look at how to design the concrete and reinforcement.

You can find the in detail calculation here. ⬇️⬇️

→ Strip foundation on clay ←

Table of contents of blog post.

Today, we’ll look at step #6 and #7 of the step-by-step process. 👇👇

1. Calculate characteristic loads that act on strip foundation

2. Load combinations

3. Define properties of the sand/clay and concrete

4. Geotechnical design – bearing resistance verification

5. Geotechnical design - Sliding resistance verification

6. Reinforced concrete design – bending verification

7. Reinforced concrete design – shear verification

Reinforced Concrete Design Of A Strip Foundation

First of all, strip footings don’t necessarily need reinforcement. In fact, I am designing 2 buildings with strip footings at the moment, and for most of the strip footings, we are not using reinforcement.

You can do that if the geometry of the footing follows EN 1992-1-1 12.9.3. The requirements are fulfilled if the cantilever part is less than half of the footing height.

Why is that?

It’s because all the load (soil pressure and concentrated line load from the wall) can be transferred in compression, because the angle of the load is > 60°.

Now, if the soil doesn’t verify for this width, and we increase the width with the result that the angle < 60°, then we need to add reinforcement, which we’ll do in the following.

Bending verification

The point load is transferred to the foundation base with a strut and tie model, where the force of the tie will be taken by reinforcement and the struts by the compressive strength of the concrete.

Watch out here: The following strut and tie model can only be used when there is only a vertical load and no horizontal load and no moment.

Due to this strut and tie model, it can be said that the soil stress = load x soil base area.

This soil pressure creates a bending moment in the cantilevering part of the strip.

Due to this bending moment, the reinforcement of the bottom surface of the strip foundation is calculated like a RC beam.

We’ll also check the minimum reinforcement according to EN 1992-1-1 9.2.1.1 (9.1N):

The minimum reinforcement is actually greater, and we’ll therefore design the rebars for the minimum reinforcement.

We’ll also use the minimum reinforcement in the longitudinal direction.

The reinforcement calculations which you can see in the blog post more in detail result in reinforcement which looks like in the following picture.

Shear verification

In general, we want to avoid shear reinforcement. So we often rather change the geometry of the strip than add shear reinforcement.

To check if shear reinforcement is required, we’ll follow EN 1992-1-1 6.2.2.

In case, the design line load > design shear resistance without reinforcement, and you want to add shear reinforcement, we’ll follow EN 1992-1-1 6.2.3.

You can check out the step-by-step process with all the calculation steps in the blog post.

The required shear reinforcement (stirrups) is then distributed along the strip foundation in both directions. 👇👇

Conclusion

The first few times you do strip foundation designs it’s very challenging because you don’t know which verifications you have to do, and you work with soil parameters, with which I struggled a lot.

For me, geotechnical engineering was one of the courses I had the hardest time with. I simply couldn’t understand the relationships between the parameters, and I was always overwhelmed by the amount of different parameters.

How do you feel about geotechnical engineering? Let me know in the comments. ✍️✍️

It’s also very difficult to find a guide that covers both - the geotechnical and reinforced concrete design.

To be honest, I have looked many times ⌚⏱️ and didn’t find one.

So hopefully this guide will help you out when you have to design a strip foundation. 👍😃

Alright, that was it for this week.

I’ll see you next Wednesday for another structural design guide.

Cheers,

Laurin. ✌️✌️