Hello Engineers, I have a questi 2

[Eng. Tawfik Jdid]

Hello Engineers,

I have a question about a structural member known as a "Grade Beam," which is commonly used to resist differential settlement between footings.
I created a simple ETABS model to determine whether the "Grade Beam" should eliminate or resist footings settlement.
The results revealed no significant difference between soil stress or footing settlement values in the two cases (with and without "Grade Beam").
So, is it worth adding them to the footings?
Or it is a common myth that Engineers used to tell!

Much appreciated.

Eng. Tawfik Jdid

 

[BAretired]

Grade beams are commonly used to support bearing walls in my neck of the woods. Usually, they bear on piles or footings, but not for the purpose of eliminating differential settlement. Their purpose is to carry a uniform wall load from support to support.

If it is a myth that Engineers used to tell, it is not one that I have heard.

 

[Eng. Tawfik Jdid]

Thank you for your response. So, what is the common name for the member that prevents or resists differential settlement?
Are there any specific sectional requirements for that type of member that we must follow in order to avoid differential settlement?

 

[XR250]

Maybe you are referring to a strap footing?

 

[Eng. Tawfik Jdid]

No, Strap Beam is typically used when there is non-uniform soil pressure beneath a neighbor footing, and we use Strap Beam to connect that neighbor footing to an internal footing to achieve uniform soil pressure beneath the neighbor footing.

 

[BAretired]

Thank you for your response. So, what is the common name for the member that prevents or resists differential settlement?
Are there any specific sectional requirements for that type of member that we must follow in order to avoid differential settlement?

Beams are used primarily to carry load horizontally between supports. A beam occurring at Second Floor is called a Second Floor Beam. A beam occurring at grade level is called a Grade Beam.

Differential settlement is caused either by an unexpected load or by variable soil properties. There is no member whose primary purpose is to avoid differential settlement, but a continuous grade beam would reduce settlement if the reinforcement is designed to do so. Differential settlement caused by a local soft spot might be reduced but can't be avoided.

 

[Eng. Tawfik Jdid]

Many thanks

 

[BAretired]

You're welcome.

 

[TLHS]

What problem are you trying to solve? Depending on the problem, it might not show up in a typical analysis even if there will be a potential effect on the real construction

If you're trying to mitigate differential movement due to variable subgrade conditions, then a model won't show that unless you model different subgrade conditions. A variety of styles of stiff combined footings are used as semi-qualitative mitigations for subgrade conditions. If it's severe, you'll normally put things on a mat footing, but there are likely conditions with light loads where a combined grade beam style construction might make sense.

The effects of things like this also depend pretty significantly on soil stiffnesses. In really soft soils, you might reduce overall settlement by using a wide strip footing with increased bearing area, but in stiffer soils it likely doesn't do much because the load gets into the subgrade within a few multiples of your footing thickness.

Basically, it depends on what problem you're trying to solve and what conditions you have.