Ground Beam maximun length and level

[Mahi20]

Hi
I have a about 400m long building foundations (16m c/c) to be tied with ground beam.
1. What s the maximum length of ground beam to be appropriate and if discontinued any effect?
2. Is the ground beam be good at ground level or 2-3m below ground.
3. Any measure to avoid/reduce temperature stresses for (5 to 50 Deg C temp variation)?

 

[r13]

1. In general, there is no limit to the length of the grade beam. However, the length might be limited by the requirement of expansion joint for the structure it supports.
2. The grade beam shall be placed below the local frost depth, and at where the bearing strength is adequate.
3. Provide layer of thermal protection/insulation.

 

[MSUK90]

Can you send us the pictures to get a clear idea of what you are asking?

 

[phamENG]

This is a regional terminology problem for me - what do you mean by "ground beam"? Where I am, we have grade beams and continuous footings. Grade beams span between pile caps in deep foundations (driven piles, drilled piers, etc.) and a continuous footing is a shallow foundation relying on the bearing capacity of the soil. I believe I've seen ground beam used to describe both of these before on this forum.

Since you're concerned about length, I'm going to guess that you are referring to what I call a grade beam (please correct me if I'm mistaken). So...

1) Length is limited the same way the length of any beam is limited. When using deep foundations, it's important to not rely on the capacity of the shallow soils beyond using them as a casting surface (though even that may be difficult in some situations). So your length is from pile cap to pile cap. Design for strength and deflections as you would any concrete beam, but keep in mind any additional cover requirements to protect the rebar and account for imperfections in the excavation.

2) Depends on where you are. As retired13 mentions, frost may be a consideration. I'm not sure where you are, so I don't know if it is. I'm in a moderate climate, and frost depth is barely a foot down. So I can have a 2' deep grade beam stick 6 inches out of the ground if I really want to. There are other considerations, too, like expansive clay. Give us more information on where you're located and the environment and you can get better answers.

3) That has to be worked into the overall structural design and consideration of expansion joints. Once those are located, you have to manage restraint stresses vs. anticipated movement and detail accordingly.

 

[BAretired]

16m c/c is a longer than usual spacing of supports. It may be more economical to provide additional supports in order to reduce the size and reinforcement of the grade beam.

1. What is the maximum length of ground beam to be appropriate and if discontinued any effect?

With a length of 400m, you should provide expansion joints at 50 to 75 m.
Where discontinued, the beam each side of the expansion joint simply ends there.​

2. Is the ground beam be good at ground level or 2-3m below ground.

The top of grade beams can be at ground level, but usually, they are at least 150mm above grade. The bottom of grade beams will depend on the depth of beam required. If this foundation is in Singapore, frost will not be an issue. I live in a colder area of the world where temperatures can be as low as -40^oC. In my area, it is usual to provide void form under all grade beams to prevent frost heave.​

3. Any measure to avoid/reduce temperature stresses for (5 to 50 Deg C temp variation)?

That is not an exceptionally large temperature range. The provision of expansion joints will help in controlling stresses; make sure the gap is large enough to accommodate the expected temperature differential.
​

BA