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The purpose of a strap beam (or tie beam - i'm not sure of the exact expression on english) as far as I know, is to enable foundation (horizontal) stifness.

So my question is: how to model them correctly?

In case of a foundation - we add support with appropriate support K values equivalent of the soil stifness. Should we do the same for the strap/tie beam? Because there is some soil reaction which will transfer on the tie/strap beam and also it will receive some of the shear and axial forces and also bending/torsion moments from the columns.

I want to discuss case 1 and case 2, as most common use (in my experience) of tie beam/strap beam.

Strap beam - case 1

Strap beam - case 2

Xa0c
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2 Answers2

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What you are referring to is commonly known as a grade beam. Concrete grade beams tie the footings together and also resist moment transferred by columns due to wind or earthquake loads or even hydrodynamic loads in some cases. Therefore they have to support shear, moment, and axial loads and also minimal load combination prescribed by codes such as ASCE 7-10 and ACI 318. Or the applicable local codes.

Generally, they are modeled as a part of the entire structure.

As a quick illustration:

  • Case 1. The grade beam is a load-bearing part of the foundation, so it has to be designed for both direct soil reaction and also any moments from the column due to lateral loads.

  • Case 2. The grade beam may be designed for just the moment and code prescribed minimal loads. Like tie-in loads and such.

I could attach the relevant code/s if it was just a few pages, which is not.

Edit

After OP's comment, I added a diagram of the distribution of stress on the soil.

'

diagram

kamran
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  • First of all, thanks for you answer and clarification of the terms.

    In both cases in reality, there is interaction soil-structure, because even though in second case, the beam is not on the same level (foundation depth) as the footing, still - it will "lay" on soil and because of the loads and force transfer - there will be some soil reaction on it. In my opinion. We can discuss this further.

    As for the Case 1, are you suggesting that I should model it with an appropriate support which will represent the soil reaction?

     – Xa0c Jan 03 '22 at 19:56
  • @Xa0c, you are right for case 2, but the small load on the grade beam from the settlement in the soil is covered by minimum design load as per codes. As for the case 1, I would design the footings to take all their column load as a conservative approach and design the grade beam for the soil reaction equal to what pressure you calculated per square unit under the columns. this implies the columns and grade beam settle equally. Then apply the additional triangular load on the grade beam due to the moment from the column on the grade beam. – kamran Jan 03 '22 at 20:35
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Modeling a "strip beam", "tie-beam", or "grade beam" should be the same as other structural elements/beams, except two prevalent assumptions are indicated below:

  1. The grade beam is simply supported at columns. It passes the gravity load to the columns but does not produce the moment in the joint; it acts as a brace point for the column at the foundation level.

  2. The full contact with the subgrade is unreliable, so, the grade beam shall be analyzed/designed the same way as an elevated beam without the complication of soil-structure interaction.

The grade beam is acting as a tension tie to ensure uniform movement of the columns.

ADD: If you feel it is necessary/essential to engage the subgrade, then you shall discretize the grade beam and provide an equivalent compression-only spring at the nodal points. Also, you can fix the beam-column joint to get more realistic beam forces. In the end, be conservative, and be reasonable, which are keys for foundation design.

r13
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  • In my practice, most of the time the grade beam is an essential part of the moment frame and is critically important to carry the moment. Maybe it is because of California's high seismic risk, but even wind shear panels and StrongTie prefab moment frames should transfer moment to a grade beam designed for that moment. – kamran Jan 04 '22 at 01:49
  • @kamran Different zone, differ in practices I guess. The approach I posted, it penalizes the column as well as the grade beam. Most importantly, the procedure is much simple. I haven't done anything in Ca though. – r13 Jan 04 '22 at 02:13