Couple on a Grade Beam

[abusementpark]

I need to analyze what is essentially a couple on a grade beam due to column reactions from wind bracing. Can anyone guide me as to determining what maximum moment will be generated in the grade beam? I can determine a modulus of subgrade reaction from the soils report. However, the portion of the grade beam that is in uplift will be resisted by adhesion on the sides. So, i guess the main problem is that I do not really know how to model the adhesion on the sides of the grade beam as a vertical support. Any guidance would be greatly appreciated..

 

[msquared48]

Doesn't the grade beam span bvetween spread or strip footings? With these as supportpoints, you should be able to calculate the shear in the grade beam from the force couple (moment).

Mike McCann
McCann Engineering

 

[abusementpark]

No, the couple is applied in the center of a long strip of grade beam that seals the perimeter of the building. Even if there were spread footings at some point on both sides, I don't see how it would be reasonable to consider them as support points. The soil is pretty good and it is a slab-on-grade system, so I am counting on the soil to provide continuous vertical support for both gravity and uplift as described in my original post.

 

[hokie66]

Again as is common on the site, we have a problem with terminology. As Mike has suggested, the term "grade beam" is normally used for a beam below the ground surface which spans between piles or other footings. What you have is typically called a "strip footing", or possibly it an edge thickening.

Using adhesion on the sides of a strip footing is not good practice. You should rely on mass, which can include a contribution from soil engaged by the footing projection.

 

[JAE]

Using adhesion on the sides of a strip footing is not good practice. You should rely on mass, which can include a contribution from soil engaged by the footing projection.

Amen

 

[csd72]

Strip footings are usually not designed as beams.

I would treat it as a cantilever either side of each column ensuring that there is enough top and bottom steel to take the bending. Also check shear.

The bending stress is from the bearing/ the dead load resistance required to resist the loads at each column.

 

[MiketheEngineer]

A couple on a grade beam - are they smooching??

Sorry - just had to say that

 

[msquared48]

Words only an engineer could "appreciate"...

Mike McCann
McCann Engineering

 

[csd72]

They must be breaking up because they are going in opposite directions.

 

[DaveAtkins]

OK--back to the question at hand

You could model this as a continuous beam supported on springs (spring constant based on the subgrade modulus of the soil).

DaveAtkins

 

[JAE]

The couple only had a "moment" in time to spend together.

 

[msquared48]

Let's not "force" them together.

That would be "shear" stupidity...

Mike McCann
McCann Engineering

 

[kslee1000]

We have never relied upon adhesion in uplift application, because it is not reliable - will lost in time. However, just for fun, see if the procedures below works or not:

1. determine/estimate the influence area due to the uplift force.
2. compute the resistance force (adh X influence surface areas)
3. if result of 2 is less than the uplift, then add member dimension to increase dead weight or the surface areas, and go back to step 2. Otherwise, the segment of the beam under influence will stay in place, then proceed to 4.
4. reduce the uplift force by the resistance force calculated in 2. This is the force that the beam segments adjacent to the uplift region would feel.
5. in the model, release the springs in the uplift region.
6. repeat the steps 1-5 to adjust the springs until the upward movement in the uplift region is small to nil.

 

[msquared48]

Party pooper!

Mike McCann
McCann Engineering

 

[fattdad]

You may have to lower the grade beam and consider the unit weight of the soil atop the beam for your vertical reaction against uplift.

Ha on the couple!

f-d

¡papá gordo ain’t no madre flaca!

 

[abusementpark]

Hey guys, thanks for the insightful responses (well, some of you)...hehe. I just took a conservative approach similiar to the one presented by kslee100 and called it a day. Not enough time in the budget to get too deep into this one.