Strap Beam Design

[faromic]

I have a situation where exterior columns of a 7 story building are flush with the property line at one side of the building. Because of this I have eccentric footings at these locations. I want to design a strap beam at these locations, but am unsure about how to go about this. The footing at the edge of the building is 9'x9' and the other footing at the interior is 13'x13'. They are spaced at 27' on center. This leaves the strap beam length as 12.5'. THe size of the strap beam 24" deep x 36" wide. At first I modelled it as one span with a cantilever but am getting a moment of 900 k*ft. THe loads are pretty high, 500 kips on the edge footing and 1000 k on the interior footing, but the moment just seems high to me. Then I tried to model the beam as a simply supported from edge of footing to edge of footing with a length of 12.5', assuming the footing to beam connections are pin connections. I get approximately the same thing because of the conservativeness of the calculation of the moment.

I have another edge ftg. that connects to an interior ftg which is part of a mat supporting the shear wall core. The strap beam will connect the edge ftg to the mat. From RISA, the output of the moment at the area where I'm going to introduce the strap beam is 44.1 kips. This seems odd since when I compute the moment on a 13'x13 ftg (this ftg is part of the mat) I get a much larger moment. What I did was apply the 44.1 k*ft moment at the edge of the strap beam and add the moment from the analysis of the load from the edge footing.

 

[msquared48]

What happens if you model the strap or grade beam as spanning center to center of the footings? Realistically, I think this is how it would work.

Mike McCann
McCann Engineering

 

[hokie66]

The reason for the strap footing is to act as a rectifying beam for the external column footing. The eccentric column creates a moment of approximately 500 kips x the eccentricity, which is going to be more than the 900 ft-kips you came up with. There can be no pinned connections in the beam.

To reduce this bending, suggest you use a rectangular footing at the exterior, thus reducing the eccentricity.

 

[jike]

I agree with both previous comments.

A strap beam is also called a "Pump Handle" which is preventing the overturning of the exterior footing by the applied weight at the interior column (where one would normally put your hand on the handle). The moment is relatively easy to calculate as Hokie66 said 500 kips x eccentricity. I don't know that you need RISA to do that.

 

[faromic]

I agree. So at the other end of the strap beam where it connects to the interior footing, that would be assumed as a fixed connection and the strap beam is a simply supported beam spanning from center of footing to center of footing with maximum moment at the point where the edge footing connects to teh strap beam, correct?

 

[jike]

No.

The strap beam is really fixed at the exterior footing (because it is resisting the rotation there) and pinned at the interior footing.

 

[faromic]

I increased the beam size to 54 wide x 30 deep because I was exceeding rhoa max and the beam was not tension controlled. I am getting a huge moment; about 2200 k*ft for the 54" wide section. The reinforcement required is 19 in^2. Does this sound reasonable?

 

[DaveAtkins]

Yes, 2200 kf sounds correct (~500 k * 4.5'). A quick formula for determining area of steel required is Mu / 4d. In your case, using d = 26", As ~ 21 in^2. So 19 in^2 sounds correct (a little low?). You may need a lot of stirrup steel, because phi * Vc ~ 134 k, which is much less than 500 k. Also, make sure the 9' X 9' footing is adequate, because it will support more than 500 k (do the statics--I get ~ 583 k).

DaveAtkins

 

[faromic]

Doesn't the strap beam prevent the footing from receiving additional load from the eccetricity? so the load would still be 500k?

 

[hokie66]

Dave is correct, the cantilever magnifies the load on the edge footing. As I suggested earlier, it would be prudent to use a rectangular footing to reduce the eccentricity, thus reducing the beam moment and the soil pressure.

Your 2200 kip-ft is at the center of the footing. At the edge, it will have reduced, and the moment reduces linearly along the beam.

 

[faromic]

Yes, I think I'm going to do that.
thanks

 

[faromic]

is the design shear the load coming down from teh column then transferred to the strap beam?

 

[hokie66]

Just do the statics, draw the shear and moment diagrams, and you will see that the shear on the beam is much less than the column load because of the lever arm to the next column. The shear is the difference between the exterior footing reaction and the column load.

 

[DaveAtkins]

But the maximum shear is still 500 kips--in the cantilever of the strap beam. In the backspan of the strap beam, the shear will be much less.

DaveAtkins

 

[OldPaperMaker]

faromic,
It is probably time to get out one of your reinforced concrete text books and review the examples under the Footings & Foundations section.Design of Concrete Structures by Arthur H. Nilson's 12th edition has a detailed example of what he calls a "Two-Column Footing". Design of Reinforced Concrete, 7th Edition by Jack C. McCormac & James K. Nelson has an example (on page 370) that they refer to as Combined Footings.

 

[faromic]

That's exactly what I did. I have Nilson's book and took a look at those examples earlier. The reason I was asking so many questions is that I was under pressure to finish and was getting nervous.

 

[hokie66]

Dave is right that the maximum shear is 500 kips, but that is within the external footing, so the beam shear would be resisted by a much wider section than the strap. In this case, punching shear at the footing edge is probably controlling.

 

[csd72]

Just draw youself a little beam diagram with a support at the center of the pad and at the end of the strap beam (or at the resisting dead weight pad if you have one).

You will see that it is a span with a cantilever on the end.

you will have dead load on the span resisting the column load on the cantilever.

Now you can more easily understand your bending and shear.

csd