Strap Beam Design
Strap Beam Design
(OP)
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.
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.






RE: Strap Beam Design
Mike McCann
McCann Engineering
RE: Strap Beam Design
To reduce this bending, suggest you use a rectangular footing at the exterior, thus reducing the eccentricity.
RE: Strap Beam Design
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.
RE: Strap Beam Design
RE: Strap Beam Design
The strap beam is really fixed at the exterior footing (because it is resisting the rotation there) and pinned at the interior footing.
RE: Strap Beam Design
RE: Strap Beam Design
DaveAtkins
RE: Strap Beam Design
RE: Strap Beam Design
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.
RE: Strap Beam Design
thanks
RE: Strap Beam Design
RE: Strap Beam Design
RE: Strap Beam Design
DaveAtkins
RE: Strap Beam Design
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.
RE: Strap Beam Design
RE: Strap Beam Design
RE: Strap Beam Design
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