strap beam design

huangyhg

strap beam design
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.

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
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.
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.
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?
no.
the strap beam is really fixed at the exterior footing (because it is resisting the rotation there) and pinned at the interior footing.
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?
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
doesn't the strap beam prevent the footing from receiving additional load from the eccetricity?  so the load would still be 500k?
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.
yes, i think i'm going to do that.
thanks