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Beams with high concentrated loads near support
4

Beams with high concentrated loads near support

Beams with high concentrated loads near support

(OP)
According to ACI shear design procedure, the shear design value is taken at a distance d from the column face. However, due to demanding architects and owners, I often find myself with high concentrated loads at a distance less than d from the column face causing a huge peak in shear value at distance less than d. I attached a drawing of a beam with a high concentrated load near the support so you could know what I typically deal with. Increasing the beam depth would of course allow the design software to ignore this peak because it's at a distance less than d.
For these cases, do you think I should be designing for the shear value at the column face directly and neglect the effect of increasing the beam effective depth "d"?

RE: Beams with high concentrated loads near support

Design for the shear value at the column face. ACI 318-14 even shows the critical shear section (at the face of the support) for a load applied within 'd' of the support in Figure R9.4.3.2(f)

Go Bucks!

RE: Beams with high concentrated loads near support

the beam becomes a deep beam and you need to design the region following strut-tie methods.

ACI makes this pretty clear in the deep beam definition, where the condition is an "either":
1. clear spans equal to or less than four times the overall member depth; or
2. regions with concentrated loads within twice the member depth from the face of the support.

You fall under condition 2.

My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:
https://github.com/open-struct-engineer

RE: Beams with high concentrated loads near support

2
An alternative checking procedure that I support is to use shear checking provisions suitable to shear cracks steeper than 45 degrees as described in CRSI's design guide on pile caps. Strut and tie is near and dear to my heart intellectually but, as a production tool, it's painfully slow unless I've already gotten it automated somehow. As a matter of personal judgment, I will also insist on real bottom bar anchorage in this situations rather than the "standard hook past the CL of support" option. That, as a nod to what I expect the result would have been if I did STM.

RE: Beams with high concentrated loads near support

(OP)
Thank you everyone for replying, This has been very helpful.
I don't get paid well enough to do a manual strut and tie method on every similar case, and I think that contractors will give me a headache over the steel arrangement. So I will use straub46 suggestion with Kootk formula as a check, with the strut and tie method option only for very high concentrated loads. I will also write for the structural design software developer (PROKON), to consider shear values at distance d for concentrated loads, and hopefully, I could get a discount.

RE: Beams with high concentrated loads near support

Are those high concentrate forces from columns that are offset from the columns below? If so, can you either slope the column (so that it bears on the nearby column below the floor) or widen the column so that overlaps with the column below? The widened column configuration is known as a “walking column”. If you walk the column it would essentially become a huge corbel, and the column would be treated as such.

Unfortunately “there’s no such thing as a free lunch”. Whether you walk or slope the column you would now be imposing tension and compression strut forces in the slab at either end of the sloping/walking column, and you would need to deal with those forces in the slab design (which would also include following those forces in your slab diaphragm to the lateral load resisting system). Essentially you would have a strut and tie system with your sloping/walking column as the strut, and the floor diaphragm as the tie. We use this methodology all the time. Lots of times there is not enough ceiling height to accommodate a transfer girder. And transfer girders are expensive.

RE: Beams with high concentrated loads near support

(OP)
cliff234, I see what you mean, however, The original intent of this post is for cases where beams are bearing on each other. For example, CB1 bearing on CB2 in the attached file. Where CB1 location is fixed to either support a heavy brick partition on it or to be hidden within a partition below the roof and achieve higher clearance. For transfer beams supporting a new column, I agree that a strut and tie is the way to go, but I have to mention that I know engineers who have designed these beams as regular beams with high concentrated loads without any consideration to the strut and tie method.

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