Loading a column on a beam loaded on another beam 1

[VTAdam]

I am designing a building that is architecturally beautiful but structurally not so much. The face of the building recedes back 1000mm from the front and 500mm from the left side for the upper stories. I cannot take the columns for the upper floor plans all the way to the bottom. This would ruin the rooms in the first floor. Consequently, I have to load them onto beams. The situation at hand is that one of the columns will be loaded on a beam. This beam is also loaded on two other beams. This doesn't sound like good engineering to me at all. I wanted to hear from you and see if you think that this is structurally possible. Please take a look at the attachment for a visual representation. Thanks.

 

[jayrod12]

I would say yes, it is possible. but the lateral bracing required for the stability of the structure might end up being prohibitive.

Don't forget you'll need to brace each beam for it's specific condition as well. Depending on whether you are flush framing the cantilever beam to cantilever beam connection will impact what bracing you have to provide and how difficult it will be to accomplish.

It is somewhat un-nerving to say I'm going to support a cantilever beam off of another cantilever beam. But it's doable, In fact I've got two jobs under construction right now that do exactly that. One in wood, one in steel.

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[VTAdam]

Thanks. I have done it and based on my initial analyses, it looks like that it will work.

 

[hokie66]

A few things you should keep in mind in designing those transfer beams:
1) Hanging reinforcement at the intersections.
2) The non-flexural areas of the beams are strut and tie regions.
3) Not sure how many floors are supported above the transfers, but cumulative deflections could be an issue.

 

[Trenno]

Transfers happen all the time - despite being architecturally driven, they usually cost a motser and result in very large and structurally inefficient structures. Vertical continuity is something you should always go into bat for when dealing with architects.

I'm currently designing a cantilevered two way transfer that is holding up 20 floors.

Anyway... here is a good reference that may help you decipher the loads coming down onto the transfer beams and how the structure will react.

Will you be analysing/designing this transfer deck in 3D or 2D runs? If 2D, note that the beams (B2S1 & B2S3) will be 'soft supports' and thus you'll need to keep a careful eye on deflections of the system as a whole. Higher deflection will result in the transfer column seeing less load.

 

[KootK]

Great reference Trenno. I wish that there were more references out there specifically for the design of transfer slabs. At some point, I stumbled upon an excellent CSA/SAP/ETABS document related to the design of transfer floors and how modeling staged construction is often desirable. Unfortunately, I'm unable to locate it now.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

The main points are

- do not rely on a 3D analysis of the completed structure to determine the axial loads in the transferring columns. The conservative option is simply to base the column loads on tributary areas as the deflection of the transfer beams in the 3D analysis will result in the axial load being calculated on the assumption that the building was completely propped to the ground until construction is completed. The construction procedure and PT in beams will significantly affect this. The real result will be somewhere between.
- cumulative deflections are important.