Concrete Beam to Column connection 1

[woodman1967]

I am designing a project using concrete beams and columns. The beams and columns are already designed, I just need to connect them together so they resist a vertical gravity load and a horizontal thrust. Also, the columns are supported on an existing concrete wall, what would be the best way to connect these together?

I am getting the job reviewed by another engineer as I haven't designed anything in concrete since university but would like to be in the ballpark before I send the design off to him.

Thanks for your adive

 

[Structures33]

For your beam-column connection, it depends on how you designed the members.
If the concrete beams are designed as continuous over the support column, you would need to show the beam reinforcing continuous across the support and the vertical column r/f would hook into the beam. At the end condition, the beam reinforcing should have a 180 hook for full development.
For your column-wall connection - can you clarify? Are the columns already constructed? If not, you will need to epoxy dowel far enough into the wall to develop the vertical reinforcing of the column.

 

[woodman1967]

The beams and columns have not been constructed yet. The wall is existing.

The beam-column connection is an end connection. Could you clarify what you mean by a 180 hook?

Thanks for your advice

 

[Structures33]

If you have ACI 318-05 you can see a 180 degree hook on page 198. If not, it's where the longitudinal bar of the beam bends back the opposite direction - top bars would bend down and back along the bottom and bottom bars would bend up and back along the top.

 

[JAE]

Take a look at this sketch - typical beam/column connection detail.

 

[csd72]

Looks like you could do with something like this book:

http://www.1insaat.com/uploads/TrbBlogs/pdfs_1/16183_1190279138_854.pdf

 

[woodman1967]

Thanks everyone,

I appreciate your advice and the reference material.

Malcolm

 

[fa2070]

JAE,

With your sketch in mind, how do you idealize the beam at the support? Do you model it as simply-supported and neglect the bending moment in the joint? By looking at the detail it's clear that the beam-column pair doesn't qualify as a "frame", but on the other hand there's a small moment in the joint that I'm not sure if it should be ignored.
How is this bending moment dealt with in day to day design?

Thanks.

 

[JAE]

There's two schools of thought here that I've used.

First, ACI and the "right" way is to assume that the beam and column are fixed (which they really mostly are) and design as a rigid bent. ACI suggests fixing the far end of the column as rotation at its base, whether fixed or pinned, doesn't affect the beam that much.

In this case, the vertical column rebar should be developed into the beam, which I noted on the sketch.

Second, alternative way, is to design the beam as though it is a simply pinned support (which it really isn't). This creates higher moments in the beam, especially positive moment. The beam's negative moment top steel over the column, in this case, is zero, but you know it isn't. So use a percentage of the positive moment, perhaps 75%, and provide nominal top steel to control cracking. You essentially are allowing some yield, perhaps, in the top steel but since you've provided more than enough positive rebar in the bottom of the beam this is ok. Sort of a poor man's moment redistribution.

For the column, you can still estimate the moment in the column using a fixed end moment (wL^2/8) as a pseudo moment for the column.

I prefer the first way as it is more accurate. But in the "old" days when computers weren't as prevalent, most engineers designed the beams as on pinned supports and made estimates for the fixity that was really there.

 

[fa2070]

Thank you very much JAE.