×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Residential Concrete Column

Residential Concrete Column

Residential Concrete Column

(OP)
I am currently working with an Architect on his new residence.  He is using 16" dia sonotube formed concrete columns to support a porch.  They are only carrying 5.5K of load and are 6'-8" above grade.  The size is purely for aesthetics.  Do I need to follow ACI requirements for reinforcing of these columns or is there an exemption for such lightly loaded elements?  ACI requires (5)-#6 with #4 ties at 3" o.c. max.  Seems a bit much.
Thanks.

RE: Residential Concrete Column

Have not designed in ages, haven't a clue what klu/r means anymore but I bet you could get away with ties consisting of No.3 size w/spacing of 1@ between 3" to 6 " E.E bal @ 12 in o.c enclosing the 5 No.6 verts. The forum gurus will confirm so do not take my word for it.

The area of the 5 No.6 verts/Ag is about 0.01 so I don't know if you can drop it any further. ACI 318 Sec 10.9.1

Out of interest, what ACI sections did you use to come up with size and spacing of lateral reinforcement/ties?

 

RE: Residential Concrete Column

Where did you get that requirement for ties?  That seems pretty excessive.  Is it because you are in a seismicly active area?

I would go with the 1% minimum myself, but that's me.  It seems like #3 ties at 12" should be ok.  You do have the option of arranging the reinforcing in a rectangular cage if you want.  That's usually cheaper than cicular ties and vertical bars in a circular pattern.

RE: Residential Concrete Column

I believe there is an ACI provision that allows you to go down to .005 for longitundinal steel for columns that are larger than needed for asthetic reasons. Do not use less than 4 longitudinal bars. I would think that #3 at 12" oc are adequate ties for #6 longitundinal bars.

RE: Residential Concrete Column

See ACI 318-02 Section 10.8.4.  This allows you to effectively go down to .005*Ag for the column if it is larger than required with a minimum of four bars.  This is done quite often in my area for lightly loaded columns.  Based on ACI 318-02 section 7.10.5 it looks like #3 @ 12" is what you need, based on no shear.

RE: Residential Concrete Column

We usually like to add a second, additional tie at the top, located 3" below the very top tie, just to add some extra confinement where any applied moment or twisting might occur.

RE: Residential Concrete Column

We also do what JAE recommends.

RE: Residential Concrete Column

Why not try to design it as plain concrete and just place one bar in the middle for integrity?
Axial stress is less than 30 psi on the gross area. the height is only 6'-8" (if I understand correctly) What is all the reinforcing going to do other than insure that is won't break up when it falls over? Even a single bar might provide that.
Connections at the top and bottom of the column would seem to be far more critical than a 4 bar cage in the thing. Maybe 4 dowels out of the foundation would provide some fixity at the base to promote safety but above that I'd expect very little bending stress and likely well within the capacity of plain concrete. Also, the code allows reinforcement 33% greater than that needed by analysis.
I haven't look at ACI for plain concrete in a while but I would expect that there are some provisions there for such a case.

RE: Residential Concrete Column

houseguy, check the last paragraph of ACI 318 Sec 22.2.2. It states "The use of structural plain concrete columns shall not be permitted".  Chapter 22 permits use of structural plain concrete for pedestals ref: 22.2.2 (c) and 22.8

RE: Residential Concrete Column

(OP)
Thank you all for your helpful comments.  I do not typically design with concrete, with the exception of foundations, so you're insights are much appreciated.  The project is currently on hold (go figure), so I will deal with it on another day!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources