×
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

preliminary design of columns
5

preliminary design of columns

preliminary design of columns

(OP)
wuld like to know easy n most accurate way for preliminary estimate of column dimensions as i dd for the continuous beams effctv length/depth < 26 accordn to bs8110prt1. How abt for the columns?thanx

RE: preliminary design of columns

2
Consider fire (so don't go skinnier than 300mm) and always consider slenderness.

Capacity is roughly 0.5*f'c at Le/r=20 ; 0.4*f'c at Le/r=35 ; 0.3*f'c at Le/r=45 ; 0.2*f'c at Le/r=60 and 0.05*fc at Le/r=100.

This is for columns which aren't generally required to take lateral forces. Lateral forces is a completly different game.

RE: preliminary design of columns

Euler, no? Pcr = pi^2*EI/l^2 ... stress = pi^2*E/(l/r)^2

RE: preliminary design of columns

The problem is that once you have committed yourself to a certain column size, the architect is not pleased to learn that you need larger columns after his drawings have advanced beyond the preliminary stage, right?

Maybe you should start out with a low concrete strength which you know can be increased if necessary.

BA

RE: preliminary design of columns

Thanks asixth... beats my 2ksi approach... I've retained your numbers and will check with my next project.

Dik

RE: preliminary design of columns

I assume those numbers are for axially loaded only columns?

RE: preliminary design of columns

Yeah, axial loaded columns using min moment in single curvature. I find this governs most of the time, frame action from gravity or lateral loads tends to put columns into reverse curvature which I find is less detrimental than the min moment single curvature. I plotted a graph which I'll post tomorrow.

RE: preliminary design of columns

The book "Columns by Ultimate Strength Design" published by CRSI is useful too if the load and eccentricity are known.

BA

RE: preliminary design of columns

Good rule of thumb, asixth.

RE: preliminary design of columns

2
Attached are the graphs I put together. I built it up using my own column design spreadsheet. When compared to more refined column design softwares that go in-depth with the concrete compressive stress profiles the results are slightly on the conservative side which is good for preliminary design. Always good to have some small reserve capacity up your sleeve at preliminary stages.

The N40-1% refers to 40MPa concrete with 1% reo and likewise N80-4% refers to 80MPa concrete with 4% reo. Anything multi-storey I go straight for the high-grade concretes, in my region 80MPa concrete is readily available and only 25% price increase from the standard 32 and 40MPa stuff that is normally used for slabs. 28 and 56-day cylinder breaks normally go to the 100+MPa realm.

Special confinement clauses for high strength column ties does bump up the ties required.

RE: preliminary design of columns

Sometimes the size is driven by punch shear. So if you have a flat plate run some quick checks on that as well

RE: preliminary design of columns

asixth,
Can you walk through the process you went through to generate those curves?

RE: preliminary design of columns

I always try to size them so that kl/r is less than 34, so I don't have to check slenderness effects.

RE: preliminary design of columns

I used AS3600. Started with a typical sized column with a typical reinforcement. Started with k*l/r=10 and moved up in increments of 5 until I got to k*l/r=100 and then averaged the design capacity divided by area. AS3600 allows for slenderness by using a moment magnifier and a minimum design eccentricity of 0.05*D.

RE: preliminary design of columns

You can also use interaction diagrams that account for slenderness as well to get the required dimensions of a column at the stage or preliminary design.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-des...

RE: preliminary design of columns

What do you mean by "interaction diagrams that account for slenderness"? ACI recommends magnifying moments to account for the slenderness, or doing a full-blown second order analysis. Do you have a reference or an example?

RE: preliminary design of columns

The moment magnification is how ACI and many other codes allows slender columns to be designed. I wouldn't rely solely on a second order analysis to compute the "magnified moments". It will account for P-large delta and P-small delta but won't allow for the reduction in load-bearing capacity from slenderness.

http://www.scribd.com/doc/66504462/MacGregor-Slend...

The attached has a paper with generic interaction diagrams for single curvature columns with varying h/t ratios.

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