preliminary design of columns 5

[EngAnasa]

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

 

[asixth]

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.

 

[rb1957]

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

 

[BAretired]

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

 

[dik]

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

Dik

 

[ajk1]

I assume those numbers are for axially loaded only columns?

 

[asixth]

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.

 

[BAretired]

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

BA

 

[steellion]

Good rule of thumb, asixth.

 

[asixth]

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.

 

[dcarr82775]

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

 

[slickdeals]

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

 

[JedClampett]

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

 

[asixth]

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.

 

[johnbridge231]

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-design.html

 

[slickdeals]

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?

 

[asixth]

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-Slender-Columns-ACI-ASCE

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

 

[homostructural]

This publication from Access Steel might help you!

All about Eurocodes:

http://www.DesignToEurocodes.com