Stepped Columns with Single Axial Load 1

[JMcKenna]

Can anyone help me? I am trying to determine the effective Kl/r for a stepped steel column that has a single axial load. This column is not like a Crane Column with additional load at some mid-point. Most everything I have found requires the running of a program code. I am looking for a hand calculation if possible.

 

[msquared48]

Personally, I am unfamiliar with the term "stepped".

Can you clarify that, or post a sketch of the situation?

Mike McCann
MMC Engineering

 

[JMcKenna]

What I mean by Stepped is a smaller column say (W12x40 @ length=10.25') on top of a larger one (W27x84 @ length=22.0'). If I use the total length=32.25' and the r for the smaller W12x40, I obviously dont have very much capacity. And I can not use the W27x84 for the full length...so therefore I need to develop the effective length for each member.

 

[TXStructural]

Google is your friend:

http://www.aisc.org/store/p-370-stiffness-coefficients-of-beamcolumns-with-nonprismatic-members.aspx

http://www.eng-tips.com/viewthread.cfm?qid=36453

 

[ToadJones]

did you try determining "K" as if there was just some small load at the transition using DG7?
I'm not sure if it works or if the method used to make the charts converges for this situation.

It would be interesting to see it if does though.

 

[ToadJones]

after doing tiny little bit of digging using spreadsheets I have made for stepped columns (unchecked by another person) it appears that the equations converge ....ie. load at step = 0 kip and load at top = say 30 million pounds and it appears that the k-factors top out.

Maybe this is the approach to use?

Not sure.

 

[BAretired]

If you want a hand calculation, I would recommend the method outlined in thread507-267603.

BA

 

[JMcKenna]

Gentlemen, It appears that calculating the "K" value by hand is not very accomidating...therefore I will need to write a spreadsheet to help me out. It appears the article "Calculation of Effective Lengths and Effective Slenderness Ratios of Stepped Columns", can provide me with the best road map my desired result. Unless anyone has a better suggestion or link that would be helpful.

 

[ToadJones]

I think STEELtools.org has a spreadsheet that follows DG7...It may be based on the paper in your link (though your link does not work for me).
There are two ways I know of to obtain K-factors....one method is in Design Guide 7 and the other (the one I prefer) is in AIST tech. report 13.
Both yield very similar results and are quite simple.

 

[RFreund]

Here is another good thread (very similiar to BA's) as I had a similar question

http://www.eng-tips.com/viewthread.cfm?qid=288365

EIT

 

[JMcKenna]

Mr. ToadJones, I have the DG7 reference but I am curious about the AIST Technical Report #13. I tried to get a look at the appropriate section but I can't seem to find a link. I do not have a problem purchasing the book, I just want to make sure it works for me. Do you have a copy of this chapter I can take a look at before buying the book?

 

[ToadJones]

If you have DG7, I would just use it.
The method yields results that are essentially identical.
AIST 13 is for the design of mill buildings.

I am not comfortable with copying the literature and posting it here.
Besides it has numerous pages of tables similar to APP B of DG7

 

[JMcKenna]

No Problem, Thanks for your help.