×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*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.

Buckling un-braced length of uniformly loaded axial member

Buckling un-braced length of uniformly loaded axial member

(OP)
Hi all,

I did a search and didn't find any discussion so I'm starting this new thread. Hope I'm not creating a duplicate topic. For buckling un-braced length of a column with point load at the top, it is clear that the un-braced length equals the length of the column (assuming pin-pin top and bottom). My question is what is the un-braced length when the column is loaded uniformly throughout it's length and not a point load at the top? The axially force & stress diagram would increase linearly. Intuitively, I sense the two loading conditions would affect the buckling behavior of a column differently but I'm not sure how to take the uniform axial load into consideration in an analysis. I understand a conservative assumption to treat it as a point load at the top can be made.

Thanks

D2

RE: Buckling un-braced length of uniformly loaded axial member

If you have a copy of "Theory of Elastic Stability" by Timoshenko and Gere, this topic is covered nicely in Chapter 2. If you don't have a copy, I highly recommend getting one.

RE: Buckling un-braced length of uniformly loaded axial member

I second CANPRO's reply. Also, how, exactly, are you getting an axial load applied like that? Just curious...

-5^2 = -25

http://www.eng-tips.com/supportus.cfm

RE: Buckling un-braced length of uniformly loaded axial member

Quote (swearingen)

I've checked beams like this before when they're supporting grating panels that wouldn't act as a diaphragm...seismic weight of the grating goes to each joist, each joist applies a seismic axial point load to its girder, check the girder as a beam-column.

RE: Buckling un-braced length of uniformly loaded axial member

see post dated 15Nov 2010..COLUMN EFFECTIVE LENGTH....

RE: Buckling un-braced length of uniformly loaded axial member

Applied loads do not, in themselves, brace things.

However, if the applied load is coming into the column via some structural element that has strength and stiffness capable of bracing the column either nodally or relatively to some other stable structural fixity, then it may indeed brace your column.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Buckling un-braced length of uniformly loaded axial member

JAE, completely agree with your statement but I think the OP was asking a different question. I think what dougseason was getting at is this:

• Typical buckling load of a column is determined assuming that a point load is applied at the top of the column, and axial stress is uniform through the length of the column (self-weight neglected)
• The end conditions effect the k value, if pin-pin k=1.0
• If the same column is loaded uniformly along its length, the load increases as you approach the base of the column - in this scenario the column can support more load before it buckles
• If you take the total load supported by the column in the 2nd scenario and then back-cacluate for the k value using traditional methods (assuming all the load is applied at the top of the column) - you'll get a k value < 1.0
• The OP was wondering how to calculate the effective k value if the column was uniformly loaded vs point loaded at the top
• Terminology is important here - the unbraced length of the column isn't changing, the loading conditions are changing. If you want to relate this change in capacity to traditional methods and assumptions, you end up with a k value that changes with support conditions and loading conditions. Since we're used to calling the product of k*L the effective length, its easy to mix up the terminology

RE: Buckling un-braced length of uniformly loaded axial member

This question is a good one and a bad one at the same time.

It's a good question because it points out how silly the concept of effective length really is. This is a good example why AISC has moved more towards the Direct Analysis Method, which does a better job of capturing the geometric non-linearity of the structure. Now, for use in the Direct Analysis method, I would say that the effective length is really the same for the loaded at the top and the uniformly loaded method.

RE: Buckling un-braced length of uniformly loaded axial member

Ref Teknisk Ståbi p.130, an pin-pin ended axially loaded column with 'alpha x N' axial force at top and 'N' at bottom (varying linearly, alpha < 1) the critical buckling length is 'l x sqrt((1 + 0.88 x alpha) / 1.88)', where 'l' is the length of the column.

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.

Resources

eBook â€“ How to Choose the Correct Corrosion Testing Method
When designing a metal component, engineers have to consider how susceptible certain alloys are to corrosion in the final productâ€™s operating environment. In a recent study by NACE (National Association of Corrosion Engineers), it was estimated that the direct and indirect costs of corrosion in the United States is approximately 6.2% of the GDP. In 2016, that cost exceeded \$1 trillion dollars for the first time. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!