What is the Column Effect in Trusses?

[Femme_physics]

What's the "column effect"?

Reading about trusses, it says

Often, compression members must be made thicker than tension members because of the buckling or column effect that occurs when a member is in compression

I'm not sure what's the "column effect". There's no wiki entry and Google seems to show me many results that don't provide the definition...

 

[Studiot]

Take a single sheet of paper.

Stand it on edge on the table (you can hold it) and pressdown gently on the top.

What happens?

The paper bows or bulges around about the middle. This is called buckling.
The more you press, the more it buckles. Also the taller and thinner the structural element the more easily this happens.

Obviously a stretched string, (which is a tension device) cannot do this. The more you stretch the straighter the string becomes.

It is clearly undesirable for a structural component carrying compression, such as a column or wall or whatever, to buckle under the compression load. Worse this can happen suddenly without warning.

The posh (correct) term for the effect is 'buckling instability'.
The term for how tall and thin the element is is 'slenderness ratio'

go well

I don't know what you book you are reading but this one makes really easy going.

http://www.abebooks.co.uk/search/sortby/3/an/A+J+Francis+/tn/+Introducing+Structures

 

[Femme_physics]

So buckling and column effect are the same thing?

I'm reading Hibbeler's Statics 12th ed.

 

[AlephZero]

I've never heard the term "column effect" used in my own field of mech engineering, though we are certainly are concerned about buckling.

Readiing a few results found by Google, I think the "column effect" means the fact that the transverse (sideways) stiffness of a column (a bar carrying a compressive load) is reduced as the compressive load increases.

"Buckling" occurs when the transverse stiffness reduces to zero, and the column can "flip out sideways" into a curved shape without any sideways load being applied to it.

Usually you want to avoid the load ever reaching the buckling condition, because the structure would then fail in some way. But the reduction of transverse stiffness occurs to some degree for any level of compressive load, so if you have a structural member with combination of compressive and transverse loads (a so-called "beam column") the stiffness reduction can not be ignored for loads that would not cause buckling.

 

[Femme_physics]

I see. I'll just take it to mean a pre-buckling condition or something. Thanks :)

 

[EngAB]

It might be column imperfection (Side effects), that during fabrication, pouring (if concrete), etc, there might be small deviation from what was required such as:
small eccentricity, little sloping, non straight column due to opening in form work, etc...

Look for column side effects and imperfection

Regards

 

[Studiot]

If the column loading is either eccentric to the axis or not parallel to the axis then additional considerations come into play. This is not, however elementary stuff.
Civil and building engineers use 'the middle third rule' to avoid this problem (google it if you like).

Secondly most building codes have some sort of strength reduction factors to apply when the slenderness ratio exceeds somewhere between 10 and 15. Again this is not elementary stuff.