Convention for tension and compression

[charliealphabravo]

So I just discovered today that the guys in my office use an opposite sign convention for tensile and compressive member forces. Is it a Canadian thing or perhaps a mechanical thing to refer to tensile member forces as positive? I understand the convention is relative but I think this is the type of thing that can cause Mars landers to end up as smoking wrecks if you are not aware of it

 

[JStephen]

Where a sign is required, I normally take tension as positive, but hadn't thought about that being mechanical or Canadian in nature. Where a column is loaded in compression & bending, I'll normally calculate both compression and tension as positive quantities.

In most of my applications, tension is membrane tension in a shell or something similar, and as far as I know, the positive for tension is fairly universal there. In the past, I've used some structural analysis software, but was thinking it took tension as positive for structural members also.

A couple of days ago, I was using the beam equations in the AISC green book and then also some out of Roark's Formulas for Stress and Strain. One uses positive deflection as upward, the other downward. And Roark uses tension = positive for circular rings and whatnot.

 

[msquared48]

This also depends whether you are talking about a member or a global system.

Mike McCann
MMC Engineering

 

[TLHS]

Unless I'm doing something that actually requires direct manipulation of a lot of values I generally don't use signs for tension or compression. I write a T or C after them to avoid confusion. If I do something tabulated on a computer or somewhere else where I need to use signs, I'll summarize it somewhere on the sheet.

 

[IDS]

I'd say it was more a case of being a structural engineer thing to have tension as negative. I'm used to that convention, and often use it, but really it makes more sense to have tensile strains and stresses as positive, since they represent an increase in length.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BUGGAR]

Everybody in our office does it differently. It sure keeps us on our toes when we peer review calcs, though.

 

[Agent666]

I use whatever convention the software I am using uses in hand calculations, so at least the software output is consistent with my hand numbers.

Otherwise it's handy to add a T or C just to be clear as TLHS mentioned to rem9ve any ambiguity.