knowing when members in bracing are in tension or compression 3

[WendyStruct]

Is there a way of knowing, just by seeing the member on the brace or looking at the steel shape, if the member is in tension or compression? for example, my professor would look at a WT shape in a diagonal horizontal bracing and say: that member is in tension....HELP!

 

[MotorCity]

Every situation is different of course, but in general:

X- bracing: Each brace takes tension only

Single, diagonal brace: Takes tension or compression (depending on load direction)

Chevron brace: One brace in tension, the other brace in compression (depending on load direction)

Hope this helps.

 

[KootK]

1)Tension. You pretty much have to assume that all bracing members will take tension unless you know both the overall structure and the nature of the loading well enough to say otherwise.

2)Compression. With some sections like cables, rods, and small angles, you can pretty safely say that the designer's intent was for them not to resist appreciable compression based on the fact that their slenderness ratios would render them useless for resisting appreciable compression. There are exceptions to most rules, however, so it's best to have an understanding of your structure in which your bracing exists in addition to knowledge of the cross section.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cal91]

If you're wondering about trusses, under normal gravity loading the top chord is in compression and the bottom chord is in tension.

The web members will be in tension or compression. If you were to draw a "shear diagram" for the truss, then the rule is as follows:

When there is positive shear, members with their right side up are in compression while members with their left side up are in tension.

When there is negative shear, the situation is vice versa.

 

[Leftwow]

Depends on the direction of your load vectors. You can't just look at nodes and elements and say "oh, that's in tension". There is a reason the beam is in tension or compression. In order for your bracing to be loaded besides the dead load, there has to me some type of potential deflection in the overall structure.

 

[Lomarandil]

Just a clarification, Cal's rule about trusses assumes a simple span (no continuity or cantilevers)

 

[JStephen]

Imagine the deflected shape of the structure, members that get longer are in tension, members that get shorter are in compression.

 

[WendyStruct]

My question is more specifically about bracing. How to know when members used for bracing are in compression or tension. So for what I'm getting from the comments, it would depend of the load path? how it gets transferred to all members? And then, usually for X bracing both members are tension only?

 

[allgoodnamestaken]

In X bracing, one member is assumed to go into tension and provide resistance, and the other is assumed to fail in compression and provide no resistance.

Or at least, that's how I've always seen it done.

 

[sbisteel]

Checking slenderness is always a good way to see what's going on, values between 200-300 mean tension only bracing.

 

[BAretired]

If the direction of load is known, it is possible to determine whether a diagonal bracing member acts in tension or compression using the method of sections. In the case of X-bracing, one member will act in tension while the other will act in compression provided it is not too slender.

BA

 

[DaveAtkins]

On the other hand...you can design an X-brace to have one tension member and one compression member, equally loaded.

The only way to "instantly" tell if a member is designed for tension only, is to follow KootK's advice. A slender member (cable, rod, small single angle) won't be able to take compression.

DaveAtkins

 

[cal91]

Wendy, another thing to consider is that, in typical buildings, braces don't have ANY forces (of any significance) unless there is an earthquake happening or strong winds blowing.

When this situation occurs, the braces will undergo stress reversals, acting in tension when forced one way and acting in compression when forced the other way.

Of course, if KL/r > 200 then the compression force will be negligible.

It's quite impressive your teacher had the composure to note the axial force of a brace during an earthquake! Joking


Lomarandil, thanks for the clarification. I should've said in sections of positive moment the top is C and bottom is T, vice versa for negative moment

 

[WendyStruct]

Ok, I'm not as confused as I was before. Thank you for the help!

 

[msquared48]

So this string has relieved your tension?

Mike McCann, PE, SE (WA)

 

[Teguci]

The professor can say whatever he wants. He's the professor. However, if I were in front of my client looking at a building frame with a significantly proportioned WT for diagonal bracing, I would not identify that as a tension only brace without further substantiation.