Bracing stiffness requirements. AISC 360 Appendix 6

[VirL]

Hello
I have a column, and i want to considering the purlin as bracing. I'm using AISC 360 Appendix 6, to check if the purlin is strong enough. From this Appendix I get a stiffness requirement.. and I'm not sure what i should use for comparison? Maybe the axial stiffness of the purling? EA/L?

 

[BAretired]

What you should do is determine whether or not the purlin has sufficient strength to resist a particular buckling force without excessive deformation. I don't know what AISC360 Appendix 6 requires as a force, probably about 2% of the column load.

I would be very surprised to learn that the purlin is not sufficient for the job.

BA

 

[VirL]

I have no problem with the strength, because, as you point out, it's small. The AISC design guide 7 points out that the stiffness controls the design when the the stiffness is not derived from axial stress only. As my purlin has also a wind load, I suppose the stiffness might control the design.

 

[BAretired]

I assumed your purlin was bracing the column by means of axial stiffness which means that the strength requirement will control according to the note you just posted within and just below the orange rectangle.

If it is bracing the column in some other way, you must calculate the purlin deflection under the specified load, then ensure that it will not continue to deflect further, resulting in potential column buckling.

BA

 

[VirL]

Thank you very much!

 

[BAretired]

You are very welcome.

BA

 

[JAE]

Isn't the "axial" bracing stiffness of a horizontal purlin dependent upon the "things" that the purlin is dependent on?

For example....if I was using a horizontal wall girt to brace a column from buckling in the plane of the wall (and girt) the girt stiffness is only as stiff as the things keeping the girt from moving along its axis....such as stiffness of attached wall panels, or the stiffness of the next column down the line, etc.

 

[BAretired]

JAE,

If the column is braced by a girt and the girt is relying on something else to prevent it from moving axially, then the bracing stiffness is not derived from axial stresses only. It is derived from the stiffness of the wall panels, cross bracing or other elements which restrain the girt from moving axially.

If two columns are joined at mid-height by a girt with no stiffening element such as cross bracing or panelling, then neither column is braced at mid-height.

BA

 

[JAE]

Well yes. That was my point

 

[human909]

Hang on a second...... Lets run back a bit. You really have given insufficient information here.

I have a column, and i want to considering the purlin as bracing.

What sort of bracing. What is the context... Without clarifying what sort of bracing some people might start jumping to conclusions. Furthermore what sort of purlin (or do you mean girt?)

There are multiple interpretations of what you have described. Some answers here could lead you in quite the wrong direction depending on the actual scenario.

 

[BAretired]

Perhaps it would be best if VirL could provide a sketch illustrating the exact situation.

BA