How to calculate a "skip weld"? 2

[EngineerofSteel]

I have an existing 2.5" STD WT column, 14.8 ft high. It supports a roof which will be extended. The moment at the top of column is 1.94 kip-ft. I will weld a channel section to the existing pipe column. The section modulus of the pipe column satisfies the resistance to the moment. The channel is needed to bring (KL/r) under 200.

My question is: How do I calculate the skip weld needed to join the channel to the pipe?

fb in the member= 1410 psi

Or perhaps there is another method to satisfy KL/r<200?
Or another technique to join the channel to the pipe so as to act as one piece?

Hmmm... If joined only at top & bottom, can I simply sum the radius of gyration for both sections and use this as the denominatot in KL/r?

Thanks in advance, DD

 

[UcfSE]

Try shear flow: thread725-124984

 

[WillisV]

Jumping before to root of your question - if the pipe column works as a beam column with no additional section there is no reason that you have to increase its section just to get a KL/r of < 200. Note that the AISC is very clear in their specification wording (See B7 for Wide Flanges and HSS Spec Section 2.3 for HSS/pipes) that the Kl/r limit of 200 is PREFERRED (but it is not required).

Refer to the commentary of B7 which states "the advisory upper limits on slenderness...are based on professional judgement and practical considerations of economics, ease of handling, and care required to mimimize inadvertant damage during fabrication, gtransport, and erection."

In this case its already fabricated, transported, and erected - therefore I would strongly consider using professional judgement to overrule the PREFERRED KL/r limit as long as strength considerations were satisfied.

 

[UcfSE]

Before you override the AISC recommendation, remember that the equations provided by the AISC code are approximate and have some assumptions buit in, as you would expect. If you increase your slenderness ratio beyond their recommended limit, those assumptions will not be met. The amount of inaccuracy this causes will depend on just how far from the recommended limit you are. You will also be on your own for any liability and can assess how comfortable you are with that and go from there.

 

[EngineerofSteel]

Wow.

WillisV, you answered a question I have been asking for a long time... how are other firms putting up pole barns with 2.5" Std Wt pipes???? Now I know.

Please help with the B7 reference... I will need to convince some others who must okay my work (I have EIT only.) Do you mean AISC green book, 9th edition?

You are correct.. the column is already in place!

I have a follow up question: Some months ago (and I've forgotten the source) I reviewed an engineer's work, who showed in his calcs that an HSS column exceeded the KL/r minimum. THEN, he noted something like, "So, check the _____." And he then had another calculation which showed that KL/r wasn't a controlling factor, and his column was adequate.

Is anyone familiar with such a follow-up calculation? Or have an idea as to what was checked?

UcfSE,
There are many pole barns here in California's San Joaquin valley which have been in place 50+ years. I have heard that case law considers "generally accepted engineering practice." So, I think liability here is buffered. So, I am comfortable with reasonably exceeding 200. I will be more comfortable if I can show a "back-up" check, such as the one I saw -somewhere- a few months ago.

Thanks to both!

 

[EngineerofSteel]

More...

B7 in Specifications & Codes... aka 5-37:

the secondary check used to ensure the decision to exceed KL/r<200 for compression-controlled member sizing is Eqn (E2-2). This calculates an allowable stress. So, for compression members, this question is answered.

Still looking for the commentary which mentions: "considerations of economics, ease of handling, and care required to mimimize inadvertant damage during fabrication, transport, and erection."

 

[UcfSE]

That is pretty interesting, DD. I can come up with checking another aspect of column design, such as local instability, and saying that controls over global stability therefore don't worry about kL/r. That doesn't mean that is correct thinking of course, just throwing out ideas. If the column also has tension and the tension controls the design, then the compression kL/r doesn't apply. You would go to the recommended maximum kL/r for tension members - 300.

I'm glad you checked your reasoning before just jumping over 200. That was all my goal really was. Some people hear something, on the net for instance or by the water cooler, and take it for the absolute truth and then get in trouble. I hit some of the same problem you do at times. I have my EIT only so anytime I come up with rules or such I have to know where I found it or no one buys it.

 

[WillisV]

DairyDesigner,

The commentary and the section B7 I was referring to are in the 3rd Edition LRFD manual. Commentary reference sited is located on page 16.1-182.