How do you determine the weld size for a built-up column?

[smitty9898]

I'm trying to determine how to calculate the required weld size for a built-up column. This is not a typical built-up column (i.e. lattice, etc.). This built-up column consists of three plates to form an "I" section. There are welds between the web and the flanges.

Obviously, the welds sizes could be determined if it was a beams by analyzing the shear flow. However, I'm not sure how to determine the load on the welds as shown in the attached sketch. Seems pretty straigtforward, but I could not find any info in textbooks or the internet. Any help would be greatly appreciated. Thanks,

 

[smitty9898]

If you notice on the Aslani + Goel equation, the force on the intermediate connecter is independent of the applied load.

Therefore, I think it can be assumed that there is no load on the weld until the buckling load is reached? Not sure.

 

[ToadJones]

Size end continuous weld for MQ/I
Size intermittent weld for VQ/I

M=SxFy * allowable factor (say 0.66) with Sx being for the entire section.

To get V:

M=PL/4 (simple beam point load analogy)

P= 4M/L

V= P/2

Use Blodgett's percentage charts to size your intermittent weld based on the size of the continuous weld.

If your application is very light, this is probably a conservative approach.

I tried it for 20'-0" long column with 1/2" x 8" flanges and 1/2" x 7" web of A36 steel and got continuous 1/4" fillets at the ends for 12" and shear flow of 1.5 k/in along the length...pretty low numbers

 

[ToadJones]

You cannot load a column without any eccentricity. It is virtually impossible. Even simple shear connections will induce a small moment in the column.

 

[SAIL3]

BA--you bring up some interesting points, all good.
To my simple mind, the concept of stability is as follows:
Stability is not a load condition...it is more a capacity condition which is measured by Kl/r.
In using Kl/r of the entire X-section, it assumes that all elements of the X-section act together as one unit or has the capacity(reserve) to resist the particular mode of buckling under consideration.
If the potential buckling mode of the WF is in the strong direction, it implies potentail curvature in that direction and hence the development of this shear flow "V".
Wheather the col. will ever have to use this capacity is not the issue.
If, for some reason, one does not choose to use the full deveopment of the flanges in calculating this "V" as an upper bound, the question still remains...what value of this "V" would one use so you can sleep at nite?.

 

[ToadJones]

this column will have shear flow as it will be impossible to load without inducing bending.

 

[BAretired]

I agree with Toad that it is impossible to load the column without bending, but bending does not produce shear flow. Shear produces shear flow.

A column with hinged ends, loaded axially will develop a small bending moment if the member is curved, but it will not develop horizontal reactions at the hinges. The only shear it will have is P*[α] where P is the axial load and [α] is the angle in radians from a plumb line.

However, I was not aware of the Aslani Goel equation and will have to think about that a bit more.

BA

 

[ToadJones]

BA is correct.
However, if there is an eccentric load, there will be shear.

 

[ishvaaag]

This time and a rare one I have to partially diverge with BAretired. You might (theoretically) load the column without eccentricity, but what those dealing with buckling came to understand is that there must be "some" initial eccentricity causing what is then called "buckling". Giotto in the Quattrocento draw the column breaking that way, and in the 1700's Euler gave the basic numbers to it. Upon the bending of the column, it would be rare that a constant moment develop, in fact it is stated as P·e with e varying (and zero if and where hinges), and as soon as there is moment variation there is shear transfer between the web and flanges. This is just to remember the drawing with the stresses in the most stressed part of the buckling member of the early XXth century.

The determination is precisely by the numerical methods referred by BAretired. I purchased the book by Godden someone recommended me and made some Mathcad 2000 worksheets on it. I post a pair of them for constant section members, one gives the elastic critical load Pcr and the other an in-plane Bend Buckling Strength "Pn" that still lacks the influence of local and torsional buckling.

In them it is clear that an e can be determined and is key precisely to the evaluation of the critical or limit loads on buckling.

 

[csd72]

I think what the OP meant was that the question referred to the component from a purely concentric load as he understood the shear flow component.

I dont think they actually meant that there was no eccentricity.

I understand the reason for the qustion as it is not very intuitive but I think BA hit the nail on the head with his first post.