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

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

(OP)

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,

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,

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

If,a ssditionally there are other meaningful stresses standing (say, pass a normal load) a interaction stress etc need be accounted to meet the interaction, as in any weld.

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

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

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

For buckling overall, the deformed shape of a buckled column creates a moment curve along the length. The critical buckling load P times e. I believe this creates a shear along the length which, using VQ/I allows you to develop a weld pattern - like PAStruturalPE suggests - full length at the ends and stitch welds along the main length.

I'm away from my books right now but I believe you should NOT treat this as a pure axial loaded column (i.e. the welds only in longitudinal compression). They will see shear if the column buckles and you must ensure that the parts work together in order to use the r value for the whole shape.

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

Thanks to all who posted. The responses are of great help. Here is my response.

I don't believe it is possible to determine numerical values for the curvature of a buckling column. I can determine the theoretical buckling shapes for a buckling column (i.e. "k" values), but an actual maximum deflection of a buckling column cannot be determined. The maximum deflection is required to determine the moment on the column (i.e. the M=Pe equation).

I understand that there are eccentricities due to lateral loads, initial curvature, connection eccentricities, etc. I can determine the shear loads on the welds from these eccentricities. However, I'm trying to determine the shear load on the welds from buckling only.

I also understand that there can be localized buckling of the plates, if the weld spacing is too large. This can be solved.

At first I thought this would be simple, but I'm finding out this problem may not be solvable. See reference to Eurocode below. It says nothing about accounting for shear from buckling.

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

by default, covered VQ/I hear flow for any load that won't overwhelm the total section as proportioned. Once that is met, I believe as long as you meet AISC's requirements for welding of built-up compression members that you're covered (kl/r of any component < 0.75*kl/r of gross section).

As far as the buckling consideration, if buckling is a failure, is there a reason to design a weld to resist those forces if the gross section is unable to? Additionally, and maybe I'm remembering my mechanics incorrectly, but I don't believe that buckling of a column is associated with a finite lateral displacement. The column buckles because the lateral displacement is infinite (theoretically) because the section is unable to resist the moment (Pe) once an initial displacement occurs).

Am I mistaken with any of that?

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

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

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

What for a more exact determination of the required size of the welds even on rational simplification asks (if more precision is required) for a definition of how the load at the ends is being applied, and a rational evaluation of the change of the stresses whilst growing to the critical state.

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

I still don't know how a moment can be computed from a buckling load?

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

I think your statement "I don't believe that buckling of a column is associated with a finite lateral displacement." is correct. Once the overall column begins to buckle, its going to fail. The eccentricity is irrelevant at that point, which is why I don't believe there is a calculable moment from the buckling load. Not sure?

I'm thinking the necessity for welds along the length is for the following: prevent local buckling of each individual plate (so that the buckling load for the overall section can be reached), resist moments/shears from lateral loadings, and resist moments from physical eccentricities (i.e. load eccentricity, imperfections in plumbness, etc.) I might be way off, I'm still looking into it.

I'm trying to analyze an existing column that was built prior to AISC requirements.

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

EIT

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

EIT

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

Critical loads can be determined by numerical methods or, in simple cases, by closed form theoretical solutions as found in numerous text books.

I don't believe there is a shear requirement in the weld, but I agree with those who say that the spacing of the welds has to be such that individual plates have a kL/r less than the main member to prevent individual plate buckling.

For intermediate columns, the situation is a bit different because inelastic buckling is involved and short columns don't buckle.

BA

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

This still leaves the question of the overall stability of the column as awhole.

To me, the concept of stability infers the ability to recover from or absorb a destabilizing event...lets say in this case an initial lateral deflection "e".

Addressing all of the local buckling concerns does not guarantee this.

To this end, the min shear requirement from VQ/I mentioned by JAE

still leaves the question of what value of "V" to use in designing the welds.

Here is how I would try to approach it, right or wrong.

To get a handle on the value of "V", I would look at the col. as a bm and assume a unifrm load on this beam that would represent, say, 25% of the capacity of the bm(without any axial load).

From this, I would get a value of "V" and then design the welds for the shear from VQ/I plus the axial load in the col.

To check if this assumption of 25% is reasonable, I would get the corresponding deflection of the above bm under this uniform load and evaluate it. If it turns to be equal to:

l/500....not good, try a loading of 40%

l/250...maybe

l/125...ok

What I am looking for here is a certain level of robustness or margin of safety.

Ofcourse,in the real world,I would do as PAStructural suggested and move on.

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

I thought there was a way to determine a force difference between the flange and the web - assuming you engage in Af x Fy of the flange and ensure that the welds can transfer that force to the main body of the column.

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

If the load is applied to the web and not to the flanges, then welding is necessary in the end regions to transfer the load from the web to the flanges. Those welds will carry shear, but there is no shear flow because V = 0 for the full length of the column.

Why does it not guarantee this? If the load is below critical load, then I believe it does guarantee this because every fiber of the built up member has the same strain energy as the equivalent fiber of a solid beam section. The bult up member cannot buckle.

BA

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

ht

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

Modern Steel Construction Q & A link

They refer you to an AISC Engineering Journal article from 1992:

"Analytical Criteria for Stitch Strength of Built-Up Compression Members" (Engineering Journal, 3rd quarter, 1992)

This can be downloaded from AISC's website

AISC Engr Journal 3rd Q 1992 link

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

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

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

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

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

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

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?.

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

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

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

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

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

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

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.

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

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.