AS4100 - Weld design for fabricated welded column
AS4100 - Weld design for fabricated welded column
(OP)
Hi everyone,
If we need to design a welded column, subject to axial load, how do we determine the design loads for the welds connecting the flanges to the web?
I know for a beam that is subject to shear and bending moment, the weld is designed using the shear flow, f=VQ/I, and it's easy enough to find the design load for the weld as I usually have a clearly defined shear V*, or in alternative, we can just design the weld based on the web's full shear capacity. However, for a column that is not subject to significant shears, doing so would be overly conservative.
So my question is, how would we approach this when we do not have a clear shear force to design to? One idea I have is to design the weld based on 2.5% of each flange plate section capacity in compression, making sure I develop enough force to prevent its out-of-plane buckling - for example, if my flange plate has a section capacity of 1000 kN and a maximum unrestrained height of 300mm before the plate "member" capacity starts being influenced by buckling and becomes lower than the section capacity, I design a weld that can take 250 kN every 300mm of length, for example, 2 x 6CFW SP 125/175 HIT/MISS.
I would do this for both flanges and also for the web (although the web will rarely govern the design as it will have double welds and it has usually a smaller section area than the flanges)
Any thoughts on this? Is there any code provision taht can help with this (could not find any)?
Thanks
If we need to design a welded column, subject to axial load, how do we determine the design loads for the welds connecting the flanges to the web?
I know for a beam that is subject to shear and bending moment, the weld is designed using the shear flow, f=VQ/I, and it's easy enough to find the design load for the weld as I usually have a clearly defined shear V*, or in alternative, we can just design the weld based on the web's full shear capacity. However, for a column that is not subject to significant shears, doing so would be overly conservative.
So my question is, how would we approach this when we do not have a clear shear force to design to? One idea I have is to design the weld based on 2.5% of each flange plate section capacity in compression, making sure I develop enough force to prevent its out-of-plane buckling - for example, if my flange plate has a section capacity of 1000 kN and a maximum unrestrained height of 300mm before the plate "member" capacity starts being influenced by buckling and becomes lower than the section capacity, I design a weld that can take 250 kN every 300mm of length, for example, 2 x 6CFW SP 125/175 HIT/MISS.
I would do this for both flanges and also for the web (although the web will rarely govern the design as it will have double welds and it has usually a smaller section area than the flanges)
Any thoughts on this? Is there any code provision taht can help with this (could not find any)?
Thanks
Talk To Other Members
RE: AS4100 - Weld design for fabricated welded column
You could check out AS 5100.6 Clause 10.4.1, which is intended for laced/battened compression members, but specifies a minimum shear force for the column to be designed to. I would think it would be applicable to your case also.
RE: AS4100 - Weld design for fabricated welded column
Also I presume that your 250kN value was a typo as that is 25% not 2.5%.
Really this is more about ensuring there is a robust and more than sufficient capacity for suitable shear flow. Gusmurr's suggestion sounds like a good start.
RE: AS4100 - Weld design for fabricated welded column
Thanks for the tip on AS5100, will chack that out
human909
Yeah I agree the code approach to restriant is not the best and buckiling in reality is based on stiffness, nonetheless the 2.5% approach is generally used with buckling restraint of members based on the code, and I was trying to find some code-based way to design these welds. Will check the AS5100 clause suggested by gusmurr
And yes, that was definitly a typo - I meant to write 10000 kN capacity, not 1000.
RE: AS4100 - Weld design for fabricated welded column
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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: AS4100 - Weld design for fabricated welded column
The required restraint force becomes smaller as stiffness increases, approaching a minimum requirement asymptotically.
Added: don't max out your b/t ratio because there must be some reduction in local buckling capacity at the flange tip due to non-continuous restraint at the web. Surely not theoretically correct, but using diagonal distance for 'b' might be an adequate shortcut.
RE: AS4100 - Weld design for fabricated welded column
-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: AS4100 - Weld design for fabricated welded column
I'd hazard a guess that intermittent welds are less likely to be used for welded column sections because engineers are fundamentally cautious and conservative and not always the ones bearing the cost of designing welded columns.
Labour in Australia is pretty damn expensive so considering intermittent welds might seem suitable.
RE: AS4100 - Weld design for fabricated welded column
-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: AS4100 - Weld design for fabricated welded column
A smaller continuous weld should be cheaper than an equivalent (larger) intermittent weld, so intermittent would only be considered if you're at the minimum permitted weld size.
RE: AS4100 - Weld design for fabricated welded column
With a different design environment, different labour costs, different fabrication setups and different codes the economics are not directly comparable.
As far as I'm concerned I avoid welded columns and beams due to their very high cost. I've certainly seen other engineers use them naively without regard for their cost.