HSS column at base plate with no workable flat 2

[Eng456123]

Hypothetical/Academic question:

Say we have an HSS3x3x1/4 fillet welded all around to a base plate. HSS is subject only to flexure. How do you calculate the capacity of the weld when the HSS has no workable flat?

 

[TRAK.Structural]

Figuring out the exact capacity from flexural loading seems like a complicated geometry problem.

As a lower bound you could use Blodgett's welds as lines method with a geometry of a square that fits within the actual shape of a weld around the tube.

 

[Eng456123]

@TRAK Thanks. Agreed. That seems like a reasonable solution, thanks!

A coworker suggested sizing the weld to develop the capacity of the HSS? But I was struggling to see where he was going with it or how you would actually calculate it. We started in Table J2.5 where it says the base metal is governed by J4. But J4 gives you a capacity, which you would need to compare to another capacity (of the weld) which is back to square one...?

 

[DrZoidberWoop]

With small square & rectangle HSS members, you'll have to use engineering judgement or market knowledge to estimate a reasonable effective weld length/workable-flat (or google the shape and give a conservative guess).

There are a few quick and dirty approaches to welding HSS like this.

1) CJP and avert your gaze from mean looks from fabricator.

2) Size the fillet weld for the required flexure via a conservative flange force method (ignoring 2 walls of the HSS weld group)

Ff=Moment/hss_d and D_req = Ff/(weld_factor*estimated_effective_weld_length)
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D_max_sixteenths = phi_r*0.6*Fu_hss*design_wall_thickness_hss/weld_factor (variant of AISC Eq 9-2)
where: phi_r = 0.75 (LRFD) or 0.5 (ASD) and weld_factor = 1.392 (LRFD) or 0.928 (ASD)

3) You could also do an elastic method calc with the full weld group (Alex Tomanovich spreadsheet kicking around the internet).

4) Apply D_max_sixteenths and ask for EOR verification. If not acceptable, propose to use thicker hss for required loads.

 

[RPMG]

50/1.67 KSI * 1/4" = 0.928 k/in * D * (1.5 for 90 deg), D = 6, so 3/8" fillet weld all around would develop the strength of 1/4" thick tube steel.

 

[centondollar]

Sizing a weld for the force resultant, assuming it to be under pure shear, is conservative (see any textbook on steel design for the derivation), so just equate the HSS yield force (area*yield stress) to the strength of the weld in shear (weld strength * perimeter * throat thickness) and solve for throat thickness - this provides (in theory) a full strength connection.

 

[bones206]

RPMG - Not sure the directionality increase factor would be permitted in this case, as it is technically a rectangular HSS section.

 

[Deker]

Agree with bones. Also not permitted for welds loaded eccentrically; OP said the HSS is subject to flexure.

 

[SteelTubes]

You might find the Steel Tube Institute's Effective Weld Length Calculator helpful.

https://steeltubeinstitute.org/hss-effective-weld-calculator/