Checking for welds that connects two UB section

[learntopay]

Hi all,

How can I check the weld size (red hatch) for a twin UB section as shown below? Should I consider moment in my checking? The welds are along the member to join the two UB sections.

 

[centondollar]

Vertical shear is transferred by the webs. In theory, the bending and shear capacity is not affected by this weld, but in practice, there is no point in creating a box section without achieving a closed section's torsional characteristics, so I'd recommend a full penetration weld at the reds spots in your figure. Half-V or full-V, welding the topside and turning the section to weld the other side.

For a full-penetration weld, no separate strength check is needed for the weld.

 

[learntopay]

Hi centondollar,
thanks for the response. We initially proposed single side full penetration butt weld, however the contractor changed to partial butt weld. There are moments and shear force from the member output, should I check the welds for moment induced tension together with the shear?

 

[centondollar]

To make a box section (torsion constant, warping constant and torsional shear flow of a box), you need to close the section. Choosing a type of full pen weld that ensures a complete joint penetration full-strength joint is another matter.

 

[icebloom]

You can try look at mechanics books for shear flow formula. Imo its not really worth the effort in messing around trying to calculate this - could you just tell the contractor that PPBW is insufficient and to revert to FPBW? Not sure how things work where you are but if you're the engineer then contractor should ultimately defer to you unless you're proposing something that really isn't feasible.