SDS screw shear capacity

[Vrpps EIT]

Hi All,

When (3)-2x6 are sandwiched together with SDS screw connection and subjected to an axial load, for determining the shear capacity is the total load to be divided by the total number of screws in the connection or only row-wise (2-screws per row). So each row has to resist that shear load or they are combined together? Attached a pic.

Thank you

 

[Vrpps EIT]

 

[jayrod12]

How is the load being delivered? Potentially speaking, if nicely delivered via an adequate bearing system, then the fasteners only need to prevent the members from buckling individually which is significantly less demand than 1/3 (or 2/3) of the total axial load. I feel more like the 2-5% of the axial load would be appropriate in that case.

If the load were being delivered into only one of the plies, then at a minimum some of the fasteners would see 1/3 of the total shear load, and some may see 2/3, but that would only be for the first little bit until the load were appropriately delivered to the other plies.

 

[Vrpps EIT]

The load is transferred via double top plates at the top of these piles. so will be distributed into the three pieces in the pile.

ok, so whatever the Axial Load (F) is, is it to be divided by the total number of screws (6) or the number of screws in a row (2)to determine how much is each screw going to take up the load?

Thank you

 

[phamENG]

Piles, or columns? If you need piles, I would recommend getting actual piles to be driven as it'll be hard to get dimension lumber treated to the level that piles are. They'll deteriorate fast. I'm guessing you mean columns, though, since you talk about a double top plate.

So, assuming you have good load transfer through bearing (load applied concentrically and the top plate is NOT in bending), then you're going about it wrong. As jayrod mentioned - preventing individual buckling and forcing it to act as a unified compression member is what you need. It's not a straight shear calculation. Not sure what code you're designing to, but NDS 2015 has the equations you need in section 15.3.

 

[Vrpps EIT]

I am sorry I should have been specific, it is actually a splice connection where the middle 2x6 is overlapped by the 2 exterior 2x6, hence the exterior (2) 2x6 doesn’t run all the way down from top to bottom plate it is just in the middle section where the interior 2x6 needs to be spliced.
Not a built-up post, it is just a single 2x6 (middle) stud which needs to be replaced at the damaged section and then spliced by two exterior 2x6 with the screws.
Apologize for wrong portrayal

 

[jayrod12]

what phamENG said in terms of NDS section, but since you're in Canada, see the built-up column nailing requirements, Fig 3.2 in the WDM 2010 edition. Where I am we're behind the times accepting the 2015 code.

Note that this again is for equally loaded plies. If you had a beam framed into the side of one ply, then all bets are off.

Ugh, splice is entirely different, because now if it starts to buckle laterally, you've created a hinge. Honestly for the cost of 10 bucks, replace the stud full height. Do it on both sides if you really need to.

 

[phamENG]

How much of the original is damaged? It actually doesn't change the situation much. As long as you've stopped whatever was causing damage to the original stud and what's left is still intact, it becomes spacers in your built up column. Just make sure that the new studs are full height and tight to the sole plate and top plate.

 

[dhengr]

Vrapps EIT:
It looks to me like your sketch shows three rows of screws, with the rows spaced vertically at 8” o/c, and with 2 screws in each row. The important thing is that the 3 – 2x6’s be cut to length correctly/accurately, so that the load is delivered through the double top pl. to all three of the 2x6’s at once, a good and proper bearing detail. Then, as Jayrod12 suggests, all three 2x6’s carry the load equally and the screws just hold the three 2x6’s together as a single column unit. It would be wise to drive every other row of screws from the opposite face, since the holding power is greater under the head than at the screw tip. If one of the 2x6’s is cut long, then the top pls. impart the load to that stud, and shear in the screws needs to distribute the col. load to the other two 2x6 members. In any case, compression perpendicular to the grain on the double top pl. might be the controlling design consideration at this bearing detail. Screws are fairly good at distributing this kind of load pretty uniformly, whereas bolts need more joint movement to bring the bolts into solid bearing in holes which may not be perfectly aligned. Thus, only one or two bolts might start transferring the load, at first, until the others are brought into bearing in their holes. This is not so much the case with screws.

 

[Vrpps EIT]

It is an exterior stud wall assembly the middle stud is 10' heigh and the damage (due to moisture deteriorated) is 6" to 8" heigh in the middle but not to the full width (5.5") of the stud. So the concept was to cut out the damaged section and then replace it with new 2x to whatever width it is damaged and laminate this new section with 2x6 studs on the exteriors both the side to the new section and screw or bolt them if needed. It is drywalled in the inside and plywood sheathed on the exterior. Yeah replacing as a whole is easier but just for learning am curious about how to perform calc. to check for the connection!!

 

[Vrpps EIT]

The overlap length of the splice studs will be 4'(overall)

 

[phamENG]

Oh. Never mind. Don't do that. You need to go full height. Trying to pass the load in and out of the original stud through some little scabbed on sticks of lumber is a fool's errand.