Wood Post - Holdown Shim Fastening

[WAstruc10]

Let's say we have a shearwall endpost holdown which utilizes SDS screws, but it's installed misaligned by one stud's width, so a short piece of 2x6 shim is needed to fill the gap. Do you think it would be acceptable to simply lengthen the SDS screws by 1.5" and assume that the shim is now acting as an integral part of the endpost? Or do I really need to add extra fasteners (in addition to those used in the holdown) between the shim 2x6 and the continuous endpost studs? And to take the theoretical question one step further, what about for a through-bolted holdown? The latter would seem to allow shim slippage, while the bolts are subjected to undesirable bending, but for some reason having the SDS screws through all the plies "feels" better, even though I think the theory should be the same. It seems pretty conservative to have 20 SDS screws in a holdown and then require 20 more, located above the holdown, just to make the shim integral...but maybe that's what it takes? Thanks for your input.

 

[MiketheEngineer]

Double posting is frowned upon. Have one removed.

 

[woodman88]

Simpson holdowns assume a solid wood member for the holdown design values. To use a multi-ply 2x post Simpson states "To tie multiple 2x members together, the Designer must determine the fasteners required to join the members without splitting the wood. See page 28 for SDS values." This is additional to the holdown connection requirements. So at the least, I would require a full height stud and attach it per Simpson's note.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[WAstruc10]

I realize Simpson leaves it up to us to design the attachment of the shim stud to the typ stud. In this case the shim stud can't be full height because it's below a window sill. My question is, based on principles of mechanics, why wouldn't simply using longer SDS screws do the job? It seems to be a question of shear and moment at each shear plane. So if the shear capacity works between steel plate and stud, and also works for stud-to-stud, the matter of shear seems to be adequately resisted if the screws are of adequate length. And the moment due to shim width, which is trying to pry the shim off the continuous post, is a pretty low value with one 2x shim, so maybe a few extra screws above the holdown would seem warranted if we don't want to take any withdrawal tension in the upper holdown screws themselves. To put it another way, if we take it to the extremes...let's say 1/8" plywood shim, I think we could all agree that this works fine without additional fasteners. But if it's a 12" thick shim with really long screws that engage the continuous post with same embedment as assumed in the Simpson tables, it seems that the moment issue is worsened while the shear issue still works fine (without additional fasteners). I have a feeling some may disagree so please weigh in...and again, I'm not really asking for the conservative stance (I understand that - add more screws, or tell them to relocate the holdown), but for a logical reasoning for why additional fasteners would be required instead of just longer fasteners (and if possible, why this small diameter fastener (screw) scenario feels so different from a bolted holdown scenario, where I certainly would be putting in more fasteners to keep the shim from slipping).

 

[woodman88]

Looking at just the SDS shear values. The shear (for DF/SP) for 14ga or greater side plates is 420#. The amount you can transfer to the shim. The shear for 2x side plates is 350#. This is the amount you can transfer from the shim to post. So you lose 70# per SDS at a minimum to the post, which with 20 of them is 1400# total lost of shear.
You will need to check with Simpson to see what they say, but the catalog states “Simpson Strong-Tie Company Inc. does not guarantee the performance or safety of products that are modified, improperly installed or not used in accordance with the design and load limits set forth in this catalog.”

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.