Wood Post Base Connection 1

[LearningAlways]

Hello.

I am helping a friend design a deck on his roof. He's using Arid Deck, a metal deck system that allows for drainage within its cavities. He lives in a log cabin and would like to use wood posts for his railing. Arid Deck sells a rail system but that is more money than he's willing to spend.

I'm trying to look into how to connect a 4"x4" wood post to the framing below the metal deck. I figured it would be simple enough, Simpson should make some kind of moment resisting connection that will sit on top of the deck and bolt through to the wood frame below. But no, all of their connections that are post installed have a disclaimer that say these connections do not prevent rotation at their base and should not be used unless top of post is supported/braced (i.e. this is not a moment connection).

But I am assuming that horizontal railings that connect to each base would not provide enough stiffness to the top of the post that it the bottom of the post would be considered pinned. It is probably more likely fixed at bottom with a spring at the top.

Any ideas about how to resolve this? There is a connection that in my mind would be enough to prevent base rotation (despite what Simpson has on their brochure). It is the Simpson CPTZ. It looks like what I've seen used on Glulam columns (whether these columns were considered fixed I do not know).

http://embed.widencdn.net/pdf/plus/ssttoolbox/rdzmii8bis/C-C-2019-p070-071.pdf

Feedback on this connection? I will say the Arid Deck post bases are rather thick which makes sense as they are moment connections and most other connections I've seen aren't as beefy.

 

[msquared48]

How about connecting the 4X4 to the side of the rim joist, rather than on top, assuming you have one? Should be able to accomplish this with lag screws.

Mike McCann, PE, SE (WA)

 

[hokie66]

The first step would be to ascertain whether or not the roof framing can resist the moment you want to apply to it.

 

[LearningAlways]

msquared48: No rim joists in this case.

hokie66: let’s assume the metal decking can take the loading as the company sold railing is made to sit on top of it and be bolted through it to blocks underneath. I’ll have to look at the specs again but off the top of my head I’d say that the deck is fine.

All I’m asking for is a post installed anchor attaching to wood that can only be installed from the top of the deck(as there is very little room underneath the deck). If a sketch would help I can provide that as well.

 

[hokie66]

Yes, a sketch would help. I am having trouble visualising how the aluminium decking relates to the roof framing.

 

[HouseBoy]

LA
I've spec'd this before and it seems like a good product (and I THINK it is what you are asking about).
Sketch/photos would help.

https://shop.titanbuildingproducts.com/structural-post-anchor-solutions.php

Steve

 

[gte447f]

In general, a moment connection at the bottom of a wood post is a difficult connection to design and/construct. A moment connection to concrete is easier than a moment connection to wood. Simpson actually does now have a moment connection post base to concrete (MPBZ). The reason such a connection is difficult to design for connection to wood base material is because of the widely varied (infinite) number of configurations of wood base material members that you might encounter. You might want to attach to wood beam, joist, rafter, blocking, etc. of various sizes and lengths and connections to other framing. All of these factors could limit the capacity of any post base that gets attached to them.

The connector that HouseBoy linked above looks ok, but they just say that it tests for 500 lbs in concrete. They have a wood attachment kit but no details on configuration or the capacity of the wood members it is being attached to. I personally would not spec it for attachment to wood. I might spec it for attachment to concrete if they had an ICC report or some kind of test report.

 

[Ron]

Thaler Products has a wide variety of manufactured supports that can be used for your application. Further, they are included with flashing to assist with waterproofing the roof penetration....

Thaler Products

Disclosure: I have no affiliation with Thaler. I have specified and used their products.

 

[LearningAlways]

Thank you all for the help! I work in the precast industry and wood design is something I am vaguely familiar with.

Attached is a sketch that was previously permitted by the City. This was done in 2008 and before construction could be completed (only the dormer was finished) the economy fell out and the deck project was put on hold. Now my friend needs these drawings updated and resubmitted. The new deck is now a lot smaller. About the same depth but is not only 13' wide instead of covering his entire roof as is shown in the sketches. Since I will be stamping these drawings (the first time I've ever used my stamp!) I've picked them apart to make sure it is safe.

I'd say 90% of this design is straightforward. The only part I am struggling with are the railing post connections; in particular those that are nearest to the dormer. My friend wants to attach the railings to the side of the rim joist (you were right msquared48, my terminology was incorrect) which works except at the posts nearest to the dormer/house; everywhere else there is enough height between the roof and deck to be able to attach blocking. One idea that he came up with is attaching handrailing directly to the house instead of using or attaching to a rail post.

Also, can someone explain the purpose of the 3x5 angle in detail 2/A2? It seems that wood blocking would be a better way to support the posts than the detail shown. Don't know if this is a common detail or not...

Again, thanks to those kind enough to give support and feedback.

 

[gte447f]

LearningAlways, I suggest you get yourself a copy of American Wood Council's DCA6 "Prescriptive Residential Wood Deck Construction Guide" if you don't already have it, and see if you can make your guard rail post attachments per the details in DCA6. I think it is a free download from AWC. There is a detail for installing 4x4 posts to a rim joist for when the rim joist is perpendicular to the floor joists and when the rim joist is parallel to the floor joists. In your case, your levelers should act just like floor joists in the DCA6 details. Unfortunately, the DCA6 details may not work at the shallow end of your deck where you only have 3.5" structure depth.