DECK GUARD RAIL POST FRAMING - MOISTURE AND TEMPORARY LOAD 1

[PT99]

This is 4x4 deck railing posts using pressure treated lumber and calling for (unspecified brand) wood sealing products, with IBC loading.

Can you use Cm (moisture) factor as 1.0.
Table A says to use 0.85 for Fb, when moisture content exceeds 19% - so the question is, will the sealer keep the moisture content below 19%. Of course the owner will have to seal the lumber regularly and hopefully the lumber is below 19% when sealer is applied.

With Cm of 0.85 and Cd (Duration of 1.25), it doesn't pass typical 4 foot railing span with D-Fir#2 and Moment Arm = 3.5 ft (to account for 36 inches + distance to support bolts).

C incise = 0.8, C size = 1.5, Cd =1.25 Cm = 0.85
so Fb' = 900 x 1.275 = 1147.5 psi. With Cm =1 and Cd = 1.25 it does pass.

That brings up second question, can you consider a railing force as a temporary load and use 1.25.

 

[RPGs]

 

[Eng16080]

so the question is, will the sealer keep the moisture content below 19%. Of course the owner will have to seal the lumber regularly and hopefully the lumber is below 19% when sealer is applied.

Seems like a lot of ifs. I would use CM=0.8 to be safe.

That brings up second question, can you consider a railing force as a temporary load and use 1.25.

This is really the bigger question. Most engineers seem to use CD=1.6 for deck posts. Some use 1.0, and apparently others (like yourself) use 1.25. I don't know the correct answer on this, but it's been debated a lot here. If you don't use CD=1.6, I can say you'll have a difficult time getting the connection to work. FWIW, most of the deck holddown connectors on the market assume CD=1.6 in their published load capacities (although I'm not sure there's much justification for that). If you're confident that the railing would only be subjected to the design load for a total duration of 10 minutes over the course of it's life, then the table shown above allows you to use CD=1.6.

 

[XR250]

Is it really total duration and not individual durations? If so, 2 months of snow does not seem realistic.

 

[lexpatrie]

1.6 isn't allowed, it's a LIVE load. This is illogical (maybe), but oh well.

Also, if you are dealing with DFL (incised) to me that suggests you're out west, and out west, they sometimes like to use "green" lumber in construction. That will affect C[sub]M[/sub], as well. (the M is a capitol). Yes, I know that's being overly picky, but there it is.

I would also point out that C[sub]D[/sub] = 1.6 is "maxed out" in the argument, there's nowhere to go above that. That's fine in holdown calculations for tension elements and the like, but when it comes to a guardrail I'm not a fan anyway, even if it could be justified with a literal read of the code, which it can't.

On the 1.6 duration "from fastener manufacturers", I think that's for a "misuse" of the product, not what it's intended for, particularly if we are talking about a Simpson DTT which is for securing the deck laterally to the house, not for a guardrail connection which it is not, to my knowledge, tested for. You can use stuff "off label" but you have to make adjustments as needed if the duration of load is wrong, i.e. if I use hurricane ties for collar tie connections but the duration of load has to be adjusted you can't use the "stock" value without adjustment.

 

[phamENG]

XR250 - yep, total duration. It's 2 months of design snow load over the course of a building's life. It's all about time at maximum loading.

A guardrail may not see 10 years worth of 200lb point loads, but 10 seconds isn't very practical, either.

 

[jjl317]

Wow - learned something today - I had always thought the load durations were the length of time during incidence - not total cumulative.

Ignoring the statistical tweaks, it's hard to imagine during a structures' lifetime that you will hit the 50 year 3 second gust 200 times (10 minutes = 600 secs / 3 sec per gust)?

Crazy

 

[phamENG]

A hurricane or two.

 

[Eng16080]

On the 1.6 duration "from fastener manufacturers", I think that's for a "misuse" of the product, not what it's intended for, particularly if we are talking about a Simpson DTT which is for securing the deck laterally to the house, not for a guardrail connection which it is not

The Simpson DTT is intended to be used to secure a deck railing post. Below is p. 315 from the 2024 catalog. Under "Allowable Tension Loads" notice below it the load duration factor "(160)". I don't necessarily agree with using this load duration factor either, but this is what they use. If you use 1.0 or 1.25 instead, now you're stuck with reducing the load that the connector can take (not that most engineers would even be aware of that, I imagine).

 

[XR250]

It's 2 months of design snow load over the course of a building's life. It's all about time at maximum loading.

I can see this particular load case being un-conservative.

 

[phamENG]

I can see this particular load case being un-conservative.

Perhaps that's why (or, at least, partly why) ASCE has overhauled the snow load maps to bring them in line with reliability design?

I'm not in snow country, so it doesn't bother me. It really hasn't snowed here in about 3 years.

 

[lexpatrie]

If I recall correctly, the FAQ article I wrote refers to testing that said the 4x4 doesn't really work, in terms of allowable stresses.

https://www.eng-tips.com/faqs.cfm?fid=2862

 

[phamENG]

Hate to say it, lex, but I'm 99% sure nobody reads FAQs on this site.

 

[Eng16080]

Based on my own calculations, a 4x4 post is usually adequate with CD=1.6, but is marginal or insufficient with CD=1.0.

 

[dik]

"nobody reads"... most people don't read...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[lexpatrie]

Yeah, Pham, I hear you on that. That's fine, I like making them. I don't do it for ego points or anything.

I feel like I learn something, and if somebody ten years from now finds it, it's all good, or it's the scribbling of a madman.

I've seen one get referenced, but I've been writing my own personal items, not adapting to the actual frequently asked questions on the site. (Unless you want to count C[sub]b[/sub], but the FAQ entries can't have any images in them and that really hampers things, a normal forum post will a) attract commentary, b) eventually "freeze" so you can't update it (I know, make a new one, but still, I don't really like making an FAQ that's just a revise-enabled link to a forum post.)

And yes, there was a sale on an lifetime unlimited parenthesis marks a few weeks ago, and I bought it. Groupon is the best!

Eng16080 - If I recall correctly the original DTT was intended to secure the deck through the exterior wall, (that requirement appeared circa 2009?) and to me that makes sense as C[sub]D[/sub] = 1.6 given the origin of those loads (wind/seismic, as there wasn't research on live lateral loads (from occupants) until later on). I think the DTT2 they expanded the use, (maybe?, partly in response to the Woeste research pointing out the typical "nail and forget" approach wasn't strong enough) and the ESR covers that use. You can factor it down on the presumption that the listed (tested) load is not connection limited and comes directly from testing (said adjustment would be conservative if it is "fastener limited", as I recall, I usually check in with Simpson Technical support to confirm that approach is valid and haven't exactly specified one on a deck - I usually note the code requirement and say "contractor provide connection per IRC XXX.X" at ends of deck).

Lateral Loads Generated by Occupants on Exterior Decks, Parsons, Bender, Dolan, Woeste, Structure Magazine, January 2014.
(I'll NOT reference the FAQ from here...that's a direct link to the 2014 article, which is kind of reprint from a Wood Design Focus article as I recall).

ETA - Well, that timeline of events isn't perhaps supported "in the record" as the guardrail article was 2005.

Strong Rail-Post Connections for Wooden Decks, by Joseph Loferski and Frank Woeste, P.E., with Dustin Albright and Ricky Caudill, Journal of Light Construction, Feb 2005.

I don't have a date for when the 1,500 lb connector appeared in the IRC. Maybe someone else feels like tracking that down, I think it wasl also revised downward on the load requirement at some point.

For those of you receiving eighteen notifications that I posted a reply (because I'm editing and expanding again and again, I somewhat apologize).

 

[Eng16080]

If I recall correctly the original DTT was intended to secure the deck through the exterior wall, (that requirement appeared circa 2009?) and to me that makes sense as CD = 1.6 given the origin of those loads (wind/seismic, as there wasn't research on live lateral loads (from occupants) until later on). I think the DTT2 they expanded the use...

Yup, I think this checks out in terms of using the same connector for a new purpose.

From "Prescriptive Residential Wood Deck Construction Guide - Commentary":

 

[lexpatrie]

Yeah except in my mind I think the DTT was after that article came out. Maybe I'm wrong.

I can track it down if people really care in my stack of old Simpson catalogs.

My recollection insists that the DTT2Z (Z for z max i.e. galvanizing) was purpose built, if you would, for the 36" guardrail height and the deck lateral tie requirement. I don't have documentation to back that up (I'm outside listening to my dog bark at ... probably nothing).

 

[Eng16080]

I'm not sure the history of the DTT, tbh. The question still remains, though, whether CD=1.6 makes sense for designing the railing post connection.

 

[lexpatrie]

Based on a literal read of the code section, no, because it's a live load with duration = 1.0.

On the whole "standard of care" matrix (where one is expected to use similar skill to other "peers" in an undefined "area" there's some room for argument that 1.6 "works" as there are some prescriptive (non engineered) guides like the DCA that "use" it for non-engineered designs.)

Note that a non-engineered "guide" that isn't code or code referenced is not exactly another local engineer or SEA putting it forward.

I wonder if anybody has tried to get a 1.6 duration into the code and it's been refused. That would really hinder this "standard of care" argument.