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.

 

[phamENG]

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

I don't entirely agree. (I'll be referencing the 2015 NDS here - it's what's on my shelf within reach.)

The body of the code directs you to use Table 2.3.2 for C_D OR Figure B1. The title of table 2.3.2 is "Frequently Used Load Duration Factors, C_D". The title presents some level of ambiguity. Not a lot, but some.

It also gives you the option to use Figure B1 in Appendix B. This figure does not indicate what a live load would be. It's up to the engineer to determine the expected cumulative time the member/connection/assembly will spend at that peak load.

So the standard of care, I would think, would be placed on how you determine that time period. What level of rigor is required to break from the "frequently" (but not universally) used time period? My guess is a whole lot more than any of us would actually spend and certainly a lot more than anyone paying one of to design a deck would be willing to pay for.

 

[lexpatrie]

In a liability lawsuit this is not a particularly great location to start the argument slash defense. And the failure of a guard will probably involve severe injuries and or death. Similar to deck and balcony collapses, so expect to stare down that prong of legal strategy from the plaintiffs expert. Duty and Standard of Care, generally accepted principles of mechanics and the most conservative (i.e. safe) reading of the code.

In a non engineering sense, there isn't a lot of reward for a particularly high, or undefined risk. Which kind of is an engineering sense.

If there's been an attempt to formalize C_D = 1.6 and it's been refused by the code amendment process, I think that would be a definitive death for this sort of argument.

As I haven't been on one of these since say, 2010, I haven't been tracking it.

"Guardrail, by other."

If I really wanted to go down this road with a higher duration of load, I'd be looking for an actual letter from the relevant building official in the jurisdiction to allow it very clearly and upfront, and I'd probably (a la Florida) put it on the drawings specifically as a deviation from code that was "accepted as safe by the building official." Then it's clear exactly what is going on.

The NDS isn't all that fruitful here, if you ask me. It's the definition as a live load in the building code, (and ASCE 7, as I recall). And it's probably been a live load since it first appeared, too.

Also, as I'm back inside by my little trove of Simpson catalogs, the DTT2z first appears around the 2009 catalog, and it's absent from the 2007. So I think the DTT1 is the newer product (for the deck attachment to the house). I don't have a 2008 Simpson Catalog that I know of.

 

[lexpatrie]

FAQ for the deck lateral load connector to the structure, looks like it started at 1,500 lbs in 2009 IRC, and remains there, unless you want to provide four, then it (more recently) dropped to 4 x 750 Lb connections (ASD).

https://www.eng-tips.com/faqs.cfm?fid=2912