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Deck posts?

MikeC

Silver Member
Joined
May 17, 2012
Messages
230
Location
NW Pennsylvania
Sizing of deck posts.

Plan is a 6 ft attached deck - about 12 ft above grade. Everything in the plans looks good except for 4x4 posts holding the deck up. While I don't like it, I can't find anyting in the IRC about posts and this is backed up by R301.1.2. Do I require an engineer to figure this out or is there somewhere that I can get a common answer for my problem?
 
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By attached, you mean attached to the existing residence?

If you are the AHJ inspector, would you be comfortable

approving / "signing off" on this project, when you do not

know about the loads on those 4 x 4's or other areas

of this 12 ft. above grade deck?.......Personally, I would

not be so comfortable......There are a lot of components

to consider besides the 4x4 posts themselves: means

of attachment, ...type & size of fasteners, ...bracing,

...guardrailing, ...[ possibly ] different types & sizes

of brackets or other structurally loaded components,

etc.

I vote for an RDP to design this deck!

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Last edited by a moderator:
I would have no problem signing off on 6x6 posts, but I don't like the current design. The problem is that I have nothing to base my dislikes or likes on. I was hoping there was something in writing that I was missing. The good news is that a DP would also address the lateral movement problem with the deck, preventing me from being the bad guy.
 
Some times we have to be the bad guy

I suggest you find $495.00 and invest in http://www.strucalc.com/ it is well worth it. Use it to verify your suspicions to require more info or engineering.

DO NOT USE IT TO PROVIDE A SOLUTION TO THE PROBLEM.

jmho
 
"the building codes . . . specify only that a post must be at least a 4x4. This isn’t meant to imply that 4x4 posts are good for all conditions, merely that there’s no condition where less than a 4x4 is acceptable.

Published by the American Forest and Paper Association (AF&PA), the DCA6 (Design for Code Acceptance #6) Prescriptive Residential Wood Deck Construction Guide is an alternative to the IRC that local authorities may allow, but they may require it to be used in its entirety (the document is available free here.).

Another document that provides a flexible post-sizing table is Wood Decks: Materials, Construction, and Finishing (Kent A. McDonald et al., The Forest Products Society, 1996). Tables 6 and 7 in this book show a range of tributary areas and post sizes; they were developed by the AF&PA and should still be valid for most designs. The one caveat is that the AF&PA has reduced its recommendations for the maximum post height. While the tables allow posts to reach a height of 17 feet in some instances, the AF&PA now recommends going no higher than 14 feet."

http://www.deckmagazine.com/article/250.html

Francis
 
mtlogcabin said:
Some times we have to be the bad guyI suggest you find $495.00 and invest in http://www.strucalc.com/ it is well worth it. Use it to verify your suspicions to require more info or engineering.

DO NOT USE IT TO PROVIDE A SOLUTION TO THE PROBLEM.

jmho
$495! I have already managed to come up with $500 that wasn't budgeted. If I do it again the city clerk will probably have a stroke.
 
I'll bet there is a connector/contraption to achieve the required bearing using a 4X4, but 6X6 post do a better job IMHO:

R502.6 Bearing. The ends of each joist, beam or girder shall have not less than 1.5 inches (38 mm) of bearing on wood or metal and not less than 3 inches (76 mm) on masonry or concrete except where supported on a 1-inch-by-4-inch (25.4 mm by 102 mm) ribbon strip and nailed to the adjacent stud or by the use of approved joist hangers.
 
The code references the NDS or accepted engineering.

You can work it out longhand, you can have a computer ask for the variables and work out the same formula, or you can use a table which has used the same formula and a fixed set of variables. It's all using accepted engineering. Whether you use this to check or design is another issue.

This is sort of a tutorial column calc I did awhile ago, suggestions for improvements are welcome;

http://www.timbertoolbox.com/Calcs/Simple_column.htm
 
Not enough information given to make an informed comment (post spacing, bracing, etc.).

Here is a free design suite that will allow you to check to see if the design passes. http://www2.wwpa.org/TECHGUIDE/DesignSoftware/tabid/859/Default.aspx. Download the full (free) version and use the post/stud spreadsheet.

Also, R301.1.3 (IRC 20009) does not require a RDP. What it says is ..." these elements shall be designed in accordance with accepted engineering practice." (DRP beat me to it)
 
If the concern is about capacity, 4x4 PT SP No. 2 are going to be more than adequate for vertical loads in common residential applications - more than 10,000 lbs. Slenderness ratio at twelve feet is another issue ... but one ignored by the IRC.
 
I would not be as concerned about the 4" x 4"'s ability to support the beam, as I have not seen any deck failures caused by a cracked support column (not to say it can't happen). I do get concerned when I see 4" x 4"'s distort and warp due to drying - something you don't see on a 6" x 6". Maybe you could explain your concerns to the builder and for the small increase in $ they will agree to go with the 6" x 6"'s - definitely cheaper than hiring an engineer.
 
We limit 4x4 deck posts to 8'. This may be local amendment. 6x6's can go to 14'. All decks over 8' must be laterally braced in both directions.
 
A 4x4 can go 14 feet high before it hits the slenderness ratio limit of 50. The allowable load has to be reduced once it goes over 9 feet high because of slenderness. A 12 ft. high #2 SYP 4x4 can support 3500 lb. Other species would have a lower capacity.

I share NH09's concern about warping, and would probably use a 6x6 if I were designing the deck.
 
You guys made this about as clear as mud. I think I am gonna stick patially to the original plan. RDP plans -OR- 6x6 posts (accepted practices from AWC). This is also one of those places where I will also pull out R502.2.2.3 from the 2009 IRC.
 
The slenderness ratio is an issue (and this is why you see 4x4's bent, not from drying) and one must consider other factors such as there might be a 40lb/sf occupancy load but if you are in a 80lb/ft ground snow load, and of course the tributary area. . . . the failure will not be catastrophic, but will be a failure in bending over time.

I typically require 6x6 unless the deck is a half-floor or less off the ground.
 
brudgers said:
What code provision do you cite?
I site "submit to me engineering that indicates that I can approve 4x4's as being compliant in this installation"
 
Gut reaction, you can check it yourself or have the client retain an engineer to show that if they are spaced <8' apart on this deck the posts are at less than 50% capacity, slenderness accounted for. I'm pretty sure this is not one for "accepted engineering" to fail. There seems to be a great deal of empirical know how saying this is a bad idea, and I fully agree, I've had 6x6's twist 45 degrees in that kind of span. Is the grading up to accepted engineering or what is being missed?
 
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