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Deflection limits for decks
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Re: Deflection limits for decks By  George Roberts

Posts: 0

10/23/2007 12:16:00 PM

I believe you meant: "The span tables for joists used in decks are [b]allowed[/b] to have a maximum deflection of L/240".


L/360 for live load deflection and L/240 for total deflection.


   

Re: Deflection limits for decks By  Paul Sweet

Posts: 0

10/23/2007 4:41:00 PM

IRC Table R301.7 gives live load deflection limits. A deck would be a floor, so it is limited to L/360 deflection under a 40 PSF live load (from Table R301.5).


   

Re: Deflection limits for decks By  DRP

Posts: 0

10/24/2007 1:21:00 PM

George, I don''t care how much the span table deflects :)

devil''s advocate... there is no plaster finish on the underside of a deck.


   

Re: Deflection limits for decks By  tsmith

Posts: 0

10/24/2007 2:15:00 PM

>there is no plaster finish on the underside of a deck

Nor is there a plaster finish anywhere else in most houses. R301.7 gives the same 1/360 limit for "floors and plastered ceilings." Meaning all floors, and any ceilings which have a plaster finish. It doesn''t say "floors with plastered ceilings beneath them."

I don''t see any exception or footnote that would allow a deck to have more deflection than any other floor.


   

Re: Deflection limits for decks By  cboboggs

Posts: 0

10/25/2007 5:31:00 AM

I would have to agree. A deck is a floor system, so the maximum deflection would be L/360.
The L/240 is for all other structural members.


   

Re: Deflection limits for decks By  ClassAGuy

Posts: 0

10/25/2007 7:49:00 PM

The )code) deflection limit of lenth/360 historically comes from limit for "brittle finishes". Plaster, drywall. Decks with just joists and decking, l/240 is the limit for live load deflection, l/180 Live + Dead. No one says these are "good", just minimum. A floor, with drywall or plaster, the limits are, for live load, L/360, and live + dead, L/240. This has been the case since Columbus sailed the seas of blue. Knowledgable designers use much, much more restrictive limits.


   

Re: Deflection limits for decks By  Builder bob

Posts: 0

10/25/2007 8:17:00 PM

Will design, i appreciate the tidbit of info.....BTW your signature lacks one....."BFH". private mail me if you need to know what that means.


   

Re: Deflection limits for decks By  tsmith

Posts: 0

10/26/2007 5:09:00 AM

Will, the finish business is an old familiar fact, but it''s not in the code as far as I can see. Exactly what are you citing?

The IRC has already been quoted. The IBC (which wouldn''t apply to a SFR) allows the greater deflection for roofs supporting non-plaster finishes, but, like the IRC, it gives one deflection limit for "floors" period. I don''t see any exception for decks or for floors which have different finishes on the underside. Both I-codes just say "floors."


   

Re: Deflection limits for decks By  cboboggs

Posts: 0

10/26/2007 6:55:00 AM

Tsmith is correct. I am aware of the brittle finish logic, that is not included in the code, therefore, doesn''t factor in.


   

Re: Deflection limits for decks By  jackson2112

Posts: 0

10/26/2007 7:02:00 AM

BB,

Here, we have the BFH 9000. :D It''s an improved version of the older BFH 5000. The " 5000 " model just didn''t git-r-done. :D


   

Re: Deflection limits for decks By  clearly

Posts: 0

10/26/2007 10:01:00 AM

May I offer another take on where the L/360 came from? A quote for the Wood Structural Design Data book as published by the National Forest Products Association:

"Deflection limits are expressed as a fraction of the span and the selection of an appropriate limit has generally been a matter of judgment on the part of the designer. Originally, it was believed that a limitation of l/360 of the span, in inches, was required to avoid the cracking of plaster. However, more recent research has demonstrated that other factors have equal or greater bearing on the tendency of plaster to crack in service. The limit of l/360 has continued in use, mainly for the purpose of providing comfortable floors free from excessive vibration under impact."

BYW, For those who choose not to use L/360 for allowable deck deflection, what table do you use from the 2006 IRC to evaluate your deck joists?


   

Re: Deflection limits for decks By  tsmith

Posts: 0

10/26/2007 10:19:00 AM

clearly, I would also surmise that l/360 has persisted in part because it''s about the most deflection that some common floor finishes can tolerate without noticeable cosmetic failure. Which is consistent with the quote you posted -- that particular deflection has become common and generally accepted practice.

But if you read the fine print recommendations on more upscale floor finishes, such as any kind of tile or even some types of hardwood flooring, they really want l/480 or less. You may notice that the vendors don''t exactly go out of their way to publicize this info, because they know perfectly well that hardly any floors actually meet their criteria. Making me wonder if they don''t plan to use this to weasel out of warranty claims. The Tile Council, in particular, has backed off their recommendations to the point of making them hopelessly vague.

Of course, it''s a Catch-22. The tile contractor can''t be responsible for the structural design of the floor, and practically no houses have an actual professional "designer of record" who is competent to coordinate structural engineering design parameters with the homeowner''s interior decor preferences. And no spec builder is going to provide l/480 or l/600 floors just in case the homeowner decides on marble floors.


   

Re: Deflection limits for decks By  ClassAGuy

Posts: 0

10/29/2007 8:50:00 PM

Presuming BFH refers to "Builder From Hell", which would be a builder who knows the code at least as well as the plan reviewer and building inspector, I''ll take that as a compliment.

I asked a good friend of long standing, who was first certified as a plan reviewer and building inspector in 1975 by ICBO, and remains so today under ICC, what she thought the deflection limit for decks was intended to be. Without prompting from me, she said l/240 under live load, assuming no brittle finishes. Her justification was the "other structural members" of the IBC section.

I made my earlier post partly out of historic interest, as I read fairly widely regarding development of fire codes and other building codes. I don''t claim to be an authority, but I find historical perspective is useful in assessing current regulations.

For those interested, here is an interesting link regarding the subject: [URL=http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd054_e.html]Canadian Building Code[/URL]

I note that AITC (old copy from 1976) uses same criteria as 1979 UBC, ie "Roof member supporting plaster", and "Floor Member", at l/360. AISC (also old, legacy copy, nearly as old as my marriage) suggests l/360 limit for "floors supporting plaster".

My understanding, going back to dim recesses of education, was that excessive deflection led to lath and plaster falling on people''s heads, and was limited by earliest codes in major American cities, ie NYC, Boston and Chicago.

But, as some would say - BFD!!


   

Re: Deflection limits for decks By  tsmith

Posts: 0

10/30/2007 5:41:00 AM

Will, my personal opinion is that if you don''t mind bounciness, there shouldn''t be a functional problem with an l/240 deck.

But the code doesn''t allow it, and hasn''t for at least a decade based on your citations. I don''t see any code basis for saying a deck is "other" than a "floor."

Your link bears out just what I mentioned, that l/360 is not adequate for some conditions and finish materials. Also, with long-span engineered lumber systems, that can result in an unacceptable floor "feel." Manufacturers are coming out with methods for "rating" floor performance, taking into account other factors besides deflection.


   

Re: Deflection limits for decks By  George Roberts

Posts: 0

10/30/2007 7:05:00 AM

I will kick in with Will Design/Build for Food and tsmith.

I think stress limits are more important but ...


   

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