Previous Thread
Next Thread
Print Thread
Rate Thread
Beams and spacing for 16' Span #4769 03/13/06 07:27 PM
Joined: Mar 2006
Posts: 3
H
HumanJHawkins Offline OP
Member
OP Offline
Member
H
Joined: Mar 2006
Posts: 3
Hi,

I need to build a deck over a 16' x 20' concrete cistern. It is perfectly square and the concrete walls are intact. It fills with water to various levels during the year, and I am unable to put any support posts, etc. down into the pit.

What sort of beams will I need, and how close together sould they be to support the deck. i.e. Would 4 x 8, 16" on center be enough? Do I need more? Can I get away with less?

Because of the water, I expect to use pressure treated lumber. (They will never be sitting in water, but it is a damp area.)

Thanks in advance!


--
Jeff Hawkins
Re: Beams and spacing for 16' Span #4770 03/13/06 08:22 PM
Joined: May 2002
Posts: 218
E
Emmett Greenleaf Offline
Member
Offline
Member
E
Joined: May 2002
Posts: 218
What does your local code call for ? Many areas today have sample deck plans including charts to select wood sizes for the job based on span.
4x8 running across the 16' dimension sounds like overkill even spaced 16". Check it out.

Re: Beams and spacing for 16' Span #4771 03/13/06 10:12 PM
Joined: Mar 2006
Posts: 3
H
HumanJHawkins Offline OP
Member
OP Offline
Member
H
Joined: Mar 2006
Posts: 3
Quote:
Originally posted by Emmett Greenleaf:
What does your local code call for ? Many areas today have sample deck plans including charts to select wood sizes for the job based on span.
4x8 running across the 16' dimension sounds like overkill even spaced 16". Check it out.
I hope it is overkill, as it would be nice to spend less!

The trouble with all of the sample deck plans is that they assume the ability to have supports every 4 ft. or so. In this case, I can't do that. The same is somewhat true of the codes I can find... And the span charts all include info that I don't understand, such as how much spring is to be tolerated.

I am hoping someone can give me a range such as, "2x8s 24" on center would support as much weight as most decks need, but it would be awefully springy... 2x10s 16" on center would be more firm."

Or, "4x8s 24" on center will be stronger than 2x10s." or visa versa.

The thing is, I'm not really sure this is even in the ballpark.

Thanks agan for any info.


--
Jeff Hawkins
Re: Beams and spacing for 16' Span #4772 03/14/06 07:29 PM
Joined: Oct 1999
Posts: 463
R
Roger Nair Offline
Member
Offline
Member
R
Joined: Oct 1999
Posts: 463
A link to the next county over from me, maybe good for a ballpark idea.
http://inetdocs.loudoun.gov/b&d/docs/permitguideline_/deckdetail-1/deckdetail.pdf

Locally available materials can be different as well as local regs.

Also a limited span calculator, but from an authoritative source; http://www.woodworks-software.com/us/downloads/spancalc.php

Use with care.

A good direct article on sizing by Ed Levin is in the first volume of the guild design workbook. http://store.yahoo.com/tfguild/deswor.html

Re: Beams and spacing for 16' Span #4773 03/14/06 10:55 PM
Joined: May 2002
Posts: 218
E
Emmett Greenleaf Offline
Member
Offline
Member
E
Joined: May 2002
Posts: 218
The spring you refer to is probably deflection. Whenever you span joists between their end supports the center of the span will deflect some minor distance when it is flexed by weight like someone walking on the deck. How much deflection can you tolerate so the floor does not feel springy ? Many deck codes call for a 40psf structure which will bounce when you walk on it. A rigid floor has 0 deflection but this costs in material used.
Most timber selection building charts for decks will list the nominal deflection for a given joist size. if you use 6x6 or 8x8 material for your posts spaced 5' (matches the 20' long side nicely) with the same 4x8 for the band joists (around the edge)then you will have a pretty ridid system to hang your intermediate joists from.When you put the decking on run it at 45 degree angle across the joists and the whole structure will be more rigid. Use deck screws (no nails) and a bead of liquid nails on the joist tops for a long lasting bit of work which won't squeek. The deck behind my house has two differenct support systems - the strong floor uses 6x6 posts on 5' centers with the 2x10 joists making a sandwich of the posts attachment with thru bolts. The 2x5 decking is angled so the net floor has zero deflection and tests in excess of 200lbs psf load.
choose carefully before you build.

Re: Beams and spacing for 16' Span #4774 03/15/06 01:12 AM
Joined: Mar 2006
Posts: 3
H
HumanJHawkins Offline OP
Member
OP Offline
Member
H
Joined: Mar 2006
Posts: 3
Thanks. This has set me on the right path I think.

Cheers!


--
Jeff Hawkins

Moderated by  Jim Rogers, mdfinc 

Newest Members
HFT, Wrongthinker, kaymaxi, RLTJohn, fendrishi
5134 Registered Users
Powered by UBB.threads™ PHP Forum Software 7.7.3
(Release build 20190728)
PHP: 5.4.45 Page Time: 0.185s Queries: 14 (0.100s) Memory: 3.1462 MB (Peak: 3.3984 MB) Data Comp: Off Server Time: 2024-03-29 09:17:27 UTC
Valid HTML 5 and Valid CSS