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Joists and Girder span tables for decks questions (2006IRC)

9

97catintenn

Bronze Member
Joined
Sep 25, 2012
Messages
62
Location
Columbia, TN
When framing a deck you use R502.3.1(2) Dead Load 10psf......and with PT Pine you use the #2 line right?

And then if the deck is 16' long and you use 2x8 joist (12'10"span) which table is used for the girder?

And then if I add a hip roof over it, what happens?

And then if I have a covered deck and want to close it in with windows and tile the floor...how do I calculate the drop beam/girder needed?...

I'm working with the 2006IRC, and have been trying to figure out these calcs on my own.
 
The joist wood be sized like any other joist in a non-sleeping room so yes 502.3.1(2) and yes the #2 line refers to #2 SYP

The girder would probably be sized using 502.5(1) although it really doesn't fit anywhere. A good alternative is the DCA6 deck guide.

A hip roof adds loads to both the outer girder and the sides so you probably need to look at girders to carry those loads.

Adding windows and tile....starting to build a room aren't you? Prescriptive code probably wouldn't permit it without a foundation under it to account for all the lateral loads. Soon there will be heat and a couch out there, might as well call it a living room. It sounds like you are crossing the line between a deck/porch and a room. Remember a room has walls that require bracing, which all those windows they want to add probably won't allow room for.

Even if you allow it you better start with the footings. You are adding a lot of load to what are typically undersized to begin with. And adding tile adds quite a bit of dead load to the joists which may not have taken into account the wet-use factor. I have not seen many tiled exterior decks that ended well. The deflection and shrinkage/expansion will usually cause continual cracking.

Finally, you may want to ask them to provide the calculations. Inspectors aren't designers so let them figure it out and you can check what they give you. That doesn't mean you can't be helpful but IMHO you cross a line when you design a project. But before I even go there I would be careful about permitting this as a deck. IMO if it is what you describe they need to have it designed as a room addition.
 
I am the contractor and have it permitted as converting deck to a sunroom. I know the extra weight of the tile is going to require the deck floor to reinforced with a girder. The covered deck is 12x16 and I only have three footers spaced 8' apart for the girder. In practice, I've always supported girders every 7', but with the deck floor already built 30" off grade it was very difficult to dig three footers. So, I opted to for an 8' span. It's starting to sound like it needs to be a triple 2x8.

The AHJ does not see this as being a room like new construction. No permanent foundation required, no verification of insulation in the ceiling, no need to wire electrically like a new home. I thought it would be and asked these questions during the permitting process.
 
well, what a day for this question. Pine strength values were scheduled for final review yesterday, I haven't heard the details yet but the strength is looking to drop 30%. Also, wet service design, like a deck, should include a 15% strength drop as compared to the design values used in the codebook joist tables. A roof over it removes the wet service discount and those tables would be correct. It sounds like he is viewing this as outside the building envelope, I don't think he made the right call but now you are there to try to build it well anyway. Sometimes we want the inspector to put his foot down to help with the negotiations with the homeowner on what needs to be done... just sayin. Life is a broad spectrum of wants, sometimes I want them to disappear :)

If the 12' joists are one piece and you are proposing a center girder under them at midspan that girder will carry 5/8 of the 12' wide span, or 7.5' wide, times the 8' post spacing=60 square feet of tributary area bearing on the girder. The loads would be 10 pounds per square foot dead load, 40 psf live load... 50 psf total times 60 square feet carried on the girder, 3000 lbs.

As I look at my fingers there are three purple spots, I just pound nails so take this for what it's worth, these are my calcs, this is how I would check the center girder

Design for Bending

Plug in the 3000 lb load

96" span

4.5" actual width of the triple 2x8 girder

7.25" girder depth

Next are the design values, I'm going conservative;

Fb(bending strength)-1000psi

E(stiffness)1.4million psi

Fv(horizontal shear strength)175 psi

Click enter and it passes

Tile is a deflection problem divide the span, 96" by the deflection .1727" and it looks like 1/556 deflection, pretty stiff
 
Thanks Gary,

I hadn't read all of this weeks's mail. This was forwarded from the ALSC Thursday. They have the FPL's letter available, but it sounds like dotting I's.

The Southern Pine Inspection Bureau requests that our previously submitted Design Values for Southern Pine Lumber be withdrawn from the Board of Review Agenda on 18 October 2012. We received the Forest Products Laboratory comments the evening of 16 October. Their comments include items that we have not discussed with the FPL personnel. It is our opinion that our submission has been properly prepared and that the only changes involve rounding in some of the cells.
Click to expand...
One thing to play with in the calc is read Gary's link, change the design values according to what it is saying and notice bending and deflection. Then change the depth of the girder to 9.25" and note those again. Going one size deeper is the cheapest way to build strength and stiffness. With tile on joists and girders there is the combined flex of both the joist and the girder rather than just the joists on unyielding walls. Gary probably knows more than me but I've seen designers run those under tile girders up to l/720 at times, very stiff.
 
Joists and Girder span tables for decks questions (2006IRC)

The IRC span tables are for "dry" applications. I am wondering if the DCA-6 manual will have the span tables adjusted due to the decrease in strength of the wood in question.

Sent from my iPhone using Tapatalk
 
The IRC span tables are for "dry" applications. I am wondering if the DCA-6 manual will have the span tables adjusted due to the decrease in strength of the wood in question.
Click to expand...
The span tables in DCA6 are based on wet service and incised lumber for west coast species. A triple #2 SYP 2x8 girder supporting one end of 12' joists, 1/2 the load, have an allowable span of 8'1". Those tables limit to l/360 deflection. hmm wonder why on a deck?
 
Interesting switch, 2@2x10 vs 3@2x8

The moment of inertia used for deflection calcs is bd^3/12

for a 2x10- (1.5"x 9.25" x 9.25" x 9.25")/12= 98.93"^4 ...at 2 ply 198"^4

for a 2x8 moment of inertia is 47.63 ... at 3 ply 143"^4

The 2 ply 2x10 is stiffer, there's more wood in the right place.
 
I used your link in post #4 and it said that the double 2x10 would be stronger :) And an added bonus is that when I support that girder with a 6x6, I can notch it for the double. Notching it for a triple doesn't work as well.

Still though, I sit back and am amazed that a double 2x10 is stronger than a triple 2x8. Good thing I didn't bet my lunch on that one.
 
That's a good bit of the reason I wrote those calcs, to play "what if". It is a good idea to follow that preliminary design up with rktekt 1's advice.

In performing the three checks in my calc, or in any rectangular beam sizing program, the calculations account for the shape of the beam, its' "section properties". In my post above I gave the formula for determining the moment of inertia, one of the key variables in the formula for deflection... stiffness. Stiffness is a qualitative rather than a pure strength issue, it's about how bouncy a floor is or how likely it might be to crack tile floors or plaster ceilings. Generally deflection controls allowable spans. A floor usually gets too bouncy well before it becomes truly weak... which is a good thing.

You replied that the beam was stronger, that is another one of the three initial checks performed on a beam and it is also related to one of the section properties. These are really cross section properties, they relate to taking a slice through the beam and looking at its' geometry. For bending equations the section modulus is a key variable, and again the depth of the beam is heavily favored;

(bd2)/6

breadth times depth squared divided by 6.

You can make a beam wider and build strength but it'll sure build strength a whole lot faster to make it deeper.

The strength we are checking here is extreme fiberstress. The extreme fiber is that strap of wood at the extreme top and bottom of the timber, in this case the bottom strap of wood is what is being focused on. Think about bending a popsicle stick to failure. As it begins to fail fibers along the bottom of the stick will begin to tear. This check determines the force in that area and limits it to safe allowable levels... where 95% of the sticks in the pile are more than twice as strong as required. Knots along the edges and slope of grain variability in lumber dictate a pretty healthy margin of safety.

For the double 2x10 the section modulus would be;

(3 x 9.25 x 9.25)/6= 43"3

For the triple 2x8;

(4.5 x 7.25 x 7.25)/6= 39.5"3

You're right, all else being equal (species, grade), the double 2x10 is stronger than the triple 2x8 as well as being stiffer.

For parts like that I stack the deck in my favor. Carefully select through the pile for pieces with a nice unblemished, straight grained bottom strap of wood.

>>

Email yesterday from ALSC, they have delayed action on SYP design values until late January.

>>

One way to double check design values for use in my calc ( I kind of shot from the hip with the values a popped off with in post #4) is to set up the problem using the awc.org span calculator and scroll to the bottom of the results. There are the adjusted allowable design values.
 
Great, but what about the exterior wall loads and roof loads? Those now come down to the outside edge/perimeter of the deck frame. I believe you were using 2x8 at 16" o.c. So the outside deck frame is a single 2x8 joist, which needs to be a beam. What size? How about the connection to the ledger board of those two outside beams? What is the piece of hardware going to be? How about the ledger board attachment to the house? There are now two point loads at each end of the ledger board holding up the exterior walls and roof? Did you get a soils report for the three new footers?
 
I'm with rktect here. Lots of attention paid to stiffening the floor which is great but no attention to roof, bracing and footings. This was existing deck being converted to add a lot of load. On the other hand, at least a permit was issued and cat is attempting to cover all the bases. I think the inspectors should be asking these questions.
 
I agree. 2006IRC, it should be on a full perimeter foundation or design is required.
 
You guys lost me at applying the span tables to PT lumber. Unless something remarkable has happened to the process of preservatively treating lumber recently, a PT 2 x is not the same strength as an untreated 2 x of the same species and grade. Also most PT would be stretching the standard to grade at #2... JMHO, always willing to be enlightened.
 
Treating lumber does not lower its' strength, incising lumber does but on the east coast and in SYP you don't see incised lumber. Under the same conditions the allowable spans for pt are identical to the allowable spans for untreated lumber. The base design values are reduced for wet service (>19%) and incising, not for treating with preservatives.

Do remember passing inspection is not the goal, building a good structure is.
 
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