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-   -   I have a question about deck joist. (http://www.diychatroom.com/f19/i-have-question-about-deck-joist-82751/)

max_wonder00 09-30-2010 09:28 PM

I have a question about deck joist.
 
Is there a difference in using 8 2"x10" ptw as joist resting on beams to span 14 or should I buy 14 2"x10" ptw to be used as joist?

I have a small hatchback and can only fit 8 planks of wood. I will have to rent a truck to get the 14 2"x10" ptw home. The 8 2"x10" ptw will be resting on beams evenly place on the beam with simpson strong tie hurricane clips. I am wondering if there are any structural reason/benefits to use a uninterrupted piece of wood as joists? Thanks for any help.

Thanks

CoconutPete 10-01-2010 10:04 AM

You'll need to submit plans to the town in order to get the permit issued. What do your plans call for? Does the local code allow for the 8' lengths instead of the 14' ones?

How much lumber are you buying? When I bought all my materials it totalled 3500lbs and NONE of the 3 vehicles in our family would have hauled that so I just bit the bullet and had it delivered.

jklingel 10-02-2010 02:39 AM

A 14' span is not short, and 2x10s may not hack it, depending on your load. What IS your load? You have to know that before you buy any wood. See if your lumber company has any load/span tables. Cutting the span to 8' by using a beam will help immensely (assuming the proper beam, of course). The load carrying capacity of a board is inversely proportional (approx) the cube of its length. I have 2x10s on a 12' span, 16" OC, and have had a couple of feet piled up on it many times over the years. Two feet longer may be iffy, though.

max_wonder00 10-02-2010 06:46 PM

1 Attachment(s)
Thanks guys for trying to answer my question. I think I did not write the question clearly. So I modelled it on sketchup as a visual.

My question is about the joist length I should buy. I'm going to use 2x10 for sure, but I wanted to ask if there is a difference in buying two 8 foot long 2x10 instead of one long 16 foot 2x10? Is there a difference in buying two 8' or one 16' to use as joist?:)

please see attach sketchup model

jklingel 10-02-2010 10:53 PM

Oh, now I gottcha. Nice drawing, btw. The only measurable differences would be cost and convenience, assuming you have a good sized "lip" for the 8 footers to sit on. There is some number like 2" that a board needs when it sits on a beam.

Michael Thomas 10-03-2010 08:11 AM

On thing to keep in mind: a continuous joist running across a center support will be considerably stiffer (the floor will fell less "bouncy") than the same distance spanned by two shorter joists each extending only to the center support.

Daniel Holzman 10-03-2010 09:07 AM

A continuous joist spanning across a beam (the fourteen footer you describe) behaves quite differently than two separate beams, from a mechanics standpoint. A pair of unconnected joists is referred to as two simple beams. There is zero moment at the end of a simple beam. and the maximum moment (hence the most critical section) occurs at the center of a simple span beam when it is uniformly loaded, such as the deck case.

A continuous joist has completely different properties. There will be a large negative moment at the center of the joist over the beam support. This occurs because the joist can be viewed as being loaded at two positions, namely halfway between the center of the joist and either end, causing positive moment at the quarter points (the same position as the maximum moment in the simple beam case), but large negative moment at the center. Interestingly, the positive moment at the quarter points is smaller than the positive moment for the simple beam case given the same loading.

The advantage of the continuous beam in theory is that you can use a smaller joist to span, assuming the joist can handle the negative moment at the center of the beam. You see this on bridges all the time. Look at a continuous beam bridge, and note how it is deeper at the center to handle the negative moment. The reason it is not done more often is that continuous beam structures have problems handling expansion/contraction, and the fabrication costs for a bridge for the deeper center often exceeds the material savings for the smaller beam.

In your case, you would get a stiffer feeling structure if you used a continuous beam, since the maximum positive moment is smaller than with a simple span beam, hence the maximum deflection would be smaller. However, you are not going to notice the difference, and if you cannot transport a longer joist, get the short ones, no problem. Bear in mind that the splice for the short joists must occur over a post by code.

Michael Thomas 10-03-2010 09:44 AM

IMO, whether a user will notice a the difference is a "psychological" as well as a "structural" issue, I have encountered clients who are uncomfortable on decks which "meet code" and are "structurally sound, but which feel "bouncy" to the client.

jklingel 10-03-2010 01:29 PM

I'm sure all the moment info is true, and I wish I knew how to calc all that; it is quite interesting and would be wonderful to know. However, does anyone think a 2x10 on an 8' span is going to really feel much different than the 16 footer? That is not much span for a 2x10, in my experience. Dan: What is the recommended "lip" or "catch" that a 2-by needs when sitting on a beam like this? I assume 1.5-2" is enough, as that is what joist hangers have. thanks. j

max_wonder00 10-03-2010 10:48 PM

Thanks everyone for taking the time and answering my question. I'm a first time diy deck builder and this was a question that was pestering me. I'm happy to hear that there is not much of a difference. Now I can make a decision and move forward.

jklingel 10-04-2010 01:41 AM

Happy to help. If you ever have more "iffy" questions, Dan is the guy to ask. He can CALC what has to be, not just go by experience and a rough understanding of how members work. An engineer may even crank out the numbers on something like this and find that 16' x 2 x 8 would be just as well a 8' x 2 x 10. When in doubt, over-kill! Materials are cheap compared to re-doing and throwing away weak stuff. later. j


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