I Beam help needed

[bugstruck]

Exactly. I think you would even see that in a 20' joist that wasn't engineered. However, on that 2 x material I believe a load imposed failure on an equally distrubuted load may be more likely at the the end of the member. I was a residential framing carpenter in my late teens and was running crews before 20, so that is my early experience with wood. Engineered lumber had just showed up as I was moving on to new positions. Don't sell your experience short on understanding how materials react under different situations. You and I know most field related failures in a board are more mid-span oriented and more likely to be concentrated loads, not uniform. But that is not where an engineer may anticipate a load failure on distributed loads. The failure may be end bearing related. If that is the case, the fact that you gained mid span support with a single piece of material may not be of much consequence except to mitigate defelection, which it certainly would. In other words, the failure may still occur at the end given the same weight application. I may be all wet too, but that's how I see it.

 

[Egon]

You gotta get some shear force diagrams and some moment diagrams. Then pick the deciding factor and get the section modulus required. Then pick the beams required.

Egon /forums/images/graemlins/grin.gif

 

[bugstruck]

Your giving me too much credit. YOU can get the span from me, the distributed load from me, the total load from me, the modulus from me, the concentrated load from me. the load value from me, and the load center from me. YOU can size it and space it. If I spent 6 weeks cramming it I might get close. I have no ability beyond physical experience to nail it. Can I catch you if you S---- it up? YOU bet. I can feel it.

 

[bugstruck]

Egon, sorry. After reading my post, you sure didn't deserve that reply. My sincere apologies. May have been how I feel but your way to good here for me to slam your reply like that. Consider it directed towards some other undeserving soul. 48 and stilll learning.

 

[CurlyDave]

If we go back to the joist tables Southern Pine Span Tables and in particular the 150 psf table 150 psf table , we see that No. 2 or better graded lumber @ 12" on centers meets the 150 psf requirements, and if we look at the 100 psf table, even No. 3 @ 16" on centers will span 9' 9".

In reality, Eddie's span is only 9'6" because his center support will be 12" wide.

The bottom line here is that unless he goes to the scrap heap and deliberately selects the worst boards available there, it is just not possible to build a pedestrian bridge so that it will be underdesigned using 2 x 12s on either 12" or 16" centers.

The other point to recognize is that with narrow bridges and decking sufficiently strong to cause load sharing, the proper way to determine actual beam spacing is to divide the bridge width by the number of beams. For instance, a 48" wide bridge at a nominal 16" OC spacing will actually have four beams. Effectively this has the strength of 12" spacing. This is one reason I would go with 12" decking, which I think would be better at distributing loads between beams.

 

[EddieWalker]

Dave,

Thank you for all the help, information and advice.

With the change over to building a foot bridge, I was thinking that the decking could be PT 5/4's. With blocking between the beams and 12 inch centers.

Would I be cutting too many corners using the thinner material for decking?

Still undecided on 2x12's versus 2x10's but think for a few bucks more, the 2x12's would be allot better.

Thanks,
Eddie

 

[bugstruck]

Eddie,

The span table Dave referenced indicates you are good for 2 x 10's or 2 x 12's at either 12" or 16" spacing. All (4) scenarios get you over 100PSF live load with PT #2 yellow pine, which is what the commodity pressure treated material the lumber yards typically stock. Dave likes the 2 x 12's for decking. I agree that they would be the best decking for structural purposes for the reason he states. The only drawback to 2 x 12's for decking is they are more prone to cupping and a little more prone to end checking. These are not big considerations though. Appearance would favor a narrower board IMO. If you go with 5/4 decking, that works OK too, but I'd probably go with 12" centers to stiffen/support them better. The most cost effective solution could be 2 x 10's at 16" OC with 2 x 6 decking or it could be 2 x 10's @ 12" OC w/ 5/4 decking.

On the blocking.... all you need to do is control the ends of each span and the center of the span. Plenty adequate.

One other recommendation is to lay the deck boards tight togeather with no spaced gaps. But you knew that already. /forums/images/graemlins/wink.gif The gaps will appear in time and the wayward drifter in high heels will thank you for it.

HTH

 

[EddieWalker]

Chris,

Thank you very much for the information. This sounds like the best way to go for both cost effectiveness and safety.

On the spaceing for the decking, I was actually thinking of openeing them up some, but unsure how much because of shrinkage. My thought was to get water off of there as fast as possible.

If I install them tight, there will be some shrinkage the will create gaps to allow water through, but not all of them will shrink the same and I could easily end up with some that remain in contact with each other.

I've had two thoughts. Put 16d nails in for spacers or 1/4 inch bolts. Now I'm wondering if putting them in tight will work better????

Eddie

 

[KentT]

Eddie.

If you're going to space them, I'd recommend nothing bigger than a #8 finish nail as a spacer. We spaced my back deck with #16's and now I've got some gaps 3/8" or more...

It will all depend on how dry your 5/4 is when you put it down...

 

[bugstruck]

Eddie,

In the old days we always gapped them with 12d nails. I think that was a carry-over from before pressure treated materials as they didn't shrink so much. Then we all started seeing the gaps the pressure treated would open up to. 3/8" and even 1/2" wasn't uncommon for 2 x 6's spaced with 12d nails. For 20+ years we have been running them tight and I guess we have at least as much moisture here as you do. They'll open up substantially within a year and quite noticably within months if they get summer sun. The water runoff won't be a problem as they really don't end up as tight as you think and they will start to open up within a week or two. Every so often you may see a pair that seem to remain tight, but if you look to the next gap it will be a little wider. It's just that one fastner held better than the other and the one edge stayed nearly parked. FYI, if you put them tight the average gap in one year will be that 12d nail, or more. In 3 to 5 years about 1/4" is usual. Many pressure treated boards sill run strong for measurement. Often 2 x 6's are around 5 9/16 to as much as 5 5/8" wide. They will shrink to about 5 3/8" after a period of drying. So the gap is coming, wanted or not.