I Beam help needed

[joebleaux]

Do a Google for Beamboy 2.2 and you will be happy.


Joe


</font><font color="blue" class="small">( Part of my lake design will have a cement spillway to handle the runoff during heavy rains. This could involve a million gallons of water, so it needs to be a substantial spillway that will not erode.

It will be built at the edge of my dam on virgin soil with a shallow drainage to the creek.

To handle large quantities of water, I need to make it 18 inches deep and 16 feet wide. I'm gonna add a few feet to this and build it 20 feet wide.

The sides will also be concrete, which will create an area that will be very dificult to get through when walking along the dam. It will also be impossible to drive over unless I build a bridge across it.

I want to put two I-beams across the span. My thinking is that I'll rest the ends of the beam on a concrete footing. In the center I'll fasten the I-beam to a concrete footing with some large bolts.

This should allow for expansion of the I-beams with the changes of tempatures. They will be spaced at the width of my tractor tires and decked with PT 2x6's.

My question is what sized I-beams can I use to span ten feet and support a 4,000 pound tractor? Lets say 6,000 pounds max weight for both beams combined?

Or if anybody knows the link to a page with the ratings for various sized I-beams. I can buy any size, but would prefer to not waste money buying something rediculous.

I've also read through all the previous posts on bridges without seeing anything that would work. I'm not going to do the trailer method or any other scrap material. Only new I-beams will be used.

Thanks,
Eddie


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[bugstruck]

Eddie,

A single 20' span is a steel solution for your application IMO. Technically you may just make 40lbs/SF loading with the 2 x 12's, depending on the exact lumber grade, but that's not enough. Those 2 x 12's @ 12" OC and broken into 10' spans would be slight overkill IMO. 2 x 10's @ 12" OC on a 10' span should get you well past 50lb/SF live load. I think that 50lb/SF live would be the minimum design criteria for people only. That's generally a design minimum for residential PT wood decks, allowing for people standing in close proximity to one another (nearly packed), which you may experience on that bridge. Derek or one of the other Engineers may know the actual loading minimums for your application. The 2 x 12's @ 12" OC over 10' may be about right for a golf cart?? Not certain what those weigh though. Maybe 600 lbs. + occupants and a cooler of cold of course /forums/images/graemlins/grin.gif

 

[drm]

Building codes vary by region but are usually very similar. Live load for corridors, stairs, and egress is usually 100 psf. It can be reduced to 40 psf for single family dwellings but this is public. I do not know off hand the load requirement for foot bridges but I would use 100 psf or determine the AASHTO requirements. I would believe that they would still govern since it is a bridge.

To use tables developed for 50 psf decrease the spacing required to be half that in the tables to support 100 psf.

…Derek

 

[CurlyDave]

I would build a 4' wide bridge with the 2x12s on 16" centers. This would require 4 of them and you could use the beam tables for 12" centers because you have 4 beams in the 4' width. Continuous beams will be stronger than two 10 foot spans.

You have been working with wood for long enough to know how to deal with slight warpage and bends.


Go for it...and send us pix.

 

[EddieWalker]

Since we're just talking here and throughing ideas around, how much difference is there from 12 inch centers to 16 inch?

I'm thinking of using 5/4 for the decking and blocking the 2x12's, but havent decided on spacing. The ends and middle for sure, but wonder if ther's any advantage to more than two points in the span?

Also, does the size of the blocking affect overall integrity? Such as 2x12's versus 2x10's, 8's or 6's?

Thanks again for all the advice and suggestion.
Eddie

 

[bugstruck]

"Continuous beams will be stronger than two 10 foot spans."

Dave,

I'm not understanding that. Do you mean that a 20' joist, that is center supported at half span, would be stronger than (2) 10' joists end to end? I wouldn't agree that there would be any difference given adequate bearing, but I just wanted to make sure you didn't convey that a 2 x 12's spanning 20' with end bearing only, are stronger than 2 x 12's spanning 10' each. I know that wasn't/isn't your intention. Just making sure a real novice didn't read it literally and make that interpretation, as I nearly did.

Or perhaps I have your comment out of context?? Wouldn't be the first time I've done that /forums/images/graemlins/wink.gif

Also, note Derek's comments on 100 PSF. I thought commercial might have different loading thresholds. Something to consider.

 

[CurlyDave]

Do you mean that a 20' joist, that is center supported at half span, would be stronger than (2) 10' joists end to end?

That is exactly what I am saying. To be very precise, a 20' beam supported at both ends and at the center will be stronger than two 10' beams spanning the same supports.

This was covered in about the third week of introductory mechanical engineering. The fact that the beam is continuous imposes constraints on the deformations of the beam near the center support which increases load-bearing capacity.

You can Google "continuous beams" and find more information on this (and a lot of companies selling software to do the calculations).

Span tables do not take this into account, and I am not suggesting that Eddie should take credit for it, just that it will provide an extra safety factor.

Back when I learned about this (dinosaurs still frolicked in Central Park), the calculations were so complex that only the largest projects took credit for the addition. Most designs used the equations for discontinuous beams and then recognized some additional safety factor.

Today, even the smallest overpass takes the effect into account because we have computers to do the calculations.

I just wanted to make sure you didn't convey that a 2 x 12's spanning 20' with end bearing only, are stronger than 2 x 12's spanning 10' each.

Saying that was certainly never my intention. It is clearly not true.

My intent was to convey that 20' beams do have an advantage over 2 - 10' beams which is probably worth the extra few dollars.

 

[EddieWalker]

It just seemed to me that a longer beam, one the goes the full distance, would have more sterngth by being longer and spread out over a larger distance. I think it's something like how the decking will increase the strength of the beams by distributing the weight over a larger area. Or at lest that's how I figure it in my mind without any actual knowlegde here.

I do the same for decks. I always use the longest beams I can get to spand the deck. If I have to use two in a run, I make sure their joint never align with the next one over and have as large an overlap as I can.

Thanks,
Eddie

 

[bugstruck]

"(dinosaurs still frolicked in Central Park)" Me makes two.

All my time around this and I wasn't aware the a single beam in a center supported situation was stronger (to a degree) and that it warrants consideration. Soon as I thought about that it made sense though. I can see the limits on deformation at the center (what would be center bearing points on end to end beams). Seems it wouldn't change or mitigate much on the deformation occurring at the ends of the 20' beam though.

If we are talking wood beams (joists) that capacity gain (from limited deformation) would be differernt than with steel. Any thoughts on that? With wood I don't know that I've ever seen it considered. As you stated, that's generally just a span, + species, + live load determination to get to a size and spacing. Now I could see it as a consideration in engineered lumber and perhaps trusses where true application calculations are more likely.

 

[JasonL]

I'm no engineer and make no claims, but I am a residential frame carpenter. We use wood Ijoists for our floor systems. The floor layouts are engineered. I am pretty sure they take into account if there is a center support with a continous joist over it. Someone explained it to me like this. (I don't know if it is right, but it made sense to me!) When one side deflects, the other span helps to resist this force. If they are spliced over the center, each span acts independently.

Jason

 

[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.