I Beam help needed

[CurlyDave]

How to make an arch????

Eddie:

The arches I drew have straight bottom sections, only the top half is curved -- sort of like a mailbox.

The way I would pour an arch is to make a form using a piece of plywood. For instance, take a full sheet of either 1/4" or 3/8" plywood and screw 2x4's on the 4' ends. Look at the natural curve of the plywood and put them on the inside of the form. Put some screw eyes in the 2x4's, maybe on 12" centers, and start drawing them closer together -- the plywood will naturally form an arch, which will be surprisingly strong, although putting a load on top of the arch will increase tension in the chains and stress on the screw eyes. Maybe the kind with a machine thread and some fender washers.

Take it easy and do it in increments. Use a come-along to pull each eye in a few inches and then maintain tension with a chain. Clearly having the same number of links in each piece of chain will keep the arch the same along the entire length of the plywood.

I bet you could easily get to a 5' span with along the 8' length of the plywood. The height of the arch would be just under 2'.

Two pieces of plywood side-by-side and you could make an 8' wide bridge.

Depending on your finances you could either make up a lot of forms or just make two and pour the bridge in sections.

If I were doing this, I would make a short stem wall about 6" high between the arches, both because I think it would look better and to let me get the forms out after the pour (diesel on the form before the pour, increase tension on the forms after the concrete has set up, and it should pop right off).

This would be a very inexpensive form. I would make one up & test it by piling sand or dirt on it before using it for concrete, just to be certain it didn't require extra bracing. There is nothing quite so exciting as having a form give way...

If you are interested, I could make up a primitive scale drawing so you could see proportions.

The bridge this technique wants to "naturally" make would have 5' spans -- you would want 3 or 4 of them for a total length you mentioned. You could try putting pieces of plywood together for other lengths of span, but as the length of the span gets greater, its height should also increase.

Your idea of facing it with rock is really good. I bet it would look sharp and cost less than an I-beam bridge.

 

[bugstruck]

Eddie,

I see your thinking about doing the bridge with arch openings. I think it would look great too, but don't underestimate that task. You could build 4 steel beam/wood deck bridges in the time it would take to form that pour properly, get all the rebar bent, laid, tied, and positioned properly. Placing and finishing the concrete would be the easy part. Then you'd have to face it if you wanted stone. Mucho, mucho work IMO, even for a 20' span.

As others mentioned, if you haven't done a lot of this type of vertical form or arch form concrete work, the form strength required to hold that pour will amaze you. The arches would need to be supported straight down with at least (4) continous arched plywood ribs or numerous posts, and the vertical forms need verticals, whalers, and kickers. More wood volume than your deck would take with steel beams. Your going to have some real depth in concrete between those arches if I'm envisioning this correctly. Maybe 36" to 42"?? I'm guessing you need at least 6 or 8" at them middle of the arch.

It's a fairly small form job by commercial standards, and maybe you've done something similiar? If so, that would put you in the group of 1 in 1,000 construction workers who'd really know how best to do this. I supervised them, so I've seen plenty, but I'm not that 1 guy. I've had the pros loose pours and spend days cleaning up the mess. Can you tell I don't take formwork lightly. /forums/images/graemlins/grin.gif

 

[EddieWalker]

Chris,

You nailed it. I have no experience with arches or any sort of concrete that isnt' stright up and down or flat on the groud. I'm totally outside my comfort zone in this type of work, which is why I'm not seriously thinking about it. I like the look and the idea of it, but don't think it's worth the expense, time and effort as abosed to I-beams and wood decking.



Dave

The arches you drew are smaller than what I'd want. I'm thinking that to achieve the same amount of flow that two 9 foot by 2 foot openings, I'd need three 8 foot by 3 foot arches. Even than I think thats gonna be less flow on a wider footprint. My simple math fails me on how much more concrete that will be, but it is intimidating.



I've been thinking about attaching the decking and came up with runng PT boards the lenght of the I-beams. Drilling and bolting them together, than I can screw the top decking down to the beams. Does this make sense?

Eddie

 

[bugstruck]

Eddie,

You could run PT boards the length of the beams (plate them). I'd go with 1 1/2" thick material over 3/4" material (or double the 1 1/2" material) and use 3" minimum screws, 3 1/2" screws would be my preference. Have the steel company punch 5/8" holes at say 27" centers, staggered, alternating sides of the beam and at 6" from the beam ends (assuming 120" long beams). Mount with 1/2" carriage bolts and draw the carriage head flush with the wood plate. You may have to counterbore the carriage heads 1/8" into the wood if that wood is excessively dry but I doubt that.

The only other wood deck alternative I can think of is to drill one hole for each board and carriage bolt each board down. That would take some more time and $ though.

You likely know this one, but I'll offer it for others. Turn the crown in the annual rings in the lumber up. That may not put the best board face up in every instance, but the cupping in that Texas sun will be in the conrtolled direction and you won't have many, if any, boards needing replacement in the early years. Doing it that way also causes the moisture to wring out of the loose side of the board first, putting some balancing tension on that top exposed face of the lumber, to counter (balance) the natural tension side of the board to a degree.

You and your projects are an inspiration for what can be done. So I ponder, how do you do it all? I'm tired just thinking about all your work /forums/images/graemlins/smile.gif I think I'll go clear coat a couple of pine ceiling medallions I made up earlier. I can handle that. /forums/images/graemlins/wink.gif

 

[Defective]

</font><font color="blue" class="small">( OK, I like this idea. Angle iron in an upside down V.

Would you weld it into position, or weld a plate on the end and bolt it in? I think the bolts would be better because I can fabricte them in the shop and install them easily in the field.)</font>

Exactly!

I'd weld them in if I had the equipment to do so in place. Otherwise, no reason not to fab & bolt. I spend 9 hours a day painting structural steel and don't see much that's not bolted together when it goes up...

 

[radair]

Yes, bolting sill plates the length of the beams gives you something to screw the deck into.

Here's a photo of a 50' long, 5' wide bridge we built. The vertical posts at spaced at 5' and are bolted into welded web stiffeners that extended out past the flanges.

 

[radair]

Another picture of the steel frame. We bolted horizontal braces into web stiffers on the inside, and used diagonal bracing bolted to the bottom flange, both at 10' o.c.

Just to give you more things to think about.

 

[EddieWalker]

Radair,

Thanks for the pictures. I'm glad to see that's how others attach their decking to the I-Beams. The thought of drilling holes for every piece of decking was a little overwhelming.

How far is the distance between the beams, or maybe I should say, how far does the decking span? I'm thinking of three foot centers and figure to span about 32 inches with 2x6's on their flat side.

Do you have much flex or movement with that much of a span?

Thanks,
Eddie

 

[radair]

The outside width is 5' and we used 2 x 8 sills, so the decking spans about 42.5". We used 2.5" thick rough sawn Tamarack (Larch) because it's very rot resistant. The stuff has held up well so far. It's not flexy at this thickness and span.

2x decking spanning 32" might be a little springy, but you could easily check it with some pieces of scrap and a couple blocks. I suspect it would be OK.

 

[CurlyDave]

Eddie:

Let me turn your thinking in a totally different direction.

How big is your 4000# tractor? How big is the 2000# implement you are thinking of carrying?

The reason I ask is that I suspect the tractor is larger than 7' long by 4' wide (Overall length & width, not wheelbase). When I calculate psf, I get 143 psf for a 4000# tractor 7' x 4', which I think is pretty conservative. It will probably be bigger than that and therefore lower in psf. The implement will probably also be under 150 psf.

I think it is perfectly possible to construct a wooden bridge capable of carrrying this load with 10' spans.

Look at this website Southern Pine Span Tables and then go to table 13 Table 13 - 150 psf live loads, 10 psf dead load .

What this shows is that if you used 2" x 12" boards, Grade #2, on 12" centers you could span 12' at a 150 psf load. Since you are only planning on a 10' span, the acceptable load would increase. The 10 psf dead load would represent the deck.

Now someone is going to jump up and and say, well, the tire loads are a lot higher than 143 psf. As much as that is true, the way these tables are calculated is that the span is rated for 150 psf on each and every square foot of the span. As long as the tractor and driver are the only things occupying the square footage under the tractor the structure will spread the load out.

A simple example. When a 100 lb woman puts all of her weight on a 1/2" x 1/2" high heel, she is exerting 57,600 psf on the area under her heel. The floor she is on is not designed for that load everywhere, it spreads the force over a much larger area and she does not fall through.

When I stand with my 200 lbs on one of my size 11's I am a load of 800 psf, if I put both feet down I am 400 psf based on contact area. Again the floor spreads my weight out.

If you could get 20' long 2 x 12s, and still use the center support, you would have "continuous beams" which are even stronger than the "discontinuous beams" span tables are calculated for.

You pick up another safety factor since with beams on 12" centers a 6' wide bridge will actually have 7 beams, so the load can be increased to 7/6 of that shown in the table. In very wide spans this one extra beam is not significant, but in this case, you have almost 17% more load bearing capability.

I would deck a wooden bridge with 2 x 12's instead of 2 x 6's because the wider boards would give better load sharing between the beams. The cost difference will be immaterial--the number of board feet is the same.

They don't have a pressure treated table for 150 psf, but the 100 psf tables for PT and non-PT appear to be the same.

I haven't priced steel lately, but around here a 20' PT 2x12 is about $25-30. Compare $200 worth of PT wood with whatever the steel would cost...