# Question & advice sought on long span support



## OilValleyRy (Oct 3, 2021)

I’ll try to explain the design scenario as clearly as possible, followed by my already considered solutions.

My plan is a triple deck layout of modular construction. Each module is 6 ft long. We only need worry about mid & top decks for this.
My primary yard consists of 2 of these modules on the too deck. Directly below that on the middle deck will be a 10 foot long uninterrupted scenic scene of the mainline snaking along the Allegheny River. It will be one of those “layout defining” scenes that are instantly recognizable.
Obviously that length span, with a module seam, is rather unusual and presents challenges. My conceptual solution is using 1 single 12 foot long support “beam” from the outer front leg from each module. Essentially, a 12 foot L girder that won’t sag is what I need. The question then becomes, what to use as that “beam?”

I considered, and ruled out the following (with reasons):
Chain link fence top rail - too weak.
Unistrut - needing 20 foot cut down with 8ft wasted material & money.
12’ PostMaster from MasterHalco - Availability issues.

I’m no civil engineer. Any ideas on material solutions would be helpful.


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## Magic (Jan 28, 2014)

For a span that long I'd take a good hard look at some 6 foot pieces of 
Aluminum angle or better yet channel. Use a doubler plate bolted on for the splice.

Magic


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## mesenteria (Oct 29, 2015)

For that span, an 'L-girder' made of 1X4 would suffice. You will need three of the 1X4's, between 10-12' as you deem necessary. The third one is ripped into two equal halves, lengthwise of course, and the other two will comprise the vertical component, or the 'web' of the girder. There is only one plate, a top plate, and those come from the ripped third length. You glue AND screw to ensure a really robust mechanical connection between the two. Use 2" screws driven down from the top, through the thinner plate and down into the4" web. I use one screw per 10".

When you support those properly on frames at either end, two of those will be able to host a good sized dancing party on them. Okay, I'm kidding, but you will find that you can get up on it to do work.


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## Lehigh74 (Sep 25, 2015)

If an L girder is good, a C channel is better. That’s what I made to span 10 feet for my staging area. I used clear (radiata) pine, ripped one in half and glued / screwed the halves to another piece. Actually, in my case, I didn’t rip them in half. The one against the wall has support in the middle (screwed to the wall), so I ripped the boards with different widths (2 3/8” and 1 1/8”). The channel for the unsupported 10” length has a 5” web and 2 3/8” flanges.


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## CTValleyRR (Jul 26, 2014)

Lehigh74 said:


> If an L girder is good, a C channel is better. That’s what I made to span 10 feet for my staging area. I used clear (radiata) pine, ripped one in half and glued / screwed the halves to another piece. Actually, in my case, I didn’t rip them in half. The one against the wall has support in the middle (screwed to the wall), so I ripped the boards with different widths (2 3/8” and 1 1/8”). The channel for the unsupported 10” length has a 5” web and 2 3/8” flanges.


Not necessarily. There is such a thing as overkill. L girder is plenty strong enough to support a layout across that width, and you save yourself the price of 2 12-foot 1x4s.


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## OilValleyRy (Oct 3, 2021)

Thanks guys. I honestly thought an L girder that length would sag without some sort of gusset or mid-span support. I like that C channel idea. In fact if I reversed orientation I could also include strong stubs to secure fascia to it. I might pay extra for birch for humidity reasons.
I’m not sure where the line is between overkill & peace of mind.


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## CTValleyRR (Jul 26, 2014)

OilValleyRy said:


> Thanks guys. I honestly thought an L girder that length would sag without some sort of gusset or mid-span support. I like that C channel idea. In fact if I reversed orientation I could also include strong stubs to secure fascia to it. I might pay extra for birch for humidity reasons.
> I’m not sure where the line is between overkill & peace of mind.


There isn't one. You have to be happy with it in your own head. It's your layout afterall. You're free to build it with 8"steel I beam if you want. A properly constructed L girder won't sag, but if you need the extra for piece of mind, than by all means do so.

Don't pay extra for birch, though. While it will be more dimensionally stable than pine, you need to seal the wood, either with several coats of oil-based stain, or with latex / water-based paint /stain and a polyurethane top coat. Otherwise you will see changes due to humidity.


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## OilValleyRy (Oct 3, 2021)

CTValleyRR said:


> There isn't one. You have to be happy with it in your own head. It's your layout afterall. You're free to build it with 8"steel I beam if you want. A properly constructed L girder won't sag, but if you need the extra for piece of mind, than by all means do so.
> 
> Don't pay extra for birch, though. While it will be more dimensionally stable than pine, you need to seal the wood, either with several coats of oil-based stain, or with latex / water-based paint /stain and a polyurethane top coat. Otherwise you will see changes due to humidity.


Most of my bench work will need to get painted backdrop color. Nothing worse on a middle or lower deck than seeing obvious supports for the deck above. Well, might be worse things, but you know what I mean.


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## CTValleyRR (Jul 26, 2014)

OilValleyRy said:


> Most of my bench work will need to get painted backdrop color. Nothing worse on a middle or lower deck than seeing obvious supports for the deck above. Well, might be worse things, but you know what I mean.


Yes, I hear that. Just keep in mind that latex paints breathe, so the wood can still absorb moisture, Either use an oil-based paint, or top coat it with polyurethane.


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## Matison (12 mo ago)

CTValleyRR said:


> Yes, I hear that. Just keep in mind that latex paints breathe, so the wood can still absorb moisture, Either use an oil-based paint, or top coat it with polyurethane.


Laminated wood with alternating grain directions is extremely stable. You will have much less movement, as each ply resists the others from doing so.


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## CTValleyRR (Jul 26, 2014)

Matison said:


> Laminated wood with alternating grain directions is extremely stable. You will have much less movement, as each ply resists the others from doing so.


Granted. That's why plywood is a thing. But it can still warp -- it just tends to twist rather than bow as each grain pattern warps in it's own direction. No matter what you build it with, sealing the lumber to prevent absorption of water is still far and away your best option. Especially when we use so many water-based products in creating landscapes.


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## timlange3 (Jan 16, 2013)

I made a box beam with 1/4" plywood. 2" wide, 3" high, and 9' long. There is less than 1/16" deflection when two HO scale SD40's cross over it. I just cut strips of plywood and glued them up using 3/4" x 3/4" x 3" blocks every 12" to give more glue surface. I would think a 2" x 4" x 12' beam would fit the bill.


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