# Structural question



## msimpson (Jan 5, 2009)

Hi all,

I am staggering toward a (semi) permanent outdoor layout -- a circle of Sunset Valley dual gauge track, 15 foot radius (30 foot diameter), no switches or other complications, and a run of about 94 1/4 feet. (Hey, there's an oak tree in the middle, so you can't be sure trains really run around in circles.)

I intend to make the deck from 24 roughly 4 foot sections of 6 1/4 inch wide artificial decking, using 4 x4 pressure treated pillars set in deck-blocks at the joins. Basically, each section covers 15 degrees of turn (a 7.5 degree angle at each end, times 24). While the track wanders from inside to outside, it should stay securely on deck.

My question concerns support under the decking. My original thought was to turn a pressure treated 2x4 on edge, so that the rafter and the deck form a T cross-section. It has been suggested that in a 45" run, I may get enough sag to have serious problems and that I would be better using steel 2 x 4 studs -- Apparently Bruce Gathman has done a nice job (on a double -tracked, two rafter set-up) with this material. Thoughts? Suggestions? Alternatives? Bitter personal experiences? Cost-benefit analysis?

The track will not support anything more than the trains, at step-over and slightly above height. Probably 18 inches on the high ground up to 36" at the lowest ground. We do have a large population of small (up to 130 lb.) deer, but I am hoping they will avoid a raised obstacle, or jump over it like they do everything else.

"Blessed be those who run around in circles, for they shall be known as Big Wheels."

Thanks in advance, Mike


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## Paul Burch (Jan 2, 2008)

Just to reinforce your thoughts. The decking will absolutely relax and sag without support. Good choice on the track.


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## williep99 (Sep 19, 2008)

If I understand your descrpition, I think the steel 2X4's will sag as well. They are designed for a "vertical" load and I dont think they would stand up unless you installed ones with 4 sides in which case the cost factor comes back in. I would opt for pressure treated 2X6's if they are in your price range. Or even some angle iron if you have the desire for the extra construction time and effort. If your deck doesn't stay intact you will be starting from ground zero in your build.

Thanks, Bill


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## Richard Smith (Jan 2, 2008)

Just be aware that the "artificial decking" isn't structural and needs a lot of support beneath. PT if good quality or cedar (painted) would be a better choice if you want it to span even a fairly short distance. You don't mention your locale but if you are in an area with very much snow you need to take that into account also. Wet snow can add a lot of weight. To span your required 4 feet for each section I'd advise at least 2x6 or maybe even 2x8 material on edge beneath. Hand pick the material your self to avoid warped or cupped wood and install "bow up". Also using deck screws instead of nailing will not only give you better connection but will better facilitate changes, modifications, repairs and reuse of materials.


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## msimpson (Jan 5, 2009)

Hi all, 

Thanks, and a little more information -- 

I live in Tallahassee in the sunny South (a/k/a Mike in Tallahassee), where the average annual snowfall is not any at all -- snow load is not a problem. Weeks of temperatures in the high ninties and low hundreds, fairly frequent rain even during droughts. We use a lot more weedeaters than snow blowers. 

To revise my previous specs -- if I run a piece of decking flat, screwed together to a piece of 2 x 6 turned on edge, to make a T, and suspend this T-cross-sectioned beam between posts on 4 foot centers -- won't that work? We use 2 x 6 s as stringers under decking on similar spacing and then let heavy guys like me walk on it, so a lousy 5 or 10 lb engine should be okay, right? 

I understand and agree with "bow up" -- just like in those fancy bridges -- and to using screws (allows disassembly and transportation to another location upon retirement). And I do not mean to be argumentative, just trying to get a feel if there are any serious problems with the idea or can I try to work this in on a budget? 

Thanks to all and for any further input. Best regards, Mike


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## Allegheny (Jan 2, 2008)

Hi Mike,

While the engines/trains are not all that heavy, the composite decking to be used as subroadbed can be quite dense/heavy, so the combination of the two will probably lead to sagging over time. You live in Florida, so snow loads are no problem, but consider the effects of sun and heat. Trex and similar composites are notorious for thermal expansion. If you have a 4' span between uprights and you fix the Trex to a vertically oriented 2x, as the Trex expands, it's going to either bow the wood upward or downward as the expansion coefficients of the two materials are different. I'd be willing to bet that some sections would bow up and others down, leading to a roller coaster effect. To avoid this, it might be better to figure out some way to allow the Trex to expand (slotted holes for where the Trex is affixed to the 2x's) or to find a support material with the same rate of expansion.

A way to add much greater rigidity to the system would be to add a second 2x4 to the bottom of the first one - i.e., affix the subroadbed to an inverted "T" of PT 2x4's. This would help keep the single 2x4 from bowing under the weight. You could even add a third 2x to make for a flat surface on which to affix the Trex. But at this point you're getting into much higher weights and costs.


The use of galvanized steel studding for support is actually a good one, but I'm not sure that the upright studs are necessarily the best choice. I have seen indoor layouts in O scale where the use of the floor channel was very successful at carrying some pretty heavy brass locos. The trick here is to give the subroadbed both vertical and lateral strength at the same time. The simplest way to do this is to pop rivet two sections back to back to form an H section with the joined portions being oriented vertically. Also consider that the resistance to vertical deflection will be proportional to the square of the depth/height of the vertical flange - so channel intended for 2x6 studding will make for a support 2.5 times stiffer than that from 2x4 channel. Also consider that the 26 gauge stuff you can buy at one of the borgs is not the only game in town. If you go to industrial supply places, you can get structurally rated steel studding and track up to 12 gauge (almost 1/8" thick), but even 20 or 18 gauge would probably be overkill.


Several years ago, I started playing with the idea of bending steel or aluminum square tubing using a heavy duty modified planetary ring roller. 1" square x 0.063" thick walled steel bent to a 15' radius is incredibly stiff and will easily handle a 15# center load on 48" support centers. The problem is that the ring roller has to be perfectly aligned to avoid rolling spirals - think of a gigantic spring! It also helps to have an electric motor to drive the thing as your arms will fall off after rolling only one 20' length!


Brian
Taxachusetts


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