Rolling Your Own Wood I-Beams for Model Railroad Benchwork

By Craig Bisgeier

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(Note:  Be sure to read the November 2003 Update to this article, which can be found by clicking here.)

Ever watch those home improvement shows on TV? If you do, you've probably seen a new product for the home-building industry that's really nifty -- Laminated wooden I-beams for floor joists. These 100% fabricated beams use much less old-growth wood and weigh less than their equivalent in standard dimensional lumber.  And, their improved stiffness and strength can span greater distances than regular dimensional lumber of the same size.

If applied to model railroad construction, large layout builders could take advantage of those same features to build rock-solid benchwork with fewer legs, less wood and longer spans between legs. The cleared space can mean better under-layout access, more storage space, even floating sections and cantilevered ends. And less wood means less expense.

But, there are some problems with these commercially made beams.   They are really, really big!  Generally, they are too heavy for our simple model railroads -- after all, we aren't building homes -- and they are not cheap. You pay dearly for that great strength and you'll never use most of it.  Getting them home is another issue -- Imagine trying to tie a few of these 24-foot babies to the family car -- or paying extra to have the lumber store deliver them.  And don't even get started on getting them around a bend in the basement stairs! 

Well, don't despair!  Take advantage of this new technology by building your own I-beams!  By fabricating lighter, thinner beams from commonly available (and transportable) lumber, we can save wood and clear out lots of space under our layouts.  And still take care of our lighter-duty needs with plenty of strength. if your long benchwork spans will only be about eight feet long, it probably isn’t worth trying, but for those who need longer spans… read on! 

What it's made of

Examining a commercially made wood I-beam reveals the web is made up of a 3/8" sheet lumber product similar to ordinary chip board, or OSB. Look at the commercial beam in the photo to the left.  The depth of this beam is approximately 10".  The flanges on either edge are made of a layered and bonded wood composite, like plywood but with the grain in each layer running horizontally in the same direction. The flanges are functionally continuous, with no obvious joints, but the webs are 8’ lengths of chip board butt-jointed and glued together.  It makes a strong structural beam capable of supporting thousands of pounds.

For my layout (and as an example), I am building plywood I-beams with a pair of 1x2’s as the flanges, sandwiching a web of 3/8" plywood.  (OSB could be substituted and is likely even cheaper.)   Dimensions of the finished beam will be a flange 1-3/4" wide by x 1-1/2" deep on each edge, and a web depth of 8", in spans up to 30' long. An example is shown in the background of the comparison photo.  This should provide strength about equal that of a dimensional 2x8. More than strong enough to support the layout weight, but much lighter. And the cost per foot should be considerably less than the comparable dimensional lumber. In fact, the materials work out to about $.61 a linear foot, less screws and glue.  The commercial beams work out to about $1.25 a linear foot.  Quite a savings.

Now, because I'm psychic, I already know some of you are thinking, "Hey, how strong can those beams be when the plywood only comes in 4'x8' sheets? Ain't you gonna have breaks in the web? Ain't that gonna weaken the beams?" Yes, and no.  There will be breaks in the web.  But that’s OK – the professional beams have joints every 8’ too. our beam's support strength comes from the tension and compression properties of the 1x2 flange boards, which is very, very strong in dimensional wood products. Our flanges may not be laminated 2x3’s, but glued and screwed to the beam web they will be more than strong enough for our needs. 

Making the I-beam

Tip:  Have the guys at the home center cut the plywood or OSB for you on their big panel saw.  You usually get two cuts free, extra cuts are about $.50.  You need 5 cuts, and the extra $1.50 is worth it -- especially if you don't own a pickup truck or van.  Tip #2:  Why waterproof glue? Someday you might have a broken water pipe or some jerk may spill a soda -- you really don’t want your main structural beams to start de-laminating as the water seeps in... Spend a few pennies extra now and avoid the heartache later.

If you didn't have the home center cut it, start by ripping down the plywood lengthwise, in strips measuring the full web dimension you want to build. For my 8" beams, I cut 8" wide strips and get six web plates from a regular sheet of 4x8 plywood. I cut mine on a table saw to get the straightest cuts possible, but you could strike a line on the panel and use a jigsaw or skilsaw, cutting carefully.

The flanges on the beam will be built up of 1x2 (actual size 3/4" x 1-1/2") boards glued and screwed to either side of the web plate, on both edges. This will yield a beam with two flanges 1-1/2" thick and 1-5/8" wide, assuming 3/8" thick web material.

Because you probably won't find furring strips in 16' lengths, you’ll probably use 8’ flange boards to straddle each joint in the Web material. You want to have 4’ of flange board straddling each break in the web. This is critical - if the joints on the web and the flanges are too close your beam could fail.  

At the ends you’ll need to cut shorter lengths to make up the difference on either side of the main flange board. For instance, if your use two 8' webs, you would center the 8' flange boards evenly across the web joint, and you’d need two extra 4’ boards to either side on the ends to make up the 16'. Mark the 8’ flange boards at their center, line this mark up with the joints in the web panels. Cut, fit and mark all flange boards before proceeding to the next step.

Getting it together

Lay your web plates for each beam on the floor butted together end to end. (Hint: Try screwing on a wood cleat to hold the webs together temporarily, or duct tape them together in the center.) Place two 8’ center flange boards above the web plates. Apply glue generously over the first flange board, spreading it evenly using a piece of cardboard. Lift the edge of the web plates and place them on the flange board, aligning them carefully and making sure the web plates are butted tightly together. (It may be helpful to support the back edge of each web plate with a piece of scrap 1x2 to keep the boards level).

Don’t hurry, but work quickly. Spread glue on the top of the web plate edge and lay down the second flange board. At this point, you should clamp the three layers together about every foot or so and check that all outer edges line up and all butt joints are tight. Uneven edges and butt joints with noticeable spaces will deform the finished beam, so take a few moments and be sure it is done right.

Drill and countersink holes through the flange plates about every 12" or so, staying 3-4" away from each flange board joint (to keep the lumber from splitting). Countersink the holes about 1/8" to ensure the screw bites into the other side of the flange. Using a power screwdriver, drive the 1-5/8" drywall screws into the flange. Be careful not to split the 1x3 flange boards by driving the screws in too far. A snug fit is enough.  (I use one of these flip-things, with a drill & countersink bit on one end and a screwdriver bit in the other.  You can see it in the picture to the right.  I recommend it highly, it saves me a lot of time.)

Once the screws are in you can start the other edge, repeating the steps above. Repeat the process for each successive web panel to be added. When all center flange boards have been applied, go back and add the filler pieces on the ends to finish the beam.

Bridge Building

Now that the main beams are done, we need to tie them together into a rigid structure that will support the load.  This will be a lot like building a deck girder bridge.   The I-beams can support a lot of weight but only if they are kept vertical and rigid.  The bracing we add will keep the beams from twisting and falling over to their sides, and also keep the whole assembly from wiggling like a hula dancer.  Only a little bracing is required to make a really strong foundation. On my own layout I'm spacing my beams about 24" apart to support a 48" wide layout surface, two feet of scenery to a side. For a set of 16' beams, I use three cross-braces spaced about 8' apart along the beams’ length, creating two rectangular boxes between the two beams.

Each cross-brace is simply a rectangle of plywood cut to fit the known dimensions, in this case 8" tall by approximately 24" wide. Glue and screw an 8" chunk of 2x4 stud to each 8" edge as a screwing block, since plywood does not take screws in its edge very well. Cut the profile of the beam flanges into the end of the braces with a jigsaw so the cross-brace edges will butt up to the beam web tightly. Then glue and screw the cross-braces to the beams using 1-5/8" all-purpose screws.

Finally we have to put in angled bracing to stop the structure from wiggling in the middle. Only a little bracing is necessary, but it is important. I originally used metal X-braces I salvaged from some old IKEA furniture, but they were expensive and weren't long enough to do the job properly.  I've since replaced them with two 4-1/2' lengths of 1x2 which is a lot sturdier, and since they each create a right triangle shape with no un-braced areas, it's very stable.  Cut them so they fit snugly into the corner of the cross-brace and up against the beam web in a corner, resting on the flange edge.  Screw them to the beams with 1-5/8" screws. Square up the cross brace and screw the other end of the brace to the opposite beam. Do this on both the top and the bottom of each rectangle in the structure, one brace on top and bottom.

Legging it out

With the bridge built, we have to consider supporting the layout. My 16’ bridge needs only four legs to support all the weight I’ll put on it. (Just to be sure, we stress tested it by placing a 340 lb. load in the center of the truss.  The total deflection at the center was just 1/4".)  I’ve located the legs at the far corners of each beam, but you could also cantilever the end or ends and set each leg four feet in from the ends to distribute the load evenly.  I wouldn’t try to cantilever more than four feet, though. On a beam longer than 16’, I’d be comfortable spacing my legs out every 16 feet or so without concern.  You might also want to pay special attention to the bracing in the cantilevered section.

For building the legs, I recommend the following practice: First, look at the cross-section illustration to the left. Red represents the I-beam, blue the leg. Make a cardboard template of your beam and use it to trace cutting lines on each leg. Then cut the notch profile of the I-beam into the top of each leg, a 2x4 stud. A good jigsaw will make short work of this. Cut carefully and try to keep the gaps as small as possible, as a tighter fit will help stability. 

The leg is bolted through the flange of the beam with 1/4" x 4" carriage bolts (one through each flange), and screwed through the beam web with 1-5/8" screws. This arrangement will (a) solidly transfer the weight of the layout from the beam to the legs, and then to the floor; and (b) keep the leg from twisting and breaking under the weight. Be sure to cut the legs long enough to reach from the top of the beam to the floor. The top of the leg should sit flush with the top flange on the beam, so as not to interfere with any joists you'll attach later.

Depending on how far apart you space your beams, you may decide to place the legs facing inside or outside of the beams. If your beams are 16" or less apart, definitely mount the legs to the outside for stability! Otherwise either way will work. Mount the legs opposite one another and cross-brace them against each other using good 1x3 boards in an 'X' pattern. The two carriage bolts should obviate the need for any end-to-end bracing, but it’s probably prudent to add them anyway.

At this time your support structure is ready, and you can start building the benchwork that supports the railroad itself. You can lay joists across the I-beams as you would in regular L-girder construction, or build a grid directly atop the I-beams. If you want a larger surface to attach your joists to, screw 1x4 boards onto the tops of your I-beams with one edge flush to the beam edge. You now have the biggest L-girders ever made! We’ve done this at Henry Freeman’s layout and it works really well.

Height considerations

Setting the beam height as tall as possible gives you several advantages. More under-layout headroom, easier access to under-mounted items like switch machines and wiring, and short risers from your grid or L-girder benchwork which saves you $$$ in extra lumber. To set the beam height, work from your 0" base layout height (Lowest track point modeled). Subtract track, roadbed and subroadbed thickness, as well as the height of the grid or L-girder benchwork resting on the beams. Finally subtract an extra inch you’ll want to run wires, switch machine linkages and other items. The height you come up with will be your leg height (remember our I-beam legs reach to the top of each beam).

Other things you may need to take into account are hidden staging yards or track that may dip under the base layout height, or scenery like ravines or rivers that dip below the lowest track level. You may need to consider setting the beam height lower for clearance in these areas. If your hidden track dips low enough, you can cut a hole in the web of the beam large enough to pass the track through without weakening the beam too much. An additional leg for support in this area might be a good idea. Don’t cut into the beam’s flange, though, as that will weaken the beam considerably.

Alternate methods

(Note:  Be sure to read the November 2003 Update to this article, which can be found by clicking here.)

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