Structural Engineering 101 (a benchwork solution)

[FJ and G]

On Saturday, I started construction of level 2 and REALLY needed advice from this forum (which was locked up for some strange reason).

In the first photo, you can see where I ripped 2x4s and created a frame. I layed 1/4 in. masonite over that and on Sunday morning the whole thing was sagging. How to shore it up without adding a middle vertical column?

I thought and thought and came up with this strategy which certainly is doing the trick. I built an identical frame about 8 inches over the first and connected with all thread. Hacksawing the all thread took all day, as you can well imagine. Is there, I wonder, a simple way to get a nut off a long all-thread.

Those nuts just take lots of time to get off.

I then added the 1/4 in masonite over that and it's strong enough now to sleep on. The structure acts just like a girder bridge, with the upper and lower parts taking the strain in a tug-push fashion.

The bad part is that I lost quite a bit of space and the layout will now become a 3 layer vice 4 layer layout.

However, what I gain is structural stability and, incidently, a center portion to store wires and transformers for power and special effects wiring (note the shelf I put in between.

To protect the lower level layout (the finished portion) from sawdust, I covered the forest and stuff with newspapers.

I may start on the top level woodwork tonight and then start putting the tracks and stuff down.

The center layout will be traditional 3-rail layout with industrial stuff and top layout will be hi-rail custom made track in wilderness scenery with forced perspective.

[lionelsoni]

Dave, it looks like the shear load at the ends of the trusses is being taken mostly by those steel angles. If the bolted connections to them ever loosen up, you could get your sag back. You might consider bolting some web plates to the inside surfaces of the chords at the corners to guard against this.

[FJ and G]

Bob,

Forgot to mention that I thought about this and came up with 2 solutions.

1. The lower end of the steel uprights are bolted to extra boards (about 12 inches worth) so that they are firmly anchored at the bottom.

2. The wood frame rests on protruding bolts from the steel uprights to bear the weight, as well as being attached by screws to the uprights. This allows all of the force to bear downward on the steel uprights and resting on the bolts mean that the weight won't tear out the screws connecting the frame with the uprights.

[Dr. John]

Here's an idea for taking the nuts off of threaded rod:

Place the end of the rod in your drill chuck. Place the nut in a pair of Vice-grips. Anchor the vice grips to a bench vice (assuming you have one) slowly use reverse power on the drill, backing the rod out of the nut (just like a VERY long drill bit.)

I am making some assumptions, number one that the rod is small enough to fit into the drill chuck. If not, never mind!

[wrmcclellan]

Dave,

This is where you need the structural guru - BUCKEYE.

I think your new shelves will also sag eventually as there is no triangulation in your new 8" high frame to shift the span loads (where it will sag in the middle) to a corner where there is firm support.

One way to combat this is to have the 8 foot long sections be thicker vertically (ie the original 2x4 or better, a 1x4 - you do not need the horizontal thickness). The deflection of your longitudinal members is dependent upon the load (including the weight of the member itself).

One approach if you are willing to accept a little interference is to run those all thread rods from top to bottom for all 4 layers of the layout at one or two midpoints in the span. This way you can redeploy your third shelf you just built.

Another approach would be to suspend the entire assembly from the ceiling (which should be strong enough to handle the 4 tier layout weight (particularly since it appears to be up against a load bearing wall). You could even use some 1/8 inch stainless steel rope braid as the center span support mechanism. Use cable clamps as the stop for each layout tier.

Regards,
Roy

[FJ and G]

Dr. John,

That's a geat idea! I'll try it for the next shelf.

Roy,

Great ideas. I don't think I want to put anything in the middle that would distract but the lateral force thing makes sense.

Hopefully, by the time it starts sagging my garden RR will be done and I can just tra***his wheeled monster (or put a lawnmower engine on it and turn it into an automobile to save gas on my truck. It would be the first layout vehicle ever. :-)).

[trigtrax]

Dave to stiffen your structure you need to built what's called a torsion box. For one thing It would be better if you used 1/4 plywood rather than masonite, Add a few more cross ribs and glue and screw the plywood to both sides of the frame. This arraingement converts all forces to sheer stresses along the glue joints.
Soundwise it might be better if you filled the empty space inside this torsion box with sheets of styrofoam.

[Buckeye Riveter]

There are several ways to stiffen this including attaching the columns to the floor so that we minimize their rotation . Increasing the moment connection from the frame to the column. Increasing the Moment of Inertia. of the main outside supports. Adding two wire supports from the ceiling.

The threaded rods look good but they don't increase the resistance to deflection very well.

I really like making the outside members into a small 4" deep warren type truss with 1x2 diagonal members. The ends of the truss should be square so that you can take advantage of the stiffenes of the angle iron columsn. Of course the truss must be unloaded when attaching the diagonals in order remove the dead load deflection of the materials themselves.

[FJ and G]

THanks Buckeye and the rest. We have some real pros here.

I'm about ready to throw in the towel and take the 3 rail trains to the garden!

I should have studied up on trusses a bit more! My designs would probably result in Tacoma Narrows #2! :-)