This is a truss bridge very similar in construction to the Arched Truss Bridge , the previous bridge that I'd built. This bridge was completed sometime after the arched bridge, in 1997 I think. It's been so long, I don't really remember! (sign of old age?)

Unfortunately, I only took a few pictures and they were quite poor in quality. The one you see to your left is two photographs that were spliced together. It is a little out of focus, as are the rest of the pics.

Total Length: Approximately 8 feet (2.4 m)

Ultimate Capacity: not tested

Structure Type: Three Span Deck Truss Bridge

I built this bridge in similar construction to the previous bridge , one difference being the shape of the structure and the piers.

Another difference is that I did not double up on the vertical members. Therefore the bridge did not use as many members per given length so I had extra beams to make it a little longer.

I did not construct a roadway for this model. If there had been one, it would have sat on top of the truss.

The columns (piers) of the bridge (the brick "towers" that make the bases) involved a new concept in my building that I'll explain below.

The piers that support the truss are columns made from basic bricks. I was afraid that the brick column would not survive any side loading, and easily break apart. (I was worried about people bumping into this large model)

If a lateral load was applied, the columns would be eccentrically loaded. This means a bending moment, as well as a downward vertical load, acts upon the stack of bricks. As you can imagine, a stack of bricks has no moment capacity as tension loads cannot be resisted. The bricks on the tension side of the column would seperate thus bringing the whole thing crashing down.

My idea was to put the column into a pre-compression state that would hold the bricks tightly together. The pre-compression "reacts" against any tension endured from an eccentric load (when a tensile load is applied, the net axial load remains in compression because the pre-compression force is greater than the applied tensile force). Although the pre-compression takes away from the capacity of the column, it is only a small percentage of the total capacity.

The images to the left show a simple cut-away view of a column. There are strings that attach the base and the top. These strings are put into tension, and thus compress the column.

The gears and axle at the base are "cranked down" to tension up the strings. The main gears are locked to prevent releasing the tension in the stings.

Images created with MLCAD. See www.ldraw.org for info on the program.

LDRAW file (DAT) of this model:

* Column: column.dat

This was my second attempt at a large TRUSS structure and I succeeded in making a good-sized creation. A the time, it was the largest static MOC (My Own Creation) I'd built.

I left it unfinished as I did not build a roadway on top of the truss. I certainly had enough pieces to build a complete, yet narrow, road made of large plates.

I would like to build another large bridge someday. I would certainly make a larger structure, as I've added many more Technic beams to my collection since 1997. Perhaps I could go for 12 or 15 feet!

This bridge was my last MOC until the Crawler Crane in 2001. 1997 to 2001 wasn't quite a "dark age" in my Lego hobby, but I really didn't produce anything of my own creation. I finshed my last year at college and began working as a structural engineer. My job kept me fully occupied until about Fall 2000. That was the REAL beginning of the Crawler Crane.