4/11/2009

My son and I built this bridge to allow Matchbox truck traffic to cross the Great Television Chasm.  (Ok, I did most of the building, but he does like to put the diagonal braces on--not bad for a child under three years old.) 

The construction method of choice was the Girder and Panel set “Bridge Mania” from Bridge Street Toys (http://www.BridgeStreetToys.com).  After completion, we got out some of my childhood "Made In England by Lesney" Matchbox cars and trucks.

I have to say, these Girder and Panel sets are great toys!  This bridge was up for three months, probably, without needing any repair.  It finally did begin to sag, as you can notice from the pictures, but there was virtually no sag when I put it up and for quite some number of weeks afterward, even with the die-cast vehicles on it.

I'm not an engineer, so I may have designed it incorrectly.  It's supposed to be a Pratt Truss, but you can see in the pictures that the braces at the very ends of the bridge started bowing quite a bit.  I thought these were supposed to be in tension, but clearly they are under compression instead!  Anybody know what I should have done to prevent the bowing?