[Alavinus]

Overall equilateral triangles are probably the best way to go if you want to remove that much matierial. But are you sure you are going to need it? My former team built a 12 foot arm last year using roughly equal pieces of 2*2, 1.5*1.5 and 1*1. After cutting the peices the arm was under 20 lbs, so we ddin't need to reduce weight. (Although it didn't look quite as cool)

Here are some possible answers

1) Bridges are built tall in order to accomodate tall trucks. Having a triangular opening at the top would force the bridge to be either unecessarily tall or have issues with tractor trailers moving their cargo. The supports underneath a bridge are not triangular because it is fairly hard to find triangular stock. Most places that I've seen have a majority of square and rectangular stock.

2) The Circular holes leave more matierial to attach to I believe. Also, when designing construction equipment, you need to have a very large saftey margin because of the things they do. (Ever heard of bumper cars with pans or bulldozers?)

3) Again triangular stock is much harder to come by. Square tube is much easier to order cheply and probably easier to make. Also, if you look at boom cranes, I have seen those as an open truss triangular design, as well as square with triangles running down the center.