Common trusses used in engineering:

Warren

Pratt

Howe

K Truss

The following figures are shown under a load. The numbers represent percentage, where the total load = 100. The numbers are rounded to the nearest 5. For absolute numbers, visit the Bridge Designer.

Red represents tension, blue represents compression, and green is no load.

On this Warren truss, each of the down arrows represents 50% of the load. Notice how the two middle pieces have no load. Notice the load is quite a bit less on the ends of the top and bottom chord than in the middle. When you build for efficiency, keep in mind that for a bridge loaded in the center, the ends can be smaller than the middle.

Now examine the same Warren truss, but with added vertical members:

The added vertical members serve to break the top chord into smaller segments, making it stronger. However, the percentage of the load has increased both on top and bottom. Also notice where the green members are now.

Pratt Truss

I want you to notice the difference between the Pratt and Howe trusses under a load. The Pratt has bigger numbers on both the top and bottom chord, but its middle compression members are shorter, and hold less than those on the Howe. But the load is still concentrated in the middle, and gets less further to the ends on both.

Howe Truss

It is a trade off between the Pratt and Howe. For the Pratt, you’d have to use bigger top and bottom chords, while on the Howe you’d have to use bigger compression members.

K Truss

The K truss looks very good on paper. It shortens the lengths of the compression members compared to the other trusses. However, one must wonder if it adds additional weight simply because of the number of members. It is really interesting to note the two green members on the K truss, in theory those pieces could be taken off. However, I had to include them to make the truss design program work. This shows only one orientation of the K truss. If I reversed the direction of the K’s, I wonder how much it would change the forces.

The one thing I don’t like about this truss is the long vertical compression member in the middle of the bridge. If that one member could be shortened or even eliminated, I think the bridge would become more efficient.The K truss would be the hardest of these trusses to build. This is something worth considering. Making a strong joint that would make the most of the switch between compression and tension of the vertical members would be difficult.

Post Tags: bridge, design, photos, tips, truss

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