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How do I Choose the Best Truss Design?
The truss is an engineering design that dates back thousands of years. A planar truss design is a simple triangle, or connected triangles, while a space truss has members that extend in a different direction to establish a third dimension. Trusses are used for framing bridges, roofs, floors, subfloors, and many other structures. The best truss design typically depends on the application.
A planar truss design is used for bridges and framed roofs. For a roof, the triangles connect the roof rafters and the ceiling joists; the load is spread between the rafters and joists. For a bridge structure, the top and bottom of the joist are parallel and are called chords. Solid girders to support the load on a bridge would add greatly to the weight and cost; the truss supports the load at varying degrees, depending on how far apart the chords are spaced.
Tried-and-true planar truss designs include the Pratt truss — with boxes that contain one diagonal member per box — and the king post truss — with one vertical member and two angled members connecting to the top of the vertical. Queen post trusses have two vertical members and a top chord comprising a box that are supported by one angled member on each vertical. Lenticular trusses, commonly seen on bridges, have the top chord of the truss running at a gentle arch, thereby giving a lens shape to each structure of the truss.
For a space truss, a tetrahedron or pyramidal shape is the most common truss design. More complicated space trusses will link and configure tetrahedrons in various ways to distribute load, torsion, compression, and tensile forces. Space truss designs can be seen in structures, such as high-tension line pylons. Also, space truss webs are used for commercial building roof structures.
Analysis of truss designs is a complex process, but usually analysis is done at the joints, or hinges, of the structure rather than along the length of the members. The load that is exerted by a passing car or train, for instance, is considered a moment in which the stresses are mainly on the joints and the stress exerted along the lengths of the members are negligible. Graphical diagrams, such as the Cremona diagram or Culmann diagram, are used to calculate the stresses on the truss members themselves. To further examine these types of loads, formulas such as the analytical Ritter method can be used.
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![Roof trusses are generally made from two by four stock.]()
By: Greg PickensRoof trusses are generally made from two by four stock.
Discussion Comments
With regards to common roof trusses, the most popular truss is the fink truss (members form a "W" shape). For spans of about 30 feet, this is the most economical truss.
As the span of the truss increases, the distance between each joint increases, thereby lengthening the top and bottom chord members. The top chord members are generally in compression so their length is limited by buckling.
Hence, as the span increases, the complexity of the truss must increase to keep the length of each panel at a reasonable level.
We have been given the stereotypical assignment in my physics class to build a bridge using toothpicks and sticky tack. We have to make it stretch across a two foot space. Then it will have a paper cup attached, and pennies will be put in until it falls. I really want to have the best design.
I have been looking at a lot of pictures of trusses used in houses and bridges. Does anyone have any suggestions about what the best design would be and how I should go about building the bridge? I think the best method would be to have something simple like either a Pratt truss or a king truss in the shape of a roof rafter.
I think one of the keys would also be to make sure I get the toothpicks as much on the same plane as possible, since I think ones that aren't aligned could pose a problem.
I have seen a lot of the power lines that use the tetrahedral truss system, but I never thought of it as being in the same category as bridge design.
As far as bridge building goes, does the shape of the chords have a significant effect on the total weight that a bridge can support. Like the article says, I see a lot of bridges with lenticular trusses. Because the metal chords are arched and not straight, does it take away a little of the overall strength of the bridge in exchange for a more aesthetic design?
Between the Pratt, king, and queen trusses, which one has the ability to be the strongest overall?
@Izzy78 - I found myself wondering whether suspension bridges are classified as using a truss design. I know that they usually have a few triangular sections near the posts that go up, but I think for the most part, the strength of suspension bridges comes from the cables running down. Does anyone here know exactly how suspension and truss bridges are different? Which design overall is stronger?
Does anyone know where the truss design originated? The article says it was thousands of years ago. I would guess it was probably something that came out of ancient Rome or Greece. Was there any famous inventor or architect behind the design, though?
I was also wondering what some of the other bridge design techniques were called. I know on smaller bridges, the lower part is often in the shape of an arch so that water can run through, but there is no truss system. All of the bridges I have seen that go across long stretches of water are trusses, though. Am I right to assume that this is the strongest design, or are they just built with trusses because they are nicer to look at?
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