A truss is a structural system composed of members designed to resist only axial loads (tension or compression). Ideally, the members see no bending moment or torsion.
In reality, the members in a truss do see a small amount of bending and twisting, however these loads are small and the axial loads (tension or compression) are significant.
How is a Truss made?
To create a theoretical truss, all members must be connected at their ends by a frictionless connection. The center axis of each member should intersect exactly at a common point.
Connecting members in this manner does not permit bending moment to be transferred through the joint (the connection point), as the ends are free to rotate.
Actually, a truss is made by connecting the ends of the members by welding, bolting, etc. The neutral axis of each member may only come close to intersecting at a center point. This means that the members will see some bending moments, however the significant loads will be axial (tension or compression).
Why?
Trusses are used because they make very efficient use of the strength of the members. They are light and use less materials than a comparable moment-resistant frame.
To help understand this, go to your pile of bricks and pick out a long, slender brick (like a 1x16 Technic beam). Try to bend it by placing your thumbs in the center and your index fingers on the ends. It will flex easily and you should get the feeling that if you pressed hard enough, you would break it.
Now, take two long axles and put them through the end holes of the beam. Wrap your index finger and middle finger around the axle, with the beam between your fingers. Do this at both ends of the beam. Try to pull or compress the beam (i.e. apply load so that it is in the direction of the length of the beam- the axial direction). You should get the feeling that it would be very difficult to break if loaded in this manner.
A truss loads each member in this way (axially).
How about some examples?
Figure 1 (to left) shows several examples of simple 2-dimensional trusses. The trusses are just simple schematics of how the members should be arranged.
The ends of the members all connect at a common point (the joint). The members are connected so that only triangles are formed.
The resulting structure does work, but the members see a substantial amount of bending. This structure is called a frame. The joints (connections of the members) must be able to transfer bending moment and consequently, the members must be sized larger to handle the moment.
A Closer Look...
Figure 4 (to left) shows a truss structure equivalent to the frame. The members are slender and lighter than those of the frame. Compared to the frame, you will notice the addition of a 5th member: a diagonal connecting two opposite corners. This also divides the square frame into two triangles.
Figure 6 (to left) shows the truss with the lateral load reversed. In this case, the diagonal member will be in compression ("squashed" axially).
An alternate to solid members:
Two cables (in the X pattern) must be used as cables cannot take compression loads. As the frame is side loaded (see figure 8), one cable will always be in tension, while the other will be slack (as it's compressed).
The next parts:
EXAMPLES
- Look at real-life trusses (actual structures that exist).
BUILDING IDEAS
- How to build a Truss with LEGO bricks.
The key to a truss is a triangle. All members must be connected so that only triangles are formed.
Figure 2 (to left) shows examples of structures that are NOT trusses. The ends of the members do not connect at common points.
Figure 3 (to left) shows a simple moment-resistant frame. The members are relatively thick, and in this case are I-shaped beams (viewed from the side).
Figure 5 (to left) shows the truss with a lateral load from the left. The diagonal member will be in tension (it is pulled axially).
The solid diagonal member (shown in figures 5 and 6) can be replaced by a X pattern of cables. Figure 7 (to left) shows the cables in place of the diagonal.