TRUSS SYSTEM
In structural engineering, a truss is an important type of structure characterised by a triangulated system of members. These members are structured and connected in a way such that they only incur axial force. The members of a truss are considered two-force members because the forces are only applied at either end of the member, resulting in either a compression or tension force. Trusses are commonly used in bridge designs due to their ability to efficient span long distances.
The joints are typically pinned connections, such that no shear or moment forces are transferred from member to member. This is a major, yet commonly misunderstood, difference between truss and frame structures. A frame member will typically take a combination of shear, axial, and bending forces; whereas a truss member will only take axial force.
TRUSS COMPONENTS AND TERMINOLOGY
Trusses typically follow a pretty general and common structure, made up of various components. Below list of the relevant parts of a truss:
Top Chord
The beam of a truss spanning the length of the truss
Bottom Chord
The bottom beam spanning the length of the truss
Pitching Point
The point where the sloping members (top chord) connects with the bottom chord
Apex/Peak
Highest point of a truss
TYPES OF TRUSS
PRATT TRUSS – A Pratt Truss has been used over the past two centuries as an effective truss method. The vertical members are in compression, whilst the diagonal members are in tension. This simplifies and produces a more efficient design since the steel in the diagonal members (in tension) can be reduced. This has a few effects– it reduces the cost of the structure due to more efficient members, reduces the self-weight, and eases the constructability of the structure.
It’s also important to note that in Pratt Truss Design, and most Truss design for that matter, is that the members have their end fixities set to pinned. This ensures that the force is all transferred in either compression or tension, with little (theoretically, none) transferred in the form of bending moment force.
WARREN TRUSS – The Warren Truss is another very popular truss structure system and is easily identified by its construction from equilateral triangles. One of the main advantages of a Warren Truss is its ability to spread the load evenly across a number of different members; this is however generally for cases when the structure is undergoing a spanned load (a distributed load). Its main advantage is also the cause of its disadvantage – the truss structure will undergo concentrated force under a point load. Under these concentrated load scenarios, the structure is not as good at distributing the load evenly across its members. Therefore the Warren truss type is more advantageous for spanned loads, but not suitable where the load is concentrated at a single point or node
K TRUSS – The K Truss is a slightly more complicated version of the Pratt Truss. Its main difference is that the vertical members have become shortened – improving its resistance against buckling. It does, however, have similar pros and cons to the Pratt Truss and although it is not widely used, it is a strong design. One of its main disadvantages is that the members don’t always behave as expected. A member may be in compression under one load scenario and in tension under another. This can mean the structure may not be able to be optimally designed
HOWE TRUSS – Howe trusses are essentially the opposite of Pratt trusses in terms of geometry. In fact, looking at a Pratt truss upside-down will visualize a Howe truss of sorts. The entire structure is still relatively the same, but the diagonal braces are now occupying the opposite or the unoccupied joints. This switch in position of the diagonal members has a very important effect structurally.
GAMBREL TRUSS – On the exterior, a gambrel truss has two different slopes, where the slope gets steeper from the center. Due to its outward-protruding shape, gambrel trusses can effectively be fitted with a hollow center, which can be used as a storage area. As such, the upper section of a barn is usually shaped in a gambrel. In the case of a barn, as the members are usually constructed with the structure acts more like a frame than a truss
FINK TRUSS – The Fink truss in its most basic form has web members that follow a V-pattern which can be repeated several times. As the top chords are sloping downward from the center, the V pattern becomes noticeably smaller. As Fink trusses rely more on diagonal members, they can be very efficient at transmitting loads to the support. Derivatives of the Fink truss include the Double Fink and the Fan truss types. Double Fink trusses are essentially Fink trusses that repeat the pattern twice on either side. If the most basic Fink truss can be characterized by a double-V, then a double fink would look like a double-W. Fan trusses are essentially Fink trusses that have their web members ‘fan out’ from the joints at the bottom, usually with the addition of vertical members.