BCGBC4010A Apply structural principles to residential low-rise constructions

Structural principles - Structural members

Roof trusses

Truss types
The following diagrams of roof trusses are shown, all of them applied to an isosceles triangle shaped truss:
•	King post - one vertical strut goes from the middle of the base of the truss to its highest point
•	Fan - three struts fanning out from each of two points that are one third of the way along the base of the truss. From each of these points, one strut goes to the highest point of the truss and the other two go to points evenly spread along the sloping sides
•	Howe - a vertical strut goes from the middle of the base of the truss to its highest point; another two from the middle of the base to the middle of each of the sloping sides; and another two from the middle of the sloping sides down vertically to the base
•	Palladian - a vertical post from the middle of the base of the truss to its highest point; another two from the middle of the base to the middle of each of the sloping sides
•	Fink -two struts fanning out from each of two points that are one third of the way along the base of the truss. From each of these points, one strut goes to the highest point of the truss and the other one goes to the midpoint of the sloping side above it
•	Pratt - one vertical strut goes from the middle of the base of the truss to its highest point; another two go from the highest point to one quarter of the way along the base from each end; another two go from these quarter way points vertically up to the sloping sides of the truss.
Referring back to the diagrams answer the following questions.

Question 1
In most types of trusses, the top chords are:
A.	Ties
B.	Struts
The correct answer is B - Struts.

Question 2
In most types of trusses, the bottom chords are:
A.	In tension
B.	In compression
The correct answer is A - In tension.
It is helpful to have an understanding of the basic shapes and loads on trusses. It means you have more information to help determine if the trusses are adequate for the job.

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