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Trusses

3-7. Trusses

A truss is a framed or jointed structure composed of straight members connected only at their intersections in such a way that if loads are applied at these intersections, the stress in each member is in the direction of it length. Straight, sound timber should be used in trusses. Figure 3-17 shows various types of trusses used in construction. (The Howe and Fink trusses are most commonly used.) Trusses are used for large spans to give wide, unobstructed floor space for such large buildings as shops and hangars. Sometimes small buildings are trussed to save material. These small trusses act as rafters and give the roof rigidity.

Figure 3-17. Types of trusses

a. Placement. After the rafters have been assembled into trusses, they must be placed on the building (see Figure 3-18 ). Assemble the first set of rafters in the end section of the building or at the center. Raise rafter trusses into position by hand and nail them into place with 16d nails. (Temporary workbenches may be built for the workers to stand on while erecting trusses.) These trusses must be temporarily braced at the end section of the building until the sheathing is applied. Knee braces are not used on every rafter truss unless needed. Install trusses as follows:

Figure 3-18. Installing trusses

(1) Mark the proper positions of all truss assemblies on the top plate. The marks must show the exact position on the face of all rafters (such as south or north) (see Figure 3-18 , A).

(2) Rest one end of a truss assembly, peak down, on an appropriate mark on the top plate on one end of the structure (see Figure 3-18 , A).

(3) Rest the other end of the truss on the corresponding mark of the top plate on the other side of the structure (see Figure 3-18 , B).

(4) Rotate the assembly into position using a pole or rope (see Figure 3-18 , C).

(5) Line up and secure the rafter faces flush against the marks.

(6) Raise, align, and nail the three assemblies into position. Nail temporary 1 x 6 braces across these three assemblies. Repeat this procedure with the other assemblies as they are brought into position (see Figure 3-18 , D). Check the rafter spacing at the peaks as the braces are nailed on.

(7) Braces may be used as a platform when raising those trusses for which there is not enough room to permit rotation.

b. Web Members. The web members of a truss divide it into triangles. The members indicated by heavy lines normally carry tensile stresses for vertical loads. Sometimes the top chords of these trusses slope slightly in one or two directions for roof drainage, but this does not change the type of truss. The necessary number of subdivisions, or panels, depends on the length of the span and the type of construction.

c. Terms. These terms should be understood before proceeding further with this lesson.

(1) Bottom chord. A member that forms the lower boundary of the truss (see Figure 3-19 ).

Figure 3-19. Truss

(2) Top chord. A member which forms the upper boundary of the truss.

(3) Chord member. A member that forms part of either the top or the bottom chord.

(4) Member. The component that lies between any adjacent joints of a truss. It can be of one or more pieces of structural material.

(5) Web member. A member that lies between the top and bottom chords.

(6) Joint. Any point in a truss where two or more members meet; sometimes called a panel point

(7) Panel length. The distance between any two consecutive joint centers in either the top or bottom chords.

(8) Pitch. The ratio of the height of the truss to the span's length.

(9) Height of Truss. The vertical distance at midspan from the joint center at the ridge of a pitched truss or from the centerline of the top chord of a flat truss to the centerline of the bottom chord.

(10) Span length. The horizontal distance between the centers of the two joints located at the extreme ends of the truss.

d. Uses. Trusses are used for large spans to give wide, unobstructed floor space for such large building as shops and hangers. The Howe and Fink trusses are most commonly used (see Figure 3-20 ).

Figure 3-20. Howe and Fink trusses

e. Support. Trusses are supported by bearing walls, posts, or other trusses. To brace a truss to a wall or post, knee braces are used as shown in Figure 3-21 . These braces tend to make a truss of the entire building by tying the wall to the roof (see Figure 3-21 ).

Figure 3-21. Knee braces

f. Layout. Use the following steps to lay out a truss:

(1) Build the truss on workbenches that are paced on a level spot on the ground.

(2) Obtain the measurement of al material from the blueprints.

(3) Lay the pieces in their correct position t form a truss.

(4) Nail them together temporarily (see Figure 3-22 ).

Figure 3-22. Truss layout

(5) Lay out the location of all holes to be bored.

(6) Bore the holes to the size called for on the blueprint.

(7) Dismantle the truss and withdraw the nails after the holes have been bored.

g. Assembly. Assembling a truss after it has been cut and bored is simple. In most cases, timber connectors are used where different members of the truss join. Assemble the truss with the split rings in place. The bolts are then placed in the holes and tightened. Place washers at the head and nut ends of each bolt. Use straight, sound timber trusses (see Figure 3-23 ).

Figure 3-23. Split rings on a truss

Curriculum design: David L. Heiserman
Publisher: SweetHaven Publishing Services