-
What is a truss?
it is a structure generally composed of straight members to form a number of triangles with the connections arranged so that the stresses in the members are either in tension or compression
-
What are typical span ratios for trusses?
1:10 to 1:20
-
Pitch trusses require more depth than flat trusses
-
What is the typical span range of a truss?
from 40ft to 200 ft
-
Panel points
loads on a truss must be placed on the panel points to avoid putting bending stresses in the top chord of the truss
-
When concentrated or uniform loads are placed between panel points the member must be designed for axial loading as well as for bending
-
Trusses act much like a beam in that there is
- compression in the top chords
- tension in the bottom chords
- and web members are either in compression or tension
-
What is typical spacing for trusses?
from 10ft to 40ft on center
-
In residential and light commercial construction trusses may be spaced
2ft on center
-
What is the typical spacing for open web steel joists?
- floors - 2 to 3 ft OC
- roofs - 4 to 6 ft on center
-
Trusses must be laterally supported with bridging along the bottom chord
in some cases diagonal bracing is required along the top chords of pitched roofs if the roof deck is not adequate to act as a diaphragm
-
Individual truss members are designed as columns if they are in compression
-
Truss members in tension must
have adequate net area, after deducting for the area of fasteners, to resist the unit tensile stress allowed by the material used.
-
The effective length of chord members in compression must be determined
Kl; where K is determined by the restraint of the ends of the member and l is the length
-
What is the K value for steel trusses?
1.0
-
For steel trusses, the ratio of length to least radius of gyration, l/r, should not exceed 120 for main members and 200 for secondary and bracing members
-
For truss members in tension the net area must be determined
this is the actual area of the member less the area of bolt holes, which is taken to be 1/8" larger than the diameter of the bolt
-
Concentric truss
the members must be symmetrical on both of side of the centrical axis in the pane of the truss. Thats why you see steel trusses built with back to back members
-
What kind of members can be used for light tension loads on a truss?
bars or rods
-
Wood trusses use steel gusset plates to connect web members to the top and bottom chord
can be clamped or nailed
-
To avoid eccentric loading in a truss you must
make sure the centrical axis of all intersecting members meet at a point
-
For steel members composed of angles, it is standard practice to have the gage lines rather than the centrical axes meet at a common point.
the gage line is a standard dimension from the corner edge of an angle to the center line of the bolt hole or holes, value can be found in the AISC manual
-
Guidelines for truss analysis
1. The sum of vertical forces at any point equals zero
2. The sum of horizontal forces at any point equals zero
3. The sum of the moments about any point equals zero
4. Forces in each member are shown by an arrow away from a joint or cut section if in tension and toward the joint or cut section if in compression
5. Forces acting upward or to the right are considered positive and forces acting downward and to the left are considered negative
6. All forces should be indicated acting in their known direction. If the Direction is unknown when beginning the analysis, show the force in tension, acting away from the joint in cut section. If the calculation of the force is negative then this indicates that the direction is reversed
7. For analysis, trusses are assumed to have pivoting or rolling supports to avoid other stresses at these points
-
The x-component of a force, horizontal, is equal to the force times the cosine of the angle the force vector makes with the x -axis
Fx = F cos a
-
The y-component of a force, vertical, is equal to the force times the cosine of the angle the force vector makes with the y-axis
Fy = Fcosb
-
What are the 3 methods that can be employed to find the forces in truss members?
- the method of joints
- the method of sections
- the graphic method
-
When should you use the method of joints?
it is useful in determining the forces in all the members or when only the forces in members near the supports need to be calculated
-
When should you use the method of sections?
when it is only necessary to find the forces in a few members, particularly ones not at or near the supports
-
The graphic method is useful for
complex trusses and avoids all the calculation inherent in analytic solutions
-
Method of joints
With this method each joint is considered separately as a free body diagram to which the equations of equilibrium are applied. Starting from one support, the force in each member is determined, joint by joint, until all have been calculated.
|
|
