1. CIEG 301: Structural Analysis
Trusses
(Chapter 3)

2. Overview Commonly constructed from: Wood Iron Steel
Trusses are commonly used in:
Roof framing
Bridges
Trusses are structures composed of numerous pin connected members joined together at their end points
Trusses are loaded only at their joints
Including self weight
Truss members are only subjected to the following forces:
Tension
Compression

3. Bridge Trusses
Gusset plates; floorbeams; lateral bracing

4. Roof Trusses
purlins

5. Classification Number of unknowns, r r = b + R
b = number of bars / members
R = number of non-parallel and non-concurrent restraints
Number of Equations of Equilibrium 2j
j = number of joints
Stability and Determinacy:
r < 2j  Not stable
r = 2j  Stable and Determinant
r > 2j  Stable and Indeterminant
Degree of indeterminacy = (b + R) -2j

6. Method of Joints Concept: Each joint of the truss is in equilibrium
Determine external forces (reactions) by drawing FBD of entire structure
Determine internal forces by drawing FBD of joints
Start with joints that have:
No more than 2 unknowns
At least 1 known force
Compression forces “push”
Tension forces “pull”

7. Zero-Force Members P
Case 1: Exactly two members sharing a joint with no applied load
Case 2: Exactly three members, two of which are colinear, sharing a joint with no applied load F E D A B C
EF –FC – BE - BD

8. Method of Sections Concept:
Each section of the truss is in equilibrium
Method:
Use equations of equilibrium to solve for external reactions if necessary
An imaginary line is used to cut the structure into two parts
The three equations of equilibrium are applied to each part in order to solve for the internal forces
Choose a “cut” that causes no more than three unknowns
Sign convention:
Draw FBD such that all forces are in tension, i.e., “pulling” on the member, or
Determine orientation by inspection
Useful when:
The forces in only a few members are needed

9. Examples…