The sliding end of a roof truss rests on brick wall through a concrete...
**Solution:**
Given:
- Maximum normal reaction on the bearing = 125 kN
- Inclination angle of the principal rafter to the main tie = 30°
To find: Factored force in the principal rafter
**Analysis:**
1. The factored force in the principal rafter can be determined by considering the equilibrium of forces acting on the truss.
2. Since the truss rests on a brick wall through a concrete pad, the normal reaction on the bearing is the maximum force that the truss can exert on the wall.
3. The normal reaction can be resolved into two components: one acting vertically upwards and the other acting horizontally.
4. The component of the normal reaction acting vertically upwards will balance the weight of the truss, while the horizontal component will resist the horizontal force in the principal rafter.
5. Since the principal rafter is inclined at an angle of 30° to the main tie, the horizontal component of the normal reaction will be equal to the factored force in the principal rafter.
**Calculation:**
Let's assume the factored force in the principal rafter as F.
1. Vertical Component: The weight of the truss can be assumed to act at the center of gravity of the truss. Since the truss is symmetrical, the vertical component of the normal reaction will be equal to half the weight of the truss.
2. Horizontal Component: The horizontal component of the normal reaction will be equal to the factored force in the principal rafter.
3. By resolving the normal reaction into vertical and horizontal components, we get:
Vertical Component = 125 kN (maximum normal reaction)
Horizontal Component = F
4. The vertical component will balance the weight of the truss, so we can assume the weight of the truss as 2 times the vertical component.
Weight of the Truss = 2 * Vertical Component = 2 * 125 kN = 250 kN
5. The horizontal component will be equal to the factored force in the principal rafter.
Factored Force in Principal Rafter = Horizontal Component = F
Therefore, the factored force in the principal rafter is 250 kN.
Hence, the correct option is (A) 250 kN.