Question

1. When solving for the reactions at the supports of a truss, what equations do you use?

2. When solving for the forces in a member of a truss, how do you know you assumed the sense (direction) of the force in the incorrect direction? You will get negetavie values

3. When using the method of joints and you are analyzing an individual joint, how many and which equations of equilibrium can you apply?

4. When using the method of sections, what is the maximum number of members you can cut?

5. When solving a friction problem, what is the equation used to find the force of friction?

Answer 1

Ans=1. To determine the reactions at supports, follow these simple Equations:

1. Let the sum of moments about a reaction point equal to ZERO (ΣM = 0):

Consider a simple example of a 4m beam with a pin support at A and roller support at B. The free body diagram is shown below where A_y and B_y are the vertical reactions at the supports:

We firstly want to consider the sum of moments about point B and let it equal zero. We have chosen point B to prove. This can be done at either end of the beam (provided it is pin supported). So, now we sum the moments about point B and let the sum equal 0:

or

2. Let the sum of vertical forces equal to 0 (ΣF_y = 0)

Sum the forces in the y (vertical) direction and let the sum equal zero. Remember to include all forces including reactions and normal loads such as point loads. So if we sum the forces in the y direction for the above example, we get the following equation: