Dimension Diagram

At the local highway department, the bridge designer is required to calculate the force in the top horizontal member of a new bridge truss. The member force is need to help determine the correct type of material and the size of the member.

What is known:

• The car mass is 1,500 kg.
• The car is located at joint 2.
• The bridge length is 16 m (4 bays).
• The bridge height is 4 m.
• Joint 1 is a pin support, and joint 5 is a
  roller support.

When the car is over joint 2, what is the force in member 6-7?

• First, solve for the support reactions.
• Next, section off part of the truss and apply the three equilibrium equations:
  
      ΣF_x = 0
  
      ΣF_y = 0
  
      ΣM = 0