Car Driving over Bridge
Click to view movie (403k)

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?


Car Driving Over a Bridge
Click to view movie 55k)

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

        ΣFx = 0

        ΣFy = 0

        ΣM = 0