• Measurement of bar forces (ties and Rafter).
  • Calculation of bar forces by the method of joints.
  • Comparison: measurement result-calculation-graphical method.


  • Resolution of forces in a single plane, statically determinate system.
  • 3 pin joints, 2 of which serving as supports one fixed and one free longitudinal.
  • 2 bars each fitted with a force gauge (compression) and one member fitted with spring balance (tension).
  • 3 element and force markers.
  • 2 rafters with compression gauges and fixed length, 1tie with variable length.
  • Different angles adjustable between bars.

This apparatus is used to measure the deformation in elements of roof truss. Whenever a load is applied to a roof truss, its members undergo a strain. This unit consists of a base structure which provides the support locations for the roof truss. Two metallic elements and one adjustable element with spring balance and chain form a roof truss in a triangular shape.

Load is applied from the top of the roof truss structure via a hanger. Each member has a force gauge (two consisting force gauge and one comprises of spring balance) connected to it which shows the forces produced due to applied load in each member.

This apparatus represents a simple roof truss bar structure. In the single plane system the bars are only subjected to compression and tension. Loads are applied only to the nodes.

The unit comprises three members that are joined together using pin joint such that the joints are free to move. A longitudinally adjustable chain permits the structure to be constructed with different angles. Two of the pin joints also form the supports (fixed and free with bearings) and are clamped to the sturdy aluminium section base frame. The external load is applied to the upper pin joint by means of weights. The bar forces occurring are measured directly on force gauge in the middle of the bar. The method of joints enables the bar forces to be determined by formulating a system of equations.