Definition and Theory
By changing the angle of the cable, the horizontal force in the compressive member changes. In a cable structure shown below, the load (P) on one end of the cable is the same as the reaction force (Pr) on the other. Force (P) can be broken into vertical (Pv) and horizontal (Ph) components. The vertical component (Pv) = (Pr) sine q and the horizontal component (Ph) = (Pr) cosine q. As the angle goes from 90° to 0° the force in the horizontal component (Ph) increases from 0 to (P).

Variables: Rh = Horizontal Reaction Rv = Vertical Reaction P = Load Pr = Reaction Force Ph = Horizontal Component Pv = Vertical Component q = Angle of Cable SFh = Sum of Horizontal Forces SFh = Ph - Rh = 0 Ph = Rh (the horizontal components are equal in this diagram)

Procedure

• Use a 1/8" x 1/4" x 36" long piece of balsa wood as the compressive test member. Be sure to place the strong axis (the 1/4" dimension) is in the vertical position. This will insure the compressive member buckles in the weak, horizontal, direction first.
• Starting with the cable at a 45° angle, load the pennies until buckling occurs. Record the information in the table. Do this for each angle as shown in the data table. Remember, the length of the compressive member stays the same in this experiment.
• Graph the relationship between Column Angle and Load on the graph provided
• Answer the questions in the lab.

Laboratory Worksheet