6.11 : Space Trusses: Problem Solving

A space truss is a three-dimensional counterpart of a planar truss. These structures consist of members connected at their ends, often utilizing ball-and-socket joints to create a stable and versatile framework. Due to its adaptability and capacity to withstand complex loads, the space truss is widely used in various construction projects.

Consider a tripod consisting of a tetrahedral space truss with a ball-and-socket joint at C. Suppose the height and lengths of the horizontal and vertical members are known.

Assuming that a tensile force is applied at joint D, a free-body diagram that includes all the reaction forces at A, B, and C joints can be drawn to determine the force acting on members BC and BA.

The moment equilibrium condition at joint C is applied, considering the distances expressed in position vectors in three dimensions.

Simplifying further and using the force equilibrium conditions, the vector components along i yield F_Ay as 6 N and along k give F_Bx as -7.2 N. Finally, equating the j coefficients gives the value of F_Ax as 6 N.

Now, consider the free-body diagram at joint B to calculate the forces F_BC and F_BA. The forces F_BD, F_BC, and F_BA can be expressed using position vectors. The force equilibrium condition at joint B is applied.

Equating the coefficients of the i, j, and k unit vectors to zero yields the forces along BC and BA as zero.