a$)$ A new truss bridge that recently opened for use collapsed suddenly. When an investigation was carried out it was discovered that the orientation of the supports caused the structure to be statically unstable externally. Explain in detail, what conditions must be in place for a structure to be unstable externally.

b$)$ The method of joints and method of sections are the two accepted methods to resolve problems with trusses. Explain in what instances it is best to use both methods or use them separately.

c$)$ When calculating reactions and the directions of axial forces in truss members it is customary to assume their direction. The method of sections was used to resolve a truss to determine the force in the members. Explain what the negative sign indicates for the magnitude and what effect this has in the calculations if there is a need to calculate the force in another member.

d$)$ From a material usage standpoint why is a truss better in some instances?