Truck chassis assembly forms the structural backbone of a heavy commercial vehicle. The main function of the truck chassis is to support the aggregate components and payload acting on it during operating conditions. The chassis is subjected to vertical loading (Bump), fore and aft loading (braking and acceleration), lateral loading (Cornering) and torsional loading (Articulation). Out of the above load cases, the vertical load (bump load) decides the maximum load carrying capacity of the chassis when the vehicle traverses uneven surfaces. This paper explains the importance of theoretical bending stress calculation of truck frame side member (FSM) subjected to vertical load which will help the designer to arrive at the major section of FSM during initial stage of the design. Different concept proposals of FSM can be evaluated using these calculations quickly. Based on the experience in validation of Finite Element analysis results of similar type of vehicles with test results, the calculated theoretical bending stresses are compared with those obtained from numerical analysis. Finite element method (FEM) is applied for the numerical analysis of truck chassis assembly using commercial software MSC/Nastran.