In the design of continuous beams, say bridges, achieving an economical layout is one of the primary objectives,
and requires simultaneous consideration of cost and layout variables. This study aims to figure out an explicit methodology
for the optimum layout design of multi-span reinforced concrete beams under dynamic loadings in order to optimize the total
cost. The aim is to determine the optimum spans lengths for continuous beams under dynamic loadings considering
the involved cost elements. For this purpose a new methodology is developed in order to shift from the traditional design
space to a new one. Then, an objective function for the cost and layout optimization problem is formulated in the new space.
The proposed method employs action effects of the beams as alternative design variables in lieu of the cross-sectional ones.
This ability enables the method to be easily used in realistic structural optimization problems rather than simple models,
and helps the layout design procedure to take less time, in an iterative optimization process. In order to solve the formulated
optimization problem, an Ant Colony Optimization algorithm is proposed. The robustness of the methodology is illustrated
through a number of examples.