en Optimal design Optimal design پژوهشي Research <span style="font-size:11.5pt"><span style="text-justify:inter-ideograph"><span style="text-autospace:none"><span new="" roman="" style="font-family:" times="">This paper presents a computational framework for optimal design of non-prismatic reinforced concrete box girder bridges. The variables include the geometry of the cross section, tapered length, concrete strength and reinforcement of box girders and slabs. These are obtained by the enhanced colliding bodies optimization algorithm to optimizing the cost and again CO₂ emission. Loading and design is based on the AASHTO standard specification. The methodology is illustrated by a three-span continuous bridge. The trade-off between optimal cost and CO₂ emission in this type of bridge indicates that the difference of costs, as well as CO₂ emissions in the solution with both objectives is less than 1%. However, the optimal variables in the cost objective are different from the variables of CO₂ emission objective.</span></span></span></span><br> &nbsp; optimal cost, optimal CO2 emissions, RC box girder bridge, non-prismatic, ECBO algorithm. 55 72 http://ijoce.iust.ac.ir/browse.php?a_code=A-10-66-362&slc_lang=en&sid=1 L. Mottaghi 180031947532846002565 180031947532846002565 Yes Civil Engineering Department, Imam Khomeini International University, Qazvin, Iran A. Kaveh 180031947532846002566 180031947532846002566 No School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran R. A. Izadifard 180031947532846002567 180031947532846002567 No Civil Engineering Department, Imam Khomeini International University, Qazvin, Iran