## 8 Conclusions
are the lightest structures for all spans longer than 7 m. The CFSHEA structures are, in the case of all imposed loads, with the exception 2 kN/m\({}^{2}\), lighter than the CFSIPE structures for all spans above 17 m;
* When the energy life-cycle cost objective function is used for the energy efficient case: the CFSIPE structures exhibit the cheapest designs for spans of up to 12 m for low- and medium-imposed loads (up to 6 kN/m\({}^{2}\)), and they are also the cheapest for spans of up to 10 m for higher imposed loads (above 8 kN/m\({}^{2}\)). The CFSWIS structures are the most competitive designs for all spans exceeding the mentioned spans;
* When the energy life-cycle cost objective function is considered for the energy inefficient case: the CFSIPE structures are the cheapest designs for spans of up to 15 m for low-imposed loads (2 kN/m\({}^{2}\)), for spans of up to 13 m for medium-imposed load (4 kN/m\({}^{2}\)), and for spans up to 12 m for higher imposed loads (above 6 kN/m\({}^{2}\)). The CFSWIS structures are the most favorable structures for all spans longer than those mentioned;
* When the energy life-cycle cost objective function is used for the high energy consumption case: the CFSIPE structures are the most favorable designs for spans of up to 10 m for all imposed loads, while the CFSWIS structures are the most favorable for spans over 15 m. All composite structure alternatives (with IPE, HEA, and welded profiles) exhibit nearly the same optimal ELCC costs for spans between 10 and 15 m.  
We also studied how different objective functions affect the amount of the calculated costs and mass of the structures. It was found for all of the CFS structures that were considered (with all types of steel I sections) that the mass optimization exhibits 1.20-times more expensive structures on average, the energy life-cycle cost optimization for the energy efficient case exhibits 1.50-times more expensive composite structures on average, the energy life-cycle cost optimization for the energy inefficient case exhibits almost two-times more expensive structures on average, and that the energy life-cycle cost optimization for the high energy consumption case creates structures that are almost three-times more expensive on average when compared to the composite structures obtained in the direct cost optimization (DC). Similarly, it has been established for all the CFS structures considered here that the direct cost optimization of the structures exhibits 75% higher masses on average, the energy life-cycle cost optimization for the energy efficient case exhibits 50% higher masses on average, the energy life-cycle cost optimization for the energy inefficient case exhibits 30% higher masses on average, and that the energy life-cycle cost optimization for the high energy consumption case creates masses that are 20% higher on average compared to the optimal masses obtained in the mass optimization. Based on the above comparisons, we can conclude that the higher (more wasteful) the energy consumption case considered in the objective function is, the lower the obtained masses of the CFS structures are. Higher energy consumption cases exhibit a few times higher energy life-cycle costs compared to the costs obtained in the direct cost optimization. Recommendations for the optimal design of CFSs have been presented based on the performed energy life-cycle cost optimization (the energy efficient case is considered). An engineer or researcher can use the recommendations to establish the optimal structure for a desired span and uniformly imposed load. Conceptualization, S.K. ; methodology, S.K., U.K. and T.Z. ; formal analysis, S.K., U.K. and T.Z. ; writing--original draft preparation, S.K., U.K. and T.Z. All authors have read and agreed to the published version of the manuscript. The authors acknowledge the financial support from the Slovenian Research Agency (grant number P2-0129). Not applicable. Data Availability Statement: The data presented in this study are available upon request from the corresponding author.