## 8 Conclusions  
The present paper deals with the mass and the direct (self-manufacturing) cost and energy life-cycle cost optimization of steel-concrete composite floor structures. Mixed-integer non-linear programming (MINLP) was employed for the optimization. Since a number of individual MINLP optimizations were performed for different design parameters, such as different loads, spans, concrete and steel classes, welded, IPE and HEA steel profiles, and different energy consumption cases, multi-parametric MINLP optimization was applied. Different objective functions of the structure such as mass, direct cost and energy life-cycle cost objective functions were defined for the optimization. Moreover, three different energy consumption cases were proposed for the energy life-cycle cost objective: an energy efficient case (50 kWh/m\({}^{2}\)), an energy inefficient case (100 kWh/m\({}^{2}\)), and a high energy consumption case (200 kWh/m\({}^{2}\)). In each optimization, the defined objective function of the structure was subjected to the design, load, resistance, and deflection (in)equality constraints, which were defined as being in accordance with the Eurocode specifications. The plastic moment resistance of the CFS structures and the neutral axis located in the concrete plate were taken into account. The optimal results based on the above different criteria are then compared to achieve competitive composite designs. These results are also compared with the results calculated in the direct cost optimization, presented by [PERSON] et al. [14]. Comparative diagrams have been developed which make it possible to design the competitive spans of CFSs for three different types of steel I beam sections (welded, IPE and HEA profiles):  
* When the mass objective function is defined: the diagrams show that the CFSIPE structures are the lightest structures for spans of up to 7 m, while the CFSVIS structures \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**HEA**} & \multicolumn{4}{c}{**IPE**} \\ \hline q & & & & & & & & & \\ (kN/m\({}^{2}\)) & L = 5 (m) & L = 10 (m) & L = 15 (m) & L = 20 (m) & L = 25 (m) & L = 10 (m) & L = 15 (m) & L = 20 (m) \\ \hline  & 81.88/282 & 91.53/310 & 124.74/278 & 160.23/356 & 204.66/343 & & 68.52/252 & 76.56/209 & 109.71/177 & 161.04/194 \\
2 & 11 C50/60 & 12 C45/55 & 10 C45/55 & 13 C50/60 & 12 C50/60 & & 10 C30/37 & 8 C50/60 & 6 C50/60 & 6 C50/60 \\
2 & 260 S235 & 300 S235 & 450 S235 & 700 S235 & 900 S235 & 270 S275 & 330 S235 & 450 S235 & 600 S235 \\  & 4.340 & 4.464 & 3.941 & 5.211 & 5.043 & & 3.407 & 3.286 & 2.501 & 2.532 \\ \hline  & 86.88/331 & 100.26/337 & 139.16/330 & 185.98/299 & 232.65/397 & & 71.94/254 & 85.34/190 & 127.28/273 & 236.09/300 \\
4 & 13 C50/60 & 13 C50/60 & 12 C50/60 & 10 C50/60 & 14 C50/60 & 10 C30/37 & 7 C50/60 & 10 C50/60 & 9 C50/60 \\
2 & 260 S235 & 320 S235 & 500 S235 & 700 S235 & 1000 S235 & 4 & 270 S275 & 330 S275 & 550 S235 & 600 S235 \\  & 4.401 & 4.396 & 4.059 & 3.729 & 4.946 & & 2.904 & 2.381 & 3.459 & 1.496 \\ \hline  & 91.06/335 & 107.65/391 & 150.11/379 & 195.35/391 & 264.16/363 & & 74.42/350 & 91.81/288 & 142.53/347 & 305.53/403 \\
6 & 13 C30/37 & 15 C20/25 & 14 C50/60 & 14 C50/60 & 12 C50/60 & & 14 C20/25 & 11 C20/25 & 13 C50/60 & 12 C50/60 \\
260 S235 & 320 S235 & 550 S235 & 800 S235 & 1000 S235 & & 270 S235 & 360 S235 & 600 S235 & 600 S235 \\  & 3.342 & 3.382 & 4.259 & 4.530 & 3.884 & 3.254 & 2.553 & 3.876 & 1.102 \\ \hline  & 93.37/405 & 113.60/417 & 163.334/364 & 218.33/466 & 302.23/354 & & 76.87/256 & 101.35/270 & 156.27/307 & 364.84/501 \\
8 & 16 C30/37 & 16 C20/25 & 13 C45/55 & 17 C50/60 & 11 C50/60 & 8 & 10 C50/60 & 10 C20/25 & 11 C50/60 & 15 C50/60 \\  & 260 S235 & 340 S235 & 550 S235 & 500 S235 & 1000 S235 & & 270 S275 & 360 S235 & 600 S235 & 600 S235 \\  & 3.532 & 3.254 & 3.834 & 4.058 & 2.590 & & 2.831 & 2.899 & 2.542 & 0.806 \\ \hline  & & & & & & & & & & \\ \end{tabular}
\end{table} 
Table 6: Recommended optimal design for the CFS system with standard steel sections (ELCC EEC).