Regarding the average number of strands in relation to the span,
Fig. 12 illustrates a linear correlation for the span length of the bridge.
Results are quite similar for both objective functions. Fig. 13 shows the
relation between the concrete strength and the span length. There is a
clear trend toward increasing concrete strength as the span lengthens,
so as to lighten the load. If emission is optimized, concrete strength
values decreased compared to cost objective. Despite the fact that
high-strength concrete is usually used for precast–prestressed concrete,
note that concrete strength for short spans tends to be the minimum
proposed (25 MPa for slabs and 35 MPa for beams). However, for the
maximum span length studied, even with the concrete strength increase,
it is far from the highest (100 MPa). Regarding the ratio of the
mean spacing between beams, there is no a clear tendency with the
span length, but in every case the spacing is less than half of bridge
width (Fig. 14); what is more, there is no clear difference in values for
the spacing between the cost-optimized and the emission-optimized