Modular construction joist made up of a high-strength plastic material, particularly suitable for realizing grids, and closure members subjected to pedestrian and vehicle traffic

The grid comprising a plurality of intermediate joists and two end joists, an intermediate joist comprising a an isosceles trapezium or of a circle sector whose straight-line edge is turned upwardly, an upper stringer (201) on an upper edge of said body (101), a lower stringer (301) on a lower edge of said body (101), a cross-section of said body (101), upper stringer (201) and lower stringer (301) being in the shape of a double T with the lower stringer (301) of a width less than a width of said upper stringer (201), a first and second parallelepipedal support (401,401') on each end of said body (101), said first and second supports (401,401') having a width equal to said width of said upper stringer (201), first and second symmetrically arranged raised portions (500,501') extending from said body (101) and downwardly from said upper stringer (201), pins (3,3') extending from a side of each of said first and second supports (401,401') and first and second raised portions (501,501'), conjugated seat (4,4') defined on a side of each of said first and second supports (401, 401') and each of said first and second raised portions (501,501'), each of said first and second supports (401,401') and first and second raised portions (501,501') has an opposing pin and seat and an end joist having a parallelepipedal shape (1') of the same width as that of one of the intermediate joists and pins (3,3') and seats (4,4') positioned on a single side to be coupled with an intermediate joist and laterally counterposed recessed portions (6,6') which are realized in order to render said end joists lighter.