the grid assembly 1 said mat 8 being structured with an undulated formation 8 1 in four directions as shown in fig7 such that it allows absorbing without stress the dimensional variations of the grid assembly 1 in the adaptation to the shapes of the application surfaces of the structure the gaps of said grid assembly 1 are covered with corresponding sheet plates 9 or the like for the uniform support of the mat 8 on the grid assembly 1 which plates according to a practical embodiment fig3 and 5 are arranged in a hinged attachment on diagonal ties 4 of the mentioned grid assembly 1 and are supported with freedom to slide on telescopic crosspieces 10 arranged in the areas with the corresponding opposite vertices this arrangement of the closure of the gaps of the grid assembly 1 is non limiting and can be any other arrangement with the same effect in which the closing plates 9 are secured in the mounting allowing the dimensional variation of the gaps of the grid assembly 1 such that the hinged attachment for securing said plates 9 can be provided on any of the sides there of on the corresponding tie 4 of the grid assembly 1 whereas the other two sides are free to slide the modular formation of the grid assembly 1 of the proposed structure can.