it will be readily appreciated that this system allows considerably flexible design positioning and implantation of a structure when organizing a show or an event if it is desired to use the hall with the modular platforms 1 in low position on bare ground all the girders 2 2 39 resting directly on the ground 15 via the lower sections 37 37 39 38 38 39 a particularly resistant foundation is obtained allowing in particular the circulation of lifting machines trucks transporters it is thus possible to bring into a large hall elements for forming an internal structure useful in the case of using the hall for an exhibition modifications may then be made very rapidly by simple remote control for individually actuating each platform to the desired height and automatically stabilized the platform 1 in its position each platform remaining parallel to the ground 15 and stable in any position its positioning in space being controlled in stable manner by the stabilizers 26 26 39 which each have one anchored to the ground tiers may therefore be formed as shown in fig9 in any geometrical configuration or angle allowing in particular two halves of the hall to face each other mutually the audience looking towards the centre or on the contrary the whole of the hall facing a spectacle taking place on one side or the other it will be readily appreciated that the extension and positioning of the seats 40 in their active position.