may be adjustable in tension end parts 24 and 26 are thus each self supporting structures and each maintains its membrane in a stressed condition at all times it is noted that bracing structure is required to provide rigidity and tension to the end parts 24 and 26 and variations on the support structure are therefore permissible so long as those criteria are maintained thus the bracing structure may vary for different environments 0039 fig7 and 8 illustrate details of pivot assemblies 28 30 pivot post 60 comprises pipe 62 which passes through the structure 39 s concrete pad 64 and is embedded in concrete pile 66 pipe 62 may be filled with concrete 68 brackets 74 and 76 are fixed to structure member 36 each bracket includes a cylindrical sleeve 78 in assemblies structure 10 the sleeves 78 are slid over post 60 to form the pivot each horizontal structural beam 40 is provided with wheel assemblies 48 and 50 the wheel assemblies support the weight of the end parts so that the pivot assemblies do not carry the weight of the end parts additional wheel assemblies may be used if required connecting and or sealing means not shown are preferably provided between beams 39 and 41 in the closed position and between beams 37 and 43 and beam 45 main part 32 of structure 10 when the end parts are in the closed position in order to actually activate opening or closing of the assemblies 24 and 26 around pivot assemblies 28 and 30 tow bar attachments are provided.