No mechanical amusement riding device shall extend more than three feet below the ground level unless the sides and bottom of all pits are built of concrete; all pits shall be provided in the bottom with drains connected to the sewers. If pits are too deep to drain to the sewer by gravity, a syphon, automatic electric pump or other device shall be installed in the drain connection. The structure shall be of wood, steel or other serviceable material substantially fabricated and braced.