It is very difficult to produce a radius on the top edges of a precast seating unit. A radius can only be tooled into the edge or created by rubbing the edge with an abrasive stone. Consider a chamfer instead.
The sight lines and footprint constraints of many stadiums and arenas demand that the leading edge of upper seating tiers extend over the tiers below. Such cantilevering raker and bleacher framing presents designers and builders with special challenges. This problem exists regardless of the choice of support framing material, whether it is precast concrete, cast-in-place concrete or structural steel. In order to maximize construction economy every effort should be made to build a mechanism, prior to the precast's arrival on site, designed for the stable gravity support of the precast bleacher unit.
A detail of a full end support (available in detail section of web site) where the precast is provided with a full end support at the raker beam. From an erector's perspective this represents the quickest, most efficient framing method. From the precast concrete manufacturer's standpoint, this is the lightest, most economical "tub" section available framed in single, double or triple riser units.
Designers should verify that the stadia seating units have support on both ends, typically to a precast raker beam. Support must also be provided when large holes are cut into the seating units at mid-span. These openings are commonly required when a recessed camera platform or ADA designated area is needed. Tub seating units will need proper bearing so that they can be set in place, released from the crane line and then connected to the support structure.
A pin that is field "shot" onto a steel support bearing or a pin that is placed in a field drilled hole in a precast support raker works best. This type of connection allows for field tolerances. The hole in the horizontal seat surface is typically grouted to lock the connection in place; a cover sealant can be used to protect the connection. A free or slide type variation of the pin connection can simply be made by placing a rectangular tin cap over the pin prior to the grouting of the hole in the horizontal seat surface.
These are typically spaced across the front nose of the stadia unit above and pinned down into the upturn of the stadia unit below. A hidden connection can be achieved by casting a threaded insert into the bottom surface of the lower stadia unit. When erecting the stadia units, threaded rods are installed into the cast-in inserts in the upper stadia units and grout is poured into the sleeves in the lower stadia units. When the stadia units are set on each other, the connections are automatic and the connections lock as the grout in the void hardens.
It is important to recognize that precise dimensional coordination must occur in sports facility design. The forming methods used and repetition of precast components create a consistent quality in a controlled environment. Designing, detailing, and producing one internally coordinated material streamlines erection of the structure.
A total precast concrete system provides a single source supplier. There is only one schedule and one field construction management which minimizes coordination conflicts. The precast concrete components are cast off-site regardless of weather constraints. Talk to a MAPA precaster regarding schedules and estimates.