sets have the advantage, over drum sets, of eliminating the need for blocks. To avoid lateral drift of the batten as the lift lines pay out of the grooved drums, the helical groove orientation on the drums of the line shaft may be alternated between drums to balance competing fleet angles. However the elimination of drift by this method is typically compromised by limited batten travel. Line shaft hoists can also use yo-yo type devices instead of helically grooved drums. Yo-yo hoists are typically used where lighter loads are imposed (e.g., for operating an Austrian puff curtain). Because yo-yos lines are wrapped over themselves, the velocity and travel of the lines are relatively difficult to accurately control. Point hoist Point hoists, also known as spot line winches, control a single lift line and are commonly used for automated spot rigging or flying rigs. A point hoist may operate in solitude, or in unison with other point hoists to comprise a line set. Chain hoists, more commonly referred to as chain motors, are the most common form of point hoist, especially with touring musical shows (e.g., rock-and-roll shows), but are relatively slow. Chain motors can be mounted at the grid to hoist a load from above, or mounted at the load to "climb" towards the grid. Point hoists using wire rope (GAC) are common, and steel band point hoists are also used. While generally more expensive than chain hoists, wire rope and steel band point hoists can operate at relatively high speeds. Wire rope spot line winches may be configured to pay out to the side (horizontally), for use in conjunction with a loft block, so that the position of the relatively heavy winch can be static and only the loft block need be spotted above the pick point. Infrastructure Fly system infrastructure consists of the relatively permanent load-bearing and load-transferring structures of a stage house. The infrastructure, generally fabricated of structural steel members, is sized by a structural engineer during the design of a new theatre, or theatre renovation. Rigging system infrastructure ultimately limits a fly system's capacity. Building codes generally require that fly system beam design meets the L/360 rule: beams shall not deflect by more than the length of a span divided by 360. For example, a head block beam should not deflect more than under the system design's maximum loading condition. Beam design using the L/360 rule