Ground support

A ground support system carries a truss on towers that stand on the ground, usually with a sleeve block and motors that lift the grid up the towers once it has been built at floor level. It is the answer in three situations: a venue with no usable rigging points, a venue whose points will not take the load, and outdoors, where there is nothing overhead at all.

The trade is floor space and engineering for independence from the building. Towers occupy footprint, need bases or ballast, and need clear space around them where nobody is seated.

Outdoors it stops being a rental item and becomes a temporary structure. The governing standard requires engineering documentation sealed by a registered design professional, a named qualified person responsible on site, and an operations management plan that states what happens as the weather changes — including a wind speed at which the structure must be modified or struck, and the time it takes to do it.

That last point is the one that is consistently underestimated. A plan that says the roof comes down at a given wind speed is only real if somebody has measured how long bringing it down actually takes, and if an anemometer on site is being read rather than a forecast on a phone. In South Florida, in season, that is the difference between a plan and a hope.

Staging and rigging — owned inventory, our own crew, and a site survey before anything is specified.

• Box truss The square aluminium lattice everything hangs from — rated for a load in addition to its own weight, and only as strong as the span it is asked to cross.
• Chain motor The electric hoist that lifts truss and scenery into the air on a loop of chain — rated by capacity, and not by itself a licence to leave a load hanging over people.
• Rigging plot The drawing that says what hangs where, how heavy each point is and how high it trims — submitted to the venue weeks ahead, not produced on the day.