Rigging points vs ground support

A convention hall or a theatre has structure overhead that an engineer has already rated. The production hangs from it, and the open questions are how much weight and at which points.

A park, a beach, a closed street or a parking lot has nothing overhead. The structure arrives on the truck, stands on its own base, and carries the lighting, the video and the loudspeakers from the ground up.

Crew size, truck count, the drawing attached to the permit and the ballast all follow from which of those two the site is. So does the answer to how high the truss can go.

The F34 load span tables published by Global Truss carry three sets of allowable loads on one sheet. One is headed as limited by strength and L/180 deflection. One is strength only, with no deflection limit. One is strength reduced per ANSI E1.21, and that is the reduction that applies outdoors.

L is the span. On a thirty-foot span, L/180 allows two inches of movement at mid-span. A figure quoted without its table carries no such limit with it.

The sheet also sets conditions a load figure on its own does not carry. The self-weight of the truss is already included in the allowable loads. Shock and dynamic forces are not included. A uniformly distributed load applies only where it is spread evenly across the entire span. The loads shown are not additive.

Two further conditions decide what a venue can actually offer. Every load has to attach at a truss panel point. Bridles are not permitted unless a licensed structural engineer approves them in writing.

An offer of a couple of points anywhere along that beam is therefore not the arrangement the table describes. Global Truss states on the same sheet that the tables are for general reference only, and that a rigging plot should be reviewed by a licensed structural engineer.

Columbus McKinnon publishes its Lodestar electric chain hoists as a table of models rather than a single rating. At one horsepower the same motor size appears four times. Half a ton at 32 feet a minute on a single chain fall, one ton at 16, two tons at 8 on two falls, and three tons at 5.5 on three falls.

Half a horsepower repeats the pattern one step down — a quarter ton at 32 feet a minute, half a ton at 16, and a full ton at 8 on two falls.

A call for a one-ton motor has named a capacity and left the trim time open. Where the truss moves between acts, the feet-per-minute figure is the one that governs the schedule.

The hoist hangs as well. Columbus McKinnon publishes the three-ton model at 158 pounds and the two-ton at 131 and 130 pounds across its two voltage codes, before any of the load the hoist is there to lift. Twenty points of the three-ton model is over a ton and a half of hoist on the structure.

On the rating itself, Columbus McKinnon states that the Lodestar meets ASME B30.16 and exceeds H4 duty requirements. Each hoist is inspected and tested to over 125 per cent of rated load before it ships.

Prolyte publishes the MPT tower at a maximum height of 8.00 metres and a maximum loading capacity of 1,000 kg. That capacity is conditional. Prolyte states that it depends on the tower height and size, the applied load, and how the tower base and top are restrained, and refers the reader to its own Blackbook for the detail.

The fixed figures are the ones a load-in plans around: a self-weight of 115 kg, an H30V mast section and a CCS6 coupling system in EN-AW 6082 T6 aluminium. The maximum handwinch load is published at 750 kg.

The sleeve block takes 30 and 40 series truss on all four sides through bolted couplers, and S36R or S36V truss through an adapter plate. Prolyte describes the block as fully bolted rather than welded.

A hung point inherits a rating somebody calculated for the building years ago. A tower carries a rating the crew completes on site, through the base, the restraint and the ballast. That gap is where the engineering drawing for a permit comes from.

The standard named in that third table is ANSI E1.21, published by the Entertainment Services and Technology Association. The 2023 edition is titled Temporary Structures Used for Technical Production of Outdoor Entertainment Events, supersedes the 2020 edition, and is aligned to the 2024 International Building Code.

Its published scope is a minimum level of design and performance parameters for the design, manufacturing, use and maintenance of temporary ground-supported structures used in the production of outdoor entertainment events. The abstract states that the document does not address fire safety and safe egress issues.

A reviewer at a permit desk usually asks about both. A structural calculation answers the first. Crowd routes, exits and the fire lane are answered elsewhere in the same application, and a production that brings only the first answer goes back round.

The Global Truss sheet names the codes behind its own numbers: the 2018 International Building Code, ANSI E1.21-2020, the Aluminum Design Manual 2015 and AISC 360-16. A load figure is only as current as the documents under it.

Indoors, the questions are the building’s own rigging capacity and where its points fall against the stage. Then whether the house holds any of the work exclusively, and whether the plot needs a bridle the truss manufacturer will not allow without an engineer.

Outdoors, the questions are the ground under the base, the ballast, the restraint at the top of the tower, the wind exposure, and the drawing the city wants attached to the special-event permit.

A park site answers almost none of that from a published table, so the survey comes first. The same stage plot can be a four-point hang in a hall and a pair of towers with ballast on a lawn.

Both arrangements finish with a truss at a trim height over a stage. The published tables say what each one carries. The site says which one is available.