Rigging & Staging calculator
Truss Load & Span Calculator
For a truss on two supports, each support carries a share of the load determined by lever arms: a load close to one end loads that end harder. This calculator sums a distributed load (truss weight plus cable) and up to three point loads into the reaction force at each support. It computes load distribution only, never a pass or fail.
Truss self-weight plus evenly distributed cable and fixtures. 12 inch box truss alone runs 6 to 8 lb/ft.
Static load distribution only. This does not evaluate truss capacity, deflection, dynamic loads, or hardware. Anything that flies over people gets signed off by a qualified rigger or engineer.
Formulas
Right support reaction
R2 = ( w×L×L/2 + Σ(Pi × xi) ) / L- w:
- distributed load in lb per ft
- L:
- span in ft
- Pi, xi:
- each point load and its distance from the left support
Left support reaction
R1 = total load - R2How it works
This is textbook statics: the sum of vertical forces is zero and the sum of moments about any point is zero. Taking moments about the left support isolates the right reaction, and the left reaction is whatever remains. A point load a quarter of the way along a span sends 75% of itself to the near support.
The distributed load term covers what crews forget: 40 ft of 12 inch truss is a couple hundred pounds before a single fixture hangs, and multicable runs add real weight per foot. Enter truss plus average rigging weight as lb/ft and lumped items (movers, PA bumpers, video sleds) as point loads.
What this deliberately does not do: check the truss. Allowable span and load tables are published per truss model by its manufacturer, and deflection, chord buckling, and connection hardware are engineering questions. Use these reactions to plan motor and house-point loads, then verify capacity against the manufacturer tables and put a qualified person on anything overhead.
Worked example: 40 ft span, 11 lb/ft truss and cable, two 250 lb movers at 10 ft and 30 ft
- 1.Distributed: 11 × 40 = 440 lb acting at midspan (20 ft).
- 2.Moments about left: 440×20 + 250×10 + 250×30 = 8800 + 2500 + 7500 = 18,800 lb·ft.
- 3.R2 = 18,800 / 40 = 470 lb. R1 = (440 + 500) - 470 = 470 lb.
Each motor sees 470 lb; symmetric loads split evenly, as expected.
Share of a point load reaching the near support
| Load position on span | Near support | Far support |
|---|---|---|
| Midspan (50%) | 50% | 50% |
| 1/3 point | 67% | 33% |
| 1/4 point | 75% | 25% |
| 1/10 point | 90% | 10% |
Field notes
- Motors are rated by capacity classes (quarter-ton, half-ton, one-ton); reactions near a class limit deserve a bigger motor, not optimism.
- Cantilevers (load beyond a support) reverse the math and can unload the far support; that geometry is out of scope here and worth an engineer.
Frequently asked questions
How do I calculate how much weight each truss motor carries?
Sum moments about one support: each point load times its distance to that support, plus the distributed load times half the span, divided by the span, gives the far reaction. The remainder lands on the near support.
How much weight can a truss hold?
Only the manufacturer load table for the exact truss model and span answers that: allowable load falls quickly as span grows. This calculator tells you where your load goes, not whether the truss accepts it.
Do I need to include the truss weight?
Yes. Self-weight plus cable is often the largest single "fixture" on the stick, and it always splits between the supports whether or not it makes it into the plot.