Truss Types — Flat, Triangular, Box, and Circular
Truss Types — Flat, Triangular, Box, and Circular
Quick Answer
Aluminum stage truss comes in four primary configurations: flat (ladder), triangular, box (square), and circular. Box truss supports the highest multi-directional loads and spans the longest distances, making it standard for concert rigs. Triangular truss offers rigidity with lower weight for permanent installations. Flat truss serves low-load applications like lightweight drape lines. Connection systems use either conical connectors or bolted plates.
Summary
Selecting the correct truss profile determines the structural safety, load capacity, and footprint of a stage rig. This guide examines the mechanical differences between flat, triangular, box, and circular truss, comparing their load ratings, connection methods, and span characteristics for professional rigging applications.
Who This Article Is For
- •Technical directors specifying temporary structures.
- •Production managers calculating roof system capacities.
- •Rigging crews handling daily ground support assembly.
- •Rental companies managing truss inventory.
What You Will Learn
- •The physical differences between 6082-T6 and 6061-T6 aluminum alloys.
- •How truss shape dictates rotational stiffness and load capacity.
- •The mechanics of conical versus spigot connection systems.
- •How to accurately interpret manufacturer span tables and derate for dynamic loads.
Introduction
A production team plans to fly a 12-meter span loaded with 20 moving profiles, 15 wash fixtures, and three line array hangs. The total distributed weight approaches 1,200 kg. Hanging this load on a 290mm triangular truss risks structural deflection and eventual failure, whereas a 400mm box truss handles it with an adequate safety factor. Understanding truss geometry is not about aesthetics; it is about managing physical forces—tension, compression, and shear—across unsupported spans. The correct profile prevents catastrophic failure in overhead applications.
Engineering Fundamentals of Stage Truss
Aluminum Alloys: 6082-T6 vs. 6061-T6
Stage truss relies on extruded aluminum alloys. The two standards are 6082-T6 (European standard) and 6061-T6 (North American standard). The "T6" designation indicates the metal has been solution heat-treated and artificially aged to maximize tensile strength.
- •6082-T6: Higher tensile strength (approx. 340 MPa) and better corrosion resistance. Commonly used by brands like MILOS.
- •6061-T6: Slightly lower tensile strength (approx. 310 MPa) but highly workable for complex extrusions.
Anatomy of a Truss Section
A typical section consists of three main elements:
- 1.Main Chords (Tubes): The thick outer pipes bearing the primary tension and compression loads (usually 48mm-50mm diameter).
- 2.Bracing (Diagonals): The smaller internal webbing welded between chords to prevent shearing (usually 16mm-20mm diameter).
- 3.Connectors: The end plates or receivers joining sections together.
Wall thickness directly correlates with load capacity. A 50x2mm chord handles standard loads, while heavy-duty touring truss uses 50x3mm or 50x4mm walls.
Flat (Ladder) Truss
Flat truss consists of two main chords connected by ladder-style bracing in a single plane.
Load Capacity and Mechanics
Flat truss lacks depth in the Z-axis, meaning it provides almost zero resistance to lateral (horizontal) bending. It only supports vertical loads when oriented correctly.
When to Use
- •Grid ceilings in studios with frequent hanging points.
- •Lightweight banner frames.
- •Low-profile pipe-and-drape replacement.
Limitations
Never use flat truss for long unsupported spans or heavy automated fixtures. Twisting (torsion) forces from moving heads will bend the truss.
Triangular Truss
Triangular truss introduces a third chord, creating a 3D structural profile with distinct top and bottom planes depending on orientation.
Structural Characteristics
The triangular shape provides excellent rotational stiffness compared to flat truss. However, orientation dictates its capacity:
- •Apex Up: One chord on top, two on bottom. Better for lateral stability.
- •Apex Down: Two chords on top, one on bottom. Preferred for hanging fixtures directly on the bottom chord to maintain center of gravity.
Typical Applications
- •Trade show booths and exhibition stands.
- •Permanent architectural lighting installations.
- •Low-clearance venues where saving 100mm of vertical height matters.
Box (Square) Truss
Box truss features four main chords, forming a square cross-section. It is the workhorse of the live event industry.
Multi-Directional Load Capacity
The symmetrical geometry of box truss means it resists deflection equally in both X and Y axes. This allows it to handle the immense torsion generated by large moving heads (like the ROBE MegaPointe) and the lateral wind loads experienced by outdoor roof systems.
Size Variants
- •200mm / 290mm: Standard exhibition and lightweight stage use.
- •390mm / 400mm: Concert touring, heavy video walls, long spans up to 18m.
- •520mm / 760mm+: Pre-rig truss, mother grids, and massive outdoor roof structures.
Circular (Curved) Truss
Circular truss sections are rolled through a mechanical bender to create specific radii.
Construction and Geometry
Manufacturers roll the main chords independently, then weld the bracing. Because the extrusion process changes the aluminum's grain structure slightly, curved truss often has a lower Safe Working Load (SWL) than straight truss of the same profile.
Radius Options and Assembly
Circles are defined by their outer diameter (e.g., 2m, 4m, 8m) and divided into segments (quarters, eighths). An 8m circle might require 8 pieces of 45-degree arcs.
Applications
- •Nightclub centerpieces holding dynamic lighting.
- •Theatrical set frames.
- •Custom architectural ceiling pieces.
Connection Systems
Conical (Spigot) Connectors
The standard in Europe (MILOS, Prolyte). It uses a solid aluminum bullet (spigot) inserted into the hollow chords, secured by a tapered steel pin and an R-clip.
- •Mechanics: The tapered pin creates a friction lock, drawing the two truss sections tightly together.
- •Advantage: Fast, hammer-driven assembly. Self-aligning. Minimal tool requirement.
Bolted Plate Connectors
Common in older systems and North American brands (Thomas, Tomcat). Relies on heavy end-plates bolted together.
- •Mechanics: Relies entirely on the tensile strength of the bolts.
- •Advantage: Difficult to assemble incorrectly. Handles high shear loads well.
- •Disadvantage: Slower assembly, requires wrenches, plates can bend if dropped.
Compatibility Warning
Never mix truss from different manufacturers, even if the connectors appear identical. Micro-variations in pin hole alignment or alloy composition invalidate all engineering certifications.
Load Ratings and Span Tables
Understanding manufacturer data is non-negotiable for safe rigging.
Key Load Types
- 1.Uniformly Distributed Load (UDL): Weight spread evenly across the entire span (e.g., 20 identical wash lights spaced 0.5m apart). This yields the highest total capacity.
- 2.Center Point Load (CPL): A single heavy mass hung precisely in the middle of the span (e.g., a large mirror ball or center cluster). This yields the lowest total capacity.
- 3.Third/Quarter Point Loads: Discrete heavy loads placed at specific intervals.
How to Read a Span Table
Manufacturer tables state the SWL based on span length. For a MILOS M290 box truss spanning 8 meters:
- •The table might show a UDL of 73 kg/m (total 584 kg).
- •The CPL for the same 8m span drops to 292 kg.
- •As span length increases, load capacity drops exponentially due to the truss's own self-weight.
Safety Factors
Reputable manufacturers calculate SWL with a safety factor, typically 8:1 or 10:1 for overhead lifting in entertainment, according to standards like DGUV V17 or ANSI E1.2.
Ground Support vs. Flying
Flown Rigs
Truss suspended via chain hoists (like CODASTAGEMAKERS) from venue structural points.
- •Requires precise calculation of bridle angles and point load distribution.
- •Forces are primarily tensile.
Ground Support
Truss resting on the floor via base plates or built into tower systems.
- •Introduces massive compression loads on the vertical tower legs.
- •Requires heavy steel base plates (minimum 1m x 1m for tall towers) to prevent tipping.
- •Outdoor structures require guy wires and ballast (water tanks or concrete blocks) to counteract wind uplift.
Common Mistakes
- 1.Ignoring Cantilever Limits: Hanging heavy loads past the pickup points on a flown truss. The cantilever section usually has only 10-20% of the capacity of the main span.
- 2.Hammering Pins Vertically Without Support: Banging conical pins into truss resting directly on concrete damages the chords. Always prop the truss up on wooden blocks.
- 3.Over-Torquing Bolts: On plated truss, crushing the aluminum end-plate by over-tightening steel bolts.
- 4.Disregarding Dynamic Loads: A 50 kg hoist moving at 10m/min exerts significantly more force than a static 50 kg weight. SWL must be derated for dynamic movement.
Best Practices
- •Log Every Use: Track how many times truss is used outdoors in corrosive environments.
- •Inspect Welds Annually: Look for hairline fractures around the bracing nodes (the heat-affected zone).
- •Use Correct Slinging: When wrapping truss with synthetic spansets, wrap around the main chords at the node points (where bracing meets the chord). Do not choke the center of a bracing tube.
- •Match Truss to the Job: Don't spec 400mm truss for a simple drape line; don't use 290mm for a massive video wall header.
Decision Matrix: Truss Profile Comparison
| Feature | Flat (Ladder) | Triangular | Box (Square) |
|---|---|---|---|
| Main Chords | 2 | 3 | 4 |
| Rigidity (Vertical) | Low | High | Very High |
| Rigidity (Lateral) | None | Medium | Very High |
| Storage Volume | Minimal | Medium | Maximum |
| Best For | Light drape, grids | Exhibitions, low ceilings | Concerts, long spans, heavy loads |
| Typical Max Span (290mm) | 4m | 10m | 12m-16m |
ART-R Recommendations
For professional rental and integration, ART-R recommends the MILOS truss ecosystem.
- •Exhibitions & Corporate: MILOS M290 series (Box or Triangular) provides the best balance of weight and capacity.
- •Heavy Touring & Roofs: MILOS M390 and M400 series handle massive point loads from line arrays and automated fixtures.
- •Accessories: Always stock OEM conical connectors, base plates, and book-corners to maintain structural certification. Do not use third-party pins.
FAQ
Can I mix 290mm box truss from MILOS and Prolyte?
No. Even if the conical connectors fit, the alloys, wall thickness, and welding techniques differ. Mixing brands voids manufacturer warranties and structural certifications.
What is the difference between spigot and conical connectors?
In modern entertainment rigging, the terms are often used interchangeably to describe the solid aluminum bullet and tapered pin system.
How do I calculate wind load for an outdoor truss structure?
Wind load calculations require knowing the surface area of everything attached (lights, banners, roof skin) and local wind speed data. This requires software analysis by a structural engineer.
Can I hang fixtures on the diagonal bracing?
Never. Diagonal webbing is designed only to resist shear forces between the main chords. Hanging fixtures on bracing causes the tube to bend and fail.
What happens if a conical pin is bent?
Discard it immediately. A bent pin indicates it has been subjected to extreme shear force beyond its yield point and its structural integrity is compromised.
Why does my 10m truss span bow slightly in the middle?
All truss deflects under load. Manufacturers provide allowable deflection figures in their span tables. If the deflection exceeds the specified tolerance, the truss is overloaded.
Can I powder-coat my aluminum truss black?
Yes, but the curing process must not exceed 150°C, or the heat will anneal the aluminum, destroying the T6 temper and significantly reducing the load capacity.
How tight should the R-clips be on conical pins?
The R-clip simply prevents the tapered pin from backing out. It does not bear load. It should slide in easily once the pin is fully seated.
People Also Ask
What is the maximum span for 290mm truss?
Depending on the manufacturer and wall thickness, a 290mm box truss usually maxes out around 12 to 16 meters with a very minimal point load capacity.
How much does aluminum truss weigh?
Standard 290mm box truss weighs approximately 6 to 8 kg per meter. Heavy-duty 400mm truss weighs 10 to 14 kg per meter.
Is steel truss better than aluminum?
Steel truss is heavier and can span much longer distances with higher loads (e.g., arena mother grids). Aluminum is standard because it is lightweight enough for crews to assemble rapidly.
Can I use scaffolding clamps on truss?
Yes, standard half-couplers (cheseboroughs) are designed to grip 48-50mm aluminum chords without crushing them.
What does SWL stand for?
Safe Working Load. It is the maximum load the manufacturer certifies the truss can hold under specific conditions, including a built-in safety margin.
Related Articles
- •Selecting a Lighting Console — Features That Matter
- •DMX512 Protocol Explained — Addressing and Universes
- •Choosing Chain Hoists for Stage Rigging
Related Categories
Downloads
- •MILOS M290 Span Tables (PDF)
- •ART-R Rigging Safety Checklist (PDF)
- •Conical Connector Assembly Manual (PDF)
Glossary
- •Chord: The main continuous outer tubes of a truss section.
- •Node: The point where diagonal bracing intersects with a main chord.
- •Torsion: Twisting force applied to the truss axis.
- •Shear: Force causing two adjacent parts of a structure to slide parallel to each other.
- •Deflection: The degree to which a structural element is displaced under a load.
- •Spanset: A synthetic round sling used to choke truss for lifting.
- •Guy Wire: A tensioned cable used to stabilize an upright structure.
- •Bridle: Two or more rigging legs meeting at a central point to distribute a load between structural beams.
- •Cantilever: A beam anchored at only one end, extending outward.
Conclusion
Selecting between flat, triangular, box, or circular truss is an engineering decision based on load distribution, required spans, and directional forces. Box truss remains the industry standard for overhead rigging due to its multi-axial stability. Strict adherence to manufacturer span tables, correct connector assembly, and routine inspections ensure structural integrity and crew safety.
Schema Recommendation
- •Article
- •FAQPage
Image Suggestions
- 1.Hero Image: A wide shot of a massive MILOS outdoor roof structure being assembled, showing heavy 400mm box truss and lifting hoists.
- 2.Technical Diagram: Cross-section illustration showing the differences in chords and bracing between flat, triangular, and box truss.
- 3.Product Close-up: Macro photograph of a conical connector assembly (spigot, tapered pin, R-clip) fully seated in a truss chord.
- 4.Application Photo: A large circular truss suspended in a nightclub, rigged with ROBE LEDBeam fixtures.
- 5.Comparison Visual: An infographic explaining Uniformly Distributed Load (UDL) versus Center Point Load (CPL) on a truss span.