What is a space frame?
A space frame is a structural system built from linear members connected at nodes into a repeating three-dimensional pattern, usually pyramidal, so that the assembly acts as a single surface rather than as separate beams.
Three distinctions worth keeping clear, because the terms get used loosely:
- Space frame vs truss. A plane truss carries load in one plane and needs bracing out of it. A space frame is three-dimensional, so it resists load from any direction without secondary bracing. Strictly, a bolted ball-jointed space frame is a space truss — its members work in tension and compression — and in practice the two names mean the same system.
- Single-layer vs double-layer. A single-layer grid is one surface of members, and it relies on curvature for stiffness — that is a gridshell, and it lives on the free-form structures page. A double-layer grid has top and bottom chord layers joined by diagonals, so it works as a deep plate and can span flat. Space frames are usually double-layer.
- Space frame vs portal frame. A portal frame is a series of parallel two-dimensional frames, each carrying its own strip of roof. A space frame is one structure carrying the whole roof. This is the comparison that matters commercially, and it is worked through in detail in our space frame vs portal frame article.
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Plane truss Carries load in one plane and needs bracing out of it.
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Double-layer space frame Top and bottom grids joined by diagonals into pyramids. One structure carrying the whole roof, from any direction, without secondary bracing.
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Portal frames A series of parallel two-dimensional frames, each carrying its own strip of roof.
This system covers 20 – 200 m spans, double-layer flat or curved.
- Span, double-layer flat or curved
- 20 – 200 m
- Grid module
- 1,500 – 3,000 mm
- Members at one node
- Up to 18
- Deflection limit, roof · glazed
- L/250 · L/300
How the load actually travels
In a portal frame, load has one route: through the rafter, into the column, into the footing. Remove a member and the frame fails.
In a space frame the network is redundant by geometry. Failure of one member does not collapse the structure — load redistributes through alternative paths in the network. That property is not a safety bonus bolted on afterwards; it is what the triangulated pattern does inherently, and it shows up in three places:
- Robustness. An accidental impact, a fabrication defect or a corroded member degrades the structure rather than ending it.
- Point loads. Hanging a services run, a lighting rig or plant from a space frame spreads the load into the network. On a portal frame the same load goes into one rafter, which then has to be sized for it. Large travelling cranes are the exception: a portal frame with its own crane gantry is often the more direct answer.
- Seismic behaviour. Members that yield shed load to healthy members rather than initiating collapse. Combined with lower mass — and therefore lower inertial force — this is a large part of the case for a space frame roof in a seismic region.
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Portal frame Load has one route: rafter, column, footing. Lose a member on that route and the frame fails.
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Space frame, in plan The network is redundant by geometry. Remove one member and its load redistributes through the members around it.
The span where a space frame starts to win
Roughly 30 m and above. Below that a portal frame is usually the right answer and we will say so.
At those spans a space frame is typically 25 – 40% lighter than an equivalent portal frame hall. The worked comparison on a 60 m clear-span production hall, set out in full in the article:
| Measure | Trussed portal frame | Space frame |
|---|---|---|
| Roof steel | 65 kg/m² | 44 kg/m² |
| Total tonnage | 351 t | 238 t |
| Difference | — | 113 t lighter, 32% |
The saving compounds downward, and that is the part that changes the budget. Lighter roof means smaller reactions, which means smaller footings and less excavation and concrete. It means the crane is on hire for less time. Erection typically runs 25 – 40% faster than site-welded portal frames, with a smaller crew and tighter quality control, because members and nodes arrive CNC-fabricated and are bolted rather than welded in position.
None of those line items appears in a steel tonnage comparison, which is why space frames are frequently dismissed on a first-pass rate that only counts the roof.
Structural depth, grid module and “how thick is a space frame roof?”
Two numbers define the geometry:
- Grid module: 1,500 – 3,000 mm. The spacing of the nodes on each layer. A tighter module gives more, smaller members and more nodes; a wider module gives fewer, heavier members and fewer nodes. Node count drives fabrication cost, member weight drives material cost, and the optimum sits somewhere between.
- Structural depth — the distance between top and bottom chord layers. This is what “roof thickness” means on a space frame, and it is not a fixed ratio. It is governed by deflection.
The deflection limits on this system are the way to work backwards: L/250 for a roof, L/300 where the roof is glazed. For a 60 m span, L/250 is 240 mm of permitted deflection. The depth required to stay inside that, at a given grid module and member range, comes out of the analysis rather than a rule of thumb — which is why depth, module and member sizes are resolved together rather than in sequence.
Three practical consequences for an architect:
- Deeper is lighter, up to a point. Increasing structural depth reduces chord forces and therefore chord sizes. Past the optimum the diagonals grow long enough for buckling to govern them, so depth is optimised rather than maximised.
- Depth eats headroom or building height. The trade is between structural efficiency and the volume you are heating and cladding.
- Glazing tightens it. L/300 instead of L/250 allows about 17% less deflection, so the frame needs 20% more stiffness, and on a glazed roof it usually governs. Decide early whether the covering is glazed, because it changes the frame.
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Module and depth The module is the node spacing on each layer, 1,500 – 3,000 mm. The depth — the roof’s thickness — comes out of the deflection check.
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Shallow Less lever arm between the layers, so the chords carry more force and grow.
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Deep Chord forces fall — but past the optimum the diagonals get long enough for buckling to govern them. Depth is optimised, not maximised.
Sections through a double-layer grid. Forces indicative only.
Members and nodes
Member sections: CHS 33.7 – 168.3 mm, wall 2.6 – 8.0 mm. Circular hollow sections, in S235 / S355 to EN 10025-2, hollow sections to EN 10219.
CHS is used rather than open sections for three reasons: it has the same buckling resistance about every axis, which suits a member that may be loaded from any direction; it presents the least surface area per unit of capacity, which matters for coating cost across thousands of members; and it connects cleanly to a spherical node.
Node system: bolted spherical ball-and-sleeve, up to 18 members.
The node is the system. A machined steel sphere is drilled and tapped on the axes the geometry requires; each member terminates in a cone and sleeve, and a single high-strength bolt draws it into the ball. The consequences:
- Angular variation is absorbed in the drilling, not in the member. The same ball type serves many different geometries.
- Up to 18 members can meet at one node, which is what makes a dense three-dimensional network buildable at all.
- Bolted, not welded. Site welding a triangulated network overhead is slow, weather-dependent and difficult to inspect. Bolted assembly is fast, repeatable and checkable — and it is what allows the erection speed above.
- Millimetre accuracy is a prerequisite, not a luxury. Members and nodes are CNC-fabricated because a redundant network accumulates error. A small tolerance on each of several hundred members becomes a structure that will not close.
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One member into the ball The tube ends in a cone and a sleeve, and a single high-strength bolt draws it into a tapped hole in the ball — bolted, not welded.
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Many members, one ball The angles are drilled into the ball, so the same ball type serves many geometries — up to 18 members meeting at one node.
Simplified to the working parts.
Roof coverings, and why they change the frame
Options on this system: sandwich panel, standing seam, glass, polycarbonate, membrane.
This is the choice architects most often make last and should make early, because on a long-span roof the structure is substantially sized by what it carries:
| Covering | Weight | Effect on the frame |
|---|---|---|
| Membrane (PTFE, PVC, ETFE) | Lightest | Structure sized by wind uplift and snow rather than dead load. Uplift can govern — a very light roof has to be held down. |
| Polycarbonate | Light | Translucent, with little dead-load penalty. Finite service life; check the UV warranty. |
| Standing seam | Light–moderate | Long runs with few penetrations, good for large simple roofs. |
| Sandwich panel | Moderate | Insulation and weathering in one, fast to fix. The default on industrial halls. |
| Glass | Heaviest | Changes everything. Dead load rises sharply and the deflection limit tightens to L/300. Budget for both — and for the glazing performance itself, covered under free-form skylights. |
Where dead load rather than snow or wind is driving the member sizes, pricing a lighter covering before the steel is fixed is worth doing. The saving compounds the same way the weight saving does. Where the lightest possible roof matters more than a stiff one, a membrane on a cable structure may be the better system altogether.
Flat, curved and domed
The system covers double-layer flat or curved geometry.
- Flat grids are the industrial default. They span without needing curvature for stiffness, which is what the second layer buys.
- Barrel vaults and domes add curvature, which lets load resolve partly into the surface. A curved space frame can go lighter than a flat one across the same span, and a domed space frame is among the most material-efficient long-span roofs available.
- Free-form surfaces are a different problem. When the geometry stops being describable by a simple rule, the node family stops repeating and the structure becomes a gridshell — that work is on the free-form structures page.
The practical boundary: if the surface has one radius, or two, the space frame node system handles it economically. Once every node resolves at a different angle, you are in free-form territory and the cost model changes.
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Flat grid The industrial default. The second layer is what lets it span without curvature.
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Barrel vault One radius. Curvature lets load resolve partly into the surface, so it can go lighter than a flat grid across the same span.
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Dome Among the most material-efficient long-span roofs. Once every node resolves at a different angle, it is a free-form gridshell instead.
Erection: ground assembly or falsework
Two methods, both on the specification.
Ground assembly and lift. The frame — or a large section of it — is assembled at ground level, where work is fast, safe and easy to inspect, then lifted into position. Fastest and highest quality, but it needs crane capacity and a clear footprint for assembly.
Piecemeal on falsework. The frame is built in position on temporary support. Slower and needs falsework design, but it works where craneage is limited or the space below cannot be cleared.
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Ground assembly and lift Built at ground level, where work is fast, safe and easy to inspect, then lifted. Needs crane capacity and a clear footprint.
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Piecemeal on falsework Built in position on temporary support. Slower, and the falsework is designed — but it works where craneage is limited or the floor below cannot be cleared.
The choice is usually made by site conditions rather than preference, and it should be made before fabrication, because it affects how the frame is split into sub-assemblies and where the site joints fall.
The Arian Motor Factory in Arak is a clear-span space frame over a production hall, erected from prefabricated members and nodes.
Corrosion protection
Hot-dip galvanised to EN ISO 1461, or powder coat.
Galvanising deserves particular attention on a space frame, for a reason specific to the geometry: the structure has an enormous surface area relative to its weight — hundreds or thousands of small-diameter members, each with an inside and an outside. A coating applied by brush or spray reaches the outside. Hot-dip galvanising reaches everything the zinc touches, including the inside of vented hollow sections, and it protects cut edges sacrificially.
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Applied coating Reaches the outside of each member. The inside of a hollow section stays as it was.
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Hot-dip galvanised Zinc reaches every surface it touches, including the inside of vented hollow sections, and protects cut edges sacrificially.
That matters most in the conditions these buildings are often in: production halls with process humidity, coastal sites, and anywhere with condensation cycling. Powder coat is appropriate where appearance governs and the environment is benign, and the two are sometimes combined — galvanised then coated, where both service life and colour are required.
Fire
Exposed steel above an occupied or industrial space needs a fire strategy, and on a space frame it needs deciding early rather than late, for one reason: protecting thousands of small members is disproportionately expensive compared with protecting a few large ones. The surface-area-to-weight ratio that makes galvanising attractive makes intumescent coating costly.
The available routes — unprotected by calculation, intumescent coating, or a sprinkler-based strategy — depend on the occupancy, the height, the escape distances and the governing code, so the answer is project-specific and comes from the fire engineer rather than the frame supplier. What matters here is that it is asked at concept stage, because a decision to protect can change the member sizing and always changes the budget.
What to decide, and when
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Concept
The clear span required and why — what has to fit under it. The roof covering, at least in category. Whether the roof is glazed, because that tightens the deflection limit. Seismic and snow data for the site, or just the location.
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Developed design
Grid module, structural depth, and the geometry — flat, curved or domed. Support conditions and where the frame lands. The fire strategy. Any point loads the frame has to carry: cranes, services, lighting, plant.
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Technical design
Node schedule, member schedule, corrosion protection specification, the erection method and the sub-assembly split that follows from it.
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Before tender
A defined grid, depth and covering. A space frame tendered on span alone gets priced on assumptions about all three, and the prices will not be comparable.
