----------- | ------------- | -------------- | ---------- | ------------ | | Span range | 10 to 150 m | 20 to 200 m | 10 to 40 m | 15 to 100 m | | Erection speed | Fast (bolted) | Slow (welded) | Medium | Medium | | Moment resistance | Low | Moderate | Low | Moderate | | Cost per node | High | Medium | Low | Medium |

Member sizing

Chord members

Chord members carry the primary axial forces (tension in the bottom chord, compression in the top chord for gravity loading). Circular hollow sections (CHS) are most common due to uniform buckling behavior and efficient node connections.

Sizing approach: Determine max chord force from analysis, select trial CHS, check compression per applicable standard, verify local buckling (D/t limits), check combined axial and bending if eccentricities exist.

Typical chord sizes:

Web (diagonal) members

Web members transfer shear between top and bottom chord layers. Forces are typically 30 to 70 percent of chord forces but can approach chord magnitudes near supports. Web forces reverse under wind uplift — check both tension and compression.

Simplified strip method

For preliminary sizing before FEM analysis: treat a central strip of one-module width as a beam, distribute moment to two directions (Mx = My = wL^2 / 16 for square grids), compute chord force = M / d. This method can underestimate forces near supports by 30 to 50 percent — detailed FEM analysis is always required for final design.

Span-to-depth ratios

Deflection limits: Roofs L/200 to L/250 full load, L/360 live load. Floors L/360 total, L/480 live load. Canopies L/180 (ponding may govern).

Loading considerations

Dead loads

• Self-weight: 0.15 to 0.30 kPa (spans under 30 m), 0.30 to 0.60 kPa (30 to 60 m), 0.60 to 1.00 kPa (over 60 m). Iterate from preliminary sizing.
• Roof cladding: 0.05 to 0.15 kPa (metal deck) or 0.30 to 0.60 kPa (concrete on metal deck)
• Services, ceiling, insulation, lighting: 0.20 to 0.65 kPa combined

Live loads

• Roof live load: 0.96 kPa (20 psf) per ASCE 7-22; 1.0 kPa minimum per EN 1991-1-1
• Snow: per ASCE 7 Chapter 7, EN 1991-1-3, or NBCC. Drifting creates unbalanced loading that can govern design.
• Floor live load: 2.4 to 7.2 kPa per ASCE 7 Table 4.3-1 or EN 1991-1-1

Wind loads

Wind often governs for lightweight roof space frames:

• Net pressure coefficients per ASCE 7 Chapter 30 or EN 1991-1-4
• Uplift reverses web member forces — check all members for force reversal
• Long-span roofs susceptible to aerodynamic instability; wind tunnel testing for spans over 60 m
• Asymmetric patterns can produce member forces exceeding full-span loading

Seismic loads

Space frames are lightweight, so seismic forces are often low. However, seismic weight includes all supported dead load. Horizontal forces require chord-plane bracing. R-factor per ASCE 7 Chapter 12.

Thermal loads

A 60 m frame in a 40 degree C range moves 28.8 mm. Supports must include sliding bearings on all but one fixed point. For spans over 100 m, thermal forces can govern connection design.

Support settlement

Differential settlement of 10 to 25 mm can increase forces near settled supports by 15 to 30 percent. Limit total settlement to L/500 and differential settlement to L/1000.

Stability analysis

Overall stability

Eigenvalue (linear) buckling analysis should yield a buckling load factor exceeding 4.0 for preliminary checks. A factor below 2.0 indicates the structure is too shallow or too lightly braced.

Member stability

Follows standard compression provisions: AISC 360 Chapter E, AS 4100 Clause 6.3, EN 1993-1-1 Clause 6.3.1 (buckling curves a-d), CSA S16 Clause 13.3. K is typically 0.7 to 1.0 for web members and 1.0 for chords.

Second-order effects

Required per AISC 360 Chapter C, AS 4100 Clause 4.4, EN 1993-1-1 Clause 5.2, or CSA S16 Clause 8. P-delta (member bow) is captured by the column buckling formulation. P-Delta (structure sway) uses the direct analysis method or amplified first-order analysis.

Progressive collapse

Failure of a single diagonal can propagate in a zipper-type collapse. Concern for public assembly occupancies, corrosive environments, and low-redundancy forms. Mitigation: design all members for at least 50 percent of maximum chord force, perform member removal analysis per GSA or DoD UFC 4-023-03, and provide redundant load paths.

Comparison with conventional framing

Choose a space frame for: spans over 40 m needing column-free space, architectural feature roofs, high service integration, or prefabricated bolted erection advantages.

Consider alternatives for: rectangular plans with one dominant span direction, spans under 25 m, budget-constrained projects, or heavy floor loading.

Worked example — double-layer grid member sizing

Configuration: Square-on-square, 36 m x 36 m plan, 3 m module, 1.8 m depth (L/d = 20). Four corner supports. Steel: 350 MPa.

Loading: Dead = 1.0 kPa, Live = 1.0 kPa. Factored (LRFD): 1.2 x 1.0 + 1.6 x 1.0 = 2.8 kPa.

Step 1 — Total load: W = 2.8 x 36 x 36 = 3,629 kN. Reaction per corner = 907 kN.

Step 2 — Chord force (strip method): Effective strip load per direction = 2.8 x 3.0 / 2 = 4.2 kN/m. M = 4.2 x 36^2 / 8 = 680 kN-m. Chord force = 680 / 1.8 = 378 kN.

Step 3 — Top chord (AISC 360 Ch. E): Trial CHS 114.3 x 4.0 (A = 13.82 cm^2, r = 3.89 cm). KL/r = 77.1. Fe = 332 MPa. Fcr = 224.6 MPa. phiPn = 279 kN < 378 kN required. Insufficient.

Step 4 — Revised: CHS 139.7 x 5.0 (A = 21.21 cm^2, r = 4.77 cm). KL/r = 62.9. Fcr = 263.6 MPa. phiPn = 503 kN > 378 kN. OK. DCR = 0.75.

Step 5 — Bottom chord tension: phiTn = 0.90 x 350 x 2,121 / 1,000 = 668 kN > 378 kN. OK.

Step 6 — Web member (near support): Max web force ~0.60 x 378 = 227 kN. Length = 2.34 m. Trial CHS 76.1 x 3.6: KL/r = 91.1, phiPn = 130 kN. Insufficient. Revise to CHS 88.9 x 4.0: KL/r = 77.5, phiPn = 215 kN. Marginal — use CHS 101.6 x 4.0 or reduce module.

This example shows that compression buckling governs both chord and web members. The strip method provides reasonable preliminary sizing, but FEM analysis is essential because it underestimates forces near supports.

Code provisions for space frames

EN 1993-1-8 Section 7 covers hollow section joints (CHS and RHS) including T, Y, K, and KK joints — directly applicable to space frame nodes. EN 1993-1-6 provides shell buckling rules for single-layer domes and barrel vaults.

Common mistakes

1. Assuming all members carry equal force. Members near supports carry 2 to 3 times the force of mid-span members. Corner-supported grids concentrate forces at corners. Always use FEM analysis — the strip method is only for preliminary sizing.

2. Neglecting progressive collapse. A single diagonal failure can propagate through adjacent members. Redundancy checks and member removal analyses are recommended for public assembly occupancies.

3. Ignoring thermal expansion. A 60 m frame in a 40 degree C range moves 28.8 mm. Supports must include sliding bearings on all but one fixed point. Fixing all supports induces thermal forces that can fracture connections.

4. Under-estimating self-weight. Self-weight of 0.15 to 0.60 kPa must be iterated: size members, calculate actual weight, resize.

5. Forgetting force reversal under wind uplift. Compression members under gravity may go into tension under uplift, and vice versa. Check all members for both conditions under all load combinations.

6. Ignoring differential settlement. Even 10 mm differential settlement can increase member forces by 15 to 30 percent in these highly indeterminate structures.

7. Using inappropriate effective length factors. Nodes provide partial rotational restraint, not true pins. Use direct analysis method (AISC 360) to avoid assuming K-factors, or calibrate K for the specific node type.

8. Omitting ponding checks. Frames with slopes less than 1:24 are susceptible to rainwater ponding — a positive feedback loop that can lead to collapse. Check per AISC 360 or dedicated ponding analysis.

9. Neglecting construction sequence effects. Locked-in forces from staged erection with temporary supports can differ significantly from final-state analysis. Construction stage analysis is required for spans over 40 m.

Frequently asked questions

What is the maximum span for a steel space frame?

Double-layer grids have exceeded 150 m spans. Single-layer domes have exceeded 200 m (the Louisiana Superdome spans 207 m). Most commercial applications range from 20 to 80 m. Practical limits depend on loading, support conditions, and budget.

How much does a steel space frame weigh?

Self-weight ranges from 0.15 kPa for short spans to over 0.60 kPa for 50 to 80 m spans. Node weight adds 5 to 15 percent.

What is the difference between a space frame and a space truss?

A "space frame" is any 3D framing system. A "space truss" implies pinned joints with axial-only forces. The terms are used interchangeably since most space frames have nominally pinned joints. If joints have significant moment resistance, include bending in analysis.

Do space frames require fire protection?

Yes. Options include intumescent paint (1 to 3 mm thickness), spray-applied fire-resistive material (SFRM), or a fire-rated ceiling. The large surface area of many small members makes fire protection costs significant.

Can space frames support floor loading?

Yes, with appropriate span-to-depth ratios (12:1 to 18:1) and stricter deflection limits (L/360 to L/480). Floor-type space frames are heavier and deeper than roof frames. Vibration serviceability is a key consideration.

What software is used for space frame analysis?

General-purpose FEA software: SAP2000, ETABS, STAAD.Pro, Robot Structural Analysis, Midas Gen, Strand7. Requirements: thousands of members with pinned or semi-rigid joints, linear and nonlinear buckling analysis.

How are space frames erected?

Four common methods: (1) Ground assembly and crane lifting (spans up to 50 m). (2) Scaffold-supported in-place assembly (40 to 100 m). (3) Push-out or slide-in from ground level (very long spans). (4) Unit-by-unit bolted assembly in the air from man-lifts or platforms.

What maintenance do space frames require?

Regular corrosion inspection at node connections, bolt tightness checks, deflection monitoring, and protective coating maintenance. In corrosive environments, inspections should not exceed 2-year intervals.

Are space frames cost-effective?

Under 30 m, conventional trusses or portal frames are typically cheaper. Between 30 and 60 m, space frames become competitive for square plans. Over 60 m, space frames are often the most economical column-free solution. Nodes account for 25 to 40 percent of total cost.

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Related references

• Truss Analysis — planar and 3D truss analysis methods
• HSS Connections — hollow section joint design per AISC and EN 1993
• Deflection Control — serviceability limits and deflection calculation
• Steel Buckling — member and system buckling behavior
• HSS Section Properties — CHS and RHS dimensions and properties
• Roof Framing — steel roof system types and design considerations
• Roof Loading — snow, wind, and rain loads on steel roofs
• Fatigue Design — fatigue assessment for dynamically loaded structures
• Corrosion Protection — coating systems for exposed steelwork
• Floor Vibration — vibration serviceability for steel floor systems
• Frame Analysis — methods for structural frame analysis
• How to Verify Calculations — verification and quality control checklist

Run these calculations

• Truss Analysis Calculator — model and analyze 2D and 3D trusses
• Beam Capacity Calculator — steel beam flexural and shear capacity
• Column Axial Load Design Check — compression member design
• Section Properties Database — lookup section properties for standard shapes
• Load Combinations (ASCE 7) — generate factored load combinations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. Space frame design requires comprehensive finite element analysis performed by a qualified structural engineer. The site operator disclaims liability for any loss arising from the use of this information.