Steel Space Frame Design — Double-Layer Grids & Nodes

Steel space frame design: double-layer grid geometry, node types (ball, plate, tube), member sizing, support layout, deflection control, and progressive collapse considerations.

What is a space frame?

A space frame is a three-dimensional truss system where members are connected at nodes in a regular geometric pattern. The most common configuration is the double-layer grid — two parallel planes of chord members connected by diagonal web members. Space frames distribute load to supports in two directions simultaneously, achieving very long spans with minimal depth and weight.

Typical span-to-depth ratios range from 20:1 to 30:1. A 60 m span space frame might be only 2.5 m deep. This efficiency comes from the three-dimensional load path: every member participates in resisting applied loads, unlike a planar truss where only in-plane members contribute.

Common configurations

• Square-on-square — top and bottom chord grids are square, with the bottom grid offset half a module in both directions. The simplest and most common layout. Module sizes typically 1.5-3.0 m.
• Square-on-diagonal — bottom grid is rotated 45 degrees relative to the top grid. Provides better load distribution and shorter web members but creates more complex edge conditions.
• Triangle-on-triangle — triangular grids in both layers. Maximum stiffness but more members and nodes, increasing fabrication cost.

Node types

• Ball (Mero-type) node — a solid or hollow steel sphere with threaded holes. Members connect via cone-head bolts threaded into the sphere. Clean appearance, allows members at multiple angles, but limited moment transfer. Suitable for spans up to 120 m. Ball node diameters typically range from 60 mm to 350 mm.
• Plate node — flat gusset plates welded to a central tube section. Simpler fabrication than ball nodes, lower cost, but bulkier appearance. Common for spans under 40 m.
• Hollow tube node — cylindrical or prismatic hollow section with stub tubes welded to receive chord members. Good for architecturally exposed applications. Requires careful weld design for fatigue.

Worked example — member force estimation

Configuration: square-on-square, 36 m x 36 m plan, 3 m module, 1.8 m grid depth (span/depth = 20). Four corner supports. Factored uniform load = 2.0 kPa (includes self-weight, services, and live load).

Total factored load: W = 2.0 x 36 x 36 = 2,592 kN. Reaction per corner support = 2,592 / 4 = 648 kN.

Using the simplified strip method for preliminary design, treat a central strip of width 3 m (one module) as a beam spanning 36 m with UDL = 2.0 x 3.0 = 6.0 kN/m (total) but supported on a 2D grid, so the effective strip load is approximately 6.0 / 2 = 3.0 kN/m for each direction.

Mid-span moment in one direction: M = wL^2 / 8 = 3.0 x 36^2 / 8 = 486 kN-m. Chord force = M / depth = 486 / 1.8 = 270 kN.

For 270 kN compression in a chord member spanning 3.0 m: using CHS 76.1 x 3.2 (A = 7.32 cm^2, r = 2.58 cm), KL/r = 300/2.58 = 116. With Fy = 350 MPa, phiPn = 0.9 x Fcr x A. Fe = pi^2 x 200,000 / 116^2 = 147 MPa. Fcr = 0.877 x 147 = 129 MPa. phiPn = 0.9 x 129 x 732 / 1000 = 85 kN. Insufficient — a larger section such as CHS 88.9 x 4.0 (phiPn approximately 140 kN) or CHS 114.3 x 3.6 (phiPn approximately 220 kN) is needed.

This demonstrates that space frame chord members are governed by compression buckling. Detailed computer analysis (typically using finite element methods) is essential because the simplified strip method can underestimate forces in members near supports by 30-50 percent.

Code provisions for space frames

EN 1993-1-8 Section 7 specifically covers hollow section joints (CHS and RHS), including T-joints, Y-joints, and K-joints. This is directly applicable to space frame node design.

Common pitfalls

• Assuming all members carry equal force. Force distribution in a space frame is highly dependent on support layout. Members near supports carry 2-3 times the force of mid-span members. Corner-supported grids concentrate forces near corners.
• Neglecting progressive collapse. If one diagonal web member fails (corrosion, impact, fatigue), the load redistributes to adjacent members. If those members are near capacity, a zipper-type progressive collapse can propagate. Redundancy checks or member removal analyses are recommended for public assembly occupancies.
• Ignoring thermal expansion. A 60 m space frame exposed to 40 degrees C temperature range experiences 60,000 x 40 x 12 x 10^-6 = 28.8 mm thermal movement. Supports must include sliding bearings or flexible connections on all but one fixed point.
• Under-estimating self-weight. Space frame self-weight is typically 0.3-0.6 kPa for spans of 30-60 m. During preliminary design, using too low a self-weight estimate leads to under-sized members in detailed analysis.

Run this calculation

• Portal Frame Calculator
• Section Properties Database

Related references

• Truss Analysis
• HSS Connections
• Deflection Control
• How to Verify Calculations
• steel beam capacity calculator

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. The site operator disclaims liability for any loss arising from the use of this information.