Bolted Sphere vs Welded Sphere Grid: Which to Choose

Spatial grid structures are the defining product of our company. As a manufacturer holding the Special-Class qualification for bolted sphere grid fabrication — the highest tier in China's steel structure qualification system — we have produced over 3,000,000 square metres of grid structures per year. But grid structures come in two fundamentally different node types: bolted sphere and welded sphere. The choice between them is not aesthetic; it is a structural engineering decision driven by span, load, site conditions, and construction schedule.

This article provides the engineering comparison that project owners and design engineers need to make the right choice. We cover node force capacity, assembly methods, field welding requirements, quality control, and the decision matrix we use when advising clients.

1. The Bolted Sphere Node System

The bolted sphere (also called Mero sphere or bolted ball) was invented in the 1940s and remains the most widely used grid structure node worldwide. The node consists of a solid steel sphere, typically forged from 45# carbon steel, with threaded holes drilled at precise angles. Steel tubular members are connected to the sphere via a bolt that passes through the tube end cap into the sphere. The bolt is tightened from inside the tube using a spanner, creating a friction-and-bearing connection.

Node Force Capacity

The capacity of a bolted sphere node is governed by the bolt tensile strength and the bearing capacity of the sphere material. For a standard M20 bolt (8.8 grade) in a 300 mm diameter sphere (45# steel, σy = 355 MPa equivalent), the design tensile capacity per bolt is approximately 160 kN. With multiple members meeting at a node (typically 6-9 members in a space frame), the resultant node capacity ranges from 250-350 kN, depending on member geometry and load direction.

For heavier loading, we manufacture larger spheres: 400 mm, 500 mm, and up to 900 mm diameter. A 500 mm sphere with M30 bolts (10.9 grade) can achieve node capacities of 500-600 kN. However, as sphere diameter increases, the weight penalty grows — a 500 mm solid steel sphere weighs approximately 515 kg, compared to 111 kg for a 300 mm sphere. This is why bolted spheres are most economical in the 30-60 m span range where lighter nodes suffice.

Assembly Speed and Quality

The defining advantage of the bolted sphere system is assembly without field welding. Members arrive at site with bolts pre-installed in the end caps. Workers insert the bolt into the sphere, tighten it from inside the tube using a box wrench, and move to the next joint. A skilled crew of 4 can assemble 200-300 m² of grid per shift. No welding equipment, no welders, no welding inspection, no paint touch-up after welding.

This assembly method also means quality is factory-controlled. Every bolt is tightened in our factory's pre-assembly area to verify fit, then loosened for shipping. The sphere's threaded holes are machined on CNC indexing fixtures with angular accuracy of ±0.05°, ensuring that members meet at the correct geometry. Field assembly errors are limited to bolt tightening, which is verifiable by torque wrench.

Limitations of Bolted Spheres

Bolted spheres are not suitable for every project. Three limitations constrain their use:

• Node capacity ceiling: For point loads exceeding 300 kN per node (heavy concentrated loads, very long cantilevers, suspended equipment), the bolt diameter and sphere size required become impractical. A 900 mm sphere weighs 3,500 kg — lifting it into position requires a crane that may not be available on site.
• Member access for bolt tightening: The bolt is tightened from inside the tube. This means the tube diameter must be large enough for a worker's arm and wrench — typically Ø76 mm minimum. For small-diameter members, bolted spheres are not feasible.
• Dynamic and fatigue loading: Bolted connections under cyclic loading can loosen over time. For structures with significant vibration (railway platforms, bridges, equipment support), welded spheres or welded joints are preferred.

2. The Welded Sphere Node System

The welded sphere node consists of a hollow steel sphere formed from two spun hemispheres welded together at the equator, with tubular members welded directly to the sphere surface using full-penetration groove welds. There are no bolts — the connection is monolithic.

Node Force Capacity

Because the connection is a full-penetration weld, the node capacity equals the member capacity — there is no bolt or thread to limit the force transfer. A welded sphere with Ø219 × 10 mm members welded to a 500 mm × 16 mm sphere can transfer the full axial capacity of the member (approximately 1,100 kN in compression). With larger members (Ø273 × 12 mm), node capacities can reach 1,500 kN or more.

This higher capacity makes welded spheres the standard choice for:

• Spans from 60 to 120 m where member forces exceed bolted sphere limits
• Heavy point loads (suspended mechanical equipment, scoreboard rigging)
• Seismic applications requiring welded connection ductility
• Long cantilevered structures where bolt loosening under wind vibration is a risk

Field Welding Requirements

The defining cost of welded spheres is field welding. Every member-to-sphere connection requires a full-penetration groove weld, executed in the field, in position (overhead, vertical, or horizontal). This means:

• Certified welders must be on site, with valid qualifications for the position and process being used (typically FCAW or SMAW for field work).
• Welding procedure specifications (WPS) must be developed and qualified, including preheat and interpass temperature control for thicker materials.
• Non-destructive testing (UT or RT) of 100% of full-penetration welds on primary members, per GB 50661 or AWS D1.1.
• Surface preparation and coating after welding: the weld area must be blast-cleaned to Sa 2.5 and coated with the same system as the shop-applied coating, with a 50 mm overlap onto the existing coating.

Assembly speed is slower: a crew of 4 welders with a helper can complete approximately 80-120 m² of grid per shift, compared to 200-300 m² for bolted spheres. The total installed cost per square metre is typically 15-25% higher than bolted spheres for the same span, but for spans above 60 m, bolted spheres may not be feasible, making the comparison moot.

Advantages of Welded Spheres

Welded spheres offer several advantages beyond raw force capacity:

• Monolithic connection: No bolts to loosen under vibration, no threads to corrode. The welded joint is as strong as the parent material and does not require maintenance.
• Unrestricted member diameter: Small-diameter tubes (Ø60 mm and below) can be connected, enabling denser grid patterns for architectural effect.
• Architectural freedom: Because members are welded, not bolted, the geometry is not constrained by bolt access requirements. Curved grids, irregular node configurations, and non-standard angles are all possible.
• Seismic performance: Welded connections can be detailed for ductility, with the weld and surrounding base metal forming a continuous load path that performs better under seismic loading than bolted connections that may slip.

3. Technical Comparison Matrix

The following matrix summarizes the engineering parameters that drive the selection between bolted and welded sphere systems:

4. Decision Criteria: When to Use Each System

Based on our experience fabricating and erecting grid structures for sports venues, transportation hubs, industrial buildings, and public facilities, we have developed a decision framework. The following questions should be answered in sequence:

Step 1: Determine the Span

The span is the first filter. For double-layer grid structures (the most common type):

• Span < 30 m: Bolted sphere is almost always the right choice. The loads are light enough that M20 or M24 bolts provide ample capacity, and the speed advantage of bolted assembly dominates the cost equation.
• Span 30-60 m: Bolted sphere is preferred. Use larger spheres (Ø300-400 mm) with M24 or M27 (10.9 grade) bolts. If there are heavy point loads or seismic requirements, consider welded spheres for the primary load path.
• Span 60-90 m: Either system can work. Bolted spheres require Ø400-500 mm spheres with M27-M30 bolts. Welded spheres become more competitive because the cost premium shrinks relative to the total structure cost. The decision shifts to site-specific factors (welder availability, construction schedule, seismic zone).
• Span > 90 m: Welded spheres are typically required. At these spans, member forces exceed bolted sphere capacity, and the monolithic welded connection provides the stiffness and ductility needed for long-span behaviour. For spans approaching 120 m, consider whether a beam-string system (prestressed cables supporting the grid) might be more weight-efficient.

Step 2: Evaluate Site Conditions

If the site has limited access for welding equipment, no reliable power supply, or a very short construction window, bolted spheres are preferred — they can be assembled with nothing more than wrenches and a small crane. If the site is remote or environmentally sensitive (reducing the risk of welding sparks and fumes), bolted spheres eliminate field welding entirely.

Step 3: Assess Loading Conditions

Heavy snow loads (>2.0 kN/m²), suspended equipment loads, or significant seismic activity (design acceleration > 0.2g) push the decision toward welded spheres. The higher node capacity and monolithic connection behaviour provide margins that bolted connections cannot match.

Step 4: Consider the Project Schedule

If the construction window is compressed (a common scenario for transportation hubs and public buildings with fixed opening dates), bolted spheres' 2.5x assembly speed advantage can be decisive. A 10,000 m² roof that takes 50 shifts with bolted spheres takes 100+ shifts with welded spheres — a difference of weeks that may determine whether the project meets its deadline.

Step 5: Lifecycle and Maintenance

For structures with a 50-year design life, consider that bolted connections require periodic inspection (every 5 years for torque verification, per our maintenance recommendation). Welded connections are maintenance-free but cannot be disassembled. If the structure may need to be relocated or modified in the future, bolted spheres are the only option.

5. Hybrid Systems: When Neither Alone Is Right

In practice, many of our projects use hybrid systems that combine both node types. The most common hybrid approach is:

• Welded spheres for the primary load path: The perimeter members, main span girders, and heavily loaded nodes use welded spheres to achieve maximum capacity and stiffness.
• Bolted spheres for infill members: The interior grid members, which carry distributed loads at lower forces, use bolted spheres for assembly speed.

This hybrid approach optimises both cost and performance. The welded primary members carry the major forces with monolithic connections, while the bolted infill grid is assembled quickly and can be replaced if damaged. We have used this approach on several stadium and airport terminal projects, achieving both the architectural requirement for a uniform visual appearance and the engineering requirement for efficient force transfer.

6. Manufacturing Capabilities at Permanent Steel Structure

Our manufacturing facility in Xuzhou is equipped for both bolted and welded sphere production. For bolted spheres, we operate CNC sphere drilling centres with 5-axis indexing that machine threaded holes at angles accurate to ±0.05°. For welded spheres, we have hydraulic spinning machines for hemispheres up to 1,500 mm diameter, automated submerged-arc welding (SAW) for equator welds, and robotic FCAW stations for member-to-sphere welding.

Our Special-Class qualification for bolted sphere grid manufacturing — one of only a handful in China — means our welding procedures, quality systems, and inspection protocols are audited to the highest standard. Every grid structure we fabricate goes through digital pre-assembly: the entire structure is modelled in 3D, member lengths are verified against the model, and any interference or tolerance issue is resolved before steel is cut. This digital pre-assembly process, combined with our CAD-CAM integrated production line, ensures that field assembly proceeds without fit-up problems.

To learn more about our bolted sphere grid product, visit our bolted sphere grid product page. For welded sphere specifications and capacities, see our welded sphere grid product page.

7. Common Misconceptions

In our consultations with project owners and design engineers, we frequently encounter three misconceptions about the bolted vs welded sphere decision:

Misconception 1: "Welded is always stronger, so it's always better." Welded spheres have higher node capacity, but for spans under 60 m, the bolted sphere capacity is more than adequate. Over-specifying welded spheres adds unnecessary cost (15-25% premium) and construction time (2.5x slower assembly) without any structural benefit.

Misconception 2: "Bolted spheres are not suitable for seismic zones." Bolted spheres can perform well in seismic zones when properly designed. The key is to ensure that the bolted connection does not become the weak link — the bolt capacity should exceed the member yield capacity, which is the capacity design principle. In practice, this means using higher-grade bolts (10.9 or 12.9) and larger spheres in seismic zones.

Misconception 3: "You can't mix bolted and welded nodes in the same structure." Hybrid systems are well-established and common. The key engineering consideration is differential stiffness: welded nodes are stiffer than bolted nodes, so the load distribution analysis must account for this. Our structural design software handles this automatically.

Conclusion

The choice between bolted and welded sphere grid systems is a multi-variable engineering decision. Span is the primary driver, followed by loading conditions, site constraints, schedule, and lifecycle considerations. Bolted spheres excel in the 30-60 m span range with moderate loads, offering fast assembly, factory-controlled quality, and no field welding. Welded spheres are the choice for 60-120 m spans, heavy point loads, and seismic applications where monolithic connections and maximum node capacity are essential.

The right answer for your project depends on factors that cannot be evaluated in a general article. If you are designing a grid structure and need guidance on node type selection, contact our engineering team. We will review your span, loading, site, and schedule, and provide a recommendation based on 25 years and 3,000,000 square metres of grid structure experience.