Bolt ball space frames are widely used in large-span structures, but many existing systems no longer meet current seismic requirements. Seismic retrofit improves structural safety, enhances earthquake resistance, and extends the service life of the structure.
This article systematically reviews the general technical framework for bolt ball space frame seismic retrofitting from three perspectives: seismic performance characteristics, core retrofit design methods, and key construction implementation points, providing a practical reference for engineering applications.
Seismic Performance Characteristics of Bolt Ball Space Frames
Connection Features of Bolt Ball Joints and Their Seismic Relevance
Bolt ball joints are part of a highly statically indeterminate spatial structural system, and their connection behavior is closely linked to seismic performance. The joint connects members to the spherical node via high-strength bolts, forming a three-dimensional load-bearing system. The stiffness and ultimate bearing capacity of the joint directly determine the internal force distribution and energy dissipation mechanism of the entire structure under seismic action.
Integrity Advantage of Spatial Grid Structures
Spatial grid structures possess excellent overall integrity, which is a natural advantage in earthquake resistance. Proper grid division and joint layout enable efficient force transfer and redistribution during an earthquake, making full use of the collaborative behavior of all members and preventing local failures from escalating into progressive collapse.
Typical Failure Modes of Bolt Ball Space Frames in Earthquakes
Post-earthquake investigations have revealed the following typical failure modes of bolt ball space frames:
- Joint failure: bolt fracture or spherical node tearing under high-stress cyclic loading;
- Member instability: overall flexural buckling or local buckling of compression members under dynamic loads;
- Support failure: shear fracture or anchor bolt failure of supports due to insufficient deformation capacity;
- Overall collapse: progressive collapse under rare earthquakes caused by insufficient structural redundancy.
Basic Requirements for Seismic Design
One of the most important characteristics of a seismically resistant structure is ductility. The seismic design of bolt ball space frames must meet the following requirements:
A clear multiple-line seismic defence system;
Adequate deformation reserves in joints and members;
Compliance with current seismic design codes and spatial structure regulations.
Core Design Methods for Seismic Retrofit of Bolt Ball Space Frames
Adding Bracing Systems: Optimizing Load Transfer Paths
Bracing systems are a key measure to enhance the lateral stiffness and energy dissipation capacity of space frames. In seismic retrofit, optimizing the arrangement of column bracing and horizontal bracing can create an effective seismic bracing system. In particular, eccentrically braced frames use diagonal brace members to stiffen the frame, enabling it to act as a truss and significantly improving overall stiffness and energy dissipation. The bracing layout should be tailored to the existing structural characteristics to ensure that the upgraded structure meets the target fortification level.
Enhancing Joint Ductility: Strengthening Techniques for Bolt Ball Joints
Joint ductility directly governs the overall seismic performance of the structure. For the weak points of bolt ball joints, the following strengthening measures can be applied:
- Joint reinforcement: increasing bolt diameters, adding stiffening ribs, or locally welding steel plates to boost joint capacity;
- Detail optimization: improving the geometric details of the joint region to avoid stress concentration and enhance plastic rotation capacity;
- Material upgrade: replacing original components with higher-grade steel (e.g., Q390, Q420) to increase strength and toughness in the joint zone.
Isolation and Energy Dissipation Technologies: Active Control of Seismic Response
Isolation and energy dissipation are “active” measures to improve structural seismic performance. Base isolation has been widely recognized as one of the most effective seismic mitigation methods. In the retrofit of existing space frames, the following can be applied:
Isolation bearings: installing lead-rubber bearings, friction pendulum bearings, etc., at column tops or supports to lengthen the natural period and substantially reduce input seismic forces;
Dampers: adding viscous dampers, metallic yield dampers, or friction dampers at critical locations to increase the damping ratio and effectively dissipate seismic energy.
Structural Redundancy Design: Building Multiple Seismic Defence Lines
A multi-line defence system is an essential principle to prevent collapse under rare earthquakes. Seismic retrofit design should establish a clearly layered defence mechanism:
- First line: the bracing system yields or deforms first, dissipating a large portion of seismic energy;
- Second line: joints and members enter the plastic stage, continuing to dissipate energy while maintaining structural stability;
- Third line: the overall structural redundancy provides the final safety reserve to prevent collapse.
Key Construction Implementation Points for Seismic Retrofit
Preliminary Feasibility Assessment of the Retrofit Scheme
Before finalizing the retrofit scheme, a comprehensive evaluation of the existing structure is essential, including:
Structural condition survey and damage detection (cracks, corrosion, bolt loosening, etc.);
On-site construction condition assessment (space limitations, working platforms, lifting conditions);Technical and economic comparison of multiple retrofit alternatives to select the optimal solution.
Fabrication and Installation Accuracy Control of Retrofit Members
The fabrication accuracy of retrofit members directly affects the strengthening effect. During installation, strict control is required over:
- Dimensional deviations of fabricated members (preferably within ±1 mm)
- On-site positioning and alignment errors
- Final tightening torque of high-strength bolts to achieve the specified pre-tension.
Structural Safety Monitoring During Construction
Retrofit construction is often carried out while the structure is still under load, making real-time monitoring indispensable. Dynamic monitoring of stresses and deformations in critical members and joints should be implemented to ensure that the structure remains safe throughout the construction period.
Performance Verification and Acceptance After Retrofit
Upon completion of the retrofit work, the effectiveness of the strengthening should be verified through static load tests, dynamic characteristic tests (e.g., ambient vibration testing), and finite-element re-analysis. Acceptance should be conducted in accordance with current codes and standards.
Conclusion
Seismic retrofit of bolt ball space frames must be based on tailored schemes that consider the specific structural characteristics, existing damage conditions, and the required seismic fortification targets. A scientifically sound retrofit design not only effectively improves the seismic safety level of the structure but also extends its service life. It is important to emphasize that seismic retrofit is not purely a technical issue – it also requires a balanced consideration of economic costs, construction feasibility, and operational disruptions, so that safety, economy, and practicality are achieved simultaneously.
Yunjoin recognizes that professional design and construction capabilities are the key to successful project delivery in the field of spatial structure seismic retrofitting. Leveraging years of technical expertise in bolt-ball spatial structures, the company provides a full-chain seismic retrofit service for projects in earthquake-prone regions – from inspection and assessment → scheme design → member fabrication → construction and installation. Whether for the strengthening of existing space frames or the seismic optimization of new projects, Yunjoin offers professional and reliable technical support, helping clients achieve both structural safety and investment efficiency.
