Residual Vertical Bearing Capacity of Circular Hollow Sandwich Steel Tube Concrete-Filled Long Columns After Lateral Impact

Literature Overview

This research investigates the residual vertical bearing capacity of circular hollow sandwich steel tube concrete-filled long columns following lateral impact events. The sandwich steel tube configuration, which consists of an inner steel tube, a concrete core, and an outer steel tube, provides enhanced structural redundancy and damage tolerance compared to conventional CFST columns. The study is particularly relevant for applications where columns may be subjected to accidental lateral loads, such as vehicle collisions, falling objects, or blast events, and must subsequently maintain their vertical load-carrying capacity to prevent progressive collapse.

Core Technical Points

The lateral impact event causes local and global deformation of the column, including denting of the outer tube, crushing of the concrete core, and potential buckling of the inner tube. After the impact event, the column must continue to support the vertical gravity loads from the structure above. The residual vertical bearing capacity is defined as the ratio of the post-impact load capacity to the original undamaged load capacity, and it is a critical metric for assessing the damage tolerance and progressive collapse resistance of the structure.

The sandwich configuration provides several advantages in this context. The outer tube absorbs the initial impact energy through local deformation and plastic hinge formation, protecting the inner tube and concrete core from direct damage. The concrete core provides confinement to both tubes and prevents local buckling under the combined lateral and vertical loading. The inner tube serves as a redundant load path if the outer tube is severely damaged.

Manufacturing and Material Considerations

The sandwich steel tube is fabricated by welding an inner steel tube to an outer steel tube with the concrete core cast in between. The welding process must ensure a continuous bond between the inner and outer tubes, typically achieved through intermittent or continuous fillet welds at regular intervals along the column length. The weld quality is critical, as any lack of fusion or cracking could compromise the load transfer between the tubes and reduce the overall structural capacity.

For the inner and outer tubes, high-strength steel grades such as Q460 or Q690 are commonly used to maximize the load-bearing capacity within limited dimensions. The concrete core is typically a high-strength concrete with compressive strength of 60 to 100 MPa, which provides excellent confinement properties and damage tolerance. The combination of high-strength steel and high-strength concrete creates a synergistic effect where the steel confines the concrete and the concrete prevents steel buckling.

Engineering Practice Implications

This research has direct implications for the design of structures that must withstand accidental loads, such as bridges over highways, buildings near roadways, and industrial facilities where equipment may fall. The ability to predict the residual vertical bearing capacity after a lateral impact event is essential for assessing the risk of progressive collapse and for designing appropriate mitigation measures.

From a design perspective, the sandwich steel tube configuration offers a practical solution for achieving damage tolerance without significantly increasing the structural weight or cost. The additional outer tube provides a sacrificial layer that absorbs impact energy while protecting the primary load-bearing elements. This approach aligns with the progressive collapse prevention philosophy, where the structure is designed to maintain its vertical load path even after localized damage.

Key Questions and Reflections

The research highlights the importance of considering post-impact performance in the design of critical structural elements. However, several questions remain open. The interaction between the lateral impact damage and the subsequent vertical loading is complex and may depend on the sequence and rate of loading. The residual capacity may also be affected by the duration of the post-impact loading, as time-dependent effects such as creep and fatigue could further reduce the capacity.

Additionally, the repairability of damaged sandwich columns after a lateral impact event is an important practical consideration. The ability to inspect the internal damage, particularly in the concrete core and inner tube, is limited by the opaque nature of the sandwich configuration. Non-destructive testing methods such as impact echo, ground-penetrating radar, and ultrasonic tomography may be required to assess the extent of internal damage, but these methods have limitations in terms of accuracy and resolution.

This study contributes valuable insights into the damage tolerance and progressive collapse resistance of sandwich steel tube concrete-filled columns. For engineers designing structures that must withstand accidental loads, the findings provide a basis for developing design guidelines and detailing recommendations that ensure adequate residual vertical bearing capacity after lateral impact events.