Seismic Performance of Concrete-Filled Steel Tube Oblique Grid Tubular Structures

Literature Overview

The paper by Shi Qingxuan, Wang Feng, Sang Dan, Wu Chaofeng, and Wang Peng (2018, Journal of Vibration and Shock, Vol. 37, No. 7) investigates the seismic performance of concrete-filled steel tube (CFST) oblique grid tubular structures using both static pushover analysis and dynamic nonlinear time-history analysis with the PERFORM-3D finite element software. The study examines yield paths, lateral displacement and inter-story drift angles, shear lag effects, and damage distribution patterns in the oblique columns under seismic loading.

Oblique grid tubular structures are an advanced structural system that utilises inclined columns arranged in a grid pattern to form a tubular lateral force-resisting system. The use of CFST columns in such systems leverages the high strength-to-weight ratio and excellent energy dissipation capacity of CFST members, making this system particularly suitable for tall buildings and large-span structures in seismic regions.

Analytical Methodology

Numerical Modelling Approach

The use of PERFORM-3D, a fibre-based finite element software specifically developed for nonlinear analysis of reinforced concrete and composite structures, is appropriate for this study. The fibre-based modelling approach allows for the capture of material nonlinearity in both the steel tube and the concrete core, which is essential for accurately predicting the seismic response of CFST members.

Key Findings

Load-Transfer Mechanism and Lateral Stiffness

The CFST oblique grid tubular structure exhibits a clear and rational load-transfer path, with high lateral stiffness and good spatial working performance. The inclined columns act as diagonal braces, effectively resisting lateral loads through axial deformation. The primary and secondary joints (nodes) play a critical role in balancing the internal force differences among the oblique columns, resulting in inter-story drift at joint levels being smaller than at non-joint levels.

This finding has direct implications for structural design: the joint regions in oblique grid tubular structures are inherently stiffer and more robust than the inter-joint regions, which means that the critical sections for seismic damage are the mid-height regions of the oblique columns between joints. This is consistent with the general principle in seismic design that the most vulnerable locations should be identified and detailed accordingly.

Shear Lag Effect

One of the most interesting findings is the behaviour of the shear lag ratio along the height of the structure. The shear lag ratio transitions from positive to negative values as the height increases, and with the development of plasticity, the position where the negative shear lag first appears shifts downward.

The shear lag effect in tubular structures refers to the non-uniform distribution of axial stresses in the corner columns and flat columns of the tube. In a positive shear lag condition, the corner columns carry more load than the flat columns, which is the typical elastic behaviour. The transition to negative shear lag indicates a redistribution of forces, with the flat columns becoming more heavily loaded relative to the corner columns. This redistribution is a hathe writing systemark of structural plasticity and indicates that the structure is entering a state of force redistribution that can either be beneficial (by engaging more of the structural capacity) or detrimental (by overloading specific members).

Damage Distribution

The type of ground motion and the acceleration amplitude have a significant influence on the damage index of the oblique column layers, but have a relatively minor influence on the damage distribution pattern. This is a practically important finding because it suggests that, while the severity of damage depends on the specific earthquake scenario, the pattern of where damage occurs is relatively predictable and can be used to guide structural detailing.

Engineering Practice Implications

Design Considerations for CFST Oblique Grid Tubular Structures

Based on the findings of this study, several design recommendations can be made:

1. Joint design: The joints should be designed to accommodate the high internal force differences between oblique columns. Moment-resisting joints are preferred over shear-only connections to ensure the full lateral stiffness and strength of the system is mobilised.
2. Column detailing: The mid-height regions of the oblique columns between joints should receive additional lateral support or reinforcement, as these are the most vulnerable locations for seismic damage.
3. Shear lag mitigation: The design should account for the shear lag effect, particularly in the upper storeys where the transition to negative shear lag occurs. This may require additional bracing or modified column section properties in the upper levels.
4. Ductility requirements: The CFST columns should be designed to achieve adequate ductility, with the concrete core providing confinement to the steel tube and the steel tube providing confinement to the concrete core. The slenderness ratio of the columns should be limited to prevent brittle buckling failure.

Welding and Fabrication Considerations

From a fabrication and welding perspective, the oblique columns in this structural system present unique challenges. The inclined geometry means that the columns are not vertical, which complicates the fabrication and erection process. The welding of joints between oblique columns and the horizontal beams or diaphragms requires careful planning to ensure full penetration and adequate weld quality. The use of CFST columns requires that the concrete filling be completed before the column is erected in its final position, or alternatively, the column may be erected hollow and filled on-site — the latter approach is more common for tall structures but requires careful management of the concrete placement process.

Study Insights and Reflections

The study provides valuable insights into the seismic behaviour of CFST oblique grid tubular structures, but there are limitations that should be acknowledged. The analysis is purely numerical, and while PERFORM-3D is a well-validated software for nonlinear analysis, numerical models are always subject to modelling assumptions and material model uncertainties. Ideally, the numerical findings should be validated against experimental data from shake table tests or cyclic loading tests of scale models.

The finding that the damage distribution pattern is relatively insensitive to the type of ground motion is encouraging from a design perspective, as it suggests that the structural system has a consistent vulnerability pattern that can be addressed through rational design. However, the sensitivity of the damage index to ground motion characteristics underscores the importance of using multiple ground motion records in the seismic assessment of such structures.

Conclusion

This study contributes significantly to the understanding of the seismic performance of CFST oblique grid tubular structures, providing practical guidance on load-transfer mechanisms, shear lag behaviour, and damage patterns. The findings support the use of CFST oblique grid systems in seismic regions, provided that appropriate design and detailing measures are taken. Further research combining experimental validation with advanced numerical modelling is recommended to refine the design recommendations and extend them to a wider range of structural configurations and loading scenarios.