Seismic Performance of Concrete-Filled Steel Tube Diagonal Grid Tube Structures

Structural Configuration and Analytical Approach

This study by Shi Qingxuan and colleagues from Xi'an University of Architecture and Technology, published in 2018 in the journal Vibration and Shock, investigates the seismic performance of concrete-filled steel tube (CFST) diagonal grid tube structures using the PERFORM-3D finite element software. The research is supported by the National Natural Science Foundation of China (grant 51478382) and several Shaanxi Provincial research grants. The paper is classified under TU973.3, which pertains to the seismic design of building structures.

Diagonal grid tube structures represent an advanced structural system in which the exterior columns are arranged in a diagonal (battlement) pattern rather than a vertical alignment. This configuration creates a spatial grid that provides exceptional lateral stiffness and load-carrying capacity, making it suitable for super-tall buildings and long-span structures. The use of CFST columns in this system combines the high strength and ductility of concrete-filled steel tubes with the geometric advantages of the diagonal grid configuration.

The analytical approach employed in this study combines two types of nonlinear analysis:

• Static pushover analysis (elastic-plastic analysis): Used to determine the yield path, lateral displacement capacity, and inter-story drift angles of the structure under gradually increasing lateral loading.
• Dynamic time-history analysis (elastic-plastic time-history analysis): Used to evaluate the structure's response to realistic earthquake ground motions, including the effects of different earthquake wave types and acceleration amplitudes.

Key Performance Indicators

The study evaluates the seismic performance of the CFST diagonal grid tube structure through several key performance indicators:

Yield Path and Load Transfer Mechanism

The static pushover analysis revealed that the CFST diagonal grid tube structure has a clear and well-defined load transfer path. Under increasing lateral load, the diagonal columns yield progressively, with the yielding sequence following a predictable pattern that reflects the structural hierarchy. The primary (main) joints — where the diagonal columns intersect with the floor beams and ring beams — effectively balance the internal force differences between adjacent diagonal columns, resulting in smaller deformations at the joint levels compared to the non-joint levels.

This finding is particularly significant because joint regions in diagonal grid tube structures are inherently complex due to the multi-axial force conditions and the geometric discontinuity created by the diagonal column arrangement. The fact that the main joints can effectively balance the internal force differences suggests that the CFST columns provide sufficient stiffness and strength to maintain the integrity of the joint region under seismic loading.

Shear Lag Effect Analysis

One of the most interesting findings of this study concerns the shear lag effect in the diagonal grid tube structure. The shear lag ratio — which quantifies the non-uniformity of stress distribution across the perimeter of the tube — was found to transition from positive to negative shear lag as one moves up the height of the structure.

Positive shear lag (at lower levels): The stress concentration occurs near the corners of the tube, where the diagonal columns are located. This is the expected behavior for a tube structure under lateral loading, where the corner columns carry a disproportionately large share of the shear force.

Negative shear lag (at upper levels): The stress distribution reverses, with the mid-span regions of the tube walls carrying relatively more shear force than the corners. This phenomenon is attributed to the changing boundary conditions and the progressive yielding of the diagonal columns, which redistributes the shear forces.

The study further found that as the structure undergoes progressive plastic deformation, the location at which negative shear lag first appears shifts downward. This means that under severe seismic loading, the negative shear lag effect extends to lower levels of the structure, potentially affecting the overall structural behavior in ways that are not captured by linear elastic analysis.

Dynamic Time-History Analysis Results

The dynamic time-history analysis was conducted using multiple earthquake wave types and different peak ground acceleration (PGA) values to evaluate the structure's response under various seismic scenarios. The key findings include:

1. Earthquake wave type sensitivity: Different earthquake wave types (with different frequency content and duration characteristics) had a significant effect on the damage values of the diagonal column layers. This is expected because the diagonal grid tube structure has specific natural frequencies and mode shapes that may resonate with certain earthquake wave characteristics.
2. PGA sensitivity: Higher PGA values led to proportionally higher damage indices, as expected. However, the relationship between PGA and damage was nonlinear, with damage increasing more rapidly at higher PGA values as the structure entered the inelastic range.
3. Damage distribution pattern: While the magnitude of damage varied significantly with earthquake wave type and PGA, the overall damage distribution pattern remained relatively consistent. This suggests that the structural configuration itself governs the damage pattern, while the earthquake characteristics primarily affect the severity of the damage.

Engineering Design Implications

The findings of this study have several important implications for the seismic design of CFST diagonal grid tube structures:

• Joint design: The main joints should be designed with sufficient strength and ductility to accommodate the large internal force differences between diagonal columns. The study's finding that these joints effectively balance force differences is encouraging, but designers should ensure that the joint details can withstand the expected seismic demands.
• Shear lag considerations: The transition from positive to negative shear lag means that the stress distribution across the tube perimeter changes with height and loading level. Designers should account for this variation when sizing the diagonal columns and ring beams, rather than assuming a uniform stress distribution.
• Performance-based design: The damage distribution patterns identified in the study can be used to establish performance objectives for different seismic intensity levels. For example, the structure should be designed to remain elastic under frequent earthquakes, with limited inelastic deformation under design-level earthquakes, and without collapse under rare maximum credible earthquakes.
• Earthquake wave selection: The sensitivity of the structure to different earthquake wave types suggests that engineers should use a suite of earthquake waves with diverse frequency content and duration characteristics in their seismic analysis, rather than relying on a single or two earthquake waves.

Study Insights and Critical Reflection

This study contributes to the growing body of knowledge on the seismic performance of advanced structural systems. The use of CFST columns in diagonal grid tube structures is particularly attractive because CFST columns offer high strength-to-weight ratio, excellent ductility, and fire resistance compared to hollow steel tube columns. The study's findings confirm that this combination provides a structurally efficient and seismically robust system.

The shear lag analysis is particularly noteworthy because it reveals a complex phenomenon that is not well understood in existing design codes. The transition from positive to negative shear lag and its dependence on the loading level and plastic development highlights the limitations of linear elastic analysis for this type of structure. Engineers designing CFST diagonal grid tube structures should use nonlinear analysis methods to capture these effects accurately.

The study also demonstrates the value of combining static pushover and dynamic time-history analysis in evaluating seismic performance. The pushover analysis provides insight into the load transfer mechanism and failure sequence, while the time-history analysis captures the dynamic effects and the influence of earthquake wave characteristics. Together, these two analysis methods provide a comprehensive picture of the structure's seismic behavior.

In conclusion, this study provides valuable insights into the seismic performance of CFST diagonal grid tube structures, with particular emphasis on the shear lag effect and the influence of earthquake wave characteristics. The findings support the use of CFST diagonal grid tube structures for super-tall buildings and long-span structures in seismic regions, provided that the design accounts for the complex nonlinear behavior identified in the study. Future research should focus on experimental validation of the analytical findings and the development of simplified design methods that capture the essential features of the structure's seismic behavior.