Progressive Collapse Resistance of Steel Tube Concrete Frames with Steel Rebar Truss Deck Plates

Literature Overview

This paper by Zheng Long and Wang Wenda from the School of Civil Engineering, Lanzhou University of Technology (published in Journal of Natural Disasters, Vol. 32, No. 3, 2023, pp. 36-47, funded by NSFC Projects 51268038, 51778274) investigates the progressive collapse resistance of steel tube concrete (STC) column-steel beam-steel rebar truss deck plate systems under edge column removal scenarios. Using ABAQUS/Explicit dynamic analysis, the study evaluates the vertical bearing capacity, failure modes, deck plate concrete damage patterns, and key steel member axial stresses. The research quantifies the contributions of three resistance mechanisms: main beam flexural mechanism, main beam catenary mechanism, and deck plate mechanism.

Core Technical Findings

The study reveals that the STC column-steel beam-steel rebar truss deck plate system exhibits good progressive collapse resistance under edge column removal conditions. The key quantitative findings are:

The consistent ~30% contribution of the deck plate mechanism throughout the loading process is a significant finding that challenges the traditional assumption that deck plates are secondary elements in progressive collapse resistance. This suggests that deck plate design should be given greater consideration in progressive collapse-resistant design.

Progressive Collapse Mechanism Analysis

The progressive collapse resistance mechanism evolves through distinct stages as the load increases after edge column removal:

1. Initial loading stage: The main beam flexural mechanism and deck plate mechanism provide the primary resistance. The removed column's load is redistributed to adjacent beams through bending action, while the deck plate acts as a membrane to distribute loads laterally.
2. Intermediate loading stage: As the beams approach their flexural capacity, the catenary mechanism begins to activate. The beams develop axial tension as they deflect downward, creating a catenary-like load path. The deck plate mechanism continues to contribute through membrane action.
3. Maximum capacity stage: All three mechanisms contribute simultaneously, with the flexural mechanism still dominant at 42.7%, followed by the deck plate mechanism at 33.5%, and the catenary mechanism at 23.8%.
4. Post-peak stage: The system may experience degradation as the beams and deck plate reach their ultimate capacity. The remaining resistance depends on the reserve capacity of the steel members and the ductility of the STC columns.

Deck Plate Mechanism and Damage Patterns

The steel rebar truss deck plate mechanism operates through a combination of:

• Membrane action: The deck plate acts as a tension membrane, distributing loads from the removed column area to adjacent columns and beams.
• Diaphragm action: The deck plate transfers lateral loads to the vertical structural elements, enhancing the overall structural integrity.
• Composite action: The interaction between the steel deck plate and the concrete topping creates a composite element with enhanced tensile capacity.

The damage patterns observed in the FEA analysis reveal:

• Concrete tensile damage concentrates in the negative bending zone on the upper surface and the positive bending zone on the lower surface.
• Rebar truss tensile stresses are highest in the same zones, indicating that the truss reinforcement effectively resists the tensile forces generated by the deck plate membrane action.
• The galvanized steel base plate develops tensile stresses that reflect the load transfer mechanism from the damaged area to the undamaged area.

Engineering Practice Implications

For engineers designing progressive collapse-resistant structures, this study provides several important insights:

• Deck plate design: The deck plate should be designed with adequate tensile capacity to contribute to progressive collapse resistance. This includes appropriate rebar truss configuration, concrete topping thickness, and connection details.
• Beam design: The main beams should have sufficient flexural capacity to redistribute loads from removed columns. The flexural mechanism provides the largest contribution at maximum capacity, so beam design is critical.
• Column design: STC columns provide the necessary ductility and strength to support the redistributed loads. The steel tube confinement enhances the column's ability to sustain large deformations without brittle failure.
• Boundary conditions: The effectiveness of the catenary mechanism depends on the boundary conditions of the beams. In the edge column removal scenario, the boundary conditions for some beams (e.g., G6 beam) may be unfavorable, limiting their catenary capacity.

From a steel tube manufacturing perspective, the STC columns used in these frames require:

• High-quality steel tubes: The steel tubes must have good mechanical properties and be free of manufacturing defects to ensure reliable confinement of the concrete core.
• Proper concrete fill: The concrete must be properly vibrated to ensure full fill and good bond with the steel tube inner surface.
• Weld quality: Any welds in the steel tube (e.g., at column-beam connections) must be of high quality to prevent premature failure under cyclic loading.

Study Insights and Reflections

The finding that the deck plate mechanism consistently contributes approximately 30% of the total vertical resistance is a paradigm-shifting result for progressive collapse engineering. Traditional progressive collapse design has focused primarily on the beam-column frame, treating the deck plate as a secondary element. This study demonstrates that the deck plate is a primary contributor to progressive collapse resistance, and its design should be given equal consideration.

The quantification of individual mechanism contributions provides a valuable tool for engineers to evaluate the effectiveness of different design strategies. By understanding how much each mechanism contributes, engineers can make informed decisions about where to allocate structural resources. For example, if the goal is to maximize the catenary mechanism contribution, the beam design should focus on providing adequate axial tension capacity and favorable boundary conditions.

The ABAQUS/Explicit dynamic analysis approach is appropriate for progressive collapse simulation because it can capture the dynamic effects of column removal and the subsequent load redistribution. However, engineers should be aware that explicit dynamic analysis can be sensitive to time step selection, material model calibration, and boundary condition modeling. The study's emphasis on ensuring the accuracy of material properties, damage definitions, and modeling methods is commendable and reflects good engineering practice.

One limitation of the study is that it focuses on a specific frame configuration and loading scenario. The progressive collapse resistance of different structural systems, such as moment frames, braced frames, and composite systems, may exhibit different mechanism contributions. Future research should extend the parametric study to include different structural configurations and loading scenarios to develop a more comprehensive understanding of progressive collapse mechanisms.

In conclusion, this paper provides valuable quantitative insights into the progressive collapse resistance of STC frame systems with steel rebar truss deck plates. The finding that the deck plate mechanism contributes approximately 30% of the total resistance challenges traditional design assumptions and has significant implications for progressive collapse-resistant design practice. Engineers should consider integrating deck plate design into their progressive collapse assessment and design processes.