Axial Compression Bearing Capacity Analysis of FRP-Concrete-Steel Tube Composite Solid Square Short Columns

Literature Overview

The paper by Hou Yulin, Zhao Junhai, and Cao Xueye from the School of Civil Engineering, Chang'an University, published in Industrial Construction in 2016, presents a theoretical analysis of the axial compression bearing capacity of fiber-reinforced polymer (FRP)-concrete-steel tube composite solid square short columns. The authors propose a novel composite column configuration that combines the advantages of FRP confinement, concrete core strength, and steel tube structural support. The study introduces the concept of dividing the sandwich concrete into effective confinement zones and ineffective confinement zones based on the different confinement effects, and employs a unified strength theory to derive a theoretical expression for axial compression bearing capacity.

Theoretical Framework

The proposed FRP-concrete-steel tube composite solid square short column consists of a steel square tube outer shell, an inner concrete core, and FRP sheets wrapped around the steel tube. The steel tube provides the primary structural support and initial confinement, while the FRP sheets provide additional confinement to the concrete through the steel tube. The key innovation in the theoretical model is the recognition that the confinement effect is not uniform throughout the concrete section, with the corners experiencing different stress states compared to the mid-face regions.

Effective Confinement Zone Analysis

The authors divide the concrete section into two zones based on the confinement effectiveness. The effective confinement zone is the region where the FRP and steel tube confinement is fully developed, typically located near the corners where the steel tube provides direct lateral restraint. The ineffective confinement zone is the region where the confinement is reduced due to the distance from the steel tube walls, typically located at the mid-face regions of the square section.

To account for the square section geometry, the authors propose an equivalent circle transformation, converting the square FRP sheet arrangement into an equivalent circular FRP arrangement for analytical convenience. This transformation is based on the principle that the confinement pressure distribution can be approximated by an equivalent circular geometry with appropriate correction factors.

Unified Strength Theory Application

The unified strength theory is employed to account for the effects of intermediate principal stress, material tensile-to-compressive strength ratio, and corner radius on the bearing capacity. The unified strength theory provides a more comprehensive description of the yield behavior of materials under multi-axial stress states compared to traditional von Mises or Tresca criteria. The intermediate principal stress coefficient b and the side pressure coefficient are key parameters that influence the predicted bearing capacity.

Comparative Analysis with Existing Literature

The theoretical predictions from the proposed model are compared with experimental results from related literature, and the agreement is reported to be satisfactory. This validation is important for establishing the credibility of the theoretical model and its applicability to practical design. The comparison should be examined critically, considering the range of parameters covered by the experimental data and the sensitivity of the predictions to each parameter.

The finding that bearing capacity increases with the intermediate principal stress coefficient b is consistent with the general understanding that triaxial confinement enhances concrete strength. The finding that capacity increases with the side pressure coefficient α is also expected, as a higher side pressure coefficient indicates greater lateral confinement pressure. The non-monotonic behavior with corner radius R, where capacity first increases and then decreases, is an interesting finding that suggests an optimal corner radius exists for maximizing bearing capacity. This behavior can be attributed to the competing effects of corner radius on stress distribution and confinement effectiveness.

Engineering Practice Implications

The proposed FRP-concrete-steel tube composite column concept has potential applications in retrofitting existing concrete structures, where FRP wrapping is combined with steel tube installation to achieve significant strength and ductility improvements. The theoretical model provides a basis for predicting the enhanced bearing capacity of retrofitted columns, which is essential for structural assessment and design.

From a materials engineering perspective, the composite column design leverages the complementary properties of different materials. The steel tube provides high strength and stiffness, the concrete core provides compressive strength and fire resistance, and the FRP provides lightweight confinement with high tensile strength. The challenge lies in ensuring proper bond and load transfer between these materials, particularly at the steel-FRP interface where differential thermal expansion coefficients may cause debonding under cyclic or thermal loading.

Study Insights

This research contributes to the growing body of knowledge on hybrid composite columns that combine multiple confinement mechanisms. The use of unified strength theory to account for multi-axial stress effects represents a more rigorous approach compared to simplified confinement models. The identification of an optimal corner radius for maximizing bearing capacity provides practical guidance for the design of FRP-confined square sections. However, the theoretical model should be further validated with dedicated experimental programs covering a wider range of parameters, particularly for longer columns where buckling effects become significant, and under cyclic loading conditions relevant to seismic applications.