Effect of Creep on Axial Compression Stability Bearing Capacity of Hollow Steel Tube Concrete Members

Literature Overview

This paper by Wang Hongxin and Zha Xiaoxiong from the Department of Urban and Civil Engineering, Shenzhen Graduate School, Harbin Institute of Technology, published in the Journal of Harbin Institute of Technology in 2009, investigates the influence of concrete creep on the stability bearing capacity of hollow steel tube concrete (HSTC) axially loaded members. The authors derive a critical bearing capacity formula for HSTC members and use finite element analysis to calculate the free creep of plain concrete, unloading creep of the core concrete, and longitudinal creep of the member. By modifying the tangent moduli of both steel and concrete after creep occurs, the authors compute the reduction rate of stability bearing capacity due to creep.

Theoretical Framework and Methodology

The study employs a rigorous theoretical framework that begins with the derivation of the critical bearing capacity formula for HSTC members, accounting for the composite action of the steel tube and the hollow concrete core. The finite element analysis is then used to compute the time-dependent deformation components, and the material moduli are updated to reflect the post-creep state.

The methodology follows a logical sequence that can be applied to other composite structural members:

1. Derive the initial critical bearing capacity formula for the HSTC member based on elastic stability theory.
2. Calculate the free creep strain of plain concrete using established creep models.
3. Calculate the unloading creep strain of the core concrete, which occurs when the concrete is unloaded from its self-weight stress.
4. Calculate the longitudinal creep strain of the entire member under sustained load.
5. Update the tangent moduli of both steel and concrete to reflect the post-creep material state.
6. Compute the reduced stability bearing capacity using the modified moduli.
7. Calculate the reduction rate as the ratio of capacity loss to the original capacity.

Key Findings and Parametric Analysis

The parametric analysis reveals several important relationships that govern the creep-induced reduction in stability bearing capacity:

The hollow ratio effect is particularly significant and counterintuitive at first glance. A higher hollow ratio means less concrete core, which reduces the total creep deformation. However, it also reduces the overall stiffness, which could potentially increase the sensitivity to creep. The net effect, as found in this study, is that higher hollow ratios result in less creep influence on bearing capacity, because the reduced concrete volume dominates the behavior.

The sustained load ratio finding is of direct practical importance. The authors recommend that when the permanent load component of the total axial force is 30% or greater, a creep reduction coefficient should be applied to the member bearing capacity. This 30% threshold provides a clear, actionable criterion for engineers to determine when creep effects must be considered in design.

Design Recommendations and Engineering Practice

The practical design recommendation from this paper is straightforward but important: when the permanent load component exceeds 30% of the total axial force in a hollow steel tube concrete axially loaded member, the bearing capacity should be multiplied by a creep reduction coefficient. This coefficient accounts for the time-dependent reduction in stability capacity due to concrete creep.

For pipeline engineers, this finding has implications for the design of steel tube concrete columns used in pipeline support structures, particularly in applications where sustained loads are significant, such as:

• Support columns for elevated pipeline bridges where dead load is a major component of the total load
• Foundation columns for offshore platforms where the self-weight of the structure represents a significant portion of the design load
• Long-term load-bearing members in storage facilities where sustained pressure loads are present

The recommendation to use a creep reduction coefficient is conservative and practical, but engineers should be aware that the 30% threshold is based on the specific conditions studied in this paper. For different geometric configurations, material properties, or environmental conditions, the threshold may differ. Engineers should exercise professional judgment and consider the specific conditions of their application when applying these recommendations.

Study Limitations and Future Research Needs

The study has several limitations that engineers should consider when applying the findings. The creep models used are based on established concrete creep theories, but the accuracy of these models for lightweight or high-strength concrete may be limited. The finite element analysis assumes perfect bond between steel and concrete, which may not reflect actual construction conditions where partial debonding can occur. Additionally, the study does not consider the combined effects of creep and other time-dependent phenomena such as shrinkage, thermal effects, or fatigue, which can all contribute to long-term capacity degradation.

Future research should investigate the combined effects of creep and other time-dependent mechanisms, develop more refined creep models for specific concrete types used in steel tube concrete applications, and conduct long-term experimental validation of the theoretical predictions. The development of simplified design procedures that incorporate creep effects without requiring complex finite element analysis would also be valuable for practical engineering applications.

This paper provides a rigorous and practical framework for considering creep effects in the design of hollow steel tube concrete axially loaded members. The identification of key parameters and the establishment of a clear design criterion for when creep effects must be considered represent significant contributions to the field. Engineers should use these findings to ensure the long-term integrity and safety of steel tube concrete structures subjected to sustained loads, while remaining aware of the limitations of the study and the need for further research to refine the design methodology for specific applications.