Vulnerability Study of Steel Tube Concrete Frame-Buckling-Restrained Brace Systems under Coupled Wind and Seismic Actions

Literature Overview

This paper by Liu Yang, Li Hongnan, Li Chao, Zheng Xiaowei, and Zhang Hao, published in "China Civil Engineering Journal" (Vol. 52, No. 2, 2019, pp. 56-65), investigates the vulnerability of steel tube concrete (STC) frame-buckling-restrained brace (BRB) structural systems under coupled wind and seismic actions. The research was conducted at Dalian University of Technology and Shenyang Jianzhu University, and was supported by the National Key R&D Program of China (Grant 2016YFC0701108) and the National Natural Science Foundation of China (Grants 51738007, 51808099).

The study addresses a critical issue in structural engineering: the combined effect of wind and seismic loads on structural systems. Traditionally, wind and seismic loads are considered separately in design, but in reality, structures may experience both loads simultaneously, particularly during extreme weather events that coincide with seismic activity. Understanding the coupled effect of wind and seismic loads on structural vulnerability is essential for developing more robust and reliable design approaches.

Core Technical Points

Structural System Description

The study focuses on an STC frame-BRB structural system, which combines the high strength and stiffness of steel tube concrete frames with the energy dissipation capacity of buckling-restrained braces. This hybrid system offers a balance between strength, stiffness, and ductility, making it suitable for applications where both wind and seismic resistance are important.

Simulation Methodology

The research employed OpenSees finite element software to conduct nonlinear dynamic time-history analyses under four loading scenarios:

Seismic action alone
Seismic action coupled with 1-year return period wind
Seismic action coupled with 10-year return period wind
Seismic action coupled with 50-year return period wind

Wind speed time histories were simulated at different structural heights, accounting for the vertical variation of wind speed. The seismic demand analysis method was used to generate fragility curves for the structural system under each loading scenario.

Key Findings

The study revealed two important trends:

Effect of Wind Intensity: As the wind action increases (from 1-year to 50-year return period), both the structural response and vulnerability increase. This indicates that the coupled wind-seismic effect is significant and should not be neglected in design.
Effect of Seismic Intensity: As the seismic intensity increases, the influence of wind action on structural vulnerability gradually decreases. This suggests that for very strong earthquakes, the wind contribution becomes relatively minor compared to the seismic demand, but for moderate earthquakes, the wind contribution can be significant.

Loading Scenario	Structural Response	Vulnerability
Seismic only	Baseline	Baseline
Seismic + 1-year wind	Slight increase	Slight increase
Seismic + 10-year wind	Moderate increase	Moderate increase
Seismic + 50-year wind	Significant increase	Significant increase

Fragility Analysis

The fragility curves generated for each loading scenario provide a probabilistic assessment of the structural system's performance. The fragility curves show the probability of exceeding specific damage states as a function of seismic intensity, for each wind-seismic coupling scenario. The comparison of fragility curves across different loading scenarios quantifies the additional vulnerability introduced by wind coupling.

Technical Analysis of Coupled Wind-Seismic Effects

The coupled wind-seismic effect is a complex phenomenon that involves the interaction of two distinct types of loading: the quasi-static or slowly varying wind load and the dynamic, high-frequency seismic load. The interaction between these two loads can lead to nonlinear effects that are not captured by separate consideration of each load type.