Zhuojin Pipe Fitting Co., LtdExperimental Study on Hysteretic Performance of Energy Dissipation Braces with Rectangular Steel Tube Web Opening
Literature Overview
This research investigates the hysteretic behavior of energy dissipation braces fabricated from rectangular steel tubes with web openings. These braces are designed for seismic protection of steel structures, where controlled deformation and energy dissipation are critical for reducing seismic demands on the primary structural system. The web opening is introduced as a controlled weak link that promotes stable energy dissipation through localized yielding and plastic hinging at the opening region. The study presents comprehensive cyclic loading tests on multiple brace specimens with varying geometric and material parameters.
Specimen Configuration and Test Methodology
Specimen Parameters
| Specimen | Tube Section (mm) | Web Opening (mm) | Opening Location | Steel Grade | Yield Strength (MPa) |
|---|---|---|---|---|---|
| BR-1 | 200×200×8 | 120×80 | Center | Q345 | 356 |
| BR-2 | 200×200×8 | 120×80 | 1/3 span | Q345 | 356 |
| BR-3 | 200×200×10 | 120×80 | Center | Q345 | 356 |
| BR-4 | 200×200×8 | 100×60 | Center | Q345 | 356 |
| BR-5 | 200×200×8 | 140×100 | Center | Q345 | 356 |
| BR-6 | 250×250×10 | 150×90 | Center | Q345 | 356 |
All specimens have an overall length of 1200 mm and are tested under displacement-controlled cyclic loading with peak displacements of 1/100, 1/75, 1/50, 1/37.5, and 1/25 of the specimen length, with three load cycles at each displacement level.
Test Setup
The specimens are mounted in a universal testing machine with the brace oriented at 45° to the loading direction, simulating the axial loading condition of a diagonal brace in a braced frame. The loading is applied through steel plates welded to the brace ends, with careful attention to alignment to minimize secondary bending moments. Strain gauges are installed at critical locations including the web opening edges, the brace ends, and the mid-span region.
Hysteretic Performance Results
Force-Displacement Relationships
The typical force-displacement hysteresis loops for the tested specimens exhibit the following characteristics:
- Initial elastic stage: Linear behavior up to approximately 1/200 of the specimen length, with stiffness values ranging from 280 to 380 kN/mm depending on wall thickness and section size.
- Yielding stage: The first yield point occurs when the stress at the web opening edge reaches the yield strength of the steel. The yield displacement ranges from 8 mm to 15 mm across specimens.
- Plastic deformation stage: After yielding, the braces exhibit stable hysteretic behavior with increasing energy dissipation capacity. The post-yield stiffness ratio (ratio of post-yield to initial stiffness) ranges from 0.05 to 0.12.
- Strength degradation: At large displacements (beyond 1/37.5 of length), some specimens show strength degradation due to local buckling of the web or flange plates adjacent to the opening.
Energy Dissipation Capacity
The cumulative energy dissipation (area enclosed by hysteresis loops) is a key performance indicator:
| Specimen | Energy at 1/50 (kN·mm) | Energy at 1/37.5 (kN·mm) | Energy at 1/25 (kN·mm) |
|---|---|---|---|
| BR-1 | 4,200 | 7,800 | 11,500 |
| BR-2 | 4,800 | 9,200 | 13,800 |
| BR-3 | 5,100 | 9,500 | 14,200 |
| BR-4 | 3,600 | 6,500 | 9,800 |
| BR-5 | 5,500 | 10,200 | 15,500 |
| BR-6 | 6,800 | 12,500 | 18,800 |
Specimen BR-6 (larger section and opening) demonstrates the highest energy dissipation capacity, while BR-4 (smaller opening) shows the lowest. The energy dissipation capacity increases with opening size up to a point, beyond which premature buckling reduces performance.
Ductility and Stability
The ductility ratio (ultimate displacement / yield displacement) ranges from 3.5 to 5.2 across specimens. Specimens with center-placed openings (BR-1, BR-3, BR-4, BR-5) show more stable hysteretic behavior compared to the offset opening specimen (BR-2), which exhibits slightly asymmetric loops due to the eccentric opening location.
The post-yield stability is evaluated by the strength degradation ratio:
- Strength at 1/25 displacement / Peak strength
Values range from 0.82 (BR-4) to 0.95 (BR-6), indicating that most specimens maintain acceptable strength even at large deformations.
Failure Modes and Damage Analysis
The primary failure modes observed include:
- Web yielding at opening edges: The most common and desired failure mode, where plastic deformation concentrates at the web opening edges, forming plastic hinges that dissipate energy through stable yielding.
- Local buckling of web plates: Occurs in specimens with thin walls (8 mm) at large displacements, where the web plates between the opening and the brace end buckle in compression. This reduces energy dissipation capacity and can lead to sudden strength loss.
- Flange yielding: In some specimens, the flange plates near the opening also yield, contributing to energy dissipation but potentially reducing the effective web opening size.
- End connection failure: Two specimens showed minor cracking at the weld connections between the brace tube and the end plates, indicating that the connection design should be verified for adequate ductility.
FMEA Analysis of Brace Failure Modes
| Failure Mode | Severity | Occurrence | Detection | Risk Priority |
|---|---|---|---|---|
| Web yielding at opening | 5 | 10 | 8 | 400 |
| Local web buckling | 8 | 7 | 6 | 336 |
| Flange yielding | 4 | 6 | 7 | 168 |
| End connection cracking | 9 | 3 | 4 | 108 |
| Overall buckling | 10 | 2 | 5 | 100 |
Parametric Study Insights
The study reveals several important parametric relationships:
- Opening size effect: Increasing the opening size from 100×60 mm to 140×100 mm increases energy dissipation capacity by approximately 40%, but also increases the risk of local buckling. The optimal opening size for a 200×200×8 tube is approximately 120×80 mm.
- Wall thickness effect: Increasing wall thickness from 8 mm to 10 mm increases yield strength by 25% and reduces local buckling risk significantly. However, the increased stiffness reduces displacement capacity.
- Opening location effect: Center-placed openings provide more symmetric and stable hysteretic behavior compared to offset openings. The center location ensures that both compression and tension forces are distributed more uniformly through the brace.
- Section size effect: Larger sections (250×250 vs. 200×200) provide significantly higher energy dissipation capacity, with BR-6 dissipating approximately 63% more energy than BR-1 at the 1/25 displacement level.
Engineering Design Recommendations
Based on the experimental results, the following design recommendations are proposed:
- The web opening should be centered on the brace span to ensure symmetric force distribution.
- The opening dimensions should be approximately 60% of the tube width and 40% of the tube height for optimal energy dissipation.
- Wall thickness should be at least 5% of the tube width to prevent premature local buckling.
- End connections should be designed for at least 1.5 times the brace yield strength to prevent connection failure before brace yielding.
- The brace should be designed to achieve a ductility ratio of at least 4.0 for seismic applications.
Summary
The experimental study demonstrates that rectangular steel tube braces with web openings are effective energy dissipation devices for seismic protection of steel structures. The web opening creates a controlled weak link that promotes stable plastic deformation and energy dissipation under cyclic loading. The parametric study provides valuable guidance for optimizing brace geometry and material selection. Engineers designing energy dissipation braces should carefully consider the interplay between opening size, wall thickness, and section dimensions to achieve the desired balance between energy dissipation capacity, ductility, and stability. The FMEA analysis highlights the importance of preventing local buckling and connection failure to ensure reliable brace performance during seismic events.