Lacing and Battening for Built-Up Compression Members
Built-up compression members are widely used in steel structures where a single rolled section is insufficient to resist compressive loads. These members are formed by connecting two or more structural components together using lacing systems or batten systems so that the entire assembly behaves as a single compression member.
General Requirements for Built-Up Compression Members
- Different components are arranged to achieve nearly equal radius of gyration about both axes.
- Components are connected together to act as a single compression member.
- Lacing systems and batten systems are commonly used for connections.
- Lacing is generally preferred for eccentric loading conditions.
- Battening is preferred for axially loaded compression members.
What is a Lacing System?
A lacing system consists of diagonal bars connecting the individual components of a built-up column. These diagonal members transfer shear forces and maintain the spacing between the main members.
General Specifications of Lacing System
| Specification | Requirement |
|---|---|
| Lacing Members | Flat bars, angles, channels or tubular sections |
| Uniformity | Lacing system should remain uniform throughout the member |
| Direction of Single Lacing | Opposite sides should have same direction |
| Tie Plates | Provided at ends and interruptions |
Types of Lacing Systems
1. Single Lacing System
- Diagonal bars placed in single direction.
- Simpler fabrication.
- Used for lighter structures.
2. Double Lacing System
- Diagonal bars cross each other.
- Provides better rigidity.
- Suitable for heavy compression members.
Design Specifications of Lacing System
Proper design of lacing systems ensures adequate rigidity and stability of the built-up compression member.
| Design Requirement | Specification |
|---|---|
| Increase in Slenderness Ratio | Increase by 5% |
| Angle of Inclination (θ) | 40° ≤ θ ≤ 70° |
| Maximum Slenderness Ratio of Lacing Bar | 145 |
Effective Length of Lacing Bars
| Lacing Type | Effective Length |
|---|---|
| Single Lacing (Bolted) | le = l |
| Double Lacing (Bolted at Ends) | le = 0.7l |
| Welded Lacing | le = 0.7l |
Where:
- L = Length of lacing bar
- r = Radius of gyration
- KL/r = Slenderness ratio of member
Minimum Width of Lacing Bar
| Bolt Diameter (mm) | Minimum Width of Lacing Bar (mm) |
|---|---|
| 22 | 65 |
| 20 | 60 |
| 18 | 55 |
| 16 | 50 |
Minimum Thickness of Lacing Bar
Where:
- tmin = Minimum thickness of lacing bar
- l = Length of lacing bar
Transverse Shear Force in Lacing System
Lacing systems must be designed to resist transverse shear force equal to 2.5% of the axial load on the column.
Force in Lacing Bars
| Lacing Type | Force in Lacing Bar |
|---|---|
| Single Lacing | F = V / (N sinθ) |
| Double Lacing (N = 2) | F = V / (2 sinθ) |
| Double Lacing (N = 4) | F = V / (4 sinθ) |
Advantages of Lacing System
- Economical for heavy compression members.
- Provides good structural efficiency.
- Reduces overall weight.
- Allows better stress distribution.
- Suitable for eccentric loading conditions.
- Improves overall buckling resistance.
Disadvantages of Lacing System
| Disadvantage | Explanation |
|---|---|
| Complex Fabrication | Requires multiple lacing bars and connections |
| Maintenance Issues | More exposed surfaces prone to corrosion |
| Connection Detailing | Requires careful design of joints |
| Construction Time | Higher fabrication and erection effort |
Difference Between Lacing and Battening
| Lacing System | Battening System |
|---|---|
| Uses diagonal bars | Uses batten plates |
| Lighter structure | Heavier structure |
| Suitable for eccentric loads | Suitable for axial loads |
| Better stress distribution | Higher rigidity |
| More economical in many cases | Simpler analysis |
Applications of Laced Compression Members
- Steel bridge members
- Transmission towers
- Industrial steel columns
- Heavy compression truss members
- Lattice towers
- Built-up steel columns
Conclusion
Lacing systems are an efficient method of connecting built-up compression members in steel structures. Proper design of lacing bars ensures adequate rigidity, shear transfer, and buckling resistance of the member. Understanding IS 800:2007 provisions for lacing systems is essential for the safe and economical design of steel compression members.
References
- IS 800:2007 – General Construction in Steel
- Steel Structures by N. Subramanian
- Limit State Design of Steel Structures by S.K. Duggal
- Design of Steel Structures by Ramchandra
- Structural Steel Design Manuals
Disclaimer
This article is intended for educational and informational purposes only. Engineers should always refer to the latest IS codes and professional engineering practices before performing actual structural design calculations.
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