Understanding Battened Struts and Effective Length in IS800 : 1984

Steel compression members, often called struts or columns, can be built up using multiple sections connected together. Two common methods for connecting these sections are lacing and battening. Battening involves using transverse plates (battens) to connect the main components of the compression member.

Why Increase Effective Length for Battened Struts?

When a battened strut is subjected to compressive load, it tends to buckle. Unlike a solid member or a laced member which primarily resists buckling through bending stiffness, a battened member also experiences shear deformations in the battens and the main components between the battens. These shear deformations reduce the overall stiffness of the member compared to an ideal solid or a perfectly rigid connection system. To account for this reduced stiffness and the resulting lower buckling strength, the Indian Standard code IS800 : 1984 specifies an increase in the effective length of battened compression members.

IS800 : 1984 Provisions for Battened Struts

According to Clause 7.7.3 (e) of IS 800 : 1984, for battened compression members, the effective length ($L_{eff}$) is considered greater than the effective length of an analogous laced column. This increase is specifically mandated to factor in the shear effects present in batten systems.

Effective Length Increase Percentage

The code IS 800 : 1984 stipulates that the effective length of a battened strut shall be increased by a certain percentage compared to what would be used for a similar laced column or a solid column with the same end conditions and overall length. The specified increase is:

Increase in Effective Length = 10%

So, if $L$ is the actual length of the battened strut and $k$ is the effective length factor based on end conditions, the effective length for buckling calculations is taken as $1.10 \times k \times L$.

Let's denote the effective length calculated based on end conditions without considering battening shear effects as $L_{eff, base}$. The effective length to be used for design of battened struts is $L_{eff, battened}$.

As per IS800 : 1984,

$\Delta L_{eff} = 10\% \text{ of } L_{eff, base}$

$L_{eff, battened} = L_{eff, base} + \Delta L_{eff} = L_{eff, base} + 0.10 \times L_{eff, base} = 1.10 \times L_{eff, base}$

Analyzing the Options

The question asks for the percentage by which the effective length of battened struts shall be increased as per IS800 : 1984.

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• Option 1: 5%
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• Option 2: 8%
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• Option 3: 10%
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• Option 4: 12%
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Based on the provisions of IS800 : 1984, the effective length is increased by 10%.

Conclusion

For battened struts designed according to IS800 : 1984, the effective length used for buckling calculations must be taken as 10% greater than the effective length determined solely based on the end conditions. This accounts for the reduced stiffness due to shear deformation in the battening system.

Revision Table: IS800 1984 Steel Design Concepts

Additional Information: Steel Compression Members

Steel compression members are fundamental elements in structures, carrying axial compressive loads. Their design is governed by buckling behavior, which depends heavily on their geometry, material properties, and support conditions. Codes like IS800 provide guidelines for calculating their strength safely.

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• Types of Compression Members: Columns, struts, rafters (in some cases), compression flanges of beams.
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• Built-up Members: Used when a single rolled section is insufficient to carry the load or achieve the required stiffness. Examples include two angles back-to-back, two channels with lacing or battening, or plate girders subjected to axial compression.
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• Lacing vs. Battening:\n
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  • Lacing: Uses diagonal or cross flats/angles connecting main members. More rigid in resisting shear deformation compared to battens, hence no general increase in effective length for overall buckling (though local buckling between connections is checked).
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  • Battening: Uses transverse plates connecting main members. Less rigid against shear compared to lacing for the *overall* member action, requiring the 10% effective length increase to account for this flexibility in the main buckling calculation. Battens also need to be designed for the shear forces.
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• Shear in Battened Columns: The transverse shear force in a battened column arises from several sources, including the shear associated with buckling, wind loads, and accidental eccentricity of loading. The battens must be designed to resist this shear.
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