Understanding Lacing in Steel Columns

Steel columns are often constructed by connecting multiple structural sections, such as angles, channels, or plates, to form a single composite member. To ensure these individual components act together efficiently and prevent them from buckling independently, bracing systems are used. Lacing is one common type of bracing system used in built-up columns.

Lacing involves connecting the main components using diagonal flat bars, angles, or channels arranged in a single or double pattern. These lacing members are primarily designed to hold the main components at the correct relative positions and to resist any transverse shear force that might arise.

Transverse Shear Force in Lacing Design

A built-up column with a lacing system needs to be designed to resist a transverse shear force, denoted as \(V_t\). This force acts perpendicular to the longitudinal axis of the column. The transverse shear arises due to various factors, including potential bending in the column between the points of support or lacing, and accidental lateral loads or eccentricities.

Design codes provide specific requirements for the minimum value of this transverse shear force \(V_t\) that the lacing system must be designed to safely resist. This required shear force is usually specified as a percentage of the total compressive load \(P\) acting on the column.

Designing for Transverse Shear in Double Lacing

The question specifically asks about the design transverse shear for a double lacing system. In structural design codes, the recommended minimum transverse shear force \(V_t\) for which the lacing system of a built-up column should be designed is specified as a percentage of the total axial load \(P\) on the column.

For both single lacing and double lacing systems, the minimum transverse shear force \(V_t\) is typically taken as 2.5% of the total axial load \(P\).

This means that the double lacing system must be designed to resist a transverse shear equal to:

\( V_t = 2.5\% \text{ of } P \)

Which can be written as:

\( V_t = \frac{2.5}{100} \times P \)

\( V_t = 0.025 \times P \)

This value of 2.5% of the total column load \(P\) is a standard requirement in design codes to ensure the stability and safety of the built-up column under load.

Transverse Shear Requirement Summary

Here is a summary of the required transverse shear for design:

Therefore, for a double lacing system, the transverse shear \(V_t\) to be resisted is equal to 2.5% of the total load \(P\).

Revision Table: Key Aspects of Lacing Design

Additional Information: Lacing System Details

Beyond the transverse shear magnitude, designing lacing systems involves several other considerations based on design code provisions:

• Angle of Inclination: The angle of inclination of lacing bars with respect to the longitudinal axis of the column should generally be between 40 degrees and 70 degrees. This range ensures efficient force transfer and practical connection details.
• Slenderness Ratio: Individual lacing members must satisfy slenderness ratio limits to prevent buckling. The maximum slenderness ratio for a lacing bar is typically limited to 145.
• Connections: The connections of lacing bars to the main components are crucial. They must be designed to transfer the calculated forces effectively, considering shear, bearing, and tension or compression in the fasteners (rivets, bolts, or welds).
• Spacing of Lacing Points: The distance between consecutive lacing connections along the column length affects the effective slenderness ratio of the main column components. This spacing must be limited to ensure the main members do not buckle between the lacing points.
• Double Lacing Arrangement: In a double lacing system, the lacing bars are arranged on both sides of the column axis and intersect at mid-width, often with a connection plate or rivet/bolt at the intersection point. This configuration provides redundancy and can sometimes offer better rigidity, although the design shear requirement remains the same as single lacing (2.5% of P).