Channel section – outstanding (projection) length in compression: For a compression member made from a single channel, the outstanding length is measured how?

Introduction / Context:
In built-up compression members using channels, the free (outstanding) leg is prone to local buckling. Codes limit the projection-to-thickness ratio, so one must measure the outstanding length correctly to check local buckling criteria and detail transverse connections.

Given Data / Assumptions:

• Member uses a single channel or channels with one leg outstanding.
• Fasteners are placed along a line connecting the channel web/flange to a gusset or companion member.
• Outstanding length is required for outstand slenderness checks.

Concept / Approach:
The outstanding width for local buckling is the free projection measured from the free edge up to the first line of restraint (i.e., the first row of fasteners). This dimension is used in the width-to-thickness (b/t) check of the outstanding element to ensure it remains within the “compact” or “semi-compact” limits, depending on the code basis.

Step-by-Step Solution:
Locate the free edge of the channel flange (outstand).Identify the first line of restraint provided by rivets/bolts or welds.Measure the perpendicular distance between these two: that is the outstanding length.

Verification / Alternative check:
Design examples consistently define outstand b as the free projection to the first line of connection. This aligns with local buckling formulas using b/t ratios for plate elements.

Why Other Options Are Wrong:

• Half or full nominal width: do not necessarily correspond to the restrained line; connections may not be centered.
• None of these: incorrect because a standard definition exists.

Common Pitfalls:

• Measuring to the web centreline instead of the first fastener line.
• Ignoring the effect of fillets or returns; measure the flat projection.

Final Answer:
from the free edge of the outstanding leg to the first row of rivets/bolts connecting it to the gusset or companion member