Hello friends,

In this blog, I’d like to share my insights on key STAAD parameters: LY, LZ, UNB, and UNT. It took me a while to fully grasp these concepts, so I’ll aim to explain them as clearly and simply as possible. Let’s dive in, starting with LY and LZ.

LY and LZ: Effective Length in Local Y and Z Axes

In STAAD, LY and LZ represent the effective length of a member along the local Y and Z axes, respectively. These lengths are critical for calculating the slenderness of a member, which is essential for stability analysis.

• LY: Effective length in the local Y-axis.
• LZ: Effective length in the local Z-axis.

UNB and UNT: Unsupported Length for Bending Capacity

UNB and UNT refer to the unsupported lengths of the bottom and top flanges, respectively. STAAD uses these values to calculate the bending capacity of a member, which helps ensure that the structure can handle bending forces.

• UNB: Unsupported length of the bottom flange.
• UNT: Unsupported length of the top flange.

These parameters are particularly relevant when the top or bottom flange is braced, such as when plan bracing is added. In many cases, UNB and UNT are set equal to LY, but this depends on the bracing conditions of each flange.

Why LY, LZ, UNB, and UNT Matter in Space Frame Structures

For space frame structures, setting these parameters accurately is essential for a correct and reliable design. Incorrect values can lead to errors in the analysis and potentially compromise the structure's safety and performance.

Understanding Major and Minor Axis Bending

To fully grasp LY and LZ, it’s important to understand major and minor axis bending:

• Major axis bending (LZ): This can be observed in elevation views, where the beam bends along the local Z-axis.
• Minor axis bending (LY): This is visible in plan views, showing bending along the local Y-axis.

Let’s go through various cases to illustrate how LY and LZ apply under different bracing scenarios.

Case A: LY and LZ in STAAD Without Plan or Elevation Bracing

In this structure, there’s no plan or elevation bracing, so LY and LZ are simply the distances between nodes. There’s no need to specify LY or LZ explicitly in STAAD for this configuration, as STAAD will assume the full member length as both LY and LZ by default.

Case B: LY and LZ in STAAD with Plan Bracing Only

This structure has plan bracing to control horizontal deflection. Adding this plan bracing changes the deflection pattern of the highlighted beam along the minor axis. Therefore, in this case, only LZ needs to be specified as 3.5 m.

• STAAD considers the full member length as LY for the minor axis, which remains accurate here.

Case C: LY and LZ in STAAD with Elevation Bracing Only

In this structure, elevation bracing controls vertical deflection. By adding this elevation bracing, the highlighted beam’s deflection pattern changes along the major axis. Therefore, only LY needs to be specified here as 3.5 m.

• STAAD will consider the member length as LZ for the major axis, which is appropriate for this case.

Case D: LY and LZ in STAAD for Discontinuous Members

In this structure, the highlighted beam is broken in the middle, with no plan or elevation bracing. For this case, LY and LZ are both calculated as the total distance of 3.5 m (1.75 m + 1.75 m).

Please share your thought in comment section and let me correct if I am wrong somewhere.