Understanding for Assigning LY and LZ in STAAD Pro

Hello friends

In this blog, I like to share my view for STAAD Parameter of LY, LZ, UNB and UNT. I needed comparatively more time to understand this concept. I will try to explain as simplest as possible. So let’s start with what is LY and LZ.

LY and LZ is effective length along local Y and Z axis respectively, which length is STAAD use to find Slenderness of member.

UNB and UNT is unsupported length of the member bottom and Top respectively, which length is STAAD use to calculate bending capacity.

UNB and UNT applied when Top or bottom flange restrined. which is generally applicable when plan bracing added. In most of the case UNB = UNT = LY is considered. However UNB and UNT depends on member's top and bottom flange restrained.

For Space frame structure, all above parameter must be considered for errorless design.

To understand LY and LZ, we need to understand major and minor axis banding. It is difficult to explain only be STAAD deflection image so I will add caption below the image, Major axis banding for LZ and Minor axis bending for LY. In other words, we can see beam deflection along major axis from Elevation view and beam deflection along minor axis from Plan view.

Let’s understand LY and LZ be different possible cases.

CASE A: LY and LZ in STAAD for No plan and Elevation Bracing

In this structure, there is no Plan and Elevation bracing. So LY and LZ are nothing but node to node distance in structure. In other words, no need to specify LY and LZ for this type of structural arrangement. STAAD default considered member length is LY and LZ

CASE B: LY and LZ in STAAD for only plan Bracing

In this structure, there is a plan bracing to control horizontal deflection. By adding this plan bracing highlighted beam change its deflection pattern in minor axis. So for this case need to apply only LZ only which is 3.5 m.

For minor Axis STAAD considered member length as LY and which is true for this case.

CASE C: LY and LZ in STAAD for only Elevation Bracing

In this structure, there is a elevation bracing to control vertical deflection. By adding this elevation bracing highlighted beam change its deflection pattern in major axis. So for this case need to apply only LY only which is 3.5 m.

For major axis STAAD considered member length as LZ and which is true for this case.

CASE D: LY and LZ in STAAD for discontinues members.

In this structure, highlighted beam is break from middle. However there is no plan and elevation bracing so in that case LY and LZ is 3.5 m (1.75 m + 1.75 m = 3.5 m).

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