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General
Always consider a member's tendency to buckle due to a fictitious force P. UNL, LY and LZ will be the length of the buckled member.
LY and LZ are parameters to be specified in Staad for member design, for members subjected to axial compressive forces.
UNL, or UNT and UNB are parameters to be specified in Staad for member design for members in bending.
KY and KZ Per Chapter C, page 5-135, 5-137, Manual of steel construction, ASD.
Use Table C-C2.1 for approximate analysis
Use Fig C-C2.2 for rigorous analysis
- Do not attempt to give these parameters to any member by looking at it in an isometric view. Cut a section and view the frame in plan or in elevation and then give the corresponding parameters.
9a ) For Beams: Cut a plan view and give UNL and LY parameters to the beam. Then cut an elevation view and give LZ parameter to the beam.
9b) For columns: Cut an elevation view, if you can see the web of the column (as in a pipe rack bent) give LZ parameter for that column. When you cut the other view of the column and give UNL and LY.
Beams
Staad member
Actual
- There may be many joints in staad between the length of a member….1,2,3….n joints and lengths will be L1, L2, L3, …..L n-
. I have considered only 3 joints in the illustration below.
- Since a member in a space frame could be subject to axial forces and bending, LY, LZ and UNL must be specified in the parameters. If you specify LY and LZ but omit UNL your design will be erroneous. Likewise if you specify LZ and
UNL but omit LY your design will be erroneous.
- Do not confuse LY and LZ with KY and KZ; LY and LZ are the actual lengths of a member between two buckling points in feet or inches; KY and KZ are factors to account for the condition of the ends of a member, example, allowed to rotate, not
allowed to rotate, fixed , free, etc etc.
- It's difficult to draw 3D on paper; To differentiate between vertical and horizontal buckling..I have shown large amount of buckling (red) for buckling in elevation in the major axis of the member and lesser amount of buckling (blue) for out of plane
plan buckling.
9c) For columns: Cut an elevation view, if you can see the web of the column dotted; ie. if you see the entire flange width of the column give the LY and UNL parameters for that column. When you cut the other view of the column and give the LZ
parameter.
L1 L
Restraining member at
joint 2 in plan or elevation
P
P
a) If restraining member is in elevation only…. Example a
UNL = UNT = UNB = L1 + L2 UNL = UNB = UNT = 7 feet
LY = L1 + L2 LY = 7 feet
LZ = maximum of L1 and L2. Staad will consider default value, you need not give a parameter. LZ = default staad length
Member will buckle in
plan for full length
between points 1 and 3 in
the minor axis
Member will buckle in
elevation between points
1 and 2 or points 2 and 3
in the major axis
Elevation view, Buckling about plan
Buckling about elevation
Example of a frame in elevation not having a restraint in plan at joint 2 but having a
restraint along major axis
PLAN
SECTION
c) If restraining member is in plan and elevation…. Example c
UNL = UNT = UNB = L1 or L2. You need not specify the parameter in Staad because Staad will consider default length. UNL = UNB = UNT = default staad length
LY = L1 or L2. You need not specify the parameter in Staad because Staad will consider default length. LY = default staad length
LZ = L1 or L2. You need not specify the parameter in Staad because Staad will consider default length. LZ = default staad length
11 feet
Member will buckle in plan between
points 1 and 2 or points 2 and 3 in
the minor axis
Member will buckle in
elevation between points 1
and 2 or points 2 and 3 in the
major axis
Buckling about elevation
PLAN
Example of a frame in elevation having a restraint
in plan and elevation at joint 2
SECTION
20 feet
Looking beam in elevation, Buckling about plan
d) If there is NO restraining member in plan or elevation…. Example d
UNL = UNT = UNB = L1 + L2 UNL = UNB = UNT = 20 feet
LY = L1 + L2 LY = 20 feet
LZ = L1 + L2. LZ = 20 feet
Member will buckle in
plan for full length
between points 1 and 3 in
the minor axis
Buckling about elevation
Member will buckle in
elevation for full length
between points 1 and 3 in
the major axis
PLAN
Example of a frame in elevation having no restraints in plan
or in elevation at joint 2
SECTION
feet
feet
Looking beam in elevation, Buckling about plan
Example 3
Example 4
When you view the beam like this, you DON’T have to give the LY and UNL parameters.
You may have to give LZ when you view beams A and B in elevation.
Example 4a
When you view the beam like this, you DON’T have to give the LZ parameter.
You may have to give LY or UNL when you view beams A and B in plan. (as in example 1)
When you view the beam like this, give the LZ parameter.
LZ = 18 feet
You may have to give LY or UNL when you view beams A and B in plan.
PLAN
8 feet 10 feet
A B
Plan bracing
8 feet 10 feet
A
B
SECTION
SECTION
10 feet 8 feet
B
A
Example 5
In pipe racks a moment connected beam can have the bending moments as illustrated.
Therefore Jacobs procedure for pipe racks allows UNL to be half the beam length. (since apx half the beam flange is in compression and likely to buckle)
UNL = UNT = UNB = 15 feet
LY = 30 feet. Staad default value
LZ = 30 feet. Staad default value
Do not use this concept for moment connected beams in a structure. Bending moment diagrams in a structure are far more complex due to a continuous frame, irregular distribution of vertical loads..... and the entire length of beam
could be subject to a positive or negative bending moment.
30 feet
SECTION
apx 15 feet
apx 15 feet
30 feet 30 feet
moment connected beam
with no joints in Staad within
its length
Bending moment diagrams
Example 7 (Special Case)
Example 7a Example 7b
When you view the beam like this, you may have to give the LY and UNL parameters because the beam may or may not be considered as a restraint.
Example 7a
Consider beams A, B, C and D to be subject to a compressive force "P". If all beams buckle together in the same direction as shown by the blue dotted line then
LY = 28 feet for members A, B, C and D.
UNL = UNB = UNT = 28 feet for members A, B, C and D.
Give LZ later when you view beams A , B, C and D in elevation.
Example 7b
Consider beams A, B, C and D to be subject to a compressive force "P". If all beams DO NOT buckle in the same direction as shown by the blue dotted line then
LY = staad default length for members A, B, C and D.
UNL = UNB = UNT = staad default length for members A, B, C and D.
Give LZ later when you view beams A , B, C and D in elevation.
Since buckling of beams in such cases cannot be predicted, Paramaters in "Example 7a" are prefered.
A plan bracing, if provided, say between joints 2 and 4, would definetely be considered as a restraint and parameters LY and UNL need not be given.
PLAN
13 feet
A
B
15 feet
C D
PLAN
13 feet
A
B
15 feet
C
D
General
- There is no difference in parameters UNL, LY and LZ whether it is for a beam or a column. The concept is the same, look for buckling of the column between two points for a fictitious compressive load and that will give you these parameters.
Example 1
LZ = 6 feet for members A and B
LY = UNL = UNT = UNB = 3 feet for members A and B. Since this is the staad default length the parameter need not be given.
Isometric view of a pipe
rack bent
Longitudinal beam
Column for which Parameter is
necessary
Column orientation
6 feet
Equal
Equal
A
B
Example 3
Parameters for both views
LY = UNL = UNT = UNB = 4 feet for member A. Since this is the staad default length the parameter need not be given.
LY = UNL = UNT = UNB = 2 feet for member B. Since this is the staad default length the parameter need not be given.
LZ = 4 feet for member A. Since this is the staad default length the parameter need not be given.
LZ = 2 feet for member B. Since this is the staad default length the parameter need not be given.
Isometric view of a
structure column
Cross beam
Parameter not necessary
Column orientation
6 feet
2 feet
4 feet
A
B
Isometric view of a
structure column
Cross beam
Column orientation
6 feet
A
B
2 feet
4 feet
Parameter not necessary
Example 4
UNL = UNT = UNB = LY = 20 feet for members A and B.
LZ = 20 feet for members A and B.
Isometric view of a
structure column
Column for which Parameter is
necessary
Column orientation, any
way
20 feet
8 feet
12 feet
A
B
or
