Staad input parameters 2

[KenStructural]

What is the difference between Lz to UNB/UNT input parameters in Staad Pro V8i.

1 assumption: beam in consideration is wide flange
2 assumption; beam in consideration is tubular

 

[SteelPE]

I'm not not an exact expert but I believe Lz is a parameter for column buckling (due to axial load) and Unt/Unb is a parameter for lateral torsional buckling due to bending. T stands for top flange and b stands for bottom flange. If no parameters are input I believe STAAD uses the nodal spacing as the default value for these parameters. Check out the help menu of STAAD for clarification

 

[ToadJones]

"Lz" (or Ly, Lx etc) is used to calculate the KL/r for the given axis for calculating allowable/ available axial capacity (column).

UNB/UNT are the unbraced lengths of the bottom and top flanges.
UNB/UNT should change your unbraced length "Lb" or as STAAD calls it, "UNL".
It seems a bit counter intuitive.

For columns that are part of frames, it is often important to specify "LZ","LY", "UNB", and "UNT" since almost members will be designed using combined stress equations of chapter H (for AISC anyway).

In order to confirm all this to yourself, make a model and used the "TRACK" parameter set = 2.0, then look at the output file. It will show you your unbraced length and KL/r values.

 

[KenStructural]

Thanks ToadJones (Structural)now it is clear that the staad will design beam/column based on UNT/UNB/Ly,Lz otherwise the staad will use the default node spacing (member length), however my next question is on SHEAR, considering the staad model, if i use the command UNB or UNT = 3m. what is the design considerations interms of shear?

 

[ToadJones]

None that I can think of at all.

There is a parameter that allows you to define the distance between stiffeners to be considered in the design, if you have stiffeners.

You really just need to read through all of the parameters and what they mean. There really aren't that many.

In each case, simply think "if I were doing this by hand, what would I need to know?"

When I design using STAAD it is usually because it can very accurately determine primary and secondary forces in the members of a structure and accurately model the stiffness.
The code check is more or less a time saving convenience, but I still find myself doubling checking with hand calcs or spreadsheets.

If you were designing beam-column (which is essentially how STAAD code checks everything for AISC) you'd need to know unbraced lengths, materials, deflection limits, bending moments, axial loads, etc. STAAD needs to know the same things. So, if you don't know how to design a beam-column by hand, well, you don't know how to use STAAD to design a beam-column (not at all saying that you don't know what you are doing).

 

[amec2004]

Below is noted in Candaian steel code S16-09, it shall be similar to AISC LRFD 13 code as well.

Different Unbraced Length Definition in STAAD

UNT,UNB and UNL(obsolete)
For beam flexural code check as per CSA S16-09 clause 13.6.
UNT & UNB is unbraced length of compressive flange for member flexural code check.
It applies to flexural code check, and the length definition is related to compressive flange.
[red]
1) To decide which flange is top flange-->UNT and bottom flange --> UNB, turn on the memeber's local axis and the positive local y axis pointing direction is top flange UNT, the opposite direction is bottom flange UNB

2) One continuous beam may have top flange under compression in one segment and bottom flange under compression in other segment, so UNB and UNT might be different

3) Only when compresive flange get lateral restraint shall a point be considered as lateral support to reduce unsupported length. If the secondary beam doesn't restrain the compressive flange's lateral movement, the beam shouldn't be considered as a lateral support

4) UNL is obselete, in the old time STAAD didn't differentiate top and bott. flange and it caused issue stated in item 2 above. Now UNL is no longer in use but still valid in STAAD [/red]


KY KZ, LY LZ
For doubly symmetric column compression code check as per CSA S16-09 clause 13.3.1
LY or LZ is the unsupported length to prevent column from flexural buckling. It’s more like a floor beam/strut framing to a compressive column and has nothing to do with compressive flange. Like a building column in the transverse (strong axis) direction LZ= full length and in the longitudinal (weak axis) direction LY=strut distance

KT, LT
For singly symmetric asymmetric member compression code check as per CSA S16-09 clause 13.3.2
LX or LT is the unsupported length to prevent compressive member from twisting. It’s normally the full length of member length.

Usage of LX or LT, LY, LZ in STAAD
For doubly symmetric wide flange and hollow sections, compression is governed by Flexural Buckling as per clause 13.3.1 of S16-09. In this case KY, LY, KZ, LZ are used to calculate KL/r value

For asymmetric and singly symmetric sections, besides Flexural Buckling check, compression needs to be further checked against Torsional Buckling or Flexural-Torsional Buckling as per clause 13.3.2 of S16-09. In this case KT or KX, LT or LX are used to calculate KT x LT, which shown as Kz x Lz in S16-09 clause 13.3.2 formula.

KT or KX can be conservatively taken as 1.0 as per clause 13.3.2 of S16-09. KT=1.0 is STAAD’s default value.

LX or LT, the torsional unbraced length is the distance between braces that prevent a member from rotation about its longitudinal axis

LT or LX, torsional unbraced length can be taken as the length of segment where both top & bottom flange are braced.
In usual case, LT is taken as member full length unless at intermediate points both top & bottom flange are restrained from twisting.
In STAAD if secondary beams are framed to a main beam, the main beam is broken into several segments, and not both top & bottom flange are braced, the LT shall be re-defined as member full length for this member’s compression code check.

Why Defining of LX or LT Is Not Necessary in STAAD

From above we can see the usage of LT is only necessary for asymmetric and singly symmetric sections, such as single channel, single angle, WT and double angle.

Please refer to STAAD Pro-2007 Manual International Design Codes page 3-19 Steel Frame Design CSA S16-01, STAAD uses LT to check Torsional Buckling or Flexural-Torsional Buckling only when doing the member compression code check of single channel, single angle, WT and double angle.

STAAD’s code check of single channel, single angle, WT and double angle are NOT reliable due to the limitations of neglecting of eccentricity or flagging of class 4 section (class 4 doesn’t mean the member failing the design).
Using STAAD output for code check of single channel, single angle, WT and double angle is NOT safe and all these asymmetric and singly symmetric sections shall be code checked manually.

For more details on STAAD code check limitation on singly symmetric section, please check

http://www.civilbay.com/PDF/Brace-C...ty-Design-Charts-As-Per-CSA-S16-09-Rev1.5.pdf

For more details on singly symmetric section crane beam design crane girder crane runway beam design and anchor bolt design as per ACI 318-08, please check

http://www.civilbay.com

For above reason, defining of LX or LT value in STAAD in not necessary as it will not be used for doubly symmetric wide flange and hollow sections code check.

Definition of LX, LY, LZ
LX or LT, the torsional unbraced length is the distance between braces that prevent a member from rotation about its longitudinal axis
LY and LZ, the lateral unbraced length is the distance between braces that prevent relative movement of the compressive flange

Setting of LX or LT in STAAD
LT or LX, torsional unbraced length is taken as the length of segment where both top & bottom flange are braced.

The lateral unbraced length is the distance between braces that prevent relative movement of the compressive flange.

The torsional unbraced length is the distance between braces that prevent a member from rotation about its longitudinal axis.
Torsional unbraced length is taken as the length of segment where both top & bottom flange are braced.