AISC ASD PARAMETERS KX, KY, KZ, LX, LY, LZ, MAIN, TMAIN The sequence of performing code check as per AISC 9th edition in STAAD is as follows: 1. Check for the slenderness criterion. 2. Check for deflection (if provided) 3. Check for combined axial and bending. The member is designed and checked for rest of the provisions only if it passes the slenderness criterion. STAAD includes Ky, Kz, Ly, Lz ,MAIN and TMAIN parameters under design. These parameters govern the slenderness calculation and allowable axial compressive stress of a member in design. Ky and Kz are the effective length factor along Y and Z axes respectively. LY is the length to calculate slenderness ratio for buckling about local Y axis and LZ is the length to calculate slenderness ratio for buckling about local Z axis. MAIN serves two purposes in the program. This parameter helps the user to advise the program to bypass the slenderness check and also when defined with a value greater than unity helps to define slenderness limit for Compression. In a similar way, TMAIN when defined with a value greater than unity defines slenderness but not in compression, but tension. For example, MAIN 300 ALL will tell the program that the member has a allowable slenderness value of 300 in tension. That is, in short this replaces the default value of 200 with 300. Slenderness Calculation In STAAD, the slenderness check is done along both major and minor axes (Z and Y axes). The program reports slenderness ratio as KyLy/ry and KzLz/rz for all sections in the TRACK 2 design output. In steel design as per AISC ASD, clause B7 (see the picture below) of the code specifies that the slenderness value of the member should not exceed 300 for tension members and it should exceeds 200 for compression member.
The slenderness of a member is calculated as K multiplied by L divided by r, where ‘k' is the effective length factor, L is the default length of the member (unless otherwise specified) and ‘r' is the radius of gyration and it is calculated as sqrt (I/A). Design Parameter KX, KY and KZ The effective length factor is defined as per the end conditions of a member. In the program there are two provisions of defining these factors. One method is that we can ask the program to calculate the KY and KZ by clicking the CALCULATE tab in the GUI. By this the program calculates the K factor on the basis of the chart provided in section C-C2 of the code as shown below. However, this is not very reliable and users are urged to check their values.
Other method is to specify the effective length factor by the user against the Ky and Kz parameter and these values can be obtained from Table C-C2.1 of the code as shown below.
LX, LY and LZ LY is the length to calculate slenderness ratio for buckling about local Y axis and LZ is the length to calculate slenderness ratio for buckling about local Z axis. The default values that the program considers for LY and LZ are the respective member lengths. Let us illustrate the problem with the following example. See the attached STAAD file. A beam joins the column at mid height. That is, the beam restrains the column from buckling about the Z axis. The beam will split the column to two members. So for the purpose of calculating slenderness along Z axis we have to provide LZ as 5.0 meters for both top and bottom members of the column whereas, when the slenderness about the Y axis need to be considered, then LY has to be provided as 10 meters.
Consider the following STAAD input file ======================= STAAD SPACE START JOB INFORMATION ENGINEER DATE 27-May-08 END JOB INFORMATION INPUT WIDTH 79 UNIT METER KN
JOINT COORDINATES 1 0 0 0; 3 1.5 0 0; 4 0 10 0; 5 1.5 10 0; 6 0 5 0; 7 1.5 5 0; MEMBER INCIDENCES 2 1 6; 3 3 7; 4 6 4; 5 7 5; 6 6 7; DEFINE MATERIAL START ISOTROPIC MATERIAL1 E 2.05e+008 POISSON 0.25 DENSITY 77 ALPHA 1.2e-005 DAMP 2.8026e-044 ISOTROPIC STEEL E 2.05e+008 POISSON 0.3 DENSITY 77 ALPHA 1.2e-005 DAMP 0.03 ISOTROPIC CONCRETE E 2.17185e+007 POISSON 0.17 DENSITY 23.5616 ALPHA 1e-005 DAMP 0.05 END DEFINE MATERIAL MEMBER PROPERTY AMERICAN 2 TO 5 TABLE ST W10X68 6 TABLE ST W8X18 CONSTANTS MATERIAL STEEL ALL SUPPORTS 1 3 FIXED MEMBER OFFSET 6 START 0.05 0 0 LOAD 1 ULTIMATE SELFWEIGHT Y -1 LIST 2 TO 6 PERFORM ANALYSIS PARAMETER 1 CODE AISC
LY 10 MEMB 2 TO 5 LZ 5 MEMB 2 4 TRACK 2 MEMB 2 4 unit kip inch CHECK CODE MEMB 2 4 print memb prop print material prop FINISH ======================= Let us consider the member number 2. In this case the program calculates the slenderness as follows. We have assumed the value of K as 1.0. LZ as 5.0 meters', "r" the radius of gyration is equal to sqrt of I divided by A K = 1.0 LZ = 5.0 = 500 cm Rz = sqrt (Iz/A) = sqrt (16399.52/129) = 11.27 (The same is reported in the detailed output result, when you provide a TRACK 2 parameter) KL/rz = 1.0 x 500/ 11.27 = 44.35 This value is denoted as KL/R-Z and in the output file. In a similar manner, the value of KL/R-Y can also be found.
Notes on slenderness calculation:1.) STAAD by default checks slenderness for all members being CODE CHECKed or SELECTed, regardless of whether or not they have an axial force. 2.) Members with zero axial force are usually checked against the slenderness limits for tension. 3.) For singly symmetric shapes such as Tees and Double Angles, the KL/r value for the Y axis is calculated by STAAD using the rules for FLEXURAL TORSIONAL BUCKLING as explained in page 3-53 of the AISC ASD manual. It is not calculated as Ky multiplied by Ly divided by ry. This is because for these type of members FLEXURAL TORSIONAL BUCKLING is the primary mode of failure and not FLEXURAL BUCKLING. In case you don't want the member to be checked as per the above criterion, you may set KX and LX to very small values, so that, flexural torsional buckling will not govern, and flexural buckling will. In that situation, KyLy/Ry will match your hand calculation. You can add the following commands to simulate that condition. KX 0.1 MEMB 1506 LX 0.1 MEMB 1506 Apart from calculating slenderness, the aforementioned parameters are also used, in arriving at the allowable stress in compression according to the clauses mentioned in CHAPTER E of the code.
---------------------------------------------------------------------------------------------------------------------------If we consider the same member 2 of the above model, and see how the program arrives at the value of FA, the allowable axial load carrying capacity. Cc = sqrt (2 ∏2 E/Fy) = sqrt (2 x 3.14 x 3.14 x 29000/36) = 127.6 The value of KL/R which is greater of KL/Rz and KL/Ry is considered for further calculations. Therefore, KL/R = 152.1 KL/Ry > Cc Fa = (12 x 3.14 x3.14 x 29732.7)/(23 x 152.1 x 152.1) = 6.62
Anonymous This post is very nicely written with examples and references. There is one quesition in my mind. Why you have not performed P-delta analysis of the structure in the example? Whether it has to do anything with applying K value? Please clarify. Thanks VD Anonymous thks for this , it's very clear. However for some reasons, in my model when KL/R is above Cc, the unity check is not calcultated. Do you know why ? er.farooqueazam thanks it was very useful to me..thanks Fri, Dec 23 2011
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Anonymous Hi all,
'One method is that we can ask the program to calculate the KY and KZ by clicking the CALCULATE tab in the GUI. By this the program calculates the K factor on the basis of the chart provided in section C-C2 of the code as shown below.' Could someone tell me how can I automatically calculate KY and KZ in Staad? Thanks, GJ ………………………………………………………………………………………………………
KenStructural (Structural) (OP)
7 Feb 12 18:35 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 (Structural)7 Feb 12 19:02 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 (Structural)7 Feb 12 19:17
"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 (Structural) (OP)
7 Feb 12 19:43 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 (Structural)7 Feb 12 21:26 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 beamcolumn (not at all saying that you don't know what you are doing).
amec2004 (Aeronautics)8 Feb 12 00:35
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. 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 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-Compression-and-Tension-CapacityDesign-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.
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