***WARNING: THE VALUE OF MOMENT AMPLIFICATION FACTORS KY OR KZ IS GREATER THAN 1.4
FOR MEMBER # XXX FOR LOADCASE- XXX. SECOND-ORDER ELASTIC ANALYSIS MUST BE PERFORMED
AS PER SECTION 4.4.2 of IS800-2007.

Question

Hello All, 
 
 Can someone guide me how to remove above warning in STADD Pro, I have refereed to https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/30240/warnings-in-is800-2007, however can someone clarify below pointwise queries- 
 1) Why STADD is showing warnings for Truss members which are only subjected to Tension loads and Not bending moment, as this clause 9.3.2.2 is for combined bending and axial compression case. Should we neglect it for Truss Members or is it a bug in STADD Pro. 
 2) What should we do for member subjected to Bending and axial loads? For Example a certain X member is failing for Load case combination Y , then should we create different Load case combination similar to Y with REPEAT Load Case option and Analyze only failing Member X under this REPEAT Load case with Pdelta Analysis? 
 3) Is there any other way to eliminate this warning, by strengthening the Member or by any other means. Do advise. 
 Thanks.

Surojit Ghosh · Accepted Answer

I am sure you have understood the reason for this warning as mentioned in the linked post. Following are few clarifications -- 
 Q1. It is not a bug. At this moment, this warning is reported for all types of member. During design, program does not consider the truss specification and design it as a general member with 0 bending forces. So though you have specified a member as truss member for which bending and interaction checks are not required, still program reports this warning. You can simply ignore this warning for truss member. 
 Q2. If this warning is reported for any member which is subjected to axial compression + bending, then as per cl.4.4.2, first order analysis + moment amplification method is not sufficient to consider the second order effects and you need to perform second order analysis. In such case, program does not terminate the design and you can still use the design results. But if you want to follow codal provisions, then you should change all load combinations to repeat load cases and perform second order analysis like pdelta analysis. 
 Q3. To remove these warnings, either you can perform second order analysis or use SET NOWARNING command which will instruct program not to print any warning in the output file. If you use this command, then you have to use the latest CONNECT Edition as there was a bug in the older version which has been fixed now. 
 If you look at the equation of Ky and Kz, you can notice this depends on non-dimensional effective slenderness ratio and ratio of applied compressive force/compression capacity. If you can increase the overall compression capacity of the member by increasing the r value or by reducing the euler buckling stress, then both lamdaY and ny value reduces which eventually decrease the ky/kz value. In other words, chance of buckling and second order effects reduce for a short stiff member rather than a ling slender section, correct?

Surojit Ghosh · Answer

1. In amendment no. 1 (January 2012), cl.4.4.2 has been modified and moment amplification factor is denoted as Ky, Kz, not Cy,Cz. You can check that for more details. Also, this Ky,Kz is not same as effective length parameter, it is calculated as per cl.9.3.2.2. 2. Currently, STAAD designs all member as general member considering correct design force, which mean, for all member including truss, capacity at relevant sections are calculated and that is compared against the actual design force. So irrespective of any member specification, this warning is reported. 
 Check this post for more details -- communities.bentley.com/.../warnings-in-is800-2007

***WARNING: THE VALUE OF MOMENT AMPLIFICATION FACTORS KY OR KZ IS GREATER THAN 1.4 FOR MEMBER # XXX FOR LOADCASE- XXX. SECOND-ORDER ELASTIC ANALYSIS MUST BE PERFORMED AS PER SECTION 4.4.2 of IS800-2007.

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