Hello, I have a quick question. I have some steel columns that are pinned at the bottom and fixed at the top. I have assigned the same section sizes (UB) to them, but some are classified as Class 4 and some as Class 1 (Eurocode). I have looked at the member forces envelopes and the distributions follow the same pattern (Moments increase linearly from 0 to some value and axial loads are within 5 kN). Is there a reason for this classification difference perhaps due to the software only being able to output envelope forces at column ends?
Please attach the related model and let us know the member number which you are investigating.
Because the model I am working with is very large, I have managed to recreate the effect in a small test model, which I have just uploaded using the File Upload tool. The columns on the LHS are fine; the ones on the opposite side are Class 4.
The model also contains some lateral beams that are pin-ended. The program also classifies these as Class 4. However, looking at this table - https://www.steelforlifebluebook.co.uk/ub/ec3-ukna/axial-force-bending-s355/ - the axial loads on these beams are nowhere near the limits for a Class 4 section (The Class 2 limit is the axial load ration compared to Npl,rd when a member changes from Class 2 to Class 3; The Class 3 limit is the axial load ratio when a section becomes Class 4).
Editing this again, as I did some more research yesterday.
Looking at https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/10645/class-4-sections-in-ram-frame
What is meant by the section being subject to compression only does not refer to points of contra-flexure in the beams or columns, but rather :
1 ) UBS used as columns that are pin-ended or the moment is much smaller compared to the axial force. Therefore, UBs used as columns should light up in RAM Steel Column rather than in RAM Frame (unless they are also pin-ended in RAM Frame).
2) Very highly compressed beams
In the case of the beam shown in the article, the section would only be considered as Class 4 if the axial compression in it was greater than 0.569*3040 = 1729.76 kN. BS EN 1993-1-1:2005 5.5.2(6) states that a cross-section is classified according to the least favourable class of it compression parts. That would mean that all UBs with a point of contra-flexure and axial compression would have to be considered as Class 4 for all checks because of their web, which is overly conservative and would unnecessarily punish light compressed sections. With an Ned of 20 kN a beam of size 457x191x62 would not even be Class 3, as the limit for that is 0.139*3040=422.56 kN.
SCI P363 (https://www.steelconstruction.info/File:SCI_P363.pdf) gives the following guidance at the bottom of page A-14 & start of A-15:
None of these cases are relevant to the columns I have in my model in which the columns are pinned at the bottom and fixed at the top, so I do not see why half of them would be Class 4 and the others Class 1. The same thing applies to the pin-ended lateral beams I have modelled.
Did you upload the model with the Secure Upload File. For some reasons I am not able to access this folder. Would you be able to attach that model to this forum post? If not, please create a Service Request at http://apps.bentley.com/srmanager/ProductSupport and attach STAAD model there.
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This issue is related to RAM Structural System, correct?
Yes. Ram Frame Steel