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Institution Forums > Q&A > Analysis of portal frame View modes: 
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piotr odron - 03/06/2013 23:23:24
   
Analysis of portal frame
Hi everyone. Can anybody help me with pitched portal frame analysis and design to eurocode please. I have to analise a portal frame plasticaly without any software, only hand calculations ( it is for my dissertation ).I am not taliking about preliminary analysis where sections are determied. Is it true that graphical method is not allowed if designed to eurocode. Can I use virtual methode? Also is the position of purlins deterimned before the analysis or after?In analysis, is the vertical imposed load treated as uniformly distributed load or point load acting on the purlin. I have tried a couple of different analysis methods and they give a different distance of the plastic hinge in the rafter from the column,also different sorces recomend different ratio of moment in the rafter to moment in the column (0.75, 0.85)  .i suppose that elastic-plastic is the best but this would mean use a software( not an option) Can any one suggest most appropriate method for this task? Thank you

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Fellow (FIStructE)
Fellow (FIStructE)
Richard Haigh - 04/06/2013 09:07:42
   
RE:Analysis of portal frame
Piotr,

This sounds like a rather difficult thing to do by hand properly, and I would question the benefit of it, as you would never do this by hand for a real building as it will take too long.  On past projects I have always used the elasto plastic method implemented in computer software such as CSC Fastrak.  Essentially this assumes elastic behaviour until you exceed the nominal moment capacity of the section (the steel yields) then you assume this point rotates perfectly plastically, and is thus a pin, you then continue the analysis with an additional pin until  another section yields and there is another pin, depending on support assumptions  you probably now have a mechanism (if pinned supports) and there is no reserve capacity so the frame fails, if you have fixed bases you would continue until you have sufficient pins to cause collapse.  A way of thinking of this is like a three pinned arch.  (Three pins OK, four pins not OK).
 
As such I would suggest a starting point would be the bending moments diagrams for pitched frames in the Steel Designers Manual (page 1144 in 6th Edition).  I wouldn't worry about purlins as for most buildings there are a lot of them quite close together (circa 1.2-1.5m) so they will be approximately a UDL for reasonably long stations and rafters.  Based on selection of section sizes for stations and rafters and stations and assuming you have a vertical UDL on roof you should be able to calculate resulting  bending moments and increase UDL incrementally until you exceed the nominal moment capacity of one section (taking account of restraints from purlins, rafter stays etc) at which point a pin will form.  You then need to check that the load applied is more than the service load on the structure (but less than ultimate factored load on the structure).   Beyond this point, you need to work out BMD etc for incremental loads with an extra pin.   The Steel Designers Manual won't help you any further but perhaps the references below the diagrams will? It also won't help with deflections as the Steel Designers Manual doesn't give defection formulae for frames.
 
I would suggest all of the above at least needs a spreadsheet to allow you to incrementally change loads and section sizes.   You should also be able to superimpose vertical and lateral loading on the frame if required.
 
Not sure that this will be of any help, but I have tried.
 
Regards
 
Richard


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Associate-Member (AMIStructE)
Associate-Member (AMIStructE)
Christopher Achilles - 04/06/2013 13:03:32
   
RE:Analysis of portal frame
I was shown a simple way of doing an elastic analysis by hand before PC's kicked in properly in early 80's). You lay the frame flat including the legs (plan dims for rafters) then plot the free bending moment diagram for the rafters based on a unit load of 10kN/m. You then make a little table with height to eaves, height to end of haunch and apex height. You then have a punt at what the base shears will be and plot the base shear moment over the free bending moment. A bit of iteration is need but you can quickly reach a point where the moment at the haunch end matches the apex haunch (may take 4-5 stabs at the correct base shear). It is a simple method which gives you a unit bending moment diagram to work with. If your actual gravity load is 8.5kN/m then you take 85% of the results/shears etc. I was shown how to do it plasticly as well but my tiny brain started to hurt and I fell asleep!

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Associate-Member (AMIStructE)
Associate-Member (AMIStructE)
Christopher Achilles - 04/06/2013 13:22:12
   
RE:Analysis of portal frame
I have just found my old training notes for Plastic Design (by hand) which may be of use. I can not work out how to attach them to this post but I could e-mail them to you (6 A4 sheets).

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Peter Zsemebri - 04/06/2013 15:23:26
   
RE:Analysis of portal frame
Hi, It is Piotr Odron who needed help with portal frame design. I have had to change my account as the old one does not work(it does not accept my e-mail for some reason). You have offered the notes for plasic frame analysis. I also have to analise a frame elesticaly so if you have any notes on plastic and elestic I would realy appraciate your help. Thank you for your help and look forward to hearing from you. My email is piotrodron@live.co.uk

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Peter Zsemebri - 04/06/2013 15:24:41
   
RE:Analysis of portal frame
thank you for help ,all

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Member (MIStructE)
Member (MIStructE)
Micheal Toner - 03/02/2015 13:29:39
   
RE:Analysis of portal frame
Christopher, could you email me your notes for plastic portal design by hand please?  To toner776@hotmail.com

Many thanks in advance.

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Associate-Member (AMIStructE)
Associate-Member (AMIStructE)
Christopher Achilles - 03/02/2015 22:40:03
   
RE:Analysis of portal frame
No Problem will dig them out and get them over to you tomorrow.

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Technician Member (TIStructE)
Technician Member (TIStructE)
Richard Lytham - 17/02/2015 11:20:13
   
RE:Analysis of portal frame
Hi Chris,

I was just looking through the forums and came across this discussion about portal frame analysis.

Would you be able to email me a copy of your old design notes?

richardlytham@leachsteel.com


Thanks in advance.

Regards

Richard Lytham

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Graduate
Graduate
Thomas Roberts - 06/03/2015 12:11:50
   
RE:Analysis of portal frame
If this is still a valid offer, i would be interested in these notes! Im particularly stumped in hand calculating the moment at the apex / ridge for the purpose of giving this information to the steel fabricator. Im a graduate engineer and have been relying on collegues to determine this using software (which isnt available to me at the moment). it would also be useful to understand the principles of what drives the moment here for future knowledge. tom_roberts.1992@hotmail.co.uk

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