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RE: Beam design

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Santhosh, it would still be helpful if you identify what material of construction you are designing - steel? concrete?  (If you are referring to beam charts, it would appear that you are designing steel?) 

You cannot design for axial plus bending loads solely using beam charts.  The AISC code provides interaction equations for structural elements with axial load and bending.  And there are design aids and software programs that will provide interaction charts for concrete elements with axial compression and bending.  As Scott and Michelle noted below, these concrete charts are provided in design aids rather than in the code.  The code just defines stress and stability limits, which the design aids are based on. 

For a material such as steel, the bending forces cause variable tension and compression stresses on the cross section and the axial forces cause uniform stresses on the cross-section.  These stresses combine algebraically .  However, the allowable stresses are different for different types of applied loads and stress effects (largely due to buckling problems), so the resulting combined stress cannot simply be compared to a single allowable stress value. The interaction equations in the code identify the allowable stress limits. 


William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Tuesday, January 24, 2006 2:00 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Beam design

Santhosh,

First, I will agree that my personal belief is that if you cannot do it by hand (FEA/stiffness method type analysis of large structures being a clear exception...doing such by hand would take way too long and could be construed as torture under anyone's standards, Geneva conventions or not) than you really should not be doing by computer.

Second, I will say that I agree with Michelle that there are likely quite a few members in your structural system that likely should have minimal to no axial load in them (i.e. what I might call "neglagable" axial load).
This is where care must be used in making use of analysis results (regardless of what analysis package you use...you appear to be using STAAD).  Analysis results will give information based upon what assumptions and decisions that you make and if you are not careful then you can end up things like axial where maybe it should not be or could realistically be neglected.

Now, I don't know what you modelled nor what assumptions that you made when creating your analysis model.  Thus, it is possible that the "beam"
members that you have in your model have significant (i.e. not neglagable) axial and that they SHOULD have those axial loads (i.e. maybe you are modeling a moment frame).  So, while it is possible that maybe you have some members that really don't have or should not have axial load, I will assume that you do have some elements that require design for both axial load and flexural loads.

So, that being the case, I will again agree with Michelle...codes do NOT have interaction charts or examples, etc.  Such things are "tools" that help engineers "apply" the code provisions.  In other words, interaction charts or exmaples are items that essentially put the code provisions to work in format that makes your life (and mine and all other engineers) easier.  And like any tool, they must be used with caution as they can be used incorrectly.  Thus, I will offer my belief again that if you cannot do a design by hand, then you should not really being using "tools" (i.e.
tables, charts, computers, etc) or at a minimum you should be using them with extreme care.

Now, as to doing the design, I would encourage you to put a little effort into learning a little bit on your own first.  This will mean picking up and reading a text book or looking at some examples in some publication.
You have not said whether your beams are concrete or steel (or something else).  So, even if someone wanted to guide you to a design procedure, they would have to give you more than one as while many of the basic principles are similar for both concrete and steel for designing a beam with axial load, the "devil is in the details" (as the saying goes).

So, I would suggest taking a look at a textbook on either concrete or stell design.  If you are looking at steel members, then the AISC manual has some examples (as well as some "tools") for designing beams with axial loads (or columns with bending loads).  For concrete, a textbook is likely your best solution, but I believe that you could look at something like the PCA Notes on the use of ACI 318.  I also believe that many concrete textbooks will have some "generic" column interaction (P-M interaction) curves that you could use as "tools".

If after you have read a little bit and tried to learn a little bit on your own, but still don't understand some things about designing for both axial loads and flexural loads, then you will likely get more helpful responses if you come back and ask specific question about specific things that you don't understand.

And while I won't say it as harshly as others, you could also take a class or two that covers such material.  While I am not a person that believes that everything needs to be learned in school (engineers need to be ready and willing to learn new stuff "on the job" that was not covered in school due to time, etc.), it is certainly a good way to learn some stuff for the first time for many people.

I will agree to some degree with those who seemed liked they responded somewhat harshly in the form that I do believe that I don't really like really general questions when asked in such a way that it appears that the person asking may not have even bothered to spend a little time trying to figure stuff out on their own (I am not sure if I consider this to be true in this case or not).  I am a strong believer that one should try to figure stuff out some on their own before asking for help.  Too many times we get lazy and just assume that someone else will "spoon feed" us something when we likely could have figured it out on our own with a little effort.

So, I encourage to read a little but feel free to come back with some specific questions if you need/want.

Regards,

Scott
Adrian, MI


On Tue, 24 Jan 2006, Michelle Motchos wrote:

> My 2cents- I agree this is a scary conversation. What happened to good 
> mentoring (but that is a completely different diatribe)...
>
> Santhosh, The code does not "give charts only for bending moment 
> capacity" (at least not here in the US) because the codes contain 
> equations, not design aids (which are included for convenience "The 
> Manual" assuming you are working in steel).  Fundamentally, you need 
> to read the code sections on combined forces and apply the equations 
> for the frame members receiving axial loads and moments. As an aside, 
> unless every member you are analyzing is part of a lateral frame it is 
> unlikely that all the framing members would be subjected to axial 
> loads in everyday design. From my experience the load is generally 
> assigned to the diaphragm in the infill framing areas and they are 
> designed for bending only. That would be a modeling issue in stadd and 
> about all I use it for is single plane truss design so I can't provide 
> much other guidance.
>
> Michelle Motchos, PE
> Stevens & Wilkinson of SC
> PO Drawer 7. Columbia SC 29202-0007
> www.sc-sc.com
> mmotchos(--nospam--at)sw-sc.com
>
> -----Original Message-----
> From: Gary [mailto:gloomis(--nospam--at)masterengineersinc.com]
> Sent: Tuesday, January 24, 2006 12:57 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Beam design
>
> That is very dangerous - letting the computer do it.  If you can not 
> design a beam manually then you should never allow the computer do it.
> How do you if the computer is right?
>
> Gary
>
> -----Original Message-----
> From: Kevin Below [mailto:kbelow(--nospam--at)genio.ca]
> Sent: Tuesday, January 24, 2006 12:17 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Beam design
>
> Santhosh, it sounds like you might benefit from the Staad 
> post-processor which does the design for you.  It is called Staad.etc.
> I don't use Staad, et least not yet, but I think it will probably show 
> you the calculations involved, and you will be able to observe and 
> understand the process.
>
> génio experts-conseils inc.
> Kevin D. Below, ing., Ph.D. en Structures 290, rue Seigneuriale 
> Beauport, QC G1C 3P8 Tél. : (418) 660-6969 poste 272 Fax : (418) 
> 660-6463 kbelow(--nospam--at)genio.ca
>
>
> -----Original Message-----
> From: Santhosh Kumar Yedidi [mailto:sant527(--nospam--at)gmail.com]
> Sent: 24 janvier 2006 00:09
> To: seaint(--nospam--at)seaint.org; ShermanWC(--nospam--at)cdm.com
> Subject: Re: Beam design
>
>
> Dear Sherman,
>
> Can you please help regarding the beam design issue. I understood the 
> procedure of how to do beam design with axial force. I am working in a 
> consulting firm. We analyze any frame structure using staad. After 
> staad analysis, we get the member end forces. All member end forces 
> also contain an axial component irrespective of being a beam or 
> column.  So all beams contain an axial component. Then our beam theroy 
> is not so useful. because codes give charts only for bending moment capacity.
>
> So my question is why those people havent thought about this issue of 
> axial force. Can you suggest some generalization, which would help me 
> to design the beams using beam theory rather than again analysing each 
> beam seperately for combined axial and moment.
>
>
> On 1/10/06, Sherman, William <ShermanWC(--nospam--at)cdm.com> wrote:
> >
> > Bill Cain: that is an unfair comment.  Rizmirza provided a valid 
> > response relating to concrete design for flexure and axial loads - I 
> > didn't learn that method in school, but learned it "on the job".  
> > And a "basic structural analysis course" will not tell you how to 
> > design steel or concrete for such combined loads if it only covers 
> > "analysis".
> >
> > Santhosh: you need to be clearer about what you are asking.  Are you 
> > looking for detailed methods for a given material vs general 
> > analysis of combined loads?  Are you talking about steel design or 
> > concrete design or another material?  Your original question is hard 
> > to answer without understanding more detail of what you want to determine.
> >
> >
> > William C. Sherman, PE
> > (Bill Sherman)
> > CDM, Denver, CO
> > Phone: 303-298-1311
> > Fax: 303-293-8236
> > email: shermanwc(--nospam--at)cdm.com
> >  ________________________________
> >  From: bcainse(--nospam--at)aol.com [mailto:bcainse(--nospam--at)aol.com]
> > Sent: Tuesday, January 10, 2006 12:20 AM
> > To: seaint(--nospam--at)seaint.org
> > Subject: Re: Beam design
> >
> > Santhosh-
> > I would strongly advise you to enroll immediately in a basic 
> > structural analysis course. This question is too basic for you to be 
> > doing any design work. Regards, Bill Cain, S.E.
> > Berkeley CA
> >
> > -----Original Message-----
> > From: Santhosh Kumar Yedidi <sant527(--nospam--at)gmail.com>
> > To: seaint(--nospam--at)seaint.org
> > Sent: Tue, 10 Jan 2006 11:08:18 +0530
> > Subject: Re: Beam design
> >
> >
> > Exactly this is the answer I am waiting for, but then when you do a 
> > frame analysis all the members have axial force value. So then 
> > everything has to be designed as a column only. Thats why I was 
> > asking
>
> > this question.
> >
> > On 1/9/06, David Topete <davetopete(--nospam--at)yahoo.com> wrote:
> > > Turn the beam 90 degrees and it becomes a column with bending.
> > >
> > > Santhosh Kumar Yedidi <sant527(--nospam--at)gmail.com> wrote:
> > > How to do beam design with axial forces acting.

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