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Re: Column Design

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Dear David Maynard,

Thank you, your answer is quite satisfying. I wanted to know how
exactly things are done. But one thing I could understand from this
discussion is taking maximum values of the end reactions may not be
correct. Since there is a chance of maximum values occuring in between
the column length.

Thanks
Santhosh


On 12/29/05, David Maynard <davemaynard(--nospam--at)vcn.com> wrote:
> OK, if I get this correctly, what you are saying is that you are getting
> your maximum axial load at one location, your maximum strong axis bending at
> a different location, and your weak axis bending at another location.  So,
> three loading considerations all at different points along the length of the
> member, and you want to know which one to design for???
>
> That's simple.  Check them all.  At each location, one loading situation is
> at a max and the other two are there, but to a lesser degree then somewhere
> else.  So, what you will end up with is three separate loading combinations
> that will look something like this:
> Location A:
> P = Max
> Mx = magnitude at location
> My = magnitude at location
>
> Location B:
> P = magnitude at location
> Mx = Max
> My = magnitude at location
>
> Location C:
> P = magnitude at location
> Mx = magnitude at location
> My = Max
>
> You check all of these load scenarios for your member with the appropriate
> design parameters (unbraced lengths, K values, and what have you), and look
> at them all as beam-column interaction.  If all combinations are acceptable,
> then you have successfully designed your member.  If one combination fails,
> then you obviously have to do a little more looking around.  Implementation
> of a spreadsheet would work very well in a case like this.
>
> I have used STAAD in the past for analysis and applied my own design using
> the results.  Within STAAD, and arguably one of the most powerful tools for
> me, is an input called "Beam Force Summary."  This output will give the user
> all of the maximums and minimums along the beam length, or a group of beams,
> at the specific location.  Along with that maximum, it will give the
> corresponding loads at that point as well.  It makes getting the loads from
> my analysis a lot more efficient with way less headaches.
>
> Now, if you want to take a conservative approach to it, you could take the
> maximum axial, strong axis and weak axis bending, and design a section based
> on those, with design parameters indicating the maximum unbraced length,
> conservative K-values, and Lx and Ly at a maximum for the length.  With
> doing this, your section will definitely be on the conservative side and can
> likely be standardized where you feel applicable.
>
> I hope this helps in answering your situation.
>
> David Maynard, PE
> Gillette, Wyoming
>
> -----Original Message-----
> From: Santhosh Kumar Yedidi [mailto:sant527(--nospam--at)gmail.com]
> Sent: Thursday, December 29, 2005 4:27 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Column Design
>
> ) Yes it means one set of forces at each end. There are two ends of
> the column for which we get a total of two sets of forces.
>
> ) We can get the bending/SF/axial diagrams for each column from the
> forces at their ends.
>
> For example, a column of height 10 m is there. We get the bending
> moment diagram in both the directions (say X and Y). Let the maximum
> bending moment in X-dir occur at  2 m from the bottom and max. BM in
> Y-dir occur at 5 m from bottom.
>
> We get the axial force daigram, Let the maximum axial force be at the
> bottom of the column.
>
> But all these are not at the same location from the bottom, so which
> section of the column is critical for design.
>
> And another thing, let us suppose we have 500 columns in a structure.
> Then we cant do bending/shear/axial diagrams to find max
> bending/shear/axial for each column.
>
> The general procedure I saw is one takes maximum of the column
> reactions for design. But I feel one should also consider the
> direction of the moments at both the ends. If they are in same dir how
> to take their effect, and if they are in opposite direction how to
> consider.
>
> Santhosh
>
> On 12/29/05, Polhemus, Bill <BPolhemus(--nospam--at)wje.com> wrote:
> > It isn't clear to me what you mean be "not possible to get the
> bending/SF/axial diagrams." If you know the forces at each end of the
> column, and you know the lateral loading on the column if any, of COURSE you
> can get the diagram. Unless this is a response spectrum analysis?
>
> >
> > When you say "two sets of forces at each end," do you mean "one set in
> each of two orthogonal directions" or should this read "ONE set of forces at
> each (of two) ends"?
> >
> > More information is required.
> >
> > ________________________________
> >
> > From: Santhosh Kumar Yedidi [mailto:sant527(--nospam--at)gmail.com]
> >
> > After frame analysis we get two sets of forces at each end of a
> > vertical member (column). But while doing design we generally require
> > only one set of forces. So can any one help which set of forces have
> > to be considered.
> >
> >
>
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