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RE: Column Design

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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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