huangyhg

center of gravity
is there a simple way to calculate the cog of a 3d structure? i am using staad's cog command, but they only take into consideration the properties - not the externally applied loads.

what have externally applied loads got to do with the center of gravity ? to all intents and purposes, center of gravity = center of mass which depends only on the density.
i must agree with englishmuffin. read staad's cog command and understand it! cog in staad commands returns the location of the cog for your structure model! it has nothing to do with the applied loads.
now i think may be what you are looking for is the cog of a group of loads applied to a structure. to answer that, you can do it by hand. i do not think staad would do that for you.
as always, please understand the software that you are using before use it!
good luck.
lutfi : sorry, but there is no such thing as "the c.o.g. of a group of loads".
like englishmuffin, i am not aware of a "c.o.g. of a group of loads" but you could have a single equivalent resultant force from a group of loads.
so how do you find the cog of a structure? what is the typical procedure?
if a body is in tranlational and rotational equilibrium under exactly three forces, there is a unique point through which they all must pass. however, this is not referred to as the "center of gravity", although it could happen to coincide with the cog in special cases. for any other number of forces acting on a body, in general all you can say is that they can be reduced to a single force (the resultant) at some arbitrary point, together with a couple.
sorry - missed your post - was posting at the same time.
the center of gravity, for all practical purposes, is the same as the center of mass.
the center of mass can be defined as follows :
"that point in a body about which the sum of the mass moments of all the individual masses constituting the body is zero".
so if there are j individual point masses denoted by m(j), and the vector position from an arbitrary origin be denoted r(j), the center of mass is located r from the origin, where :

r* sum(m(j)) = sum(m(j)*r(j))
so in the most general case, you would have to find it using calculus or numerical methods. however, if you have a body composed of masses with simple individual shapes having known centers of mass, you can find it quite simply.
by the way, there seems to be a lot of confusion about this rather basic topic, and there is presently another thread which someone has started on the subject :
by the way again :
although i don't know anything about staad, i can't believe that staad's cog command won't give you the center of gravity, if that is indeed what you are trying to find.
i am not sure what lippie is talking about. staad can certainly provide the cog of a structure. use the command print cg. that's it!

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