huangyhg

simple structural question, about section properties
can someone plese tell me whether an angle, i.e. a l shape has a plastic neutral axis. because i have just used a program which gave ( second moment of areas ) zpl,x & zpl,y, as both equal to 0 ?
find a job or post a job opening
yes, angles do have z values - check out the aisc steel manual tables for angles.
this is in metric - but you can search for others:
jae, thanks for your reply.
however, you didn't read the question properly.
the question is, do angles have a plastic neutral axis, in addition to an elastic neutral axis ?
yes, they do of course.
look in the site below, under beams -> cross sections -> angles : you'll get what you are looking for. however only plastic neutral axes locations along geometrical axes x,y are given.
prex
prex, thanks for your reply.
i can see that the person that has written that small program has included the plastic nuetral axes.
but this really still doesn't proove that they are permissable for use in angles.
i was really wondering if the codes of practice, in the usa, or europe, allow angles to have a plastic z value ?
thanks again.
bertyboy - i guess i don't understand your question. to have a value of z - this implies that the entire section has gone plastic and there is an axis where the tensile plastic zone switches to a compressive plastic zone.
what prex showed in the link is that you would have a geometrical axis (x,y) and aisc also describes principal axis - both of which can be applicable based on the nature of the bending and bracing.
it sounds like you need to dig out some paper and a pencil and try it out for yourself. i wouldn't think that would be terribly difficult. force equilibrium of the section right? tension force = compression force, solve for the location that gives these results.
check out this link, it was the first site in a google search...
i think bertyboy's question is whether codes allow the use of the plastic section modulus of angles to determine the section moment capacity.
referring to the australian code as4100, the plastic section modulus can only be used if the section is compact (ie. element slenderness limits are not exceeded). there doesn't appear to be any limitation on shape.
i can't speak on other codes.
no, he is asking if an angle has a pna. apparently his program isn't working, or something like that.
apsix, thank you for your valuable post !!!
that is indeed what i am asking !
any shape can have a plastic neutral axis, and therefore a plastic z value ( second moment of area ), but the question is, is it valid to use it for the case of angles.
bertyboy,
most handbooks do not give a z value for angles because it is not standard practice to design angles and other non-doubly symmetric shapes for plastic moments. it it very difficult to ensure stability though the whole range of plasticity of such a shape.
it's hard enough to get yielding at the extreme fibre (s) for non-symmetric shapes...
by the way, z is the first moment of area, and i is the second moment of area.
z = sum [area x (distance of the centroid of this area to pna)]
the plastic neutral axis is sometimes also called the equal area axis - you have equal cross-sectional area above and below it.
tg

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]