bolt group moment of inertia

huangyhg

bolt group moment of inertia
can anyone give a basic explanation of how to calculate the moment of inertia of a bolt group?
i need to determine the size of the rebar in an anchor bolt cage that is to be embedded in concrete.
thank you.
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i have seen two different models.
one about the centerline of the bolt pattern
and the other about the furthest bolt in the
bolt pattern.
i believe that it is fairly simple to calcuate the stress in one bar using the parallel axis theorum and my/i, though i am not sure how to select the quantity of rebar needed, i.e. the spacing.
thanks.
ix=y^2
iy=x^2
ip=ix+iy
for a bolt group, the ip is the sum of all individual ip of each bolt that makes up a group.
ipgroup=sum(ix)+sum(iy)
the force due to moment that acts on each bolt then becomes:
fx=m*y/ip
fy=m*x/ip
fres=sqrt(fx^2+fy^2)
asixth-
i believe what you are showing is for a torsional moment (or an eccentric shear) on a bolt group.  i believe the op was asking a true moi of the bolt group.
transmissioneng-
is that true?  which are you looking for?  for the moi used for calc'ing tension in a bolt, find the sum of ab*(d^2) for the bolt group about the axis in question.  d is the distance from the line of bolts to the centroid of the bolt group.  this is the parallel axis theorem that you talk about.  the actual i of the bolt is ignored.  do not add ix and iy.
i believe the solution provided by asixth applies if the moment is about an axis parallel to the axis of a bolt  if your bending moments are about an axis perpendicular to the axis of the bolt, find the center of gravity of the bolt group in each direction.  then i=σad2 in each direction.
basically i am trying to calculate the number of anchor bolts required for a steel pole with high overturning moments at the base.
thanks.