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structural steel bolted connection question

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When looking single shear bolted connections (similar
to but not exactly like single-plate shear tabs) you
have to check a nuumber of things to determine the
connection capacity.  My question is this: How do you
handle block shear calculations when you have a
connection with only a few bolts and relatively small
shear, but you have a big eccentricity?  The normal
thinking with block shear calculations in shear
connections is that you have primary load vector in on
direction and you check the block shear resisting that
applied force vector.  When you have a connection with
a big eccentricity you (potentially) could have force
couple provided by the bolt group resisting the P x e
moment where the equal and opposite horizontal forces
(comprising the resisting force-couple) are
substantially larger than the primary applied vertical
force vector.

Example: (2)-7/8? diameter A325N bolts, s=3?, e=12?,
angle = 0 degrees: Per Table 7-17 in the 3rd edition
LRFD Manual (page 7-38) C=0.24.  The capacity of my
bolt group is 21.6k x 0.24 = 5.18k (note: I didn?t
check bearing on the bolt holes yet and that actually
leads me to another issue that I?ll discuss shortly.) 
If I actually had a connection reaction of 5k I would
conclude that my bolt group is OK and I would move on
to the block shear calculation.  If I had a shear
reaction of only 5k, my block shear capacity (as
normally computed for ?normal? shear connections)
would most certainly check out as being OK.  In
reality though, this is not really a ?normal? shear
connection. Although the shear reaction is only 5k,
the actual shear in each bolt is closer to 21.6k! and
the primary force vector acting on each bolt is
horizontal (not vertical).  I would say that doing a
standard block shear check for the 5k vertical
reaction would be totally bogus.  In reality I would
have to look at each of the two bolts and look at the
block shear action on each bolt individually (because
the forces in the bolts are equal and opposite).
Likewise when I check the bearing strength at the bolt
holes (section J3.10) I would have to look at a 21.6k
(almost) horizontal force in each bolt, not a 2.5k
vertical force in each bolt.

I used an exaggerated example in describing my
question.  I used a connection with a very small
vertical shear and a relatively large eccentricity. 
The question I?m asking however is the same even when
the eccentricity is smaller and the reaction is
larger. At what point do you have start thinking about
the fact that the resultant resisting force provided
by each bolt in a single plate shear connection is not
acting (primarily) in a direction opposite to the
reaction on the connection?  At some point the
?normal? shear connection block shear calculation
seems to be no longer valid.  Additionally, it seems
obvious that when you have connections with large
eccentricities you have to look at the resultant force
on each bolt hole when checking bearing on the bolt
holes ? not just a force equal to the total reaction
divided by the number of bolts.

Any thoughts?


Cliff Schwinger

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