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RE: base plates subject to uplift

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While I generally concur with the 4 possible mechanisms for 
shear transfer at columns outlined by Rick Drake, I do not 
concur with his conclusions relating to mechanism 2, "bearing 
between base plates holes and anchor rods". It is this 
condition I frequently encounter and I do not feel it is 
appropriate to assume bearing on the anchor rods if 
oversized holes are used as allowed for by AISC. 

Shear often occurs as a result of wind or earthquake which 
are reversible loads. I would not want my base plate moving 
back and forth under changing loads the amount an oversized 
hole would allow, as I would expect degradation of the grout 
and "creaking" of the structural steel. Thus I feel that 
oversized holes do cause a problem in shear transfer. 
Unless I provide shear lugs, I tend to prefer 
standard holes (+1/16") with anchor bolts set using steel 
templates and/or use of bolt sleeves. (Another possibility 
is welded plate washers.) Alternately,  if you 
assume some bolts will go into bearing due to not being 
perfectly centered, then not all of the anchor bolts can 
be used to resist shear. 

Regarding pretensioning of anchor bolts, I tend to avoid this 
due to concern with reliability - will long-term losses negate 
the pretension force? With wind, I don't mind using column 
compression to resist shear in friction, but often light 
steel structures have low compression due to roof uplift and 
due to overturning forces due to wind. 


> Rick Drake wrote:

> ... the question as to how shear forces actually get from the
> column or brace into the foundation.  There are several 
> possible mechanisms: 1)
> friction between base plate and supporting grout or concrete, 
> 2) bearing between
> base plates holes and anchor rods, 3) shear keys, 4) bearing 
> between far edge of
> base plate and supporting grout or concrete.
> 
> Mechanism 1) is probably the initial load path, especially if 
> the anchor bolts
> have been pretensioned.  Unless the shear force is 
> accompanied by enough tension
> and or overturning moment to completely "uplift" the base 
> plate, this mechanism
> will probably resist the entire shear force.  However 
> friction cannot be
> considered when resisting code earthquake loads, and another 
> design calculation
> method must be utilized.
> 
> Mechanism 2) is usually considered in design and is probably 
> sufficient
> consideration for light shear loads.  It represents the shear 
> limit state if the
> base plate has overcome friction and has displaced relative 
> to the anchor rods.
> The anchor rods are usually checked for combined shear and 
> tension.  you could
> also check the anchor rods for bearing, but usually the base 
> plates are so thick
> that this is not a problem.
> 
> Mechanism 3) should be considered for heavy shear loads, 
> although welding and
> construction issues are raised.  If a shear key is used, it 
> is probably both the
> initial load path and the shear limit state.  If tension 
> and/or overturning
> loads are present, anchor rods need to also be provided to 
> resist tension
> forces.
> 
> Mechanism 4) requires base plate bending and/or resisting to 
> mobilize and should
> not be relied on.
> 
> In summary, I don't think that the oversize holes cause a 
> problem in shear
> transfer.  Most of the shear force will probably transfer in 
> friction, even if
> the codes do not acknowledge it.  If all of the anchor rods 
> were perfectly
> centered in the base plate holes (not very likely), the 
> column base plate would
> have to laterally displace half the annular space to mobilize 
> the bearing limit
> state.  Even with the AISC oversize holes, this distance is 
> not very much.  Of
> course in the real world, where all of the anchor rods are 
> not centered, the
> travel distance will be much less before the first rod engages.