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Re: Base Plate for Moment Frame-Shear Transfer

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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,  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.

Rick Drake, SE

Fluor Daniel, Aliso Viejo, CA


                    om                   To:     seaint(--nospam--at)                                                 
                    08/22/02 11:27                                                                                 
                    PM                   cc:                                                                       
                    Please respond                                                                                 
                    to seaint            Subject:     Base Plate for Moment Frame-Shear Transfer                   

A question came up today when designing a base plate for a steel moment
frame which resists lateral forces.  We  are considering using a standard
base plate with anchor bolts and 1 1/2" of grout.

How is shear considered to transfer from the base plate, through the grout
bed and into the footing?  Do anchor bolts act in bending, or does grout
adequately transfer the load thru interlock against steel and concrete.

Or, should a different method be considered, such as embedding the base of
the column into the footing.

Thanks in advance...

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