baseplate for a rectangular precast concrete column

huangyhg

baseplate for a rectangular precast concrete column
i am sizing up a baseplate of a 800mm by 600mm (31.5" by 23.5") precast concrete column for a compressive axial force of 5000 kn (1125 kips.) i am using an equation found on the cpci manual (i believe it's the same equation on the pci manual.) please see attachment for equation and a diagram. the value of xc is 113mm (4.45"). phi a is 0.9.
i am getting a baseplate thickness in excess of 100mm or 4" using the aforementioned equation. that is definitely not right but i am not sure where i went wrong.
does anyone know how the equation shown in the attachment is derived? what is the theory behind it? are you getting a similiarly absurd number as i am?
hmm..   
clansman
if a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." code of hammurabi, c.2040 b.c.fff">
errr...missed the part where it says that this is for an ungrouted baseplate. my load should be much less (erection loads only.) my mistake.
how would i size up the baseplate for a grouted condition?
clansman
if a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." code of hammurabi, c.2040 b.c.fff">
for a baseplate of width b and thickness t, the plastic modulus, z = bt2/4 and the resisting moment = phi*z*fy.
if the baseplate is not grouted, the factored moment, mf is pf*xc, so phi*z*fy = pf*xc.  pf is equal to the load of two anchor bolts (the greatest load on one side of the column).
this can be stated phi*fy*bt2/4 = pf*xc
so t = (4*pf*xc/(phi*b*fy))1/2
if the factored load on the column during construction is 1000kn and the load is concentric, then pf = 500kn and mf = 500*0.113 = 56.5kn-m (not quite correct, but close enough).  by the way, why is xc so large?  do you really need four inches from center of anchor bolt to edge of concrete?
this leads to t = (4*56.5e6/(0.9*600*350))1/2 = 35mm or about 1.5".  if you reduce xc, thickness will also be reduced.
for the grouted condition, there is nothing further to calculate.  the baseplate is critical during construction and will not be required to do anything when grout has been placed.
  
ba
why does a precast column need a baseplate?
it needs a baseplate to fasten it to the foundation.   
ba
dont you use grouted ducts for that? or is it too small.
this is what i have used in the past:
thank you for the great response baretired.
i found that tension due to integrity load as per cl. 16.5.2.4 on can/csa a23.3-04 governed my design.
you are right, i don't need such a large xc/xt, i reduced it and calculated a smaller baseplate thickness.
thanks again!   
clansman
if a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." code of hammurabi, c.2040 b.c.fff">
csd77, i've never seen that before. interesting.
every parkade i have seen utilized baseplates or ready-made column shoes from manufacturers like peikko for example.
clansman
if a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." code of hammurabi, c.2040 b.c.fff">
i used it on a precast residential building and the company i worked for used them all the time.
i have not used them in a seismic zone.
starter bars need to be accurately placed, and getting sufficient cover around them can be tricky but if you can get past that they work great.
different ways of doing things.  i agree with csd72, using grouted ducts is the way to go in building columns.  i have only seen bolted splices used in precast power poles.