post baseplate hold down

huangyhg

post baseplate hold down
hi guys,
i have a situation where there is a 160mm (6.5") thick slab with a post baseplate sitting in a 70mm (3") recess. i need a minimum of 110mm (4.5") embedment for the anchors which isn't possible, so my options are to either provide:
1) a backing plate on the underside of the slab, visible in the carpark under which the architect doesn't like or
2) thicken the slab locally around the column baseplate. the slab is post-tensioned.
which of the two options mentioned above would be the best solution from a constructability point of view?
a backing plate would be much better.  to thicken the slab and then bolt through the thickened section, you'll be stressing the bond when the plate is loaded.  even well done overlays will debond at times, so it's "iffy".
a backing plate could be made to unobtrusive.  you could actually have threaded studs welded to the underplate, insert through pre-drilled holes through the concrete, and bolt to the post plate.  i would use an epoxy grout to fill the pocket that was formed as that will only catch moisture and debris and cause problems with corrosion and other issues.
if you use a stainless steel underplate, you won't get corrosion stains...which is one of the things architects don't like.
ron has this subject well covered.
additional thought is to carefully arrange reinforcing around the post to take on the shears.  

definitely baseplate underneath.
the pullout "cone" problem will also be eliminated: if you add an upside down "pad" of concrete under the main slab, you'll need to go more than 4" larger on all sides:  far enough outside of the original outlines of the steel baseplate on top for a 45 degree cone 6+ inches deep to surround each anchor bolt.   
and even that messy (ugly) solution won't be relatively strong (bonding between new pad and old concrete is difficult to guarrantee.)  
and it will be relatively expensive for the forms and labor.   
i'm assuming that the slab is not yet poured.
have cast-in bolts or ferrules (threaded inserts) been ruled out?
instead of the loose backing plate you could look at the anchors being welded to the plate.
fix the plate to the formwork prior to pouring for a flush soffit finish.
i agree with the plate on the slab soffit, but if you have a 70 recess, you will only have 90 concrete to resist your post load.  not much.  i would rather see a plate both bottom and top of the slab, fabricated together with a lump of steel between, and the post site welded to the top plate.  no recess.
what is the purpose of the recess? is it going to be grouted to cover the base plate and anchors later? what the post service (what loads it takes)?
jmho:
if the slab is not yest cast,i would investigate using an embed plate ( with  appropriate anchors or welded rebar anchors)  that is flush with the top of the concrete, and welding the post to the embed after the slab is cured. this would be similiar to how a lot of precast connections are done.

the post is supporting a lightweight steel roof with a dead load of 20tonne (200kn) and wind reversal of 40tonne (400kn).
i did some checks on punching shear of the 90mm section and i though it was ok, re  
kill the architect!
mike mccann
mmc engineering
be careful what you wish for... the replacement or junior
might be worse!