Post Baseplate Hold Down 2

[asixth]

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?

 

[Ron]

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.

 

[kslee1000]

Ron has this subject well covered.
Additional thought is to carefully arrange reinforcing around the post to take on the shears.

 

[racookpe1978]

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.

 

[apsix]

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.

 

[hokie66]

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.

 

[kslee1000]

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)?

 

[lkjh345]

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.

 

[asixth]

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, remembering that the slab is post tensioned so it increases the punching shear capacity.

 

[msquared48]

Kill the Architect!

Mike McCann
MMC Engineering

 

[beton1]

Be careful what you wish for... the replacement or junior
might be worse!

 

[civilperson]

90 kips, (uplift) on 3.5" thick slab requires 100 linear inches of perimeter for shear. (10" square or bigger base away from edges).

 

[csd72]

I would definately go the embedded plate and remove the recess.

 

[asixth]

I like the architect on this job

 

[asixth]

Good point with punching shear through, when designing as a slab, the punching shear is a function of the shear perimeter, shear strength of the concrete and the effective depth, d. For a post-tensioned slab with no reinforcement, what values do I take for effective depth, the overall depth of the section? Or should I be designing as a plain concrete member.

The section is stressed so this increases the punching shear capacity quite significantly and I was satisfied that punching shear will not be an issue.

 

[apsix]

I don't know about PT slab design, but adding some mesh or bars local to the would be recommended (my me).

Is there scope for reducing the grout thickness/recess depth?

 

[hokie66]

asixth,

I wouldn't count on an allowable increase for punching shear in such a thin area, especially if there is not reinforcement passing under the post. Punching shear provisions are meant for the supporting columns, where there is a substantial amount of prestress present in the immediate vicinity.

What objection do you have to deleting the recess and welding the post to an embedded plate?

 

[msquared48]

asixth:

NO... You would like the architect on ANOTHER job!

Mike McCann
MMC Engineering

 

[asixth]

hokie66,

Surely the code provisions for punching shear are valid for concentrated loads, I image the post punching through the slab in the opposite direction to a column punching through the slab.

However, it sounds like punching shear will be an issue for the detail when looking at how many people are commenting on the concept of the detail and I am looking for the best solution.

Welding a post to an embedded plate...

This would me casting-in the plate with the slab and then site welding the column to the plate. I always thought we should try to avoid site welding wherever possible. If the recess is unavoidable, I would prefer to thicken the slab locally around the baseplate than to cast-in anchors and site weld, I think they will definatley mess things up somewhere.

 

[hokie66]

asixth,

Just because the slab is post-tensioned does not mean that significant or uniform compression exists in the slab at all locations. In order to benefit from any applicable code provisions allowing an increase in shear stress, you must be confident that the compression is actually there.

As to site welding, that is a matter of judgment, and dare I say it, prejudice. I can think of lots more problems associated with mislocated anchor bolts than with site welding to cast in plates, particularly when the location is within the body of a slab remote from control lines. I prefer to cast in a fitment with a top and bottom plate, nailable to the form, thus not susceptible to moving during concrete placement.