Baseplate Punching Through Slab

[asixth]

Hi guys

I have detailed a baseplate with some reasonable bearing loads (100kN...22kips) which appears to be punching through the soffit of the slab (I am told it is getting worse).

I have detailed post-installed anchors with a 50mm (2") cover to the soffit. As it is, the baseplate bears on levelling nuts and the grout has not been installed, so all the compression load is going into the anchors. Judging by the cracking on the soffit, I am guessing that the holes where not cleaned out before the adhesive was injected and as a result, no bond has developed between the anchor and the concrete. So the compression load is transferred to the slab through end-bearing of the anchors resulting in the cracking that is observed on the soffit.

What is the best way to remediate the problem. Would it be to pull up the column (assuming the adhesive has not bonded) and re-install the anchors, or do I just need to get the grout underneath the baseplate asap and accept the extensive crack pattern on the soffit.

If the adhesive has not bonded than it creates the issue that there will be inadequate hold down for uplift conditions.

Any help will be appreciated.

 

[doka1]

demayeng are you serious? what you attached is the failure method in tension. the original poster mentioned that the column was set on nuts...and therefore (as mentioned) the column load was bearing solely on the bolts (be it at the end or on the nuts themselves).

 

[oldrunner]

Obviously the load from the column was being supported by the bottom of the anchor bolts. We’ve designed a number of anchor bolts this way, but have always checked punching shear through the bottom of the footing or slab in this case.

To insure that the vertical down load is removed from the bolts, I would have suggested expansive grout. We’ve also used this to transfer large loads from one entity to another.

Apparently you have already cracked the bottom of the slab and that should be repaired, possibly with an epoxy grouting procedure.

 

[BAretired]

Yesterday, asixth stated:

Extra steel was not detailed for the baseplates, is this a typical practice to put additional bars beneath concentrated loads such as steel column loads.{/quote]

I believe it is usual practice to place a few extra bars each way on the bottom of the slab in these circumstances. But we don't know how much reinforcement was used. Judging by the crack pattern, not enough. So it is probable that more bottom reinforcement is needed under the column for a short distance in each direction.

The present anchors may not be adequate to resist uplift and this has to be remedied.

Probably the best measure would be to place shim plates under the baseplate, remove two diagonally opposed anchors, drill through the slab and anchor with two through bolts. Then add two angles back to back on the underside of slab anchored back up to the thicker slab with through bolts, countersunk on top to permit placement of terrazzo tile. If the two uplift bolts can engage the angles, so much the better.

The above repair is dependent on the architect and owner agreeing to the steel angles below the slab. If that is not acceptable, it's back to the drawing board.



BA

 

[demayeng]

doka1: yes i'm serious. I attached the failure mode in tension because it shows the force transfer to the concrete through the epoxy bond.

The nuts are used for levelling so the compression is not loading the underside of the nut, it's loading the epoxy bond or the end of the bolt.

Either way, when the bolts are transferring the load there's very little concrete depth between the point of loading and the slab soffit. Grouting takes the load point back to the top of the slab and spreads it out over the baseplate (not just on four points).

I'll be very interested to hear the results of the bond test. If the epoxy bond had failed and the bolts were end-bearing prior to grouting, you should be able to turn them with a spanner.

 

[rowingengineer]

I'm thinking these cracks are shrinkage cracks. Given the square cracks under matches the recess over in size. I would guess the slab is fairly well restraint at the edges, and under the recess was not properly vibrated. I would check the punching shear without the concrete help and if that is ok, then i would got with just grouting up the column. if you need more steel, to make this work, I may consider FRP (but I have stocks in FRP).

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[BAretired]

Shrinkage and flexural stresses combined, maybe. FRP may be a good solution. I have never used it, so don't have any experience. Would it be practical to apply FRP to a section, say 8' x 8' centered on the column on the bottom of the slab?

But you still need to get at least two of the anchors penetrating the slab for dependable anchorage against uplift.

BA

 

[hokie66]

RE,
What do you mean "check the punching shear without the concrete help"? The concrete is what resists the shear.

I don't think it is necessarily standard practice to add bottom bars under a steel column, provided the typical steel is enough to resist the bending. In uplift situations, adding top bars under the column is often required.

I don't think there is any research showing that FRP helps with punching shear resistance, and I doubt that it helps much.

 

[rowingengineer]

Hokie normally when you query someone’s post it normally means they’re talking out there a$$, and in this case you maybe correct. Even I am confused at to what I am thinking plus my last post was confusing at best, so I will expand my train of though and feel free to tell me to put my hat back on if I am talking **** .

But the way the ASSUMED the situation is, based on one photo and one diagram.

The theory: The recess has caused shrinkage cracks to develop around the edge of the recess.

The problems: Conveners for punching shear (but really I should call it shear) in both uplift and down thrust. The reason for my concerns are that the cracks in a picture (without any scale) look like they have been leaching a fair amount for a 6 week old building in a town where it has hardly rained. Based on this I think the crack widths maybe the same from the top of the slab to underside, so I am concerned about the ability of the concrete to transfer shear between the aggregates. Now onto the second part of the deal, Asixth has indicated that he didn’t put extra steel under the base plate, as we have discussed before the steel and concrete for a tie/strut model to form punching shear resistance (if you were to keep the spacing of the seel at say 200 crts around the recess the best amount of steel you have to resist punching is 1-N12 or something like that, so my concern is that if the concrete is cracked how well does the strut tie form, for the small floating piece of concrete. The way I see it is that you would need a large amount of deflection to get the cracks to form a compressive band form the steel sounding the recess. Thus I see it is that the concrete would need to transfer the load form this recess to the sounding concrete by agg interaction like a joint on ground.

Solution: I suggest checking the shear at the edge of the recess ignoring the concrete, and check dowel action alone, and if this is ok, everything should be great, but if not, put a piece of FRP on the bottom to ensure the cracks can transfer the agg shear interlock.

So who thinks i am barking up the wrong tree?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[asixth]

I have been looking into FRP lately (as you may be aware from some of my other posts). Unfortunately load tests where not able to be performed so I am going to drill and epoxy some additional anchors bolts to the baseplate with strict supervision. I have detailed 16 dia bars at 300 crs (#5 at 12") so it is likely that we have a minimum on 1 bar passing underneath the baseplate.

I wish I could have my time again with this slab because I would do many things differently. Grouting the baseplate earier, detailing additional bottom reinforcement for the baseplate and going to mechanical anchors.

The guys on-site tell me that the prefer using chemical anchors.

 

[hokie66]

rower,

I see where you are coming from. The difference in our thinking, and I don't know if I am correct, is that I am assuming that the design model punching shear perimeter is outside the cracked area. Asixth hasn't gotten back to us on that yet.

I think his reasoning for how the cracks formed is sound, if the cracking forms roughly a 200 square. The bolts tried to punch through individually. The cracks may be a combination punching/shrinkage/flexural, but initially caused by the high concentrated loading near the bottom surface.

Not sure of the source of the water, but recesses like that tend to stand full of water for a while. The cracking made it easy for the water to get through.

Whatever the reason for the cracks, I think reinforcement in the form of a bracket spreader on the soffit is required.

 

[asixth]

The crack pattern that you see is approximately 250mm square which is outside the 370mm crack pattern that hokie mentioned, but the force transfer would be from end-bearing of the bolts opposed to bearing through the baseplate so the 370mm punching shear perimeter would be reduced.

 

[BAretired]

asixth,

Your sketch doesn't look as if there is much room to pack grout under the baseplate. Are you planning to use a grout of flowing consistency so that you can simply fill up the recess to the underside of baseplate?

The only point in using FRP on the bottom of the slab would be to make up any deficit in bottom reinforcement. It would not help punching shear directly, but might tend to limit the spread of flexural cracking.

Is uplift a probable loading condition and, if so, do you have top reinforcement under the baseplate?

BA

 

[rowingengineer]

Asixth,
Just a side note, I would have used a cast-in plate (as long as the column isn't to high in height and weight) or cast in bolts. While contractors love chemical anchors, I am yet to see someone follow the installation method correctly. I even installed some at while back and tried to follow the manual to the word and guess what, They still failed the load test. I made the mistake of putting the chemical in the hole and putting in the anchor to quickly (sledge hammer) and most the chemical came back out.

most of us as we get more experience wish we could have our time again with some designs, I was designed a slab 200 thick with shear ties in it, I will never do that again.


Hokie,
your probably right i am stretching the plausibles a bit far. but the bolt punching isn't sitting well in my head, I have three things that don't make sense.

1. The leaching doesn't make sense, lets say the slab was poured and then the holes for the chemical anchors were drilled and the chemical was added but not correctly and the bolts installed, I would expect the chemical and bolt to reduce the amount of leaching near the edge of the square, but the main leaching seems to be in the corners.

2.The cracking pattern while mainly square, it has some really weird cracks coming from the edges, which can be explained as post punching bolt shear shrinkage cracks i guess, or flexure but only if the slab is a two way slab spanning.

3. Asixth seems to think they are getting worse, I would only expect this if the load was increasing from the first punch.

have we asked the form worker when the cracks appeared? it is his job to know these things for the first 4 weeks from the slab being poured.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rowingengineer]

A question, if you wet the top of the slab do you see the same crack pattens around the recess?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[dik]

Base plate 8x8 or 10x10? Can you post a sketch of the actual bay and reinforcing?

Dik

 

[doka1]

demayeng, sorry i see where you were coming from....and do not disagree. but there were would be cracks inside the failure cone, dont forget.

rowingengineer, you should try and set anchor bolts within 1/4" when placing a slab. Its all and well when you building a foundation with formwork to support off of, and a controlled environment. In that instance its very difficult to support the bolts, and depending on the site to survey and secure them.

when using epoxy, we typically blow out with the compressor which is usually the only issue. I probably should add, if we hit rebar upon drilling a contractor trick is to use white vinegar when drilling, it eats up the rebar. (i realize what it could compromise too, but just thought you guys should know)

 

[Prestressed Guy]

Blowing out the hole with a compressor is insufficient preparation. You need to blow the dust out of the hole by inserting a tube to the bottom of the hole. The hole also needs to be brushed with a nylon brush. Repeat both steps until no dust is removed.

Improper hole prep is the reason that adhesive anchors need continuous special inspection in high seismic areas.

 

[hokie66]

They need continuous inspection wherever they are used in tension, seismic or not. It rarely occurs, so why use them? They work fine in shear, but not in tension.

 

[rowingengineer]

doka1,
Well aware of this fact, but chemical isn't the answer, I have used cast-in ferrals nailed to a form board before, for those who are to scared to set HD bolts in a S/S slab (noramlly I detail the base-plate of the column with large oversized holes and washers such that a bit of misalignment can be accommodated for HD bolts in this situation, just because concrete love to kick things), however they the casein ferrals are more expensive.

I always spec load tests if i have no other option than to use chemical anchors in tension, but then i would use epoxy anyway.



Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it