Baseplate Punching Through Slab
Baseplate Punching Through Slab
(OP)
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.
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.






RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
What is the purpose of column, can it be relocated without pocket.
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
Any chance of getting a scale/reference on the photo? I can also see some white substance, so i take it this slab has been in place long enough to get wet a few times or more, so how old is the slab?
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
I would have the contractor install shoring directly under the column to prevent it punching through. Then weld a spreader channel to the column for temporary support while you work out what to do. Looks like you need a support bracket on the soffit to spread the load.
Those lines look to me like construction joints in the slab. If that is the case, I think the cracking started as differential shrinkage cracking and was further accentuated by the stress risers created by the setdown.
RE: Baseplate Punching Through Slab
Also, review if for whatever the reasons loads may be higher than computed. Sometimes spurious load paths appear upon the construction process; or simply, known discharge of loads (such of various floors on the lower) on stiffer floors may be being incorrectly dismissed (this is unlikely at so modest load, looks as something constructive, or damaged from the start) or there may be some impact or whatever.
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
Yes I checked punching shear, there is bottom reinforcement in the slab and the slab thickness is 170mm (7") from the bottom of the grout pad. I wouldn't think that punching shear gives such a extensive crack pattern before failure.
The groove is not a construction joint, it is an indentation that was left behind by the formwork. Typical plywood formwork was not used.
I would like to explore other options before the column is relocated, the baseplate has been grouted and they have begun laying terrazzo tiles. If the column is moved outside it's recess than the column will need to be shortened on-site.
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.
The column size is 150x150 (6"x6") and the baseplate is 250x250 (8"x8") and that is the approximate size of the "square" shaped cracking that has developed.
The steelwork was erected and took approximatley 6 weeks before the roof sheeting was installed, so water could permeate through the slab for that period that it was exposed.
It was Hilti epoxy that was used so I am in the process of trying to get some load tests done to ensure that we do have sufficient bond between epoxy and concrete to ensure that the anchors will have the required hold down capacity.
There is no question that there is a serious problem but I am not sure that the column will punch through. The baseplate has since been grouted so any additional movement will need to punch the slab opposed to the end-bearing of the bolts which I think the problem is from.
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
for construction, standing code CTE at section "12.4 Quality control of the fabrication", imposes that who directs the specific part of the works aprove before start a memory report by the fabricator upon which ...
"12.4.1 control of quality of the fabricator documentation
1 the documentation of fabrication, elaborated by the fabricator will be revised and approved by the director of the works"
"... will verify, with special attention, the compatibility among the different procedures of fabrication and between these and the used materials ..."
This direct approval normally will fall for mechanical services not on the general director of the works -except for ensuring the adjudicated responsible does- but on the director of the works for the particular mechanical services at the job, as pertains to the standing regulations, someone understood to be conversant on the permissible procedures and implications. For structural work the architect or engineer director would likely be those giving approval.
Anyway, depending on the mechanical service some might argue it falls out of the scope of the general regulations of CTE, but this is extremely unlikely since CTE is a code applicable with the utmost generality to anything done to (and in) a building that requires some kind of authority approval.
Also, the to be approved memory for quality control of the fabrication furnished by the fabricator will establish as well the quality control procedures to follow (and to be approved), specifically
"12.4.2. 2... it will be verified ...
that each operation is made in the mandated order and with the specified tools (especially in the case of the works of cutting plates and profiles),
... that the personnel in charge of each operation has the proper qualification (especially in the case of the welders),
...that an adequate process is followed that allows to identify the origin of each lack of compliance, etc."
So CTE by itself adjudicates to the quality control team (inner or brought-in) the verification of that the welders have the proper qualification, this meaning that once approved by the director of the "Proyecto Específico" (a project pertaining to one or varius mechanical or electrical services) it falls upon the quality control as devised for the occasion to verify that the welders are properly qualified.
So the best advice I can give instead of entering deep in waters that I am not familiar is to ask that, following the mandatory "Código Técnico de la Edificación, CTE" in its section "DB CTE SE-A" and quoted sections above, the memory by the fabricator identify a quality team endowed with the full acceptance of its proper qualifications to practice as such and let, er, make them certify that the welders are properly licensed to practice the defined operations.
As to the procedures themselves I am not aware of the usual procedures for this kind of work and I must not give practical advice, yet it is clear that because of the approval is a responsability of the director of the "Specific Project" for the installation (that may be different of the general director of the works) to ensure that it is in accord to proper practice.
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
See the attached file - the failure mode (prior to grouting) is similar to this. So the cumulative load transfer through the chemical bond occurs somewhere down the anchor, possibly near the bottom, giving a very low effective depth to the slab soffit. This would then create a concrete failure cone like in the picture, and any cracking caused by the concrete cone would be accentuated by it being in a positive moment zone..
Now that the column is supported on the grout and if punching shear is ok, these cracks should now just be a cosmetic issue.
RE: Baseplate Punching Through Slab
I think your reasoning is good. Epoxy anchors are rarely cleaned adequately, so I don't use them in tension.
RE: Baseplate Punching Through Slab
Will the architect permit permanent projections above or below the slab? If so, how much?
I assume that he would not allow anything projecting above the slab outside the column, but what about below the slab?
BA
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
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.
RE: Baseplate Punching Through Slab
[quote]
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
RE: Baseplate Punching Through Slab
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.
RE: Baseplate Punching Through Slab
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it
RE: Baseplate Punching Through Slab
But you still need to get at least two of the anchors penetrating the slab for dependable anchorage against uplift.
BA
RE: Baseplate Punching Through Slab
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.
RE: Baseplate Punching Through Slab
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
RE: Baseplate Punching Through Slab
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.
RE: Baseplate Punching Through Slab
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.
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
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
RE: Baseplate Punching Through Slab
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
RE: Baseplate Punching Through Slab
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it
RE: Baseplate Punching Through Slab
Dik
RE: Baseplate Punching Through Slab
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)
RE: Baseplate Punching Through Slab
Improper hole prep is the reason that adhesive anchors need continuous special inspection in high seismic areas.
RE: Baseplate Punching Through Slab
RE: Baseplate Punching Through Slab
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