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.

 

[FFENGINEER]

Did you check punching shear? Perhaps the load is higher than originally thought? I do agree it needs to be grouted.

 

[ash060]

How thick is the slab?

 

[canstructural]

Is the slab grooved / sawcut for some reason? Normally structural slabs are not supposed to....

What is the purpose of column, can it be relocated without pocket.

 

[doka1]

the slab isnt grooved, look a little closer, its the seems of the plywood falsework....did they install the grillage steel? (extra bars at support?)

 

[rowingengineer]

asixth,
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

 

[slickdeals]

What kind of reinforcing was present in the slab? How thick is the slab?

 

[apsix]

I would investigate anchor bond, especially if you require uplift capacity. Try rotating the anchors by getting a spanner onto 2 nuts locked together (the column may need to be removed for this).

 

[hokie66]

asixth,
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.

 

[ishvaaag]

If things go really bad you may keep everything shored to support and then replace cracked area till some safe distance for punching. Keep most rebar you can to ensure a new fresh, clean and competent structural joint.

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.

 

[Ron]

ishvaaag...take a look at the welder certification (8mm certification)thread in the AWS Code Issues forum...you might be able to help...

 

[asixth]

Thanks for all the replies.

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.

 

[Tomfh]

The crack pattern looks like flexure cracks.

 

[ishvaaag]

Ron,

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.

 

[ishvaaag]

Hmm, it seems I didn't place the answer in AWS forum, sorry

 

[doka1]

what are the current loads on the column? did you look into when the shores were removed for the slab?

 

[demayeng]

Could it be that the problem is just with the method of levelling and putting all the load on the anchors before grouting, not to do with a failure of the epoxy bond?

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.

 

[hokie66]

asixth,

I think your reasoning is good. Epoxy anchors are rarely cleaned adequately, so I don't use them in tension.

 

[BAretired]

This is a perplexing problem. It is not clear how it may be resolved economically. I agree with Tomfh that the cracks are flexural. Ordinarily, punching shear would not be a problem as the shear stress is quite low, but in this case, the concern is that punching shear cracks could join flexural cracks to produce a reduced section in resisting punching shear.

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

 

[hokie66]

The punching shear perimeter would be about 370 square. Where does that fall relative to the crack pattern in the photo?