add steel base plate for new column to existing post-tensioned slab

[abcwood]

i have a client who needs to add a steel shade structure to the top level of an existing p-t slab parking structure. the attachment would likely be a square steel base plate with (4) epoxied bolt into the concrete. can you provide me with guidance for the issues that need to be checked, and guidance for working around the tendons? where are they located? how can this be determined in the field? what are acceptable clearances for new bolts to existing tendons, etc.
thank you

 

[Hanif08]

Hi,

You should be able to obtain the location and profile of the post-tensioned tendons from structural drawings of existing slab. If you cannot then below is some typical info of such slabs:

-For cast in situ, PT-slab of a parking structure will be a beam-supported slab that is post-tensioned separately from beams.
-Thickness probably less than 200mm or 8in. Governed mostly by shear.
-Post-tensioning is done with unbonded tendons that are usually 0.6in or 15.7mm in size enclosed in 20mm jacket.
-Tendon spacing can be 300mm - 500mm.
-Tendons will have maximum sag in the span of slab and rise to the top at beam locations.
-Space inside the slab is quite tight because there is concrete cover and mesh of top and bottom mild reinforcement. So the tendons will have to do their dance within 100-120mm of inside slab space.

I have not come across a situation such as your. In an ideal situation if someone manages to get the bolts and epoxy in safely without damaging the tendons then it should not effect the performance of existing structure. This assuming that you will check the ability of slab to carry extra load from steel structures. Hint: Shear and point loads!

However if I were you then I will make effort to find information about tendons locations. With expansion bolts, holes will have to drilled in the slab and this process should not bust the tendons. And, if there will be several locations for hole drilling then increased risk of busting more tendons.

Damaging the tendons can cause problems, obviously if you bust more tendons then problem will be compounded both for servicibility and strength. There is not a whole lot of mild steel as pt-steel contributes very much to overall strength.

Goodluck!

 

[hokie66]

It would be helpful if you can give some more information. Firstly, where is your project located, because systems vary with location. In the US, many posttensioned slabs are ungrouted single strands, but in many countries, the strands are in ducts and bonded by grout.

If at all possible, you should obtain the drawings used to build the structure, and if you are not conversant with posttensioned design, you should involve an engineer who is.

You may be able to locate the tendons from the soffit by observing the pattern of the chairs which were used to support the tendons.

 

[hokie66]

abcwood,

Just a further thought. In adding a shade structure to the deck, I would use the simplest connections possible. I would require the shade structures to be designed with pinned bases so that the deck does not have to resist bending of the columns. With only gravity and uplift forces to contend with, I would make the column connection with a central bolt through the slab, with a plate and nut on the bottom.

 

[Hanif08]

Hi,

It is extremely dangerous to make connections with one bolt in the middle. It is already a mechnism and with slightest of imperfection or external lateral load , the structure will fall like a deck of cards. In real world we are not making structures like we see or draw on a computer - straight lines.

Pinned connection is an assumption on part of the engineer in doing global structural analysis. These assumptions determine overall stability, and distribution of stiffness and hence internal forces.

Baseplate with 4 bolts is still a pinned connection! The difference is that, in the absence of external moments, the bolts are not in tension and are designed for shear only. In a typical situation one must also consider erection phase where prior to grouting the bolts will experience forces from the weight of the frame as it is erected. Sorry can't imagine all this on bases with single bolts!

 

[hokie66]

Well I can imagine it. If the shade structure frames are designed as rigid frames, there is no structural reason for multiple connectors to the slab.

 

[lkjh345]

I can picture what hokie is describing and I agree with him provided the shade structure has at least 2 rows of columns in each direction.

You want to limit the number of penatrations you have to a PT slab to the smallest number possible to reduce the chances of hitting a tendon or bar. A single bolt per sunshade column is a good idea if the shade structure can be made into a moment frame with 'fixxed' joints at the top.

It is the responsibility of the contractor to provide temporary bracing or shoring as required to handle forces during erection.

 

[hokie66]

Thanks for the support, lkjh345, and for saying it better than I did. I think the important things are to minimize the number of holes and eliminate moment on the slab from the shade structure insofar as possible. Stability and safety during erection can be achieved by other means than 4 bolt connections. 4 bolt connections in themselves present hazards due to clumsy people like me tripping over them.

 

[Hanif08]

Hi,

All I'm saying is that in real world one has to consider imperfections in the frame due to initial out of straightness of members and erection tolerances. All these cause secondary effects on the frame even if one thinks that everything (member & loading) is straight.

On the other hand if one thinks that everything is perfect then I will go even further to say that in theory you do not even need any bolts. The friction between between baseplate and concrete will be enough.

Design of pinned column bases with single bolts may work in ideal theory but it lacks practical robustness. I'm sure we are designing structures to be used for a long time and for plausible scenarios.

As for making holes in PT-Slab, a logical pre-requsite is that you should know the location of tendon so as to avoid them. The engineer on record must have given this information to people in the field otherwise how did they install the tendons in the first place.

How far should the bolt be from tendon? I don't know a theoretical answer but my logic says perhaps the same as embedded length of bolt in concrete so that the bolt can develop capacity from surrounding concrete.

 

[apsix]

Welcome Hanif08
"in theory you do not even need any bolts"; only true if you ignore uplift and shear.

"It is extremely dangerous to make connections with one bolt in the middle."; that's unnecessarily alarmist and not true. Even if the base connection of a rigid frame was destroyed it wouldn't collapse like a house of cards.

"but it lacks practical robustness"; I would tend to agree for a situation such as a carpark. The column could be protected with bollards but then you get more bolts....

"How far should the bolt be from tendon? ... perhaps the same as embedded length of bolt in concrete so that the bolt can develop capacity from surrounding concrete"; I would like to see a little more than that, just to be sure; 1.5 x embedment?

 

[hokie66]

The robustness issue is valid, but can be addressed with a bigger bolt. Perhaps Hanif08 did not understand that my single bolt is actually a stud which goes through the slab and is anchored underneath.

 

[dougantholz]

First - why bolt it? If this is a shade structure you can use counterweights to keep it in place. A large baseplate that has a sonotube of concrete poured around it will keep this shade structure in place. You can size that pretty quickly.
Second - if it is permanent and you want a positive connection you need to have a local testing lab come out and use a pachometer. This is a simple machine that will locate rebar and cables. X-rays can also be done but they are typically expensive and can be cumbersome tests. A pachometer can be rented for probably 50 bucks for a couple of hours. Remember that these things have a little inherent inaccuracy so leave a few inches clear around things. Also know that if there is conduit on the underside of the slab it will pick that up as well.
HTH

 

[civilperson]

If it is a column, then it must have four bolts for base connection. Calling it a vertical frame member/post with diagonal braces may exempt it from the four bolt rule.

 

[structuresguy]

The OSHA 4 bolt column column requirement applies for building structures. As this is an accessory, I do not believe it would apply in this case.

I would also like to see a space frame used, and also have no problem with a single bolt at each post base, assuming that the bolt is centric to the column reaction. If eccentric, then I might consider 2 bolts, centrically located to avoid local prying on the bolts.

I am more concerned with the behavior of the PT slab with this new, and most likely, unforeseen loading being exerted onto it. Not knowing the size of the structure, or the location, I don't have a sense of teh reaction forces. But they must be considered in the behavior of the slab.

dougantholz has it correct on locating the tendons. Even if you can find original design or construction drawings, I would still make all reasonable efforts to locate the tendons. I have personally seen the results of tendon breakage, and the repair can be very costly.

 

[Bagman2524]

There is equipment out there that is used to detect rebar. I would think it could also be used to detect prestressed tendons. I'm not sure how accurate they are though.

 

[hokie66]

We have all gotten off the track of the OP, and that is because I started talking about details for connecting the shade structure to the slab. The OP has been silent. The main point I was trying to make was that he should avoid the type of shade structures which are cantilevered like a christmas tree, and also the type which have large forces imposed by tension membranes. That leaves rigid frame systems. The slab has to be checked, but a PT plate has a much better chance of being able to take additional vertical forces and shear than concentrated bending moments.

 

[hokie66]

I was intrigued by this US OSHA requirement for 4 anchor bolts for columns, so I googled it, and sure enough, it is there. Interesting that California has an exception if the columns are braced or guyed. Well, duh!