Point Load On Steel Beam And Column Through Precast Plank 1

[zurch1818]

I am am designing a rooftop equipment platform which is supposed to connect to the rooftop structural system. This rooftop is constructed with a system of precast planks, steel beams, and steel columns. I plan on putting the platform directly on building columns, so globally I don't see it being much of an increase in gravity and lateral loads from the existing condition. However, I since I don't deal much with precast planks, I'm not quite certain what to do with the connection detail that has to go through the precast plank to the steel beam and column. I feel like with the magnitude of load that I am proposing that it would locally punch right through the existing precast plank. Additionally, the building plans that have been made available to me only tell me the design loads and I don't actually know exactly what the plank is in terms of strand pattern and the number of cores within each plank.

Does anyone have any recommendations for what to do with this detail or find some way to rationalize that this load would be ok? As far as I can tell, the load is going to be right at the end of the plank. I feel like I want to grout the cell solid somehow but I don't know how to easily accomplish this. I feel like cutting a hole in the precast plank to bypass it would also help, but I am concerned that I may compromise the design capacity of the plank. Thanks for any insight someone can give me.

 

[jayrod12]

If you're aligning it directly over the column, you could just indicate that they ensure the area of plank directly between the old and new column is fully grouted. It honestly may be already.

 

[dik]

For cutting holes, coord with precaster... they can provide the product. 8" HC typically has 6 voids with the strand located between the voids... info also available from precaster.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[zurch1818]

Thanks for the replies. It's weird to me too think they can grout it solid without filling the whole span or worse yet, not completely filling the void. I assume there must be construction methods to retroactively grout a segment or a cell?

I will have double check the plans to see if they at least call out a thickness. My guess is they probably do.

 

[jayrod12]

They can core/break out grout ports and observation/vent ports to ensure it gets grouted solid. It's done all the time. If you're really concerned, you could pressure grout but that might require additional prep work to ensure the grout doesn't flow too far down the hollowcore.

 

[KootK]

1) I've spent some time as a precast engineer for what that's worth (in WI no less). I've probably designed enough hollowcore to stretch to the moon and back a couple of times.

2) Are your point loads truly over top of the beams / columns supporting the plank? Or just close to them? If you're right over top of supporting elements, then it's a bearing problem and not a punching problem. Hollow core tends to bridge over top of the void spaces in a manner that yields rather amazing bearing capacities. If the loads will be delivered via any reasonable steel base plate this way, and there is minimal uplift, I would expect that you could probably get this done without any modification at all. How heavy is your load and how big will your base plates be?

3) As jayrod12 mentioned, it's pretty easy to grout cores. If it's an interior location, the process usually goes something like this:

a) Cut two holes at the extremes of the area you mean to grout. Or a continuous slot.

b) Stuff some foam plugs or something into the cores to contain the grout to the desired location.

c) Grout.

In your case, the difficulty with grouting is probably centered around how much roofing you are willing to remove and replace. Perhaps you're already doing enough roofing replacement with this that it doesn't much matter.

4) Bypassing the precast with holes would likely work too. Planks share load response with one another. It's a case by case thing but, in most cases, you'll have enough reserve shear capacity at your plank ends that you can stand to loose a couple of webs here and there.

 

[zurch1818]

Thanks again for the replies. It's just not my area of expertise, but I am comfortable enough to work on it with some research. I usually work with telecommunication structures, but we do get the occasional building where our clients want to install on top of buildings. If I go the grouting route, I will probably call out that they need to be grouted solid and leave it up to the contractor to decide how they want to do it. I will call that means and methods. It's also interesting to me to think about just bypassing the hollow core. Once I get into the project more, I will see if that is a viable option. I didn't even know that was even on the table.

I actually haven't started the design of this platform yet, so I don't know the exact magnitude of the loads yet. I'm anticipating that it will conservatively probably be about a 10k to maybe 15k LRFD compression load. I'm guessing it will probably be a 4.5" OD round HSS for the column and the base plate would be maybe 6x6 to maybe 8x8. I can make it as big as I need to, whatever it takes to make sure it is sufficiently sized so the hollow core can deliver the load to the main structural system. The platform is going to be a simple platform with just some steel beams and be open to the air, so I'm not really worried too much about uplift. I'm anticipating that some post-installed anchorage would be sufficient to carry whatever tension that I may have.

Yes, I am going to locate the platform right on top of steel beams and get at most 1' away from the columns, preferably right on the column. It is true that is probably technically bearing and not punching shear. I was more or less worried about punching through the top of the hollow core spanning between the webs of the hollow core. If checking bearing makes more sense to you, maybe that will be where I start. Are those loads that I referenced that unreasonable for bearing to be fine? Obviously, I will still have to crunch the numbers, but it is good to know that I am not completely crazy.

Our clients are so used to punching holes into roofing and replacing it, that I don't think that will be much of an issue. Our clients are more worried about cost in terms of time than they are dollars.

 

[KootK]

1) The tables below will give you an idea of the ASD punching capacity that you can expect for different systems.

2) PCI's hollow core manual will tell you how to do the bearing check which should have gobs of capacity: Link. For plank on block buildings, you usually have to go six to eight stories before you even need to grout the plank ends by the numbers.

3) I probably would grout the cores for bearing here. That, for a few reasons:

a) You'll be messing with the roofing locally anyhow.

b) Handy for getting some decent substrate for post-installed base plate anchors.

c) At at steel beam plank connection you'll have maximum plank slope and it might be difficult to get a clean, level bearing. That, particularly in an untopped system which is common for roofs.

d) I doubt that your posts would punch through the top flanges of the planks but, certainly, that is an concern that I would be happy to obviate.

4) Note that, if you were to grout for punching shear, you'd have to grout quite a ways for that to be effective. Punching shear tests on hollow core planks produce failure frustums at pretty shallow angles relative to cast in place, non-prestressed concrete. I'd probably want to grout around the point load something to the tune of 5X the plank thickness.

I was more or less worried about punching through the top of the hollow core spanning between the webs of the hollow core.

5) The concrete over the voids tends to behave a bit like an unreinforced concrete arch bridge. Hence the rather excellent bearing capacities. If you've got stiff base plates that are large enough to either land on plank webs, or straddle them, you should be in good shape that way too.

 

[zurch1818]

Thanks for everyone's help on this. I feel much more confident with the subject matter and I am now danerous. Precast/prestressed concrete just seems so different from conventional CIP concrete (even though it is more like a cousin). I just haven't touched it since school and have tried to avoid it in my current position.