Strengthening an Existing Hollowcore Plank Roof System

[KootK]

I've been engaged to look at a hollow core plank roof that needs to go from 20 PSF to 100 PSF. And probably some additional dead load too. I'd like to round up as many options as I can for strengthening. Here's what I've got so far, acknowledging that many of these options may not actually be viable for such a heavy increase in carrying capacity.

1) FRP on the bottom.

2) Steel strapping on the bottom.

3) External post-tensioning on the bottom.

4) More bonded topping on the top.

5) Parallel beams on the bottom, perhaps acting compositely.

6) A perpendicular beam line to break up the span.

7) An entirely new overbuilt roof structure shielding the plank from load.

8) Permanent sealing and pressurizing of the space below.

9) Very strong opposing magnets on the floor and ceiling below.

Any other ideas worthy of consideration? Come one, come all... Any comments on which of the above might actually be up to the task of a three to four fold load increase?





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Koot,

Do you account for 100psf occupancy and drifting to act simulataneously? Maybe I'm a little risky but I have a hard time believing you're ever going to get both of those happening at the same time. If they ever are planning to have a soiree that would reach the 100psf level, you know damn well they'd clear the snow.

Be careful on the green roof garbage as well. The drain hole clogs and boom... density increases significantly.

 

[KootK]

No, I just didn't know shoring was an option as you earlier said the lower story was occupied.

You probably answered this earlier but are we allowed to do stuff below the existing hollow core slab?

The space below is occupied and we are allowed to do stuff below the existing hollow core. Originally, they did want to avoid interruption to the space below. The project team has accepted that may not be possible however.

Well, I still think precast prestressed or cast-in-place PT slab perhaps using the existing HC as formwork will be your best bet and getting them the system they want with the least compromises.

I originally thought so too but now I'm skeptical for the following reasons:

1) Cutting narrow slots into the plank in which to grout seems somewhat reasonable. When I imagine how much of the top "flange" would have to be cut away in order to install chaired rebar or PT, that seems pretty onerous/unreliable.

2) Just due to the geometry, I think that a lot of the effective depth / PT drape would be lost.

3) Pre-stressing an already pre-stressed thing and trying to asses its state of stress hurts my brain.

4) How the heck do I check the plank for its capacity while it's acting as formwork?

5) Given the small size of the cores, I'm not sure that PT anchorages and their associated bursting stress reinforcing could be made to fit. If I place the anchorages up in the topping slab, I wind up with self defeating end moments that tend to push the mid-span downwards.

Do you account for 100psf occupancy and drifting to act simulataneously? Maybe I'm a little risky but I have a hard time believing you're ever going to get both of those happening at the same time. If they ever are planning to have a soiree that would reach the 100psf level, you know damn well they'd clear the snow.

As usual, I've been thinking similar thoughts. And I agree with the logic completely. I thought that they might limit the extent of the patio to a 15' strip up against the roof that would either be occupied by humans or drifted snow but not both. However, are we at liberty to make that decision for ourselves under the NBCC? Drift is a permutation of "Snow", obviously, and there are specified load combinations for snow and live load acting in concert which would seem to already take this into account to some degree.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

However, are we at liberty to make that decision for ourselves under the NBCC?

I don't know of anyone that has been challenged on this stance before. And I do know I'm not the only one to use the loophole.

 

[TehMightyEngineer]

I originally thought so too but now I'm skeptical for the following reasons:

Fair enough, so if post-tensioned is out then I'd think precast, prestressed panels would be a very efficient way to go.

The space below is occupied and we are allowed to do stuff below the existing hollow core. Originally, they did want to avoid interruption to the space below. The project team has accepted that may not be possible however.

If this is the case then what are the available options to add supporting steel below the existing hollow core?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

I don't know of anyone that has been challenged on this stance before. And I do know I'm not the only one to use the loophole.

Noted and perhaps soon to be exploited.

If this is the case then what are the available options to add supporting steel below the existing hollow core?

None really other than my hanger business. We can install temporary works in the space below but nothing permanent due to headroom issues.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Here's a thought thanks to your hanger + beam business. Why can't you do this:
[ol 1]
[li]Shore the existing HC slab[/li]
[li]Cut the HC slabs similar to your hanger idea and grout the cores[/li]
[li]Install a steel plate as a web and weld on a top and bottom flange as a plate girder[/li]
[li]Stiffen the bottom flange as needed support the HC slabs bearing on the bottom flange[/li]
[li]Assuming the steel girder is taller than the existing HC slabs construct a lightweight floor topping to raise up the finished floor to above the steel beam top flange[/li]
[li]Support the steel beam(s) on the exterior brick wall or steel columns embedded in (or adjacent to) the brick wall[/li]
[li]Remove shoring[/li]
[li]Drink beer[/li]
[/ol]

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

Noted and perhaps soon to be exploited.

To be fair, the majority of our buildup around here is in excess of 100 psf....

 

[KootK]

I could do that TME. The only catch is that beam would be 60' long (no way for you to know that). Assuming a 36-ish in inch beam, that's a lot of depth etc. At that point, I'd have to think the regular steel over build much more competitive.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

#12) Remove every 2nd plank and cast a TT-ish slab and joist system around what's left. I'm holding on to every other precast plank because that solves some issues that I have regarding in plane diaphragm shear transfer, temporary wall stability, and bearing stresses generated by the walls stacked above the ends of the planks. It feels pretty wacky and, therefore, probably is.

Critical and/or constructive comments welcome. I almost went mad trying to figure out how to convey this graphically without staged construction details. Be gentle.

I've attached the same details as a PDF file for those who prefer to view that way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

PDF

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

That is a bum-load of work to cut and remove planks, then have to form and CIP new beams.

Back in the late 80's/early 90's in my area there was a somewhat common precast/CIP PT hybrid system for buildings. Precast/pre-tensioned simply supported shell beams that were trough/U-shaped, about 36" wide, then PT draped within the beam shell over multipl spans, then CIP concrete.

Is it worth a revisit to re-look at longitudinal full-length saw-cutting through to some of the hollowcores (maybe 2 or 3 per 1200mm plank width) and draping PT?

With that step created at your CMU bearing wall you appear to have access for stressing.

 

[KootK]

Yup, tons of work. She's getting a "Low" feasibility rating.

I've heard of that composite system that you've mentioned. I believe that something similar is popular in some of the former colonial countries (other than Canada). Can't remember the name of it though...

Is it worth a revisit to re-look at longitudinal full-length saw-cutting through to some of the hollowcores (maybe 2 or 3 per 1200mm plank width) and draping PT?

I considered it extensively and came to the conclusion that I'd probably never be comfortable with it personally. I'm happy to entertain any dissenting opinions on the following, reproduced from above:

I originally thought so too but now I'm skeptical for the following reasons:

1) Cutting narrow slots into the plank in which to grout seems somewhat reasonable. When I imagine how much of the top "flange" would have to be cut away in order to install chaired rebar or PT, that seems pretty onerous/unreliable.

2) Just due to the geometry, I think that a lot of the effective depth / PT drape would be lost.

3) Pre-stressing an already pre-stressed thing and trying to asses its state of stress hurts my brain.

4) How the heck do I check the plank for its capacity while it's acting as formwork?

5) Given the small size of the cores, I'm not sure that PT anchorages and their associated bursting stress reinforcing could be made to fit. If I place the anchorages up in the topping slab, I wind up with self defeating end moments that tend to push the mid-span downwards.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

I considered it extensively and came to the conclusion that I'd probably never be comfortable with it personally. I'm happy to entertain any dissenting opinions

Let me think about it some more.

How much do you know about your existing planks data, like number and type of void (eg Spancrete, Ultra-Span, etc), number and size of bottom strands etc?

 

[KootK]

Nuttin'. Just depth, span, absence of topping, and probable design loads. Even the design loads are a bit sketchy as they call out drift on one other plan only and say "typical at steps". I probably know the drift but I wouldn't want to bet my license or somebody's safety on it really. If I assumed no drift, the over stress might be a good worse than even the atrocious numbers that I stated above.

Let me think about it some more.

Please do. I'm not saying that you necessarily do this but, on Eng-Tips, I have this sense that folks sometimes give "what you should do" recommendations that tend to be a bit more aggressive than what those same folks might actually do themselves, push come to shove. I know that I'm guilty of this. My answer to everything seems to be one of the following:

1) Post-tension it!
2) Strut and Tie!

I would love to know if you yourself would actually be willing to PT this thing. I sketched out the core 1:1 and stared at it for twenty minutes to ensure a proper sense of scale. Those cores aren't huge. Also, if you had the teardrop cores as below, it seems as though you'd really have to bash away at the plank to clear yourself a decent placement lane. In my neck of the woods, the cores are sometimes wittle squares. That would be easier here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I get the desire to retain the existing diaphragm connections, and to also keep the bearing below the existing wall. But #12 seems like a shit-ton of work for little gain.

A good contractor could retain any of the diaphragm connection reinforcement when they removed the planks. So I'd still be in the remove and replace camp.

How tall is the block wall above?

a bit more aggressive than what those same folks might actually do themselves

Maybe on the odd occasion, but I usually preface that with the coverall statement that I wouldn't do this but I've seen "_________"

In most cases, I only suggest what I know. That's why I don't comment of everything and have stayed away from the PT solution. I don't have any experience with PT, or at least not enough to start commenting on it.

 

[KootK]

A good contractor could retain any of the diaphragm connection reinforcement when they removed the planks. So I'd still be in the remove and replace camp.

How do you see that happening? It's this condition. Only bars at the plank joints which may be difficult to match exactly. In the precast replacement option that I presented, I recommended a new connection angle on the underside with fire proofing. I also recommended doing alternate planks in two stages to preserve bearing but the practicality of that is questionable too.

It's three stories of like framing above. I may need to look a little harder at finding a way to work with a temporary, half bearing condition.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

That condition is why I feel it's feasible to remove the planks and retain the bars. If they were in grouted cores, as opposed to joints, they'd be harder to save. They saw-cut the joint, and then remove the panel. You don't even need to saw-cut the joint the whole depth and it will probably just break free with some persuasion from a pry bar. I think that the bars at the joint would stay in place, intact, and would just require cleaning to remove the grout. Maybe it's a pipe dream thought but I see it as feasible.

I also don't think it should be too difficult to match the H/C joints for the new planks either. Provided the current H/C planks are the nominal 4 ft wide planks that are being supplied today, I think it's reasonable that they'd be able to do what you need.

The trickiest part is maintaining the bearing for that block wall. Maybe, temporarily, you build a stud shoring wall on the inside face of the block wall, tied in to the floors above and below, and have the masonry wall tied to it as well. It puts the planks on the other side of the wall into a bit more shear load temporarily, but it would be within D of the support so I'd likely be comfortable with it.

Honestly though, I'm likely being a little optimistic on the possibility of this. I'm getting too used to letting the contractor figure out the means and methods of the temporary construction stuff that perhaps I'm losing touch with what's possible.

 

[KootK]

Thanks kindly for all the advice jayrod -- I value it.

Maybe, temporarily, you build a stud shoring wall on the inside face of the block wall, tied in to the floors above and below, and have the masonry wall tied to it as well.

Clever. I'm skeptical that much of the axial load could be drawn out of the masonry wall into the stud wall. But maybe that's okay. My main concern would be the wall bowing out in flexure under the newly eccentric load. If the existing wall could be convincingly tied back to the temporary shoring wall (made sufficiently stiff), that might alleviate that issue.

I guess if one wanted to truly take the axial load through the shoring wall, true shoring posts could be installed and jacked up a bit.

I'm getting too used to letting the contractor figure out the means and methods of the temporary construction stuff that perhaps I'm losing touch with what's possible.

Is this a case where you would be comfortable just handing it off to the contractor as means and methods? Maybe that's just the way to go. They do seem to have an uncanny ability to make temporary conditions work that baffle me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

They do seem to have an uncanny ability to make temporary conditions work that baffle me.

Precisely.

Does the owner have a contractor in mind? Do you have a contractor that owes you a favour? It's possibly time to have a discussion about feasibility.

 

[KootK]

No contractor in mind yet. There are some contractors that I think owe me a favor or two. We'll have to see if they agree.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.