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.

 

[OldBldgGuy]

I just did some repairs, which could be looked at as strengthening, to a couple of Rapidex floors (I think you're Canadian & could be familiar with them from Hamilton in the 50s through 70s).

How about rebar laid in the cores and the cores grouted with a decent strength grout? The bottom of the cores is pretty thin so the bars are in the right location in the section, and it's pretty easy to calculate capacity if you have accurate geometry.

Some of the repairs I had to do were planks that had been cut for mechanical & other openings so they were accessible from the interior of the building, but others were done by opening the ends in the exterior walls.

 

[wannabeSE]

I'll through out an idea that may be more practical than the magnets. If there is enough bonded topping on the top for compression, how about adding shotcrete with (tension) reinforcing on the bottom - all connected to act compositely.(Beware, I know very little about hollowcore)

 

[jayrod12]

I get that you're looking for any out there idea. But do you really think there's a reasonable way to get a five fold increase in capacity?

Is remove and replace not an option?

 

[msquared48]

#6 is a big NONO!

Mike McCann, PE, SE (WA)

 

[KootK]

I get that you're looking for any out there idea. But do you really think there's a reasonable way to get a five fold increase in capacity

No, not without getting pretty extreme. 3, 5, 6, and 7 would work if sufficient depth were, improbably, available. Remove and replace is an option but with a some pretty steep discincentives given that it's roof over occupied space.

#6 is a big NONO!

What are your objections? Transfer stresses? Uncontrolled cracking over the new supports? Depending on your answer, I may or may not admit to having done this previously.

@OBG/Wannabe: thanks for your suggestions. Both are easy to evaluate which is nice. If either has legs numerically, I'll pitch it.

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]

Back in the mid-90's I designed and constructed an external PT tendon strengthening system to a 2-span RC roof beam where we added a hung partition system (school classroom) to the underside of the beam AND deleted the internal column, so a significant increase in strength and serviceability demand.

External PT to hollow-core planks could be challenging.

 

[theonlynamenottaken]

I just recently did a feasibility study on increasing gravity loads on an existing hollow-core plank roof structure. I found that increasing the flexural capacity was, well, feasible, but increasing the shear strength is daunting. That renders #1-3 fruitless, and #4 too unless you have confidence that the topping shear and plank shear behave compositely and additive, instead of acting consecutively.

The seemingly simple solution is to break up the span with perpendicular beams (your #6). But you end up chasing your own tail by diverting most of the shear to regions with little or no shear reinforcement, aside from the sequential analyses for flexure and lack of existing negative reinforcement. The only method that seemed to work, with confidence, was parallel beams beneath (your #5), though even that only seemed possible by slightly lifting the planks at midspan to relieve some dead load flexure, or making the beams impossibly stiff.

As much fun as it seemed to pursue one of the strengthening options it proved cheapest and most assured to go with your #7 (circumventing the planks with an overbuild). It's very easy to confidently core the planks for putting tube or pipe columns through since the void / strand geometry is so consistent. Plus, there are a lot of unknowns as far as material properties, strand pre-tension, strand relaxation, etc that you can avoid wrestling with by going that route.

 

[dhengr]

KootK:
It never ceases to amaze me that so many people seem to think we (SE’s) can just snap our fingers and make anything work and do the impossible. So, what’s a five to seven fold (new DL + LL) increase in loading anyway; and I don’t want any additional deflection or cracking either, or much added time, labor and cost. Then, just to jerk your chain a little bit, us old older guys, just like you younger-older experienced guys, do like to see cross sections through the existing hollow core plank system, with some detail, the end bearing details, span length, plank depth, core spacing and size, etc.

I’ll go OldBldgGuy one step further. Shore the existing planks, maybe even with a little camber. Saw cut 2" wide slots, full length, in every second or third core, as needed, from above. Install PT rods/cables, harp them down at mid span, and slightly above center at the plank ends. Grout the cores full, but leave the saw cut slot/key for a final topping key. Saw cut all other cores, from above, 1" wide, into the span, however far, to pick up needed shear cap’y. Be sure any existing topping is sound, bonded well and strong enough. Pour more topping to your liking, for final section depth. Post tension the new PT cables. The PT end anchorage detailing may prove a little interesting, but maybe not too bad with nice saw cut pockets for the bearing pls. prior to grouting.

If it is a simple span, that may be something you can balance fairly simply. Plank axial loading, bearing and shear will be some conservative average of original plank concrete, new grout with good bond and the layers of topping.

 

[TehMightyEngineer]

Agreed 100% with onlyname, using the existing panels as formwork for a new PT slab might be the best way to go. Or, dhengr's idea of grouting the cores with PT cables.

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

http://www.americanconcrete.com

 

[JAE]

I've seen HC planks strengthened by breaking open the bottom of the plank, inserting additional reinforcement, and filling with a high strength, flowable grout.
But this was a case where the HC was deteriorated on the bottom and the attempt was to simply get the strength back up to where it originally was.

Adding "stuff" to go from 20 psf to 100 psf is pretty extreme and I'd also worry about the supporting members beams, columns, walls, footings.

Your option 6 could be considered but with perhaps 4 or 5 lines of perp. beams across (i.e. reduce the span by 1/5 to get a 5 fold increase in strength based on shear).
I like the remove and replace as well.

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[Ron]

Use the hollow core as a form and design a proper slab on top. Similar to dhengr's idea, you could slot the top of the hollow cores and add conventional reinforcement to reduce the thickness of the overpour. I would not go to PT because of the limited area of the cores and the potential for longitudinal shear failure during stressing.

 

[jayrod12]

This just screams for remove and replace, I get that it's occupied space below, but you could phase the construction such that there's always a portion open for business.

I assume this is an attempt to take it from roof to floor loading. Or are we talking a rooftop patio (I hate those conversions)?

 

[WinelandV]

I like #9, but it'd have to be opposing magnets all the way down.

Also, people may not like their phones and laptops getting wiped because they walked into the building.

 

[KootK]

Thanks you all.

I found that increasing the flexural capacity was, well, feasible, but increasing the shear strength is daunting.

Thanks for the detailed response. It's become clear that I'll need to pay a bit more attention to shear than I was planning to. I'd figured that I'd just divide up the spans in half and grout solid locally where I needed to. Of course, if the grouting needed to be not so local, then that would get pretty onerous, pretty quick.

using the existing panels as formwork for a new PT slab might be the best way to go.

onlyname and I had bandied around overbuild but I think that the PT suggestion is all yours. We were thinking steel. The PT might be great from the perspective of maintaining a low profile. My only concern would be the PT not sheltering the hollowcore adequately. Fine and good if the PT slab is strong enough but, if the HC cracks in half and falls down in somebody's unit anyhow, that would be a problem. Maybe dowels could connect the HC to the new PT slab.

I've seen HC planks strengthened by breaking open the bottom of the plank, inserting additional reinforcement, and filling with a high strength, flowable grout.

Where it's permissible, I assume that this kind of thing is better done topside, right? Even with topping?

I'd also worry about the supporting members beams, columns, walls, footings.

Ditto. Might not be so bad here though. It's a multi story 80's building. Around here that means stacked, bearing concrete/cmu at the demising walls between units. It makes for an architecturally dull building but beefy vertical structure and foundations relatively insensitive to additional weight, withing reason.

Post tension the new PT cables. The PT end anchorage detailing may prove a little interesting, but maybe not too bad with nice saw cut pockets for the bearing pls. prior to grouting.

I'd post tension everything if folks would let me. If I could keep negative flexure at pres-tress under control, I wonder if straight tendons below centre would be viable. Without running any numbers, my gut feel that the hollowcore is probably already prestressed near to the "right" amount based on the cross section and adding more prestress would probably yield diminishing returns without introducing drape, either as you've suggested or through external PT.

Use the hollow core as a form and design a proper slab on top.

Love it. So:

1) Shore everything,
2) Break open all of the cores.
3) Case new concrete to make a deeper, stronger composite slab.

The economics would be key to choosing between something like this or, as jayrod recommends, wholesale replacement (I hear you jayrod!).

I assume this is an attempt to take it from roof to floor loading. Or are we talking a rooftop patio (I hate those conversions)?

Nailed it. Rooftop patio on a rather grand scale.

@All: For evaluation, and based on your comments, I give you #10 for consideration:

10) Throw some new precast planks on top of the old ones. Easy, quick, and probably as light as anything else. Perhaps crane issues though.



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.

 

[JedClampett]

Koot, is it possible that there's some extra margin in the existing Hollowcore? What I've noticed is that there's standard castings that the manufacturers deliver come hell or high water. For instance, they provide a minimum number of prestressing strands no matter what. Or they were leftovers for a heavier order. You might get lucky, especially for shorter spans.
It's unlikely you've got 100 psf in there, but you might get 50 or 60.

 

[KootK]

It's very possible Jed. Tracking down plank design info will be a challenge, as always, but I'll give it the old commmunity college try.

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.

 

[irishengdave]

#6 is the only practical option!

it might crack the topping over the support but you can grout it up if it's an issue. Just add enough beams until you get the capacity in shear and moment you need. Putting in frp or adding rebar will magnify moment capacity but beware as it will increase the compressive stress in the compression block possibly leading to sudden failure of the floor. Also analyzing that combination of prestressed, and reinforced cross section would be very laborious unless you have some specialized software.

 

[TehMightyEngineer]

The PT might be great from the perspective of maintaining a low profile.

My thoughts exactly.

Maybe dowels could connect the HC to the new PT slab.

If you're not going to demo the HC panels then this would make sense. Gives you more depth for deflection/vibration/shear as well if you ever wanted to try to consider the HC composite with the new PT slab.

KootK, do you have any data on the following: HC panel depth, HC span, topping on HC?

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

http://www.americanconcrete.com

 

[KootK]

KootK, do you have any data on the following: HC panel depth, HC span, topping on HC?

New info rolled in moments ago..

- 10" HC + 1.5" topping
- 9.8 m spans
- Structural drawings available
- Full paver system contemplated
- CMU bearing walls
- Some appetite for increasing structure depth.


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.