4 Storeys of Hollowcore on Wood Stud

[Craig_H]

I've been engaged by a nearby condominium board to investigate settlement and lateral deformation of their building. It is a 4-storey apartment-style condominium building. Parkade on the ground level, and three floors of living space above. Inconveniently, their record drawings are complete except for the structural drawings, and I have gone on a wild goose chase for those with no luck (EOR is retired, firm that bought his company doesn't have his records, town has nothing, province has nothing). Building was constructed in 2006-2007.

The structural system is:
Main Floor: Slab on grade parkade, wood framed exterior walls.
2nd Floor: Steel transfer girders supporting 8" Hollowcore
3rd & 4th Floors: 8" Hollowcore on wood framed walls
Roof: Light wood trusses, gabled roof on wood framed walls.
Balconies are supported on exterior steel columns. There are slope issues due to building settlement.

This is the first time that I have personally seen hollowcore on wood studs. To be honest, I am skeptical that it will work out math-wise due to the dead load of the floor system. The residents have noticed some problematic vertical and lateral deflections. The vertical deflections are likely sensible given the wood bearing walls' propensity to settle with shrinkage (crushing?) of plates and is mainly a serviceability issue. The lateral deformation noted is excessive in some cases (up to H/200 as measured with a plumb bob on exterior columns) and I have concerns for the relatively heavy floors to cause ongoing P-Delta lateral deformation.

Without the structural drawings, I am having difficulty assessing the lateral load system for the building. Photographs taken during construction seem to indicate intermittent, discrete diagonal wood bracing in the party walls between units (diagonals at spaced approximately 24" apart). Exterior walls are sheathed, but no hold-down devices are in the photographs and the panel lengths are quite short, leading me to doubt their effectiveness. Overall, a very odd building and an odd combination of materials.

Questions:
- Are there any resources for design with hollowcore on wood stud? My Google-fu might not be working today, but I have not turned up much.
- Does anyone have a good resource for evaluating wood shear walls with discrete diagonal bracing? I have not gone through this myself and CSA O86 only covers diagonal sheathing rather than the intermittent bracing that I am facing.
- Can I get a confirmation on my concern with the degree of out-of-plumb on the exterior columns? To me, anything over H/500 is worrying. The NBCC commentaries state that rotation greater than H/150 can result in ultimate limit states being reached, and this building appears to be not far away from that value.
- Interestingly, there is little evidence of distress inside the building where walls are finished with drywall throughout. This has me perplexed, as the deformation of the exterior columns was apparent from the street. Would H/200 (total and inter-storey) racking deflection not result in cracking of drywall at the joints?

 

[hokie66]

So there are single story columns supporting the transfer girders? Is the column deformation in single bending or out-of-plumbness? Is there bracing in the Ground Floor 'parkade' level?

Yes, a very curious structural system. Have you tried to contact the retired EOR? If there are no records, he may have taken them with him. Possibly destroyed them.

 

[gte447f]

Agree with Hokie. Sounds like a very curious structural system. I have not encountered that particular combination of steel and wood vertical elements with hollow core slabs. Where was the column where you measured h/200 lateral? Ground floor at the parking garage, correct? Agree with Hokie, what is the lateral system at that level, steel braced frames, moment frames?

 

[Canuck65]

I have seen this structural system before in Ontario. The company I worked for at the time was brought in to investigate, similar to what you have been asked to do. It was not my project within the company so I can only offer limited information. The opinion in the office was that this is not a preferred method of construction, however it is cheap to construct. I believe the numbers worked out for gravity, if you ignore creep. The location was reasonably active seismically and the main upgrades after our report were to increase lateral resistance. The condo board had to spend a lot of money to deal with these issues.

 

[Craig_H]

So there are single story columns supporting the transfer girders?

The steel girders are supported on concrete columns on the main floor level. These appear to cantilever from the foundation up. I am unsure whether these form the lateral system on the main floor, or if the wood framed exterior walls (few openings) take the base shear. I'd have to compare the relative stiffness to determine. Foundation depth is unknown at this point.

The H/200 lateral deformation was measured on an exterior column that supports all three levels of balconies along with the roof above. It's a 4"x4" HSS column. Deformation measured was ~2" over ~32' height. Column visually appears as a textbook P-Delta deformity:

Distortion of the camera lens may be worsening the visual appearance slightly, but never the less it was apparent while driving towards the building.

 

[dik]

Interesting... I've only used HC with wood framing once, and it was a single storey. I didn't like the idea, but it was presented to me as a project.

Other than compression perp to the grain and long term deflection issues due to the relatively large dead load, there should be no issues other than fastening.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Brad805]

I was asked about this idea a few years ago. The biggest problem will be the connections of the HC slabs to the wood framing. It will be very difficult to evaluate without having the structural drawings or the precast shop dwgs to see what the basic plan was. From a precast HC perspective I would think the connections would mainly consist of the field guys knocking out core ends, and then putting in some type of a bolt/lag or maybe plates??? Assuming you have a topping, the knockouts will be filled when that is cast. The frequency of these connections will be very hard to assess.

 

[Once20036]

Speaking on PC shop drawings, that could be another avenue to find drawings for the building.
Have you reached out to local precasters?

Reached out to the architect whose drawings you possess?

 

[Craig_H]

Thank you for the additional comments everyone. To answer some questions:
- Once20036, I reached out to the architect earlier this week. Their staff have verbally informed me that they do not have final structural drawings in their records. Their principal partner is away until the end of this week, I am hoping to be able to chat with them at that point and obtain what I can. Good call on reaching out to local precasters, I will try that route.
- Brad805, thank you for those comments. The architectural floor types call out a 3/4" topping, but I notice that the balconies do not appear to have topping applied, and rather, it appears that a floor leveler was used to locally level the panel joints. See below.

- Canuck65, thank you for that confirmation of my gut feeling. This area is not especially seismically active, but not inactive either. Sa(0.2) = 0.28. Depending on the building mass and the ductility of the bracing in the party walls between units, this building may be seismically governed.

 

[Retrograde]

That lateral deformation for the external column - are you sure it is caused by building movement? Maybe it was built like that.

 

[Craig_H]

Maybe it was built like that.

CSA S16's tolerance for plumbness & straightness of exterior columns is H/1000. You are right that it could have been built this way, but I would be surprised to see that degree of out of plumb and out of straightness go un-noticed previously. Other balconies also exhibit similar, but slightly less intense deformation. It's not just one column.

 

[azcats]

This appears to be a pretty common type of construction up there in the north. Wonder if one of the EORs on these projects could be found and helpful. To me, concrete supported by wood just seems....odd.

[URL unfurl="true"]https://www.google.com/maps/@53.3624955,-104.0151161,3a,81y,140.49h,99.23t/data=!3m6!1e1!3m4!1sxWHVB0JFgYKSyIErOWIOfQ!2e0!7i16384!8i8192?entry=ttu[/url]

[URL unfurl="true"]https://www.google.com/maps/@53.1892312,-105.7542973,3a,90y,74.48h,99.23t/data=!3m6!1e1!3m4!1szHUAF2LEg7-PbRYlA4QCvA!2e0!7i13312!8i6656?entry=ttu[/url]

[URL unfurl="true"]https://www.google.com/maps/@52.2019503,-105.1237311,3a,33.3y,210.5h,102.38t/data=!3m6!1e1!3m4!1seq2u5KVhpGD5_PvzjtkJLg!2e0!7i16384!8i8192?entry=ttu[/url]

[URL unfurl="true"]https://www.google.com/maps/@52.8638898,-104.6121999,3a,82.7y,194.12h,96.68t/data=!3m6!1e1!3m4!1sYhR1ET3pbw2zPa9-AXozfA!2e0!7i16384!8i8192?entry=ttu[/url]

 

[Craig_H]

azcats, I bow down to your internet sleuthing skills. I don't want to dox the contractor/developer (I feel rather fortunate to be able to have open conversations in anonymity on this forum), but you've definitely found a few of their past projects.

I have a sneaking suspicion that the EOR on many of these buildings may be the same as on the one I am looking into. Retired, unfortunately. I'll hopefully be able to chat with the architect early next week and hopefully that'll bring about some explanations.

 

[XR250]

Seems like a great way for firemen to get smushed