Hollowcore flooring system issues under seismic loading 1

[moyseh]

Hi all,

My query is regarding the hollowcore flooring system which is prevalent in NZ. My understanding is that this item is an industry wide issue and there's currently work being done to investigate retrofit options. Is it fair to say that some of the newfound failure modes only happen at higher drifts and not for lower seismic zones/one storey structures etc?

Furthermore, are there any approved retrofit strategies for the negative moment failure mode? My understanding is that sometimes you can make this failure mode worse when trying to add angles for the Loss of support(LoS) failure mode.

Would be interested to get any thoughts.

 

[rowingengineer]

There is a good sesoc article on this, probably worth digging up.

 

[Agent666]

Also look into the RECAST Floors research and findings, they tested some full-size assemblies with all manner of strengthening options. They did a presentation to some of the structural groups around the country. I didn't attend but a colleague sent me the PowerPoint slides (will see if I can find again).

https://www.quakecentre.co.nz/news/recast-flooring-multi-storey-building-__I.12253__N.112

For negative moment failures I think they tested stuff like FRP, and rebated epoxied reinforcement. Also tested several configurations of catch frames from memory. Still not much in terms of concrete advice resulting from the testing yet as far as I'm aware.

Yeah you can in theory make it worse with adding support angles as it moves the support point further along the saddle bars so you run out of development if you have shorter bars.

You're aware of the requirements in the NZSEE guides for the assessment of existing hollowcore? The fact that for some modes of failure you need to put a factor of 2 on the drift means they can become an issue even for relatively low drifts. The only way to know for sure is to actually review it.

https://engineervsheep.com

 

[moyseh]

Thanks for the comments. I had a look through the guide but couldn't see anything about the factor of 2 for drifts you mentioned. If you find that please let me know.

Are these the slides you mentioned ?

https://concretesociety.org.nz/Downloads/2018/S3_Chris Poland Presentation.pdf

Cheers

 

[Agent666]

This is the clause I was referring to on page Appendix C5-25. As noted it is considered a significant life safety issue so has a factor of 2 applied to any drifts when doing the assessments. This is to ensure essentially that you can reach the MCE level event without these seating or failure modes occurring, given it is a brittle failure mechanism that lacks ductility it could occur after the ULS and before the MCE level event which would not meet the life safety objectives in the code.

That PDF is an example calculation I believe it is/was published in the SESOC journal. It wasn't what I was referring to. Will have a look for it tomorrow for you as it had a lot of photos of all the different aspects they were testing.

https://engineervsheep.com

 

[Agent666]

This was the presentation slides I was thinking of. Hopefully it gives you some ideas.

https://engineervsheep.com

 

[moyseh]

Thanks Agent666 - it seems like the industry is still in limbo about addressing a lot of these issues relating to hollowcore and the best approach forward for retrofits.

 

[Agent666]

Yeah I think there seems to be agreement on the need to address it, i.e. Engineering NZ official position on the use of HollowCore. But the "how" to address is a bit less defined. We kind of have some rather half arsed assessment methods that are not that clear to implement unless you have idealised conditions. Then there is an almost complete lack of agreement on details on how to effectively address each shortfall, at least in some consistent manner so everyone is achieving the required performance expected. In the interim you've got everyone doing 101 different things based purely on judgement.

I guess they want to get it right, given a lot of previously strengthened buildings in the past (like the addition of support angles traditionally to address primarily a seating issue) actually ended up making things worse for the negative flexure failures given what they know now on how these strengthening details actually performed in Kaikoura quake (in Wellington at least).

I remember looking at a 20 odd storey building in Wellington a few years back that had been previously strengthened by a reasonably reputable firm, and I couldn't make head nor toe regarding how some of the things they did were even addressing the root cause of the buildings issues. And because of this the owners were up for a second round of strengthening to bring things up to the currently accepted standard (the yellow book - C5 guidelines). Last I heard they just sold it as it was, and passed on the problem (seems to be a lot of this going on!).

A lot of owners are just hanging back until there is some certainty that the strengthening goalposts won't be changed again in 5-10 years, but times running out in the interim for the dictated strengthening period by local TA's, especially in Wellington where the timeframes were generally shorter I believe due to the higher seismic risk there.

https://engineervsheep.com

 

[moyseh]

Yeah totally agree with all of that. The other issue that seems to crop up is that most hollowcores tend to just have a 75 topping with mesh as the reo. Normally with the pESA method, the capacity is far exceeeded even with one-storey structures in low seismic areas. But I suppose this issue can be fixed with FRP - not so simple with the other failure modes actually associated with the hollowcore (PMF,NMF,LoS)