Hollowcore Questions

[spats]

I have a situation where one end of a 25' span, 6" hollowcore plank with 2" concrete topping is deteriorated due to water damage. My current plan is to shore the plank, cut about 7'-6" from the damaged end, add a support consisting of a steel beam and two columns, and pour back the damaged area with 8" thick CIP concrete. I have two questions:
1. I'm concerned that adding a hard support within a span when adjacent planks tied to the damaged plank with a grouted keyway want to deflect.
2. Do I need to be concerned about how cutting the plank may affect it's design? I'm not a prestress guy. The plank is basically unloaded, except for selfweight plus topping.

 

[JedClampett]

I'm assuming that the columns and beams are permanent.
1) If they don't push up on the hollow core, you should be fine. It's already deflected for it's dead loads, so if you don't disturb it and move it, it should be fine.
2) Probably not. The strands should develop in the shorter section, as you're moving the point of maximum moment away from the support. If you're worried, call a hollowcore engineer. I've never had an issue with getting information like that from them.

 

[msquared48]

7'-6" is beyond the normal of four to five feet for access manholes and open access grates with knee walls, but it still should be OK.

Is there any way to contact the plank manufacturer and ask the same question?

Mike McCann
MMC Engineering

 

[spats]

This is an old apartment building built in 1970 that is being renovated. I have no way of knowing who the hollowcore manufacturer was. I've looked at hollowcore information in the PCI Design Handbook in hope of identifying the manufacturer based on the size, spacing and shape of the cores. The closest one in looks is Spancrete, but not exactly. My typical planks are 40" wide, which doesn't match anything currently in use.

 

[UcfSE]

Coreslab uses 40-inch planks.

 

[briancpotter]

1. This should not be an issue. As Jed said, the DL deflection has already occurred, there likely won't be any problems unless the renovations is dramatically increasing the load rating for this area of the building.

2. Cutting the slab shouldn't effect the design (again, assuming your loading pattern isn't changing dramatically) - you'll now have the same amount of strand as before resisting a lower moment.

That said, it's difficult to know for sure without information about the strand pattern used. Manufacturers' tend to use a variety of strand patterns, so knowing the manufacturer alone will provide limited information - you really need a set of construction documents to know for sure.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[spats]

UcfSE - according to their website (which has NO technical infomation), Coreslab first went into the precast business in 1975. My building is 1970.

Brian - thanks for the input. The system dead load is about 70 psf, and an apartment live load is 40 psf, plus maybe a light partition load. There will never be 40 psf present, so I guess I shouldn't be too worried about cracking due to a hard support out in the span. I think I may put a note on the drawing that cracking may occur. The adjacent hallway will have linoleum, and the adjacent bathroom will have ceramic tile... that is my main concern.

 

[JedClampett]

I'd call them anyhow. Possibly they bought whoever built the sections or maybe they could give you a lead. Plus they might be able to answer the general portion of your questions.