Hollowcore Slabs supported by prestressed concrete beams

[0.125pLL]

Let's say we are installing HC slabs to be supported by prestressed concrete beams. Let's also assume that supporting beams have camber (upward curvature) due to prestress even after they are in place. How to install slabs on surface (beams) that is not level? Is it resolved by shims, varying the thickness of neoprene strips or ..?

 

[cooperDBM]

Keep in mind that the weight of the HC planks will cause the supporting beams to deflect, losing a lot of their camber.

 

[KootK]

The trick with this is to remember that what matters for this is the camber differential over the width of an individual plank (4'?) rather than the camber over the entire length of the beam. And the differential camber over the width of an individual plank will usually be very small. Where beam camber does come into play is in the way that it affects topping thickness.

 

[KootK]

If one end of a plank bears on a cambered beam and the other bears on, say, a level wall, that would present a bit of a problem for a short, torsionally rigid plank. The planks do have some ability to warp to accommodate this however. The warping is not quite as flexible as with something like a double tee of course.

 

[r13]

Shim is simple but not ideal, for that small gaps will be left behind. Neoprene should be the choice. Neoprene is compressible (check with manufacture for type) and water proof. As cooperDBM pointed out, most camber will be gone after setting the HC slab into place.

 

[JAE]

Use of neoprene or other elastomeric pads isn't common for hollow core planks -

http://www.wellsconcrete.com/wp-content/uploads/2017/03/Ultra-Span-CS-Brochure-Booklet.pdf

See also PCI details like this one:

If there's no topping, then yes, you have an issue about camber and need to study it a bit for overall flatness of the untopped floor - may or may not be an issue as others have noted about dead load.
With a topping, you don't really have a problem as the topping will be ultimately floated and perhaps troweled to a flatness/levelness independent of the underlying camber.

 

[Agent666]

I've never seen neoprene used before for seating precast, it doesn't really meet the low friction requirements for bearing so would seem to be a unsuitable option.

Like JAE posted the standard is to usually use low friction plastic bearing strips (the korolath in the screenshot). This exists around the world in many forms/product names so search for what is used/specified in your local market.

If the surface is uneven you can specify seating the units on the low friction strips placed on a bed of mortar. Then leave mortar to set and don't disturb the unit. This seems to be the accepted method in these parts.

Never use shims unless you're also using a mortar bed (you don't want the entire unit weight bearing down on one point in the final constructed state). Never use steel shims, only plastic due to corrosion concerns of exposed steel.

 

[BAretired]

I've never seen neoprene used before for seating precast, it doesn't really meet the low friction requirements for bearing so would seem to be a unsuitable option.

If the hollowcore slabs are dowelled to the support, why should low friction be a requirement?

BA

 

[Agent666]

Hi BAretired, it's to do with accommodating seismic compatibility movements, and the fact that hollowcore has virtually no shear capacity until the strands develop. If there is no ability to slide then you tend to shear off the end of the unit and compromise the gravity load carrying capacity. You'll also be required to design the seating corbel for considerably higher horizontal tension forces than if you have a low friction bearing strip.

I can't really do an explanation justice so see here for a better/fuller explanation.

https://ir.canterbury.ac.nz/handle/10092/4211

Our codes in NZ only allow the use of low friction bearing strips, we also require bars into the cores to enhance the capacity near the supports.

There's quite a well known series of tests here in NZ circles where they subjected a full mockup frame to seismic drifts and the entire bottom half of the cores just fell out of the floor somewhat unexpectedly as a result of the above effects. I worked with a few people in the past who were attending university at the time of the tests. They noted when it collapsed in the testing hall they thought the building itself was collapsing because of the horrendous noise of it coming down.

The recommendations from these tests forms part of a strategy to stop this sort of thing occurring in real structures and has been in our standards since 2006.

Read about these particular tests here.

https://ir.canterbury.ac.nz/handle/10092/12896

 

[r13]

How much is the static friction differs for the two type of bearing materials? I know AASHTO still uses neoprene bearing pad for bridges.

 

[JAE]

retired13 - yes for bridges which are large "I" girders on abutments and piers. Not hollow core planks.

 

[BAretired]

Hi Agent666, I had not thought about seismic movements. It is not a concern in my neck of the woods. But because of their lack of shear capacity in the end zones, perhaps hollowcore slabs should be avoided in highly seismic areas such as NZ.

BA

 

[Agent666]

Hi BAretired, yeah I agree. Only thing is they are generally the cheapest long span concrete floor system round these parts, so they continue to persist to be used.

But to be fair double tees didn't fair that well either in our 2011 and 2016 major earthquakes though. We had a few flange hung tees in the 2016 event in particular that fell off the seating due to beam elongation issues and delaminated from the topping because there were no ties between the tees and the insitu topping. Also quite a few almost fell off the seating or sheared through the hangers which were only discovered when the structures were eventually demolished (hanging on by a thread in some cases). Luckily the 2016 earthquake occurred around midnight so office buildings affected were empty, otherwise I'm certain there would have been deaths due to this.

We've certainly seen a revival in the use of composite floors with composite decking since these events, so business is good for those guys..

I was under the impression that the bridge bearings simply deformed in shear rather than relied on sliding like a PTFE type hard plastic bearing strip.

 

[r13]

The first paper offered by Agent666 recommended/required the use of "low friction bearing strip", as "loss of support" was one of the primary failure modes from previous practices of placing HC slab directly on concrete support, or tied/bolted to the support. However, type of the low friction material was not addressed. Below is a table of friction coefficient for comparison use.

Concrete on concrete friction coefficient: 0.75 - 0.8
Static friction coefficient of Neoprene: 0.2

 

[Agent666]

Retired13, our concrete standard calls up that the material needs to have an in-service coefficient of friction of less than 0.7. In general these are the product used locally:-

https://www.lesasystems.co.nz/products/general-construction/lesa-bearing-strips/

http://www.canzac.com/products/civil-and-precast/bearing-strip/

All high impact plastics. From memory they are around 0.6 coefficient of friction in service.

While concrete on concrete is only slightly higher at 0.8, if its poured on top of one another or grout seeps under the units support bonding the unit to the supporting beam encapsulating the unit, then something 'concrete' has to break to get it to move causing the issues.

 

[r13]

Agent888,

Good to know other's practices and concerns. Thanks for the information.