Wideslab / Hollowcore typical bearing issues

A fella called fish

This thread has been cut from here http://www.boards.ie/vbulletin/showthread.php?t=2056885945

Its mainly to deal with issue which arose ion that thread and are listed in the first 7 posts below.

Please note that while "the specification of structural elements" are prohibited by the forum charter, this thread is to deal with the theory of the structural support of concrete slabs and the ancillary issues which arise. Standards prescribed by engineering bodies may be posted in support of a point but nobody may post detailed structural information about their projects, previous projects, nor ask specific question pertaining to specific projects.
sydthebeat



You mention that he wants to put the membrane down before the units are dropped in place, so presumably he's proposing to sit this membrane on top of the inner leaf?!

As a Structural Engineer I would be more concerned about the fact that you're introducing a slip plane between the top of the load bearing inner leaf (also a wind panel) and the stabilising rigid floor diaphragm...

This floor is also required to transfer horizontal forces back to the shear walls via the frictional resistance between underside of slab and top of block. I would question how it's going to do this when the frictional resistance between slab-membrane-block is practically zero.

...unless I'm picking you up wrong and you're proposing to fix this membrane to the outer face of the inner lea only, and not beneath the slab end?

Surely your Structural Engineer is providing a structural screed above the slabs to satisfy the peripheral tie requirements of the codes and regs in any event. This screed will overlap the end of the unit, sealing it anyway, so wrapping a membrane around this seems like a pointless detail if that is the case.

BryanF

A fella called fish wrote: »

You mention that he wants to put the membrane down before the units are dropped in place, so presumably he's proposing to sit this membrane on top of the inner leaf?!

As a Structural Engineer I would be more concerned about the fact that you're introducing a slip plane between the top of the load bearing inner leaf (also a wind panel) and the stabilising rigid floor diaphragm...

This floor is also required to transfer horizontal forces back to the shear walls via the frictional resistance between underside of slab and top of block. I would question how it's going to do this when the frictional resistance between slab-membrane-block is practically zero.

...unless I'm picking you up wrong and you're proposing to fix this membrane to the outer face of the inner lea only, and not beneath the slab end?

Surely your Structural Engineer is providing a structural screed above the slabs to satisfy the peripheral tie requirements of the codes and regs in any event. This screed will overlap the end of the unit, sealing it anyway, so wrapping a membrane around this seems like a pointless detail if that is the case.

so how does this work where theres a dpc course - you just copy theat same approach here

A fella called fish

You've already mobilised the weight of the shear wall at dpc level, but it's an issue that often arises on taller buildings. A slip plane at the top of the wall panel doesn't sound good from a structural perspective..

As I said above however, I don't see the point in it if you're following the standard screed details provided by the Irish Precast Assoc. (ie ends of core are covered off with 100mm screed overlap anyway)

Poor Uncle Tom

A fella called fish wrote: »

Surely your Structural Engineer is providing a structural screed above the slabs......... This screed will overlap the end of the unit, sealing it anyway,

I don't think you are picturing this correctly, the screed would have to be poured into the cavity to cover the ends of the hollow core slabs.

sydthebeat

A fella called fish wrote: »

You've already mobilised the weight of the shear wall at dpc level, but it's an issue that often arises on taller buildings. A slip plane at the top of the wall panel doesn't sound good from a structural perspective..

As I said above however, I don't see the point in it if you're following the standard screed details provided by the Irish Precast Assoc. (ie ends of core are covered off with 100mm screed overlap anyway)

hi a fella called fish, if you want to bring this issue to a different thread id be more than happy to create one.

The restriction in the charter is to the "specification of structural elements" so we would just be chatting about structural theory, which is of course allowed and positively encouraged.

A fella called fish

Poor Uncle Tom wrote: »

I don't think you are picturing this correctly, the screed would have to be poured into the cavity to cover the ends of the hollow core slabs.

The slab requires a min of 80mm bearing. A 215mm wide load bearing wall should be provided as per IPA recommendations, meaning that the screed is turned down to cover the remaining thickness of the wall thus providing the required peripheral tie.

A fella called fish

A fella called fish wrote: »

The slab requires a min of 80mm bearing. A 215mm wide load bearing wall should be provided as per IPA recommendations, meaning that the screed is turned down to cover the remaining thickness of the wall thus providing the required peripheral tie.

Apologies - this post might verge on "specifying struct. elements". Please feel free to adjust, although I'm only trying to clarify the standard detail.

Thanks

sydthebeat

A fella called fish wrote: »

The slab requires a min of 80mm bearing. A 215mm wide load bearing wall should be provided as per IPA recommendations, meaning that the screed is turned down to cover the remaining thickness of the wall thus providing the required peripheral tie.

so do you see any problem with a wideslab bearing on a 100mm inner leaf at all.... (assume all discussion here is about a standard 2 storey dwelling)

A fella called fish

sydthebeat wrote: »

so do you see any problem with a wideslab bearing on a 100mm inner leaf at all.... (assume all discussion here is about a standard 2 storey dwelling)

Assuming the 100mm block can take the combined vertical loading and lateral wind loads (?) then it should be acceptable from a theoretical, structural design perspective.

There is however the practicality of building it. I.e you have practically zero tolerance and it's not very robust in terms of a crane swinging a PC unit onto a 100mm wall. I've never specified this detail and never would.

1100010110

A fella called fish wrote: »

Assuming the 100mm block...then it should be acceptable from a theoretical, structural design perspective.

There is however the practicality of building it. I.e you have practically zero tolerance and it's not very robust in terms of a crane swinging a PC unit onto a 100mm wall.

I'm no architect/engineer but I'm pretty sure this is how my house has been built. It was designed and has been overseen by an architect from the start, 100mm block inner leaf on which the hollowcore rests, having had the ends wrapped in air-tight membrane. I live in the North West so no stranger to being battered by the wind.

sydthebeat

A fella called fish wrote: »

Assuming the 100mm block can take the combined vertical loading and lateral wind loads (?) then it should be acceptable from a theoretical, structural design perspective.

There is however the practicality of building it. I.e you have practically zero tolerance and it's not very robust in terms of a crane swinging a PC unit onto a 100mm wall. I've never specified this detail and never would.

firstly, in my locality wideslabs are used in the vast majority, and secondly, they are primarily installed this way.
There is actually loads of tolerance as there is a cavity to play with if needed. Generally, in the installations ive seen, there is almost no more that 20mm difference along a wall length.
The first floor inner leaf begins then on the edge of the wide slab.

How does introducing a plastic barrier create a 'slip plane' in any manner different than a dpc level, or even a slate cavity closer?

A fella called fish

I've done the calcs before for this scenario and it didn't work (in theory) for standard 5N block subject to full imposed and wind loads. Of course we rarely get these loads in domestic situations and also have design factors of safety which is why most domestic buildings, even if poorly built, don't fall down.

On another job my units came to site and were erected with 45mm bearing on a 215mm wall! After rigorous checking calcs it was eventually justified, however if this happened on a 100mm wall then the out of plane loading would cause serious problems.

The 'slip plane' occurs in this instance at the one point a Structural Engineer wouldn't want it, i.e between underside of slab and top of supporting wall. It's considered as a slip plane because the coefficient of friction between concrete & plastic sheeting is negligable. So as this thread relates to structural theory, in theory this support would be considered as a roller support around the entire perimeter of the structure. Any software package or basic structural theory hand calc will let you know that this is theoretically unstable.

Personally I think that bearing PC units onto 100mm walls is madness unless you're dealing with very short spans...and even then, I would never specify this personally. I have however used 150mm blocks in the past as a compromise, however I certainly wouldn't go any less.

muffler

A fella called fish wrote: »

The 'slip plane' occurs in this instance at the one point a Structural Engineer wouldn't want it, i.e between underside of slab and top of supporting wall. It's considered as a slip plane because the coefficient of friction between concrete & plastic sheeting is negligable. So as this thread relates to structural theory, in theory this support would be considered as a roller support around the entire perimeter of the structure. Any software package or basic structural theory hand calc will let you know that this is theoretically unstable.

Im playing devil's advocate here so tell me, in a standard 2 storied house how exactly (theoretically of course) is there a structural risk when wrapping the slab ends in polythene? -100mm inner leaf say, with 90mm bearing

A fella called fish

From a purely theoretical point of view, the slab is supported on 'roller' supports on all sides.

Anyone who has any design software - input a beam with a horizontal roller at each end, it won't even do the calc for you as it issues a stability warning.

Not only that, but typically we rely on wind loads hitting the wall to be transferred to the slab - which acts as a rigid diaphragm and transfers this horizontal force back to the stabilizing shear walls. As all these forces are transferred via friction, installing a plastic sheet interrupts this load path.

This analogy would be extreme as it has ZERO redundancy (ie no ties/cross walls and zeor friction) but imagine a heavy book supported on ball bearings which are supported on your desk. Now apply a load to it and see what happens.... Basic first principles in an extreme case, but if you have a frictionless joint between wall/slab then that's how it would be considered from a strucutral/theoretical perspective.

1100010110

As stated zero arch/eng knowledge but a limited geological knowledge, and if I gather correctly, what you are saying should manifest itself as a result of any local seismic activity, therefore if one were to go up to Malin head and view any houses built in this manner they should have fallen down?

brdboard

A fella called fish wrote: »

From a purely theoretical point of view, the slab is supported on 'roller' supports on all sides.

Anyone who has any design software - input a beam with a horizontal roller at each end, it won't even do the calc for you as it issues a stability warning.

Not only that, but typically we rely on wind loads hitting the wall to be transferred to the slab - which acts as a rigid diaphragm and transfers this horizontal force back to the stabilizing shear walls. As all these forces are transferred via friction, installing a plastic sheet interrupts this load path.

This analogy would be extreme as it has ZERO redundancy (ie no ties/cross walls and zeor friction) but imagine a heavy book supported on ball bearings which are supported on your desk. Now apply a load to it and see what happens.... Basic first principles in an extreme case, but if you have a frictionless joint between wall/slab then that's how it would be considered from a strucutral/theoretical perspective.

I would think that you are mobilise the weight of the precast slab, plus the internal leaf above, and so the weight from this increases the coefficient of friction between the slab and the load bearing wall underneath, or from the slab to membrane and membrane to wall. I don't think it is correct to say there is a slip plane at this point, it is the same as the dpc/dpm scenario referenced earlier.

A fella called fish

brdboard wrote: »

I would think that you are mobilise the weight of the precast slab, plus the internal leaf above, and so the weight from this increases the coefficient of friction between the slab and the load bearing wall underneath, or from the slab to membrane and membrane to wall. I don't think it is correct to say there is a slip plane at this point, it is the same as the dpc/dpm scenario referenced earlier.

Which can be very easily calculated if you know the coefficient of friction between concrete and the membrane being installed. It 's effect will be almost negligeble though. Especially if only using 100mm walls!

The entire detail is pretty poor from a structural perspective, although I'm sure it has many other positives..but one fundamental flaw in my opinion.