Fixings into Concrete Slab 1

[KayneR]

Hi there,

So the reason I have this question is that we had to design a prop to support a tilt panel wall before it was tied into the steel portal structure.
we can size the prop and tie it into the tilt panel easy enough. We can analyse the shear for where the prop is tied into the existing concrete slab.
Where we are struggling is for the uplift component at the existing slab.
We can size what the expected uplift will be and we put the appropriate factors of safety on this. But what area of concrete can we assume to pick up beyond the fixing?
The strengths of the fixings might say 25kN, we have a 100mm slab of concrete, to pick up 25kN we would need an area of 10.4 m^2. This seems like a lot. The slab is reinforced with a high strength mesh.
The fixing manufacturers fix their products into concrete and then have supports either side so that there will always be enough resistance, may not be totally applicable.

Does anyone have rule of thumb or know where to find something that says how far beyond a fixing you can pick up load?

Note the values I have used are arbitrary, I am more looking for a better understanding of a principal rather than a solution.

 

[Settingsun]

The only place I've seen such a rule of thumb was to do with how much of the floor slab in an industrial building could be added to the pad footing weight when considering restraint to column uplift. The rule of thumb was one metre either side of the footing. Since your prop will be close to a point load, this would give you 4sq.m.

Note the different context to your situation: the pad footing is providing most of the weight with the slab just giving that bit extra for the extreme wind case (the ultimate load); the slab isn't the only resistance in this case.

The caveats given are that the effective slab area is subject to variation due to joint layout, actual cover to the reinforcement (will change the capacity significantly in your 100mm slab); random cracking due to restrained shrinkage etc. Sensitivity to these should be checked if you were going to do a yield line analysis to calculate the effective slab area rather than use a rule of thumb.

 

[KayneR]

Another time I have come across this issue was with out timber braced walls.
Say we had a 1m length of wall 2m high, it would give us 6kN braced capacity. This lead to a 12kN uplift requirement for the hold down bolts.
But in a typical residential house where the braced walls are used. There is only a 100mm slab.
I asked a researcher about this and he said due to the dynamic load case that it's not an issue. The braced wall will still achieve the required strength basically.

Sorry if my question seemed really basic, still fresh out of university and still have a lot to learn.

 

[Settingsun]

I don't do residential work and it seems like you need a bit of experience and faith to make a profit in that area - can't calculate everything.

I guess the slab in your example wouldn't only be a metre long but would extend beyond the wall in one or both directions. If it's only a horizontal load applied (ignoring self-weight and uplift), then the sum of vertical loads on the slab would be zero, ie while one side is lifting, the other side is being pushed down. It then becomes a question of whether the overall slab extent and strength are enough to provide stability against the force couple, not just a question of whether the slab weighs enough locally.

 

[Toby43]

@KayneR
I would think that the area of slab available to you would be greater than 10.4m2. As long as the concrete bound within this area of available slab can cope with the internal forces, then shouldn't be a problem. If available slab is less than 10.4m2, then overall stability is not achieved. All forces must be taken into the ground, thus a further stability criteria against sliding may be required - diaphragm action of slab on grade and frictional should suffice if slab is part of completed building.
We have historically used 125mm thick slabs for tilt jobs, which stemmed from minimum thickness requirements for temporary fixings as per your post.

As for you timber framed bracing wall - Tension and Compression couple result in 0 net vertical force, but rotationally the slab weight multiplied by its lever arm from resultant of soil pressure must be greater than moment from wall i.e. 6kNx2m height. Not necessarily a common concern for most light frame construction - I can't imagine a timber framed shearwall been able to generate enough force to globally rotate a typical slab, along with the structure above it.
I would recommend looking at post damage reports from actual events, which will give you a link into real world performance - for example the APA damage assessment report into the 2015 Rowlett, Texas tornado gives some good insight into lateral failures of timber framing.

@steveh49
Contributary slab to footing uplift - I believe Woolcock et al in their publication "limit state design of portal frame buildings" utilize similar 1m extension around edge of projected footing. I too use this rule of thumb, but always ensure the footing itself can take serviceability uplift due to its weight alone.

Toby

 

[Settingsun]

Hi Toby,
We're looking at the same book (Woolcock) so can't count this as independent confirmation. But 1m seems reasonable and I do the same as you with regard to a minimum footing weight.