Existing RC slab evaluation 5

[phuduhudu]

There is an existing roof slab which had 105mm brick partition walls under. These have been removed but now the slab spans from the edges 7m x 9m. I was asked to look at it. Including screeds the slab is 210 thick. We took out some small areas of honeycombed concrete and found undeformed 12mm bars running in both directions at approx 200mm centres. I am having someone drill through the slab so we can get the exact thickness and checking the bar spacing with a cover meter. when I do the calcs (making many assumptions0 it doesn't work as it is. I have had it propped meantime but we will need a permanent solution. The client could accomodate one central column. Any ideas on how to go about this. Carl Bauer

http://www.bauerconsultbotswana.com

 

[visitor]

Can you provide some steel beams below the slab ( the steel beams can span between the columns / walls at the boundary?
If this is possible then you can avoid the central column.

 

[muyi74]

From what you have stated, there is no way that that slab would hold. So the best option is to break the span of the slab to reduce the mid-span moments.

Incorporate UB(Universal Beam)steel section to run below the roof slab along the shorter span to break the 9m span edge into two-halves. The Stanchions and base can then be placed within the surrounding blockwalls to recieve the UB.

Don't forget to check the structural implications as the above would introduce some hogging moments which has to be taken care of. With a slab that thick, you might not need top reinforcement!!(Please check)

These would avoid the use of central columns. @#@#@#@

 

[flamby]

I suggest you recheck the slab with assumption of all corners held down ( with effective arrangment of concrete counterweights placed at top of all four corners). The slab may survive.

 

[Ron]

Carl,
Since this is a roof slab, you could angle drill near the supports (45 deg preferred) and run cables under the existing slab, post-tension, and anchor on top. While it isn't conventional in the concept of post-tensioning, it does provide supplementary structural support and alleviates the center column issue. Stress blocks on top could then be grouted and flashed into the roof without much difficulty.

In short, the cable would start on the roof, penetrate the slab at 45 deg., run under the slab, exit on the opposite side at 45 deg. and be anchored after tensioning.

You can decide, from the needed supplementation, the spacing and size of cables, but they would likely be small (1/2"-9/16&quot.

 

[Ingenuity]

carlbauer,

Ron's suggestion of external PT is becoming a very common strengthening solution (certainly in the USA), but you will have to be the judge for suitability in Botswana.

A few things with your case - the slab length is very short and any stressing using strand systems you will have immediate draw-in losses (wedge seating) that needs to be accounted for in the calcs OR use hydraulic seating of the wedges. For a 20' tendon the total strand elongation at 28 kips is about 1.5", so loosing say 1/4" in draw-in, as a % of the total force you will loose 15%!

Also, you will have some elastic shortening of the slab which will get restrained by the perimeter masonry.

You also need to consider fire rating/protection of tendons at underside.

HTH and good luck.

 

[ishvaaag]

I still see too close to the support the sole induced upwards forces to help against downwards bending. I'd rather love some beam thing plus support/s if required than that.

 

[Ingenuity]

ishvaaag,

For the external PT case, you would typically use harped tendons with say 1/3 point (or similar) deviators/saddles (most common are steel tubes) to provide some eccentricity and an increased equivalent upward PT force. In some cases (not necessarily this case) you would run tendons in both directions and balance loads accordingly.

 

[RiBeneke]

Carl,
If your Botswana codes of practice are similar to ours here in SA, then the 105 mm brick partition wall classifies as non-loadbearing, and should not have been supporting the slab anyway !
Yet another suggested fix is to reinforce with one or more concrete upstand beams above the roof slab, if the drainage can still be made to work. The upstand would need to be well keyed at the joint for shear, and cast onto anchor bolts that go through the existing slab and are fastened underneath.

 

[letrab]

In Botswana I would suspect roof loading to be approx .5 kN/m2,presuming it is inaccesible which is equivalent to 50 mm water. This seems to be an ideal opportunity to test load the slab and check deflections under 50 ,60 and 75mm.Should you be satisfied with the slab's peformance under 75mm of water(LF of 1.5) you can certify its integrity
Chances are you would not need any strengthening. safety during testing is however a concern

 

[ishvaaag]

Ingenuity, the 1/3 of span stubs as deviators makes more sense than what prior to that described.

 

[muyi74]

RiBeneke
the upstand beams would be supported by what??
@#@#@#@

 

[RiBeneke]

My description was rather brief. Sorry.
The beams would be cast on top of the slab, and bonded/anchored to the slab, forming inverted Tee beams. The web of the Tee would be in compression, but for light loads (mostly self-weight of slab) this should be ok.
The Tee beams would extend to the edges of the slab, and be supported on the existing walls (or columns) by the load passing through the thickness of the slab at the edges.
It would be important to get good bond and shear transfer through the old/new concrete joint near the ends of the beam.