e-conference on tall buildings
sub: Hollow Core Slab(HCS)
It is seen from the ongoing discussions in the forum that there is some difference of opinion among experts regarding the use of hollow core Slab
(pretensioned)and its adaptability in high rise buildings. The performance of HCS used in Newzealand,Australia and Mexico during earthquake were not much different from framed buildings using rcc beams and slabs
This may not be suitable for very tall buildings in high seismic zones; but can be used in low seismic zones for medium high buildings, say upto 10-20 floors.
I have used these HCS for medium high buildings in Bahrain which is a low seismic zone area.Middle East countries like region Saudi Arabia,UAE, Bahrain,Qatar,Kuwait and Sultanate of Oman are low seismic areas except Iran,Iraq and Turkey. HCSs are simply supported slabs resting on a beam of a framed structure. Over these slabs,there is no topping with 75 mm or 100 mm screed with or without nominal reinforcement.
Prof.Dr.C.V.R.Moorthy�s observation is valid in high seismic zones because of its unsuitability in vertical accelerations during earthquake as HCS rests on beams as simple horizontal supports.
HCSs are produced in Middle East countries under technical guidance and collaboration of European countries like Denmark,Holland,Norway and Austria.Etc. The firm in Bahrain is working under Danish collaboration and Danish engineers are working in the design department and supervision areas along with local staff.
These slabs are produced in different thickness according to the span and loading and the number of strands used for pretensioning is varied in diameter and groups(see the following table attached)
150 mm -suitable for spans upto7.0m
200 mm -suitable for spans upto9.5m
300 mm -for spans upto 18-20 m
Of late they are producing slabs with 340 mm thickness which can be used for spans upto 24 m span (all simply supported spans).I have used 340 mm slab for a Palace Building in Bahrain.
These slabs are cast in factories on a casting yard of 100 m length over steel platforms. One end is anchoring end of the other is the prestressing end and hydraulic jacks are used for stressing the strands .The strands are pretensioned as per design calculation. There is a traveling crane across the platform to which is connected a hopper and below a mould depending upon the thickness of the HCS .The moulds are interchanged for different thickness of slab.The design mix for slab is 50 N/mm2- 60N/mm2 and is based on Limit state method of design as per British code or Euro code .The traveling crane moves from one end of the platform to the other end and the concrete is supplied through the hopper to the mould below when the concreting is completed, the slabs are cut to the required length on the second day or the third day.In a factory there are 2 or 3 casting yards for different thickness of HCS.
After 5 days and after curing, the slab is removed from the platform .There is a small upward chamber to the slab due to prestressing and which will be compensated when the imposed load is applied.
Between two slabs there is a small tapering space (see the sketches below) along which one longitudinal bar can be inserted and grouted using non shrinkable grout after laying the slab in position. Finish is superior as it is cast on steel moulds .Autoclaved curing is also done in very urgent cases after removal from the casting yard. For laying at site cranes are required
(见附照片、负载表和allowable spans)
K.Gangadharan
structuralconsultant
Trivandrum
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