steveyeung
Civil/Environmental
Is flat flat suitable for a building over 30 storeys (with core/shear walls)?
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Flat Slab 1
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steveyeung
Civil/Environmental
PT?? Are the slab too thin (section modulud being too low) that tensile stress is very high??
do you have to be concerned about floor vibrations in flat plates? I'm working on a small replacement hospital project (5 story) where this type of construction is being discussed. I prefer to use a skip-joist or one-way slab and beam system to help control floor vibrations. comments?
steveyeung,
No, Span/depth ratio would be in the order of 35 to 40 (not the stupid figures some people quote) and the resulting stresses would be less than in an RC slab if it was designed properly.
What spans are you talking about and how thin do you think the slab is going to be.
archeng59
This is a different question. We have gotten away for too long without considering vibrations correctly in building floors. And some people are making them far too thin, often because they do not know how to design properly and/or understand the problems.
In a hospital it is even more important.
But there is no reason why you cannot do a vibration analysis on a PT slab system to make sure the slab in in the correct response range for any type of building.
No, Span/depth ratio would be in the order of 35 to 40 (not the stupid figures some people quote) and the resulting stresses would be less than in an RC slab if it was designed properly.
What spans are you talking about and how thin do you think the slab is going to be.
archeng59
This is a different question. We have gotten away for too long without considering vibrations correctly in building floors. And some people are making them far too thin, often because they do not know how to design properly and/or understand the problems.
In a hospital it is even more important.
But there is no reason why you cannot do a vibration analysis on a PT slab system to make sure the slab in in the correct response range for any type of building.
Numberseven
Civil/Environmental
Flat slabs can be used in the buildings mentioned. Building stability can be handled by the core and shear walls with the slab acting as a diaphragm. Spandrel beams acting with exterior columns will contribute to stability and lateral load resistance. I'm not quite shure what is the concern expressed by the question. Please explain further.
rapt,
I have a piled ground floor slab, piles at spacings varying from 3.0mm to 3.75m depending on the loading (40N/mm2 to 60N/mm2) The RC slab designed is 300mm thick with top and bottom rebar of T16mm @ 200mm.
Typically would you expect that we could provide a saving on steel using PT. Would you recommend single strand bonded PT as a proposal to ask a designer to check?
I have a piled ground floor slab, piles at spacings varying from 3.0mm to 3.75m depending on the loading (40N/mm2 to 60N/mm2) The RC slab designed is 300mm thick with top and bottom rebar of T16mm @ 200mm.
Typically would you expect that we could provide a saving on steel using PT. Would you recommend single strand bonded PT as a proposal to ask a designer to check?
dik,
They go a lot higher than 30 storeys.
Zambo,
Single strand bonded tendons are not economical. The smallest economical tendon size is normally 4 strands. VSL tried a 2 strand system in USA several years ago but it was very expensive and uneconomical (and they went broke). In normal slabs, bonded PT requires spans of about 7 metres to justify the use of 4 strand tendons and maintain reasonable tendon spacings. However with your very heavy loadings you could use 4 strand tendons in your slab.
With a slab this thick and with such short spans, I would use a mat of straight tendons top and bottom rather than draped tendons. Draped tendons would have too much losses.
You could save on concrete and steel but which would be most economical would depend on the local costs for RC versus PT.
You would also have to be wary of horizontal restraint effects in the slab from the piles.
They go a lot higher than 30 storeys.
Zambo,
Single strand bonded tendons are not economical. The smallest economical tendon size is normally 4 strands. VSL tried a 2 strand system in USA several years ago but it was very expensive and uneconomical (and they went broke). In normal slabs, bonded PT requires spans of about 7 metres to justify the use of 4 strand tendons and maintain reasonable tendon spacings. However with your very heavy loadings you could use 4 strand tendons in your slab.
With a slab this thick and with such short spans, I would use a mat of straight tendons top and bottom rather than draped tendons. Draped tendons would have too much losses.
You could save on concrete and steel but which would be most economical would depend on the local costs for RC versus PT.
You would also have to be wary of horizontal restraint effects in the slab from the piles.
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steveyeung
Civil/Environmental
Thank for all ur answers!!
In Hongkong, a lot of buildings constructed in the 70's and onwards are over 30-40 storeys. They are all (nearly 88.5%) in the form of shear wall-beam-slab construction. I just wonder what the flat slab is not commonly adopted. In the UK, there is a lot of flat slab structures.....I just don't understand why....
In Hongkong, a lot of buildings constructed in the 70's and onwards are over 30-40 storeys. They are all (nearly 88.5%) in the form of shear wall-beam-slab construction. I just wonder what the flat slab is not commonly adopted. In the UK, there is a lot of flat slab structures.....I just don't understand why....
Steveyueng
I think you will find it is because the old time contractors and local consultants like their old methods so PT is not used much in high rise. When we talked to the contractors many years ago they could not see any advantage compared to the problems having another major sub-contractor on site and did not like the idea of change.
The majority of flat slabs being done in UK are bonded PT but this trend has only happened over the last five years.
There are some consultants looking at PT high rise in Hong Kong but they tend to be connected with Australian consultants where it has been done extensively for 30 years or more.
I think you will find it is because the old time contractors and local consultants like their old methods so PT is not used much in high rise. When we talked to the contractors many years ago they could not see any advantage compared to the problems having another major sub-contractor on site and did not like the idea of change.
The majority of flat slabs being done in UK are bonded PT but this trend has only happened over the last five years.
There are some consultants looking at PT high rise in Hong Kong but they tend to be connected with Australian consultants where it has been done extensively for 30 years or more.
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