I have no problem justifying it by calculations in
meeting building code. What haunts me is the long-term serviceability. Keep
in mind the slab is pretensioned and I don't expect any cracking under service
load. The 50mm immediate deflection you mentioned will be probably canceled out
by the upward prestressing camber.
I am not a believer in the approximate methods in
the codes for deflection calculations either because they are grossly
inaccurate and not conservatively so in many cases. But I am a believer in
proper deflection calculations which allow for cracking and long term effects
logically as our software does, not simply as fudge factors.
with a short term uncracked deflection of about 50mm, I would not even worry
about calculating the cracking and long term effects. The slab is not pushing
the limits, it is way past them.
You can use any arguments you like
about the slab never being loaded etc but the end result is that you have
presumably been contracted to provide a design to the local design rules which
normally requires deflection checks for the design loads. That is what the
client is paying for.
Hope your insurance is paid up and there is no
clause in it voiding the cover because you deliberately ignored standard
design procedures and logic.
At 10:16 AM 13/05/2005, you wrote:
I know the 8" slab
is mathematically pushed to its limits for the span in terms of
serviceability and strength by calculations. Long-term deflection is a
complicated thing and I am not a die-hard believer of the formulas. But I am
interested to know the practical results of any similar
You raised a valid
concern about vibration. Theoretically the slab system has a frequency of
around 3 Hz. However, being a dormitory building floor slab, there are many
non-structural partition walls running floor-to-floor height and the
vibration will be greatly mitigated.
I would be very worried about deflections, vibrations and everything
else. It is too thin. Do the calculations.
At 06:18 AM 13/05/2005, you wrote:
I am pondering on using 8" composite
slab (3" pre-tensioned plank with 5" topping) for a 33-ft simple span
floor slab in a dormitory building. The plank will be shored at
mid span before topping. My only concern is the long-term deflection.
Has any one had similar experience?