Thanks for the comments.
1. We followed BS and therefore I mentioned the reference code. Other codes may have different requirements.
2. Slab deflections have been checked in compliance with BS (span/depth ratio) and accordingly reinforcement is increased in midspan.
3. Fire resistance - 2 hour period is considered and accordingly 30mm cover to bottom bars is provided as per code. Building is designed with sprinkler system and false ceiling is provided. For cover to top bars finishes have been taken into account as per BS.
In some of the old fire damaged structures where concrete quality was ~20N/
sq.mm and no fire-fighting system (other than hand held extinguishers) was installed, I have noted that the plaster to columns and slabs spalled, however, the concrete remained intact.
In my opinion, with due regard to safety and serviceability, in modern structures where the client is required to spend money in mandatory fire-fighting system and expensive ceilings there is need to optimize concrete elements.
Pendharkar
------Original Message------
I do not think that top finish (even though rigid) can be considered toward evaluating Fire Resistance.
Secondly, 90 mm thickness for about 2.8 m span is seen as too less, from present and feature deflection parameters. Besides this may not fit in the minimum cover for rebars at bottom and top.
Thirdly, the fire resistance is required to be ensured, of different duration of fire resistance, by providing actual integral cover to reinforcement, both at top and bottom.
In my view, the slab thickness needs to be increased, to say at least 110 mm, if not more.
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SATISH DHUPELIA , PROPRIETOR
Consulting Structural ENGINEER
STRUCT-ARCH
MUMBAI, INDIA
+919820028705
SatishSatishSatish
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Original Message:
Sent: 03-24-2018 02:57
From: Shashank Pendharkar
Subject: Fire requirement for thickness of concrete floor slab
We are designing a multipurpose commercial building of concrete. Column grid is typically 8500 mm in both directions. Main beams connect columns and secondary beam are spaced at of 2833 i.e. 1/3rd span in both directions. Floor slab is 90mm thick.
Non combustible floor finish comprising of concrete screed and tiles etc. is 75 mm thick.
Estimated fire resistance for the floor = 90 mm + 75 mm = 165 mm > 2 hrs. requirement of 125 mm.
Comments are welcome on the above method of estimating fire resistance.
Pendharkar