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  • 1.  Fire requirement for thickness of concrete floor slab

    Posted 03-24-2018 09:35 AM
    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


  • 2.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-25-2018 09:20 AM
    Edited by Tirza Austin 03-25-2018 09:26 AM
    90 MM slab seems to be too thin. From long term deflection point of view. 

    Which local fire code are you using? 

    In the structure design we are concerned with the concrete cover of the structural element only. Don't add finishes to it. 
     
    Please note that you don't need to calculate the 2 hours fire resistance in the fire resistance code. It will be given how much cover should be provided for the 2 hour resistance. 

    If you have any questions feel free to ask. 


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    Mirza Baig Aff.M.ASCE
    N/A
    Hyderabad AP
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    Original Message


  • 3.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-26-2018 07:57 AM
    Edited by Tirza Austin 03-26-2018 07:56 AM
    This is interesting. I consider fire the resistance aspect for the determination of concrete cover to reinforcement. However, much thick finishes shall be done on the floor, it tends to increase cover to fire resistance for top reinforcement but not for the bottom. This approach will work on a single dimension but not in global.  Please reconsider the floor in totality. 

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    George Okumu P.E. M.I.E.K
    Nairobi, KENYA
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    Original Message


  • 4.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-26-2018 08:57 AM
    Edited by Tirza Austin 03-26-2018 08:56 AM
    The resistant of a member usually depends on how much it is provided with. If the member is provided with large cover then it resists the fire to a  greater extent - hence thick concrete floor would resist the fire.

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    [supriya [hv] S.M.ASCE
    [Student]
    [Mysore] [India]
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    Original Message


  • 5.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-27-2018 12:06 PM
    Fire resistance of a slab has several components.  Structural capacity is obviously required, driven by reinforcement temperature but other things such as continuity can be very important.  There is also a heat-transmission limit saying that the top surface temperature of the slab cannot increase by more that 250F.  90 mm is good for one hour, just barely.  Heat transmission often governs in slabs.  Some finishes add endurance.

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    William Gamble Ph.D., P.E., M.ASCE
    PROF EMERITUS
    University of Illinois
    Urbana IL
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    Original Message


  • 6.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-28-2018 12:09 PM
    Edited by Tirza Austin 03-28-2018 12:08 PM
    Our design of 90mm thickness continuous slab is based on the provisions of BS 8110 Part 2 -1985 Section 4 Clause 4.2.5 which states that for Fire Resistance:
    "In the case of solid slabs the thickness to consider is the actual thickness of slab + any non combustible finish on top."
    Pendharkar



    Original Message


  • 7.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-28-2018 12:10 PM
    Edited by Tirza Austin 03-28-2018 12:10 PM
    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


  • 8.  RE: Fire requirement for thickness of concrete floor slab

    Posted 03-29-2018 10:22 AM
    Edited by Tirza Austin 03-29-2018 10:21 AM
    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


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