PEPSCON-2018 Structural Fire Resistance Of Concrete - Requirements And Necessity Codal Provisions

Transcription

1 PEPSCON-2018 Structural Fire Resistance Of Concrete - Requirements And Necessity Codal Provisions Dr. T.P.Tezeswi Assistant Professor Dept. of Civil Engineering NIT Warangal

2 Overview Fire Hazard Fire Protection Consequences of fire Response of structural elements to fire Codal Provisions Case study Research Needs

3 History: The First Code In 1800 B.C. the Amorite king, Hammurabi, took the throne of the new Babylonian dynasty and established an extensive law system which encompassed nearly every area of ancient life The code of Hammurabi made the builder accountable for the houses he built. If one of his buildings fell down and killed someone, the builder would be put to death.

4 History: Building Codes First modern code on record was in 1625 in what was know as New Amsterdam (New York). Addressed fire prevention and governed the types of roofing materials that could be used to protect from chimney sparks. Chicago fire of 1871 caused many large cities to create their own municipal building codes.

5 Fire Hazard The factors to consider in the development of a fire are: the probability of a fire starting the intensity of the fire the speed at which the fire may spread the existence of specific risks (toxic products). The probability of a fire starting depends on: heating systems and electrical equipment; the presence of inflammable gas certain industrial chemical processes; the presence of dust that could create explosions. spontaneous combustion of rags saturated in oil, the auto infla mmation of steel wool by batteries, etc.

6 Fire Hazard The origin of the fire danger may be: Internal and linked: to the building itself and its contents to the activities carried on therein (industrial activities) to the people who occupy the building (smokers, waste). External and linked: to adjacent buildings (via windows, for example) to neighbouring installations (liquefied gas) to gas mains beneath the public highway.

7 Fire Hazard Nominal curves were developed to test structural elements experimentally to establish a relative classification for both their resistance and reaction to fire

8 Response of Concrete To Fire 14000C Concrete melts down. Bonding of SiO2 with Al2O C 12000C Concrete melting starts. First cement paste then aggregates 11000C Concrete structurally not useful 10000C 9000C 8000C 7000C 6000C Total loss of chemically bonded water from CSH Dissociation of calcium carbonate Calcination: CaCO3 => CAO + CO2 Marked increase in basic creep Expansive quartz conversion (alpha to beta quartz) 5000C 4000C 3000C 2000C 1000C 200C Dissociation of calcium hydroxide Ca(OH)2 => CaO + H2O Start of siliceous concrete strength loss Non chemically bonded Water vapourizes Hydrothermal reactions loss of physical bound water. Simple expansion Explosive spalling

9 Response of Concrete To Fire

10 Response of Steel Reinforcement To Fire

11 Response of Structural Steel To Fire

12 Consequences of Fire in RCC : Spalling

13 Spalling Spalling can be defined as a thermal instability that occurs when concrete is exposed to fire.the dislodgement of concrete pieces during a fire is called fire spalling. During spalling in addition to material loss, reduction of concrete cover, decrease in the cross-section of the concrete member, and exposure of reinforcement to fire occurs. Steel reinforcement has a low yield point, so the exposure and sudden increase in the temperature (>500 o C) reduces the durability and leads to yielding. As a result of yielding, the integrity and the bearing capacity of the structural elements decline and in some circumstances, even structural failure has been reported.

14 Spalling Explosive spalling caused by combined thermal stresses and pore pressure Time of occurrence of different types of spalling in a fire

15 Spalling The tendency for spalling is high when : the element is made of HSC rather than normal strength concrete (NSC) the cover to the reinforcement is increased, especially more than about 40 mm the moisture content of the concrete is high the temperature rise of the fire is rapid and concrete is subjected to a high thermal gradient the concrete is subjected to compressive stress when concrete is subjected to a hydrocarbon fire compared to a standard fire

16 Spalling: Preventive Measures

17 Response of RCC Columns to Fire Fire Rating: Various parameters- thermal boundary conditions, reinforcement type, reinforcement distribution on the surface of concrete, cover to reinforcement, type of aggregate and concrete strength have significant effect. The fire rating is determined based on two failure criteria i.e., thermal and strength criteria. subjected to four side fire exposures respectively. Concrete cover = 50 mm f ck = 25 Mpa Fy= 415 MPa Aneesha Balaji, et.al., Studies on the behavior of Reinforced Concrete Short Column subjected to fire, Alexandria Engineering Journal (2016) 55,

18 RCC Columns Factors Effecting Fire Rating The axial capacity and fire resistance decrease directly with thermal boundary conditions, having maximum effect on columns with four side exposure. For the same surface area of exposure, two adjacent sides exposed to fire have a higher reduction in strength compared to two opposite sides exposed. Constructing columns as part of partition walls prevents half of crosssection getting exposed to fire that results in increasing the axial capacity significantly and hence fire rating can be enhanced by 30%. For a particular time of exposure, the four-sided exposure causes a 65% reduction in axial capacity compared to single side exposure. The fire provisions that are given in IS 456:2000 are based on the minimum cover and cross-section size for different structural members. maximum temperature in reinforcement depends only on concrete cover. Cross-section size has no significant influence, for a particular exposure condition. Aneesha Balaji, et.al., Studies on the behavior of Reinforced Concrete Short Column subjected to fire, Alexandria Engineering Journal (2016) 55,

19 RCC Columns Factors Effecting Fire Rating (contd..) The reduction in axial capacity is higher for siliceous concrete, and hence the concrete made of carbonate aggregate has more fire resistance compared to that of siliceous aggregate. Grade of concrete and steel has less effect in thermal criteria of failure but has a significant effect on axial capacity and fire rating based on strength criteria. Contradictory to normal strength design at ambient temperature, increasing the grade of concrete and steel has an adverse effect on fire rating rating based on strength criteria. The bars distributed on four faces of the column cross section provide some improvement in capacity and fire rating than those distributed on two faces for the same percentage of reinforcement. Aneesha Balaji, et.al., Studies on the behavior of Reinforced Concrete Short Column subjected to fire, Alexandria Engineering Journal (2016) 55,

20 Engineering Approach to Fire Protection Ir. J.F. Denoël, Fire Safety and Concrete Structures, FEBELCEM Federation of Belgian Cement Industry, ISBN

21 NBC-2016 Multi-disciplinary integration to be initiated right from the concept stage. Interlinking of fire alarm system, fire protection system, security system, ventilation, electrical systems, etc. Periodic validation of buildings by competent professionals through inspection of the buildings in respect of structural safety and safety of electrical and other installations and ensuring that all fire safety equipment/systems are in proper working condition. NBC-2016

22 Provisions of NBC-2016 Fire Resistance The fire resistance of an element is the time in minutes from the start of the test until failure occurs under any one of the criteria set out in 2.18(a),(b) or (c). viz., (a) Resistance to collapse(stability) (b) Resistance to penetration of flame(integrity) (c) Resistance to temp. rise on unexposed face(insulation) NBC-2016

23 Provisions of NBC 2016 Ground floor - Zero hour fire resistance (people can easily exit from the Ground floor) First Floor : With 0.5 Hrs fire resistance (for people evacuation, a time period of 0.5 Hr is required and the structure to have that fire resistance, and similarly for higher floors) Second floor : 1 Hour fire resistance Third floor : 1.5 hour fire resistance Fourth Floor: 2.0 Hour fire resistance (5th floor numbering wise) Above five floors: Fire Provisions will come into play in addition to 2 Hours minimum Fire resistance such as Sprinkler system, etc., Code prescribed fire resistance is based on time required for evacuation of occupants from building and to ensure structural stability before and during fire fighting. NBC-2016

24 Provisions of NBC 2016 In Conventional system of Framed structures, the fire requirements for structural elements such as Columns, Beams and Slabs are met with increased cover from fire resistance as detailed in IS 456:2000 Table 16A NBC 2016 and IS 456:2000 gives the Fire requirement as Prescriptive based such as providing sprinkler system Depending upon the usage of the building and as the number of floors increases beyond 5, the Fire consultant will come up with a report which is performance based calculations to substantiate the fire resistance requirement as 2 or 3 or 4 hours requirement and thereby suggest extra Fire provisions beyond Sprinkler system. NBC-2016

25 NBC-2016

26 Cover to Reinforcement IS-456:2000

27 Cover to Reinforcement IS-456:2000

28 NBC-2016

29 Minimum Dimensions for Fire Resistance IS-456:2000

30 Case Study- RCC Load Bearing Walls G+5 Residential Building As per Table 1, Point i and ix, the exterior and interior walls would require 2 hours of fire resistance if the structural members support more than one floor In NBC Table 10, corresponding to 2 hours fire requirement, the thickness of wall required is 160 mm In Table 10, a note below also states that a minimum of 1% reinforcement is to be provided In Fig.1, P.34 of IS , the wall thicknesses and its reinforcement percentages are mentioned corresponding to the fire requirements of 0.5 hrs to 2 hours

31 NBC-2016

32 Case Study Windsor Tower -Madrid, Spain Location: Madrid, Spain Fire Event: 12 February 2005 Fire started at the 21 st Floor, spreading to all floors above the 2 nd Floor. Fire duration: 26 hours Fire Damage: Extensive slab collapse above the 17 th Floor. The building was totally destroyed by the fire. Construction Type: Reinforced concrete core with waffle slabs supported by internal RC columns and steel beams, with perimeter steel columns which were unprotected above the 17 th Floor level at the time of the fire. Fire Resistance: Passive fire protection. No sprinklers. Building Type: 106 m (32 storeys). Commercial.

33 Case Study Windsor Tower -Madrid, Spain Skyscraper remained standing despite the fire that lasted for 26 hours, spreading through numerous floors. This case is an example of the excellent performance of concrete structures designed using traditional methods and subjected to an intense fire.

34 Case Study Windsor Tower -Madrid, Spain 32-storey concrete building with a reinforced concrete central core. A typical floor was two-way spanning 280mm deep waffle slab supported by the concrete core, internal RC columns with additional 360mm deep steel I-beams and steel perimeter columns. Originally, the perimeter columns and internal steel beams were left unprotected in accordance with the Spanish building code at the time of construction The building featured two heavily reinforced concrete transfer structures (technical floors) between the 2 nd and 3 rd Floors, and between the 16 th and 17 th Floors respectively. The original cladding system was fixed to the steel perimeter columns and the floor slabs. The perimeter columns were supported by the transfer structures at the 17 th and 3 rd Floor levels.

35 Case Study Windsor Tower -Madrid, Spain The building was subjected to a three year refurbishment programme of works when the fire broke out. The major works included the installations of: Fire protection to the perimeter steel columns using a boarding system Fire protection to the internal steel beams using a spray protection A sprinkler system A new aluminium cladding system The refurbishment was carried out floor-by-floor from the lower floors upwards. By the time the fire broke out, the fire protection for all steelwork below the 17 th floor had been completed except a proportion of the 9 th and 15 th floors.

36 Research Needs The following research needs have been identified: More reliable and complete thermal models for concretes of various types under various transient heating (and cooling) rates. More reliable and complete mechanical property models for concretes of various types under various combinations of stress (sustained, variable), temperature (heating and cooling), and time. More reliable and complete mechanical property models for reinforcing materials (e.g. mild steel reinforcement, prestressing steel, and alternative materials such as FRP bars, stainless steel, etc.) The development and validation of a credible, repeatable, representative, temporally and economically efficient test method and models for characterization of tendency of different concrete mixes for heat-induced spalling in fire.

37 Future... Probabilistic Hazard Approach similar to EU-2

38 Professional Needs As responsible Engineers, we need to : follow the IS codes/nbc and ensure its compliance Ensure a durable structure safe from Fire, Corrosion and stability issues. Advise the builders and contractors of the strict compliance requirement and design needs. Codes set the minimum criteria. One can follow stricter requirements at any time. They are not always perfectly clear. When two requirements are similar, implement the strictest requirement.

39 References Ir. J.F. Denoël, Fire Safety and Concrete Structures, FEBELCEM Federation of Belgian Cement Industry, ISBN Construction et incendie : les solutions,les constructions privøes et industrielles, les infrastructures», AILG Day, 19 November 2004, Physique de lincendie, Pr. J C Dotreppe John A.Purkiss, Fire safety engineering design of structures, Butterworth Hineman 1996 Aneesha Balaji, Muhamed Luquman K, Praveen Nagarajan, T.M. Madhavan Pillai, Studies on the behavior of Reinforced Concrete Short Column subjected to fire, Alexandria Engineering Journal (2016) 55,

40 Thank You!