Behaviour of precast concrete floor slabs exposed to standar This paper quantifies the thermal movements of 14 simply supported precast reinforced concrete floor slabs of 4.5 m span and 900 mm width exposed to two standardised heating regimes used in fire resistance furnace tests. The tests were designed to show the effect of varying the slab thickness, type of concrete, imposed load, soffit protection and nature of fire exposure on the mid-span flexural deflection and axial movements of the slab ends. Measured deflections showed that during the 90 min design period of fire resistance thermal bowing was dominant and the effect of the 1.5 kN/m2 design imposed load was small. The NPD hydrocarbon fire exposure caused a doubling of the flexural deflections achieved using the standard BS 476: Part 8 (now Part 20) fire exposure in the first 20 min of exposure.
Fire Safety Journal 36 (2001) 459}475 Behaviour of precast concrete #oor slabs exposed to standardised "res Gordon M.E Cooke* Visiting Professor, Department of Civil Engineering, City University, London, UK Received February 2000; received in revised form 24 October 2000; accepted 10 January 2001 Abstract This paper quanti"es the thermal movements of 14 simply supported precast reinforced concrete #oor slabs of 4.5 m span and 900 mm width exposed to two standardised heating regimes used in "re resistance furnace tests The tests were designed to show the e!ect of varying the slab thickness, type of concrete, imposed load, so$t protection and nature of "re exposure on the mid-span #exural de#ection and axial movements of the slab ends Measured de#ections showed that during the 90 design period of "re resistance thermal bowing was dominant and the e!ect of the 1.5 kN/m design imposed load was small The NPD hydrocarbon "re exposure caused a doubling of the #exural de#ections achieved using the standard BS 476: Part (now Part 20) "re exposure in the "rst 20 of exposure 2001 Elsevier Science Ltd All rights reserved Keywords: Floor slabs; Concrete; Standard "re tests; Thermal response; Structural response Introduction As part of the Building Research Establishment (BRE) Large Panel Structures research programme, the Fire Research Station (FRS) undertook a full scale natural "re test in the Ronan Point high rise block of #ats in 1984 The test was terminated because the "re exposed m long #oor slab spanning onto the external wall exhibited an unexpected high rate of increase in the mid-span de#ection after only 12 from ignition There was concern that, with further heating, the associated axial expansion * Corresponding author International Fire Safety Consultant, Lakis Close, Flast Walk, Hampstead, London NW3 1JX, UK Tel.: #44-20-7431-5372; fax: #44-20-7431-5374 E-mail address: gcooke@joinnet.uk.com (G.M.E Cooke) 0379-7112/01/$ - see front matter 2001 Elsevier Science Ltd All rights reserved PII: S - 1 ( ) 0 0 - 460 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 of the precast slab could push out the load bearing external wall panels at the wall/#oor junction causing an eccentric loading condition for the load-bearing external wall panels which might precipitate a &pack of cards' type of progressive collapse A search of the literature revealed a paucity of experimental data on axial de#ections of concrete slabs exposed to "re: while many hundreds of standard "re resistance tests have been made on #oors there has never been a requirement to measure axial de#ections and such information was therefore very rare and unpublished The interdependence of thermal bowing, axial de#ection and axial restraint was unclear It was therefore decided to proceed with a two-part programme of "re tests The "rst part, reported herein, would examine the "re behaviour of axially unrestrained, simply supported, precast concrete #oor slabs The second part would examine the e!ect of partial axial restraint on narrow strips of #oor construction For the "rst part of the programme, the author, then working at FRS, proposed, designed and supervised seven "re tests on pairs of precast concrete slabs each nominally 4.7 m long;900 mm wide Six pairs were prepared by the BRE Civil Engineering Laboratory, Cardington One pair was cut from a large #oor panel taken from the Ronan point #ats during demolition The tests were made in a standard "re resistance #oor test furnace and were designed to determine the unrestrained mid-span de#ection and axial de#ections of the slab ends at mid depth The nature of the axial de#ections measured as time progressed is shown in Figs 1(b) and (c) It was assumed that the bowing behaviour of a large #oor panel which spans in one direction will be similar to the behaviour of a narrower specimen if edge e!ects are guarded against so that unidirectional heat #ow was achieved in the narrower specimen This assumption allowed two specimens to be tested side by side in the #oor furnace, unrestrained by each other, in the simply supported condition with a span of 4.5 m This also meant that the specimens could be easily manufactured, handled and transported, and the cost of "re testing was reduced by more than 50% Precast #oor slabs of the kind tested are not used in modern #oor construction in multi-storey buildings in UK which typically comprise hollow core prestressed concrete planks with in situ topping or composite pro"led sheet steel/concrete #oor decks In addition, the tested #oor slabs were simply supported which results in maximum #exural defection representing the worst case scenario in which bene"cial rotational restraint generated by slab continuity over beams in multi-span #oors is ignored Nonetheless, the results have practical application to existing large panel precast #oor construction which has little continuity at the supports and to new single-span conventional in situ reinforced concrete construction The data can also be used to predict #exural de#ections of reinforced concrete #oor slabs having rotational restraint but this requires an assessment of the positions of contra-#exure in the #oor slab and any mitigating e!ect of membrane action arising from two-way spanning, but this is beyond the scope of this paper The test results are perhaps most useful for enabling both qualitative and quantitative comparisons to be made while changing important parameters such as the type of concrete and "re test severity Some of these test results have been presented by Cooke and Morgan in a BRE Information Paper [1] G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 461 Fig Loading scheme and measured de#ections Numerical modelling of the thermal and structural response of "re-exposed composite steel and concrete structures has reached an advanced stage in the UK, much of the impetus coming from the recent full scale test work conducted on an eight-storey building erected in the BRE large laboratory at Cardington Universities involved in modelling include Edinburgh, She$eld and City 1.1 Fire test parameters For the BRE slabs the following parameters were varied: slab thickness (150 and 250 mm), type of concrete (normal weight and light weight), live load (zero and 1.5 kN/m of #oor slab area), so$t protection (zero and two di!erent gypsum board systems) and severity of standard "re exposure (ISO 834 and the Norwegian Petroleum Directorate (NPD) temperature-time curves) Table lists the chosen combinations 462 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 Fig Comparison of temperature}time curves used in tests The BRE slabs were designed to have a 90 "re resistance assuming a live load of 1.5 kN/m when exposed to the heating conditions speci"ed in BS 476 Part 8: 1972 (ISO 834), which is appropriate for structural elements in high rise blocks of residential #ats in the UK The time}temperature curve in Part is the same as in the current standards i.e BS 476 Part 20, ISO 834 and the corresponding CEN standard A comparison of the NPD and ISO/BS temperature-time curves is given in Fig The structural design was based on BS 8110: Part 1: 1985 [2] 1.2 Fire test specimens All the test specimens were 4.7 m long by 925 mm wide and were simply supported at 4.5 m centres The "re exposed length was 4.0 m The BRE slabs used concrete mixes designed to have a characteristic cube strength of 30 N/mm The normal weight concrete (NWC) used a siliceous (20 mm #int gravel) aggregate with natural sand and had a nominal density of 2400 kg/m; the light weight concrete (LWC) contained Lytag ( pulverised fuel ash) coarse aggregate and had a nominal density of 1800 kg/m All the reinforcing steel bars were of high yield ribbed bar (Deformed Type to BS 4449) having a nominal yield strength of G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 463 460 N/mm The primary (longitudinal ) steel was mm diameter The concrete cover to the primary steel was 25 and 20 mm for the NWC and LWC slabs, respectively, being appropriate for 90 "re resistance according to UK regulatory guidance [3] The concrete side cover was 25 mm The moisture content of the slabs varied between 3.5 and 4.5% by weight The Ronan point slabs comprised a structural reinforced concrete slab of normal weight concrete nominally 180 mm thick incorporating circular voids of 110 mm diameter running longitudinally at 150 mm centres This slab was overlaid with a non-composite 12.5 mm thick layer of expanded polystyrene foam and a 65 mm thick granolithic concrete screed The screed and foam was present during the "re tests Two kinds of proprietary boarded so$t protection were fabricated and installed at the "re test laboratory by British Gypsum Ltd One slab was protected with a 10 mm thick glass reinforced gypsum (GRG) board Another slab was protected with a 12.5 mm thick Fireline gypsum-based board Both board protections were "xed to the concrete so$t using cold formed steel members which resulted in an air gap of 37 mm These protection systems were included in the test programme as they had been used in remedial work contracts on high rise #ats Details of the tests are given in Table For the BRE slabs thermocouples were attached to 50 mm diameter cylindrical cores of the appropriate concrete mix at a range of heights The sensing ends were aligned horizontally so that they would lie on an isotherm and were adhered to the core with an epoxy resin The ends of the cores were lightly bonded to the plywood Table Fire test parameters Test Specimen Thickness (mm) No of rebars Concrete Live load Heating Comments 1 2 3 4 5 6 7 10 11 12 13 14 150 150 150 150 250 250 150 250 150 250 150 150 185 185 10 10 10 10 6 10 10 10 10 NWC NWC NWC NWC NWC NWC LWC LWC NWC NWC NWC NWC NWC NWC BS 476 BS 476 NPD NPD BS 476 BS 476 BS 476 BS 476 NPD NPD BS 476 BS 476 BS 476 BS 476 BRE slab BRE slab BRE slab BRE slab BRE slab BRE slab BRE slab BRE slab BRE slab BRE slab BRE slab#so$t (1) BRE slab#so$t (2) Ronan Point slab Ronan Point slab No Yes No Yes No Yes No No No No No No No Yes Notes: Live load " 1.5 kN/m, So$t (1)"10 mm thick glass reinforced gypsum board with 37 mm air gap, So$t (2)"12.5 mm gypsum Fireline board with 37 mm air gap, NWC"Normal weight concrete, LWC"Light weight concrete, BS 476"BS 476: Part 8:1972 (ISO 834), NPD"Norwegian Petroleum Directorate (hydrocarbon "re simulation) 464 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 Fig Longitudinal section through furnace showing transducer-support frame formwork before casting the slabs so that the position of all cores and hence thermocouples from the "re-exposed face were accurately known The cores were placed at mid-span and quarter-span positions along the centreline of the slab 1.3 Test apparatus The tests were made in the "re resistance #oor furnace at the Warrington Fire Research Centre The ends of each slab were simply supported All de#ection measurements were made relative to the ends of a slab using two purpose-made hollow steel frames which rested on the ends of the slab Each frame was kept cool during a test using a continuous #ow of water so it would not itself de#ect due to a change in ambient conditions Linear displacement transducers (LDT's) were used to measure vertical de#ections at mid-span and quarter-span positions An LDT was also aligned horizontally at either end of the slab at mid depth so as to measure axial de#ection The apparatus is shown in Fig Loads were applied using A-frames, two hydraulic jacks and a system of load spreaders to approximate uniformly distributed loading indicated in Fig 1(a) In each test, two slabs were laid side by side separated from each other and from the furnace cover slabs with a #exible ceramic "bre seal so that the edges of the slabs were protected from "re and were free to de#ect during a test Fig shows the arrangement in which a pair of slabs of di!erent thickness are being tested Test results 2.1 Temperature proxles in slabs Averaged temperature pro"les within BRE slabs without so$t protection at 30 increments are given in Figs 5}7 The "gures show the e!ect of varying the "re G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 465 Fig Cross section of #oor slab specimen assembly Fig Temperature pro"les (150 mm, NWC) exposure and type of concrete The pro"les in the 250 mm slabs show a clearly pronounced moisture plateau at 1003C when steam is driven o! and also exhibit steeper temperature gradients and higher temperatures near the exposed surface when compared with the data for the 150 mm slabs Fig shows that the e!ect of the higher thermal insulation of Lytag LWC made little di!erence to the maximum temperatures attained in the concrete, but the bene"t of LWC is considerable at the depth where the reinforcing steel is normally located: the temperature at 20 mm after the 90 design 466 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 Fig Temperature pro"les (250 mm, NWC) Fig Temperature pro"les (250 mm, ISO 834) G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 467 Fig E!ect of slab thickness and concrete type exposure to ISO 834 is approximately 3003C for the 250 mm thick slab, and at this temperature the reinforcing steel would have lost none of its room temperature ultimate tensile strength [4,5] The e!ect of the hydrocarbon exposure when compared to the ISO 834 exposure is, as expected, markedly to increase the temperatures near the exposed face as shown in Figs and It has to be recognised that it is di$cult accurately to measure the temperature of concrete at the concrete/combustion gas interface because of the large temperature gradient All of the slabs tested resisted spalling throughout the full period of "re exposure and it can therefore be said that any aberrations in the temperature data are not due to spalling; such aberrations can occur due to moisture removal during the heating process and this e!ect, as previously mentioned, can and did have an e!ect It should not be concluded from this work that spalling is not a problem: it is generally accepted that spalling can occur where a large hogging moment is present (it was absent in the present tests because of the simple supports), and there is increasing evidence that high strength concrete used in prestressed planks is prone to spalling in "re 468 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 Fig E!ect of imposed load 2.2 Mid-span deyections Here some of the more important comparisons are made of measured mid-span de#ections It is assumed that the #exural de#ection of a non-loaded slab is dominated by thermal bowing, i.e the self weight of the slab has negligible e!ect upon de#ections except near ultimate failure With the exception of a 250 mm NWC slab exposed to the hydrocarbon "re which su!ered runaway de#ection at 110 min, none of the slabs collapsed during the exposure period of nominally h so the assumption that thermal bowing is dominant within the 90 design period of "re exposure seems reasonable 2.2.1 Ewect of slab thickness Fig shows the e!ect of slab thickness for three NWC and two LWC slabs, respectively, when exposed to ISO 834 The thicker slab de#ects less which is what one might expect intuitively and agrees with the theory of thermal bowing [1] which shows that thermal bowing is inversely proportional to the slab thickness The G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 469 de#ection curve for the non-loaded 180 mm thick Ronan point slab suggests that the relative magnitudes of de#ection agree with the theory of thermal bowing up to 80 min*the curve for 180 mm thickness lies between those for 150 and 250 mm 2.2.2 Ewect of concrete type Fig also shows the e!ect of concrete mix on de#ections of non-loaded slabs of 150 and 250 mm thickness, respectively, when exposed to ISO 834 The lightweight concrete slabs (incorporating Lytag aggregate made from pulverised fuel ash) de#ect markedly less than the NWC slabs This may be attributed to (a) the lower thermal conductivity of the LWC which may result in lower temperatures and lower thermal expansion in the "re-exposed structural layer, and (b) the lower coe$cient of linear thermal expansion of Lytag compared with dense aggregate, but further analysis would be needed before the relative importance of thermal conductivity and thermal expansion could be assessed Certainly the di!erence in de#ections is large*at the 90 design period of "re resistance the mid-span de#ections of the LWC slabs are roughly two}thirds those of the NWC slabs, and it should be remembered that these are di!erences in thermal bowing since the slabs were not loaded 2.2.3 Ewect of imposed load Fig shows the e!ect of imposed load on mid-span de#ections for NWC slabs 150 and 250 mm thick, respectively, when exposed to ISO 834 Within the design period of Fig 10 E!ect of heating rate 470 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 90 "re resistance it is clear that the mid-span de#ections are a!ected to a negligible e!ect by the imposed load The mid-span de#ection of the 180 mm thick Ronan point slabs exposed to ISO 834 also demonstrated the negligible e!ect of the 1.5 kN/m imposed load although it should be noted that the design imposed load is not known for the Ronan point building Hence the de#ections are dominated by thermal bowing This is an important "nding as it shows that a calculation of thermal bowing would su$ce in estimating the likely total de#ections before the onset of collapse 2.2.4 Ewect of heating rate Fig 10 shows the e!ect of exposing non-loaded NWC slabs of 150 and 250 mm thickness, respectively, to the NPD (hydrocarbon) temperature}time curve which has a higher heating rate than ISO 834 The NPD exposure results in much larger de#ections, especially in the early stage of exposure: at 20 the de#ections were almost doubled in the 250 mm thick slab Although both slabs were designed to have a 90 "re resistance for ISO 834 exposure, the slabs were able to resist collapse under the more severe NPD exposure for the 90 period and this suggests that there is a large measure of safety associated with present UK "re safety design practice for reinforced concrete slabs Fig 11 E!ect of so$t protection G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 471 2.2.5 Ewect of sozt protection Fig 11 shows de#ections of 150 mm thick non-loaded slabs of NWC exposed to ISO 834 with and without the two di!erent "re protecting boards Details of the protection are given earlier The curves show that the addition of a plaster-based so$t gives much smaller de#ections provided the protection remains in place (part of the Fireline fell down at approximately 55 min) At the 90 design period of "re resistance the de#ection of the slab with GRG protection was roughly a quarter of that of the unprotected slab 2.3 Axial deyections Axial de#ection is here de"ned as the horizontal de#ection of one end of a slab relative to the other end All the measured de#ections were made at the mid depth of the slab, Fig A positive axial de#ection corresponds to an increase in the chord length as occurs in the early stage of "re exposure due to thermal expansion Later in Fig 12 E!ect of slab thickness 472 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 the "re exposure excessive bowing leads to a shortening of the chord length and a negative axial de#ection is obtained Since the axial de#ection depends on the mid-span de#ection in the later stages of "re exposure, an understanding of the axial de#ection curves requires reference to the corresponding mid-span de#ection curves Measured axial de#ections are given in Figs 12}15 Some general comments on the results are as follows: Fig 12 shows axial de#ections for 150 and 250 mm thick, non-loaded slabs of LWC exposed to ISO 834 The axial de#ection is larger for the thicker slab, and the time of maximum de#ection is di!erent, being delayed for the thicker slab At 90 the de#ection was 5.5 and mm for the 150 and 250 mm slabs, respectively Maximum de#ections for 150 and 250 mm thick, non-loaded slabs of NWC exposed to NPD were 5.5 and 14 mm, respectively Fig 13 shows axial de#ections of the non-loaded and loaded 180 mm thick Ronan point slabs and 150 mm slabs of NWC exposed to ISO 834 The runaway axial de#ections of the Ronan point slabs are attributed to the runaway mid-span de#ections Fig 14 shows, more than in any other test, the negligible e!ect of imposed load Fig 13 E!ect of imposed load G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 473 Fig 14 E!ect of imposed load on axial de#ection, and this can be attributed to the almost identical mid-span de#ection curves Fig 14 shows that the largest axial de#ection of all the tests occurred with the 250 mm NWC slab exposed to ISO 834*the maximum positive de#ection was 14 mm In contrast, the 150 mm NWC slab exposed to NPD exhibited an early maximum positive axial de#ection of only mm followed by a reversal and large negative de#ections The "gure also shows the large e!ect of the rapid heating achieved in the NPD exposure for 150 mm thick NWC slabs Fig 15 shows that the axial de#ection of the so$t-protected 150 m NWC slab is approximately three-quarters that of the unprotected slab at the 90 design period of ISO 834 "re exposure 474 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 Fig 15 E!ect of so$t protection Conclusions (1) A series of seven furnace tests has been successfully carried out giving accurate data on mid-span de#ection and axial de#ection for 14 unrestrained reinforced concrete #oor slabs having a "re exposed length of 4000 mm The e!ect of varying the slab thickness, imposed load, heating rate, concrete type and so$t protection has been established These data enable numerical models to be validated (2) The method of test whereby two slabs nominally 900 mm metre wide structurally independent of each other are exposed to the same heat #ux means that accurate comparisons can be made The method also means that specimens can easily be manufactured, handled and transported, and the cost of testing is halved which is an important consideration for parametric research studies of the kind reported in this paper (3) Mid-span de#ections were dominated by thermal bowing during the 90 design period of "re exposure; the e!ect on de#ections of imposing the design live load of 1.5 kN/m, Fig 9, was very small and this suggests that BS 8110 is conservative (4) The higher rate of heating associated with the NPD hydrocarbon "re exposure caused almost a doubling of mid-span de#ection obtained using the ISO 834 "re exposure in the "rst 20 min, Fig 10 G.M.E Cooke / Fire Safety Journal 36 (2001) 459}475 475 (5) The e!ect of using lightweight concrete employing Lytag coarse aggregate is to reduce the mid-span de#ection associated with normal weight concrete by roughly 30% at the 90 design period of "re resistance, Fig The reduced de#ections are probably due to the lower coe$cient of thermal expansion and lower thermal conductivity of light weight concrete (6) The e!ect of so$t protection is, as one might expect, to reduce the mid-span de#ection Fig 11 shows that the mid-span de#ection of the 150 mm NWC slab with so$t protection was roughly a quarter that of the unprotected slab Fig 15, however, shows that the slab having a so$t protection which remained in place su!ers almost the same magnitude of axial de#ection at mid-slab depth as the same slab with no so$t protection, although the peak de#ection occurred much later for the so$t-protected slab Acknowledgements The author wishes to thank Professor G Cox, Fire Research Station, Building Research Establishment, for permission to publish the test results and Mr R.L Sawford and his team in the Civil Engineering Laboratory, BRE, Cardington for making the precast concrete test specimens Thanks are also due to British Gypsum Ltd for supplying and installing, free of charge, two so$t board protection systems Again thanks are due to laboratory sta! of Warrington Fire Research Centre who worked closely with the author in the preparation for and conduct of the tests The provision of funds from the Construction Directorate of the Department of the Environment is gratefully acknowledged References [1] Cooke GME, Morgan PBE Thermal bowing in "re and how it a!ects building design BRE Information Paper IP 21/88, Building Research Establishment, December 1988 [2] BS 8110 Structural use of concrete, Part Code of practice for design and construction British Standards Institution, 1985 [3] Morris WA, Read REH, Cooke GME Guidelines for the construction of "re resisting structural elements Building Research Establishment Report BR 128, BRE, Garston, 1988 [4] Holmes M, Anchor RD, Cooke GME, Crook RN The e!ects of elevated temperatures on the strength of reinforcing and prestressing steels The Structural Engineer 1982;60(1):7}13 [5] British Standards Institution, ENV 1993: Design of steel structures: DD ENV 1993-1-2: Structural "re design (including UK NAD) Expected 2001 ... &pack of cards' type of progressive collapse A search of the literature revealed a paucity of experimental data on axial de#ections of concrete slabs exposed to "re: while many hundreds of standard... and loaded 180 mm thick Ronan point slabs and 150 mm slabs of NWC exposed to ISO 834 The runaway axial de#ections of the Ronan point slabs are attributed to the runaway mid-span de#ections Fig... that it is di$cult accurately to measure the temperature of concrete at the concrete/ combustion gas interface because of the large temperature gradient All of the slabs tested resisted spalling