BS EN 15080-8:2009 BSI Standards Publication Extended application of results from fire resistance tests Part 8: Beams BS EN 15080-8:2009 BRITISH STANDARD National foreword This British Standard is the UK implementation of EN 15080-8:2009 The UK participation in its preparation was entrusted to Technical Committee FSH/22/-/10, Fire resistance testing for loadbearing elements (excluding walls) A list of organizations represented on this committee can be obtained on request to its secretary This publication does not purport to include all the necessary provisions of a contract Users are responsible for its correct application © BSI 2011 ISBN 978 580 54767 ICS 13.220.50; 91.060.99 Compliance with a British Standard cannot confer immunity from legal obligations This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 July 2011 Amendments issued since publication Date Text affected BS EN 15080-8:2009 EN 15080-8 EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM October 2009 ICS 13.220.50; 91.060.99 English Version Extended application of results from fire resistance tests - Part 8: Beams Application étendue des résultats des essais de résistance au feu - Partie : Poutres Erweiterter Anwendungsbereich der Ergebnisse aus Feuerwiderstandsprüfungen - Teil 8: Balken This European Standard was approved by CEN on 10 February 2008 CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN Management Centre or to any CEN member This European Standard exists in three official versions (English, French, German) A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the official versions CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG Management Centre: Avenue Marnix 17, B-1000 Brussels © 2009 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members Ref No EN 15080-8:2009: E BS EN 15080-8:2009 EN 15080-8:2009 (E) Contents Page Foreword Scope Normative references Terms and definitions 4.1 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.4 4.4.1 4.4.2 4.4.3 4.5 Basis and methodology of establishing the extended application General Basic principles General Basis of the extended application Mode of failure Methods of analysis Basic thermal analysis Basic structural analysis General Modelling factor Material properties Analysis of other features 10 5.1 5.2 5.3 5.4 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 5.6 Critical parameters 10 General 10 Common thermal parameters 10 Common mechanical parameters 11 Common constructional parameters 11 Specific constructional parameters for beams without applied fire protection 11 Concrete beams 11 Steel beams 12 Composite steel-concrete beams 12 Timber beams 12 Mechanically jointed beams 13 Specific constructional parameters for beams with applied fire protection 13 Report of the extended application analysis 14 Annex A.1 A.1.1 A.1.2 A.1.3 A.1.4 A.2 A.3 A.4 A.5 A.6 A.7 A (informative) Guidelines for making assessments 15 Mode of failure 15 General 15 Failure of protection system 15 Change of structural mode of failure from bending to shear 15 Change of structure mode of failure from bending to connection failure 16 Effect of material strength 16 Extrapolation of fire resistance 17 Accuracy of predictions 17 Prediction based on material laws 18 Modifying predicted temperatures 18 Deflection limits 19 Annex B (informative) The Extended Application Of Steel Beams 20 B.1 Introduction 20 B.2 Analysis of reference tests 20 B.2.1 Thermal performance 20 B.2.2 Mechanical performance 21 B.2.3 Other features 22 B.3 Model for extended application 22 BS EN 15080-8:2009 EN 15080-8:2009 (E) Annex C (informative) The Extended Application Of Timber Beams 24 C.1 Introduction 24 C.2 Extended application in the load domain (increase of load-bearing capacity) 24 C.2.1 Increasing of load-bearing capacity by higher strength class 24 C.2.2 Increasing of load-bearing capacity by increasing beam dimensions (braced beams) 25 C.2.3 Increasing of load-bearing capacity by decreasing the fire resistance 26 C.3 Extended application in the time domain: Increasing fire resistance by applied fire protection 30 Annex D (informative) The Extended Application of a Composite Steel Concrete Beam 32 D.1 Introduction 32 D.1.1 General 32 D.1.2 Reference test 33 D.1.3 Reference test 33 D.2 Analysis of reference tests 34 D.2.1 Thermal performance 34 D.2.2 Reference test 34 D.2.3 Reference test 35 D.2.4 Structural performance 36 D.2.5 Bending resistance 36 D.2.6 Assessment of reference test 36 D.2.7 Assessment of reference test 37 D.2.8 Conclusions of structural performance 38 D.2.9 Model for extended application 38 D.2.10 Extended application 38 Annex E (informative) The extended application of concrete beams 41 E.1 Introduction 41 E.2 Failure modes 41 E.3 Examples 41 E.3.1 Possible change of failure mode 41 E.3.2 Change of cross section 42 E.3.3 Change of material strength 42 E.3.4 Axial and rotational restraint 43 Bibliography 44 BS EN 15080-8:2009 EN 15080-8:2009 (E) Foreword This document (EN 15080-8:2009) has been prepared by Technical Committee CEN/TC 127 “Fire safety in buildings”, the secretariat of which is held by BSI This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by April 2010, and conflicting national standards shall be withdrawn at the latest by April 2010 Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights This document has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EC Directive(s) According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom BS EN 15080-8:2009 EN 15080-8:2009 (E) Scope This part of EN 15080 identifies the parameters and factors that affect the fire resistance of beams and need to be taken into account when considering extended application of results of beams tested in accordance with EN 1365-3 It also gives the methodology to be used when preparing an extended application, including rules and calculation methods which can be applied to establish the resultant influence of a variation in one or more parameters and to determine the field of extended application Normative references The following referenced documents are indispensable for the application of this document For dated references, only the edition cited applies For undated references, the latest edition of the referenced document (including any amendments) applies EN 338, Structural timber — Strength classes EN 1194, Timber structures — Glued laminated timber — Strength classes and determination of characteristic values EN 1363-1:1999, Fire resistance tests — Part 1: General Requirements EN 1365-3:1999, Fire resistance tests for loadbearing elements — Part 3: Beams EN 10025-1, Hot rolled products of structural steels — Part 1: General technical delivery conditions EN 10025-2, Hot rolled products of structural steels — Part 2: Technical delivery conditions for nonalloy structural steels EN 10080–1, Steel for the reinforcement of concrete — Weldable reinforcing steel — Part 1: General requirements prEN 10138-1, Prestressing steels — Part 1: General requirements EN 13501-2, Fire classification of construction products and building elements — Part 2: Classification using data from fire resistance tests, excluding ventilation services EN ISO 13943:2000, Fire safety — Vocabulary (ISO 13943:2000) Terms and definitions For the purposes of this document, the terms and definitions given in EN ISO 13943:2000, EN 1363-1:1999 and EN 1365-3:1999, together with the following apply 3.1 test result outcome of a testing process and its associated procedures detailed within EN 1365-3 (which may include some processing of the results from the testing of a number of specimens) A test result is expressed in terms of one or more fire performance parameter(s) 3.2 direct field of application of test results outcome of a process (involving the application of defined rules) whereby a test result is deemed to be equally valid for variations in one or more of the product properties and/or intended end use application(s) NOTE The direct field of application of test results are presented in EN 1365-3 BS EN 15080-8:2009 EN 15080-8:2009 (E) 3.3 extended field of application of test results outcome of a process (involving the application of defined rules that may incorporate calculation procedures) that predicts, for a variation of a product property and/or its intended end use application(s), a test result on the basis of one or more test results to the same test standard, i.e to EN 1365-3 3.4 classification process defined in EN 13501-2, whereby the fire performance parameters obtained from the results of one test, or a set of tests, or from a process of extended application, are compared with limiting values for those parameters that are set as criteria for achieving a certain classification NOTE The relevant classes and related criteria for fire resistance are specified in Commission Decisions (2000/367/EC, 2000/147/EC and 2001/671/EC) 3.5 reference test fire resistance test according to EN 1365-3 on a beam from which the test result is used for the process of extended application NOTE There may be more than one reference test 3.6 parameter aspect of the reference scenario that may vary in practice and may result in a change of the fire resistance performance NOTE Examples are the load level and the span 3.7 modelling factor factor determined for a considered relevant structural failure mode on basis of the assessment of the reference test(s), which takes into account the differences between the test results and calculated results, and which is used to adjust the results of the extended application 3.8 calculated structural resistance resistance to bending or shear of a beam in a fire test calculated at the end of the test 3.9 effective structural resistance predicted resistance to bending or shear of a beam for use in an extended application 3.10 relative resistance ratio of the bending or shear resistance of a beam in a fire resistance test to the resistance at normal temperatures calculated with all safety factors taken as unity 3.11 target classification fire resistance that the extended application is required to achieve 4.1 Basis and methodology of establishing the extended application General An extended application analysis is required when the application of a beam is not covered by the field of direct application given in the classification document of the product BS EN 15080-8:2009 EN 15080-8:2009 (E) The situation of (a) fire test(s) carried out according to EN 1365-3 will be referred to as the “reference test” and “reference scenario” The result of a test, i.e the fire resistance with respect to the load bearing capacity, will be referred as “tref,fi” If more than one reference test is available, all the tests may be used for the extended application provided that the tests all have the same mechanical boundary conditions and have all been carried out using the same fire curve NOTE It is possible that in the classification report all reference beams are classified with the same classification “Rref” although the actual test results (tref,fi) given in the test reports may differ 4.2 Basic principles 4.2.1 General It is assumed that extended application is made by appropriately qualified and experienced persons in the field of structural fire design The reference test(s) shall be well documented, i.e an insight into the performance of the test specimen(s) and the mode of failure, leading to Rref, are available Three analyses (described in 4.3, 4.4 and 4.5) should be carried out where appropriate It shall be decided whether: field of application can be extended, maintaining the classification Rref or changing the classification and if so, by how much; extension is not possible (new tests are required) Any predicted increase in fire resistance shall not exceed the lesser of 15 and 20 % of the target classification NOTE 4.2.2 This is illustrated in A.3 Basis of the extended application An adequate understanding of the structural and thermal performance, as well as an understanding of other relevant features, shall be achieved based on the scope of the required extended application For minor or obvious extensions to the reference test, the depth of analysis required may be reduced 4.2.3 Mode of failure Any assessment shall consider the possibility that the mode or cause of failure, such as structural collapse in bending or failure of a fire protection system, might change and that the mode or cause of failure in a fire test may no longer be critical if one or more parameters are changed If a change of failure mode is expected, then extended application is not possible unless additional information is available NOTE 4.2.4 For additional information see A.1 Methods of analysis When analysing the reference test(s), the rules given in the Eurocodes shall be used if applicable Additional rules are given in this standard These are also applicable in cases where the Eurocodes not fully cover the construction to be assessed Other calculation models, as well as empirical rules, shall be validated on the basis of similar tests as the reference test(s) Historic data and ad hoc tests may be used to supplement to the information of the reference test(s) BS EN 15080-8:2009 EN 15080-8:2009 (E) 4.3 Basic thermal analysis If the extended application is intended to be for a cross section of a size or shape different from the reference test(s) or for a different resistance time or another nominal fire curve, then a thermal assessment shall be made The analysis should lead to an understanding of the temperature distribution and material strength variation throughout the beam The analysis may take the form of a finite element or finite difference thermal analysis In limited circumstances, when a dimension is changed, it may be possible to show, using a simple calculation, that the temperature distribution measured in the test can be conservatively used for the modified cross section For timber beams, it may be sufficient to analyse the charring depth instead of carrying out a complete thermal analysis Where a thermal analysis is carried out, the position of the char-line shall be taken as the position of the 300 °C isotherm 4.4 Basic structural analysis 4.4.1 General The structural behaviour of the reference test(s) and of the situation to be assessed shall be analysed The depth of structural analysis will depend on the complexity of the beam and the extent of the proposed extended application For any assessment the same failure modes states should be considered as were considered for normal temperature design These include: Bending (including lateral torsional buckling) Vertical shear Horizontal shear NOTE It is not normally necessary to consider deformations See A.7 The assessment shall also include: Connections, either mechanical or glued, between parts of the construction Boundary conditions Material properties versus temperature NOTE As an illustration of the depth of structural analysis required, the report on the analysis for the vertical shear check on a steel beam might simply say, “The shear is low enough to have no influence on the bending strength - no check required” 4.4.2 Modelling factor Any assessment shall take into account the accuracy of the structural model used Models which overestimate the load resistance of the reference test(s), shall have a modelling factor applied when used to make an assessment for extended application In making any assessment, the effective structural resistance shall be determined as follows: Reff = R × kmf where Reff is the effective structural resistance; (1) BS EN 15080-8:2009 EN 15080-8:2009 (E) Annex D (informative) The Extended Application of a Composite Steel Concrete Beam D.1 Introduction D.1.1 General This example in this annex illustrates a beam made of two different materials (steel and concrete) and shows how the calibration adjustments for the thermal analysis and structural analysis are applied Two loaded beam fire tests, the reference tests, have taken place on beams of slightly different sizes (Figure D.1) The unprotected steel beams are constructed from three welded plates and are embedded into the concrete floor slab Nominal reinforcement spans over the top flanges in the 50 mm deep concrete topping All dimensions in millimetres 50 1000 160 290 20 290 Key Top flange Test 15 mm Test 12 mm Bottom flange Test 25 mm Test 18 mm Figure D.1 — Cross-section through test specimens 32 BS EN 15080-8:2009 EN 15080-8:2009 (E) D.1.2 Reference test Span Steel grade Steel strength Concrete grade Concrete strength Applied loading Self weight Fire resistance Mode of failure 4,2 m S275 Not measured 30 N/mm Not measured four loads of 120 kN 30 kN 69 Overall bending D.1.3 Reference test Span Steel grade Steel strength Concrete grade Concrete strength Applied loading Self weight Fire resistance Mode of failure 4.2 m S275 Not measured 30 N/mm Not measured four loads of 115 kN 30 kN 66 Overall bending TC4 TC3 TC1 TC2 Key TC1 Position of Thermocouple TC2 Position of Thermocouple TC3 Position of Thermocouple TC4 Position of Thermocouple Figure D.2 — Thermocouple numbers and positions 33 BS EN 15080-8:2009 EN 15080-8:2009 (E) The client requires a beam to carry normal floor loading of kN/m with a span of 10 m and a spacing to the next beams of m The required fire resistance is 60 D.2 Analysis of reference tests D.2.1 Thermal performance D.2.2 Reference test The measured temperatures at failure and at 60 The mean temperature is shown The thermocouple numbers and positions are shown in Figure D.2 No temperatures were measured in the concrete Table D.1 — Measured temperatures in reference test Thermocouple TC1 (flange 0,25 point) TC2 TC3 (30 mm up from top of flange TC4 Temperature at 60 (°C) Temperature at 69 (°C) 735 778 660 452 702 480 61 67 The temperature distribution in the reference test has been predicted using a finite difference thermal analysis program The program has been shown to predict temperatures reasonably well in similar beams and meets the criteria for advanced calculation models given in EN 1994-1-2 For the purpose of analysis, the beam is divided into a number of rectangular elements and the program computes the heat flow between adjacent elements and short time steps The predictions and measured temperatures are compared in Table D.2 Table D.2 — Comparison of measured and predicted temperatures in reference test Thermocouple TC1 TC2 Weighted mean TC3 TC4 Temperature at 60 (°C) Measured 735 660 710 452 61 Predicted 722 634 698 434 57 Temperature at 69 (°C) Measured 778 702 753 480 67 Predicted 760 671 736 462 75 NOTE The measured weighted mean = (2 × TC1 + TC2)/3 The predicted weighted mean is the mean of elements used in the thermal modelling The predicted temperatures are close to the measured values but the program appears to predict a low temperature for TC2, at the centre of the bottom flange The mean bottom flange temperature is predicted quite well The prediction of TC3 is unconservative On the basis of Reference Test 1, some modification to the predicted temperatures may be required when predicting other cases for 60 fire resistance 34 BS EN 15080-8:2009 EN 15080-8:2009 (E) D.2.3 Reference test D.2.3.1 General The measured temperatures at failure and at 60 are shown in Table D.3 No temperatures were measured in the concrete Table D.3 — Measured temperature in reference test Thermocouple Temperature at 60 (°C) Temperature at 66 (°C) 750 658 460 65 783 682 472 82 TC1 TC2 TC3 TC4 The predictions and measured temperatures are compared in Table D.4 Table D.4 — Comparison of measured and predicted temperatures in reference test Thermocouple TC1 TC2 Weighted mean TC3 TC4 Temperature at 60 (°C) Measured 750 658 719 460 65 Predicted 738 632 713 443 63 Temperature at 69 (°C) Measured 783 682 749 472 82 Predicted 775 656 743 463 75 The predicted temperatures are again close to the measured values but the program appears to predict a low temperature for TC2, at the centre of the bottom flange The mean bottom flange temperature is predicted quite well The prediction of TC3 is unconservative Conclusion of thermal performance On the basis of the above comparisons, the thermal analysis program can be used to model this type of composite beam but some small calibration corrections should be applied to the results D.2.3.2 Correction to bottom flange The difference between the measured and predicted temperatures was greatest in Reference Test and for both tests the prediction was unconservative For 60 min, therefore, any predicted temperatures should be increased by 12 °C (the difference in weighted mean temperatures) At about 700 °C, this correction could affect the flange strength by % D.2.3.3 Correction to lower web The difference between the measured and predicted temperatures was at greatest 18 °C in Reference Test and 17°C in Test For 60 min, therefore, any predicted temperatures should be increased by 18°C 35 BS EN 15080-8:2009 EN 15080-8:2009 (E) D.2.4 Structural performance Eurocode 4, EN 1994-1-2, does not cover this exact type of structure although it gives methods of computing the moment resistance of composite sections A beam of this type is torsionally stable and can only fail by bending or shear EN 1994-1-2 states that, for beams of this type, no check of shear resistance in fire is necessary D.2.5 Bending resistance The applied bending moment in Reference Test was 267,8 kNm and in Reference Test was 257,3 kNm No material strengths were measured In accordance with 4.4.3, mean properties of steel and concrete should be used in assessing the Reference Tests and in any predictions Material strengths at elevated temperatures are taken from EN 1994-1-2 Assume that for S275 steel the mean strength is 110 % of the characteristic value i.e 302,5 N/mm (275 × 1,1) and that for concrete the mean strength is 38 (30 + 8) NOTE In this example, for simplicity, the shear connection between the concrete and steel is not considered A full analysis should include this effect D.2.6 Assessment of reference test The structural model used is a simple plastic analysis of the cross-section as specified in EN 1994-1-2 The cross-section is divided into elements: 1) Concrete above top flange 2) Top flange of steel web of welded steel section 3) Upper 150 mm of steel web of welded steel section 4) Lower 70 mm of web of welded steel section 5) Bottom flange of welded steel section A material strength and temperature are assigned to each element The plastic moment resistance may be calculated by hand but, here, Excel based calculations are presented in Table D.5 and Table D.6 36 BS EN 15080-8:2009 EN 15080-8:2009 (E) Table D.5 — Calculation of moment resistance in reference test Width Depth (mm) (mm) 1000 160 160 160 20 20 290 50 15 4,6 10.4 150 70 25 Material Concrete Steel Steel Steel Steel Steel Steel Cold strength (N/mm²) Temp Centroid Force (mm) Hot strength (N/mm²) (kN) Moment contribution (kNm) (°C) 38 302,5 302,5 302,5 302,5 302,5 302,5 20 67 67 67 300 480 753 25,0 57,5 53,6 61,1 140,0 250,0 297,5 33,0 302,5 302,5 302,5 302,5 249,3 50,3 1900,0 -348,7 377,3 907,5 349,0 364,9 -47,5 Element split -11,7 30,0 127,1 87,2 108,6 Moment resistance (kNm) Compressive force in concrete (kN) Compressive force in steel (kN) Tensile force in steel (kN) 293,7 900,0 223,7 123,7 NOTE The plastic neutral axis is in the top flange of the welded steel section This element is split into elements with the upper part being in compression and the lower part in tension The calculated moment resistance is 293,7 kNm This is 9,7 % greater than the applied moment in the test (267,8 kNm) This difference is reasonable for this type of calculation D.2.7 Assessment of reference test Table D.6 — Calculation of moment resistance in reference test Width Depth (mm) (mm) 1000 160 160 160 20 20 290 50 12 2,8 9,2 160 70 18 Material Cold strength (N/mm²) Temp Centroid Force (mm) Hot strength (N/mm²) (kN) Moment contribution (kNm) (°C) Concrete Steel Steel Steel Steel Steel Steel 38 302,5 302,5 302,5 302,5 302,5 302,5 20 82 82 82 300 472 749 25,0 56,0 52,7 58,7 142,0 257,0 301,0 33,0 302,5 302,5 302,5 302,5 254,6 51,8 1650,0 -262,8 318,0 968,0 356,4 270,3 -41,3 Element split -7,1 25,4 137,5 91,6 81,4 Moment resistance (kNm) Compressive force in concrete (kN) Compressive force in steel (kN) Tensile force in steel (kN) 281.3 1900,0 137,8 037,8 NOTE The plastic neutral axis is in the top flange of the welded steel section This element is split into elements with the upper part being in compression and the lower part in tension 37 BS EN 15080-8:2009 EN 15080-8:2009 (E) The calculated moment resistance is 281,3 kNm This is 14 % greater than the applied moment in the test (246,8 kNm) D.2.8 Conclusions of structural performance The EN 1994-1-2 method of calculating the moment resistance predicted strengths 9,7 % and 14 % higher than measured in the tests These differences could be explained by errors in the assumed material properties, insufficient temperature data or could be due to some other unidentified cause The modelling factor (see 4.4.2) (kmf) is therefore: k mf = NOTE 1 1 + = 0,894 1,097 1,14 As the ratio F/R for each test is less than 1,2, the actual values not have to be limited to 1,2 D.2.9 Model for extended application The thermal and structural models used to analyse the reference tests may be used with the following corrections applied On the basis of the data presented in this example any extended application can only be for 60 Temperatures Bottom plate Lower web +12 °C +18 °C Strength Mf 0,894 D.2.10 Extended application D.2.10.1 General It is assumed that, for normal conditions, the design conforms to EN 1994-1-2 D.2.10.2 Calculation of load Floor slab Imposed load Weight of steel kN/m kN/m 0,2 kN/m The loads are factored using factors from EN 1991-1: Factored bending moment = [(3 + 0,2) × 1,0 + × 0,5] × 10 × / = 427,5 kNm It is convenient to adjust the required bending resistance to take account of the Modelling factor Using the plastic bending resistance model, the bending resistance should be 478,2 (= 427,5/0,894) Following the guidance in A.7, no calculation of deflection is required The proposed beam cross-section is shown in Figure D.3 The section size has been made deeper and the web thickness has been increased compared with the reference tests The slab width of 500 mm is based on 25 % of the 10 m span (EN 1994-1-1) 38 BS EN 15080-8:2009 EN 15080-8:2009 (E) The predicted temperatures and corrected temperatures are shown in Table D.7 and the calculation of moment resistance is shown in Table D.8 The calculated bending resistance is 503,9 kN so the design is adequate, based on the extended application analysis All dimensions in millimetres 500 300 15 50 250 25 22 350 Figure D.3 — Cross-section through required design Table D.7 — Predicted temperatures and corrected temperatures Position (Thermocouple) TC1 TC2 Weighted mean TC3 TC4 Predicted temperatures at 60 (°C) 722 731 699 470 56 Corrected temperatures (°C) 699 + 12 = 711 470 + 18 = 488 39 BS EN 15080-8:2009 EN 15080-8:2009 (E) Table D.8 — Calculation of moment resistance in design example Width Depth (mm) (mm) 2500 2500 250 22 22 350 2500 50 35 15 187 70 25 50 Material Concrete Concrete Steel Steel Steel Steel Concrete Cold strength (N/mm²) Temp Centroid Force (mm) Hot strength (N/mm²) (kN) Moment contribution (kNm) (°C) 38 38 302,5 302,5 302,5 302,5 38 20 20 56 300 488 711 20 25,0 17,5 57,5 158,5 287,0 334,5 25,0 38,0 38,0 302,5 302,5 243,9 65,6 38,0 -3328,4 1134,4 1244,5 375,7 573,8 Element split -58,3 65,2 197,3 107,8 192,0 Element split Moment resistance (kNm) Compressive force in concrete (kN) Compressive force in steel (kN) Tensile force in steel (kN) 503,9 328,4 zero 328,4 NOTE The plastic neutral axis is in the top flange of the welded steel section This element is split into two elements with the upper part being in compression and the lower part in tension 40 BS EN 15080-8:2009 EN 15080-8:2009 (E) Annex E (informative) The extended application of concrete beams E.1 Introduction This annex gives some examples for extended application of the results of one or more reference test(s) concrete beams In most cases extended application of concrete beams can be carried out by using design rules of EN 1992-1-2 E.2 Failure modes Failure modes of reinforced or prestressed concrete beams exposed to fire may be: a) Tension failure of the reinforcement in bending This is the most common type of failure and can be predicted by rapid increase of deflection b) Compression failure of concrete in bending Possible, if the compression side of the cross section is exposed to fire, like continuous beams near the supports, or thin beams c) Shear failure Possible for beams with thin webs, like I-beams d) Anchorage failure Could be possible for prestressed beams, but normally prevented by stirrups in the anchorage zone e) Spalling EN 1992-1-2 includes design rules to prevent spalling E.3 Examples E.3.1 Possible change of failure mode A typical resistance curve of a concrete beam (constant cross section) is shown in Figure N.1 If there is bending failure in fire test, the results can be extended to longer spans without risk for shear failure 41 BS EN 15080-8:2009 EN 15080-8:2009 (E) y x Key X Y Span (m) Load (kN/m) Shear Bending Figure E.1 — Typical resistance curve of a concrete beam E.3.2 Change of cross section E.3.2.1 Simple rule for bending If the average axis distance of the main reinforcement does not change Cross section dimensions, load, span and amount of reinforcement may be changed when it is verified by calculation that the tensile stress in the reinforcement does not increase NOTE When the load level in the test has been chosen to give maximum steel stress, this verification is automatically covered by normal temperature design However, decrease of the width may increase temperature in the critical areas of the cross section, and should not be done without more detailed calculations E.3.2.2 Simple rule for shear Ratio Vfi/fcdbd should not increase and average axis distance of stirrups should not decrease E.3.3 Change of material strength a) Strength class of concrete Test results are valid for all classes of normal strength concrete (≤ C50/60) with same type of aggregates (siliceous, calcareous or lightweight) assuming that load level is kept unchanged b) Type of reinforcement: reinforcing steel, prestressing bar, wire or strand Result can not be used for other types of reinforcement unless the influence of differences in thermal and mechanical properties is considered, see e.g ENV 1992-1-2 c) 42 Strength of reinforcing or prestressing steel BS EN 15080-8:2009 EN 15080-8:2009 (E) Change of strength has no influence on fire resistance assuming that load level is kept unchanged E.3.4 Axial and rotational restraint Concrete beams are normally tested as simply supported These results are on the safe side for the other cases because axial and rotational restraint increases the fire resistance NOTE Axial restraint may decrease fire resistance if it is situated above the centroid axis of the beam This should be prevented by design rules 43 BS EN 15080-8:2009 EN 15080-8:2009 (E) Bibliography [1] EN 1992-1-1, Eurocode Design of concrete structures — Part 1-1: General rules and rules for buildings [2] EN 1993-1-1, Eurocode Design of steel structures — Part 1-1: General rules and rules for buildings [3] EN 1994-1-1, Eurocode Design of composite steel and concrete structures — Part 1-1: General rules and rules for buildings [4] EN 1999-1-1, Eurocode Design of aluminium structures — Part 1-1: General rules and rules for buildings [5] ECCS-TC3: Design Manual on the European Recommendations for the Fire Safety of Steel Structures, 1985 [6] EN 1363-2, Fire resistance tests — Part 2: Alternative and additional procedures [7] EN 1992-1-2:2004, Eurocode 2: Design of concrete structures — Part 1-2: General rules — Structural fire design [8] EN 1993-1-2, Eurocode 3: Design of steel structures — Part 1-2: General rules - Structural fire design [9] EN 1994-1-2, Eurocode - Design of composite steel and concrete structures - Part 1-2: General rules - Structural fire design [10] EN 1995-1-2:2004, Eurocode 5: Design of timber structures - Part 1-2: General - Structural fire design [11] EN 1995-1-1:2004, Eurocode 5: Design of timber structures - Part 1-1: General - Common rules and rules for buildings [12] ENV 13381-7, Test methods for determining the contribution to the fire resistance of structural members - Part 7: Applied protection to timber members 44 This page deliberately left blank NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW British Standards Institution (BSI) BSI is the national body responsible for preparing British Standards and other standards-related publications, information and services BSI is incorporated by Royal Charter British Standards and other standardization products are published by BSI Standards Limited About us Revisions We bring together business, industry, government, consumers, innovators and others to shape their combined experience and expertise into standards -based solutions Our British Standards and other publications are updated by amendment or revision The knowledge embodied in our standards has been carefully assembled in a 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