TCVN 9311-6 : 2012 ISO 834-6 : 2000 FIRE-RESISTANCE TESTS – ELEMENTS OF BUILDING CONSTRUCTION – PART 6: SPECIFIC REQUIREMENTS FOR BEAMS

Foreword

TCVN 9311-6 : 2012 is identical to ISO 834-6: 2000.

TCVN 9311-6 : 2012 is converted from TCXDVN 346 : 2005 (ISO 834-6 : 2000) according to the regulation in Clause 1, Article 69 of the Law on Standards and Technical Regulations and Point a), Clause 1, Article 7 of the Government’s Decree No. 127/2007/ND-CP dated August 01, 2007 detailing the implementation of a number of articles of the Law on Standards and Technical Regulations.

The TCVN 9311 series under the general title “Fire-resistance tests – Elements of building construction” consists of the following parts:

• TCVN 9311-1 : 2012, Part 1: General requirements.
• TCVN 9311-3 : 2012, Part 3: Guidance on test methods and application of test data.
• TCVN 9311-4 : 2012, Part 4: Specific requirements for load-bearing vertical separating elements.
• TCVN 9311-5 : 2012, Part 5: Specific requirements for horizontally load-bearing separating elements.
• TCVN 9311-6 : 2012, Part 6: Specific requirements for beams.
• TCVN 9311-7 : 2012, Part 7: Specific requirements for columns.
• TCVN 9311-8 : 2012, Part 8: Specific requirements for non-loadbearing vertical separating elements.

The ISO 834 series Fire-resistance tests – Elements of building construction also includes the following parts:

• ISO 834-9:2003, Fire-resistance tests – Elements of building construction – Part 9: Specific requirements for non-loadbearing ceiling elements
• ISO/DIS 834-10, Fire resistance tests – Elements of building construction – Part 10: Specific requirements to determine the contribution of applied fire protection materials to structural elements
• ISO/DIS 834-11, Fire resistance tests – Elements of building construction – Part 11: Specific requirements for the assessment of fire protection to structural steel elements.

TCVN 9311-6 : 2012 was prepared by the Institute of Architecture, Urban and Rural Planning – Ministry of Construction, proposed by the Ministry of Construction, appraised by the General Department of Standards, Metrology and Quality, and announced by the Ministry of Science and Technology.

1 Scope

This standard specifies the procedures to be followed to determine the fire resistance of beams when tested as single elements.

Beams are normally tested exposed on three faces. Where exposure is required from four faces or on less than three faces, the test shall be carried out under the appropriate conditions. Where beams form part of a floor construction, they shall be tested as part of that floor construction as described in TCVN 9311-5: 2012 and evaluated for integrity and insulation.

The results of tests carried out in accordance with this standard may be applied to elements differing in detail from the tested specimen, providing such elements comply within the scope of application referred to in the various parts of this standard or when extrapolation is carried out in accordance with ISO/TR 12470. As ISO/TR 12470 provides only general guidance, extrapolation for particular cases shall only be undertaken by competent experts on structural fire engineering.

General guidance on the method of testing is contained in Annex A.

2 Normative references

The following referenced documents are indispensable for the application of this standard. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

TCVN 9311-1 : 2012 ¹⁾, Fire-resistance tests – Elements of building construction – Part 1: General requirements.

TCVN 9311-5 : 2012 ¹⁾, Fire-resistance tests – Elements of building construction – Part 5: Specific requirements for horizontally load-bearing separating elements

ISO/TR 12470, Fire-resistance test – Guidance on the application and extension of results.

ISO/IEC 13943, Fire safety – Vocabulary.

3 Terms and definitions

For the purposes of this standard, the terms and definitions given in TCVN 9311-1: 2012, ISO 13943 and the following apply.

3.1

beams

Horizontally installed structural building elements such as primary beams, secondary beams, joists.

NOTE: They may be integral with the structure or separate from the element which they support.

3.2

composite construction

Steel or composite steel/concrete beams supporting a reinforced concrete floor slab designed to act compositely with the beams under load.

3.3

exposed length

Length of test specimen exposed to heating from the test furnace.

3.4

span

Distance between the centers of the supports.

3.5

specimen length
Overall length of the test specimen.

4 Symbols and abbreviated terms

The nomenclature and units relevant to testing are defined as follows, in accordance with TCVN 9311-1: 2012:

5 Test equipment

The equipment used for this test shall comprise a furnace, loading equipment, restraint facilities, support frames and measuring equipment as described in TCVN 9311-1: 2012.

6 Test conditions

6.1 General

The heating and pressure conditions, furnace atmosphere and loading conditions shall conform to the requirements given in TCVN 9311-1: 2012.

6.2 Restraint and boundary conditions

The restraint facilities and boundary conditions shall conform to the requirements given in TCVN 9311-1: 2012 and to the requirements of this standard.

6.3 Loading

6.3.1 All beams shall be tested under a load calculated in accordance with the provisions of 6.3a), b) or c) of TCVN 9311-1: 2012 in consultation with the applicant for the test who shall supply the structural data necessary for the design as appropriate. The properties of the materials used for calculating the load shall be stated and the source of the data given.

6.3.2 Where specimens are designed to represent beams smaller than the full size element in practice, it is important that the size of the specimen, loading type and level, support conditions are selected to achieve the same failure mode (e.g. failure due to bending, failure due to shear, failure due to bond, or failure due to anchorage) in the specimen as in the construction it represents; i.e. the load applied during the test shall be the same proportion of the maximum load as for the practical construction. Where failure modes are uncertain, two or more tests may need to be conducted designed specifically to cover the various possible failure modes.

6.3.3 The magnitude and distribution of the load shall be such that the maximum bending moment and shear forces generated are not less than those expected in practice.

6.3.4 The loading system shall be capable of producing the required uniformly distributed load or providing the same load by point loading through a loading frame. Where point loading is employed to provide a pattern of bending moments similar to a uniformly distributed load, the points shall not be less than two and shall be spaced at intervals of at least 1 m. Where a four-point loading system is used, the points shall typically be positioned at 1/8, 3/8, 5/8 and 7/8 of the span (L_sup) measured from each end. The load shall be transmitted to the beam through loading plates no wider than 100 mm. The loading system shall not be permitted to restrict the free movement of air above the specimen surface except at the loading points, no part of the loading equipment being closer than 60 mm to the surface.

6.3.5 The loading system shall be capable of compensating for the permitted maximum vertical deformation of the specimen.

7 Test specimen

7.1 Specimen construction

7.1.1 Where the specimens incorporate a beam in combination with a representative floor or roof construction to which they are to be attached in practice and tested as an entity, such a combination may form an integral part of the test construction, in effect creating a T-beam or L-beam cross-section. For steel beams, slabs may be constructed from dense concrete or lightweight concrete. The result for one shall not be applied to the other.

7.1.2 Where the specimens incorporate a beam intended to be tested in combination with the actual floor or roof which it will support in practice, the slab thickness shall reflect the design of that practical construction. The width of the actual floor or roof shall be not less than three times the width of the beam or 600 mm, whichever is the greater. The actual width chosen shall depend on the design of the test furnace.

7.1.3 If the test construction is not representative of an actual floor or roof, the beams shall support cover slabs standardized in shape, constructed symmetrically about the beam and defined as follows: the cover slabs shall be designed and constructed as separate sections, with discontinuous reinforcement where used, to prevent any composite action between the cover slab and beam which could impart strength and stiffness to the beam. The cover slabs shall be constructed of aerated concrete having a density of (650 ± 200) kg/m³, each slab being no longer than 1 m in length and with a thickness of not less than (150 ± 25) mm. The width of the cover slabs shall be not less than three times the width of the beam or 600 mm, whichever is the greater. The actual width chosen shall depend on the design of the test furnace.

7.1.4 Beams with hollow encasement shall have the ends of the beam sealed to prevent hot gas circulation around the beam. Installation of the specimen shall be such that the encasement does not finish within the furnace or can be broken by restrained expansion in contrast to the function of the beam in practice.

7.1.5 Where beams incorporate mechanical connections along their length in practice, equivalent connections shall be provided either as in practice or at mid-span position. Where fire protection is provided at the connections, specimens incorporating connections shall be similarly protected.

7.2 Size of test specimen

7.2.1 For freely supported beams, the exposed length (L_exp) shall be not less than 4 m. The span between supports (L_sup) shall be the exposed length (L_exp) plus a maximum of 100 mm at each end of the beam. The specimen length (L_spec) shall be the exposed length (L_exp) plus a maximum of 200 mm at each end of the beam. The arrangement of a simply supported beam in a furnace is illustrated in Figure 1.

KEY

1 Support
2 Cover slab
3 Beam
4 Roller

Figure 1 – Example of simply supported test specimen

7.2.2 For beams representative of practical conditions, the exposed length (L_exp) shall be not less than 4 m in cases where the exposed length of the beam in practice is longer than the furnace length. For beams designed to have an exposed length in practice shorter than 4 m, the specimen shall be tested with the practical exposed length. The bearing length shall not exceed that in practice. The specimen length (L_spec) shall be the exposed length (L_exp) plus a maximum of 200 mm at each end of the beam.

For built-in beams, a minimum 4 m length of exposure may not be relevant, as only part of the span will be subject to hogging bending; the rest being held in a restraint frame. Therefore, when testing a restrained beam, a span length greater than the minimum 4 m undergoing sagging bending shall be chosen. If it is required that X% of the beam undergoes sagging bending, the overall length shall be L_exp= 4 x 100/X m.

7.3 Number of test specimens

The number of test specimens shall conform to the requirements given in TCVN 9311-1: 2012.

7.4 Conditioning of test specimens

At the time of the test, the strength and moisture content of the test specimen shall approximate the conditions expected in normal use. The test specimen shall include any finishes and fittings. Guidance on the conditioning of specimens is given in TCVN 9311-1: 2012. Once equilibrium has been reached, the moisture content or state of dryness shall be determined and reported.

7.5 Installation of test specimen and restraint

7.5.1 The arrangement of a simply supported beam in a furnace is illustrated in Figure 1. The test installation shall be fully representative of a level span.

Beams exposed to fire can be supported on rollers (simply supported) or made to simulate practical conditions. When supports and restraint simulate practical conditions, these shall be described in the report and the test results shall be reported as “restricted”.

7.5.2 Specimens representative of normal beams shall be tested on roller bearings. Where boundary conditions are known, the test conditions may be installed to represent practice with bearing surfaces smoothed concrete or steel plates.

7.5.3 Simply supported test specimens shall be arranged to permit unrestrained longitudinal and vertical deflection and be free from any externally induced restraint due to friction.

7.5.4 Devices to provide longitudinal or rotational restraint shall be designed to develop forces expected in practice due to thermal expansion and specified restraint requirements.

7.5.5 When testing several beams simultaneously, each shall be subjected to the specified test conditions and shall be independently loaded.

7.5.6 All joints at cover slabs and air gaps at the specimen edges shall be sealed with non-combustible material to allow movement.

7.5.7 Supports shall be packed and sealed with suitable fire resistant compressible material, to prevent hot gas affecting the boundary conditions during the test.

7.5.8 Where beam ends extend outside the furnace, the supporting beam ends shall be insulated using non-combustible material, or mineral wool blanket (100 ± 10) mm thick and of density (120 ± 30) kg/m³.

7.5.9 Specimens representative of beams which are continuous or restrained at one or both supports, shall be installed such that the angle of rotation at the support(s) towards the unexposed side is equal to that which can occur in practice.

7.5.10 When testing beams exposed to fire on all four faces, the minimum distance from the top of the beam to the floor of the furnace shall be not less than the width of the beam.

NOTE: Where tests have to be carried out with asymmetric beams, or beams restrained at one end, special arrangements will need to be made.

8 Installation of instrumentation

8.1 Furnace thermocouples

8.1.1 Thermocouples shall be installed to measure the furnace temperature and shall be evenly distributed so as to give a reliable indication of the average temperature in the vicinity of the exposed faces of the test specimen. There shall be at least two thermocouples for the first 1 m length, or part thereof, of the exposed length of the beam. These thermocouples shall be located and mounted in accordance with TCVN 9311-1: 2012.

8.1.2 The thermocouples shall be located at regular intervals no greater than 1.5 m apart at a distance of (100 ± 50) mm below the plane of the bottom flange and at (100 ± 50) mm from the edges of each face of the beam. Each thermocouple shall be oriented such that the blackened face “A” is facing the floor of the furnace or the nearer furnace wall. On each face of the beam, there shall be an equal number of thermocouples oriented towards the floor as towards the nearer parallel wall.

8.1.3 Where the height of the beam is 500 mm or greater, additional thermocouples shall be provided and positioned as in 8.1.2 but at the mid-height of the beam instead of below the bottom flange.

8.2 Specimen thermocouples

8.2.1 Where beams are fabricated from steel or where materials are used for which data on their properties at elevated temperatures are well established, measurement of specimen temperatures will aid prediction of failure and enable results to be used for technical assessment. Screwed, peened thermocouples are suitable for attachment to steel. Thermocouples shall be provided with a length of at least 50 mm of the twin conductor in an isothermal region prior to making the hot junction.

8.2.2 Thermocouples shall be located at mid span and at two other positions between the mid span and quarter point to furnace edge. Typical thermocouple arrangements at each location are illustrated in Figure 2.

KEY:

a Steel beam
b Steel joist
c Concrete beam

Figure 2 – Typical thermocouple arrangements for test specimen

8.2.3 Thermocouples positioned to determine temperature gradients in concrete elements will aid identification of failure and enable results to be used for technical assessment. Thermocouples shall be placed on each tension reinforcing bar unless there are more than eight bars where eight shall be selected to give representative temperature readings of the whole assembly (See Figure 2).

8.3 Deformation measurement

8.3.1 The zero datum of the test shall be the deflection measured after the load is applied just prior to commencement of heating and after deflection has stabilized.

8.3.2 The vertical deflection along the longitudinal axis of the beam shall be measured at mid span.

8.3.3 Deflection measurements shall be made in sufficient positions to obtain maximum deflections.

9 Test procedure

9.1 Loads and actions

The application and confirmation of load on the beam shall follow the requirements given in TCVN 9311-1: 2012 and 6.3 of this standard.

9.2 Furnace control

Measurement and control of the furnace temperature and pressure conditions shall follow the requirements given in TCVN 9311-1: 2012.

9.3 Measurement and observations

Monitoring of the test specimens for compliance with the criteria of load-bearing capacity, integrity and thermal insulation shall be carried out by measurement and observations in accordance with TCVN 9311-1: 2012.

10 Performance criteria

The fire resistance of beams shall be assessed against the load-bearing, integrity and thermal insulation criteria specified in TCVN 9311-1: 2012.

11 Evaluation of test results

The test shall be considered valid if the conditions relating to the test equipment, test conditions, specimen preparation, instrumentation and test procedure have been followed within the limits specified for items of sufficient accuracy as required, and have conformed with the requirements of this standard.

The test shall also be regarded as valid if the furnace exposure conditions relating to furnace temperature, pressure, and surrounding temperature exceed the upper limits of tolerances specified for these items in this standard.

12 Expression of test results

The results of the fire resistance test shall be expressed in accordance with TCVN 9311-1: 2012.

When a test is conducted on a test specimen which is loaded to an engineering load and is specified by the test applicant as being less than the maximum that can occur under an acceptable code, the load-bearing shall be expressed in the result in the restricted terms. Details shall be given in the test report of the magnitude of this load deficit.

13 Test report

The report shall comply with the requirements given in TCVN 9311-1: 2012.

Annex A (Informative) General guidance on the method of testing

A.1 General

In practice, beams supporting concrete floor or roof slabs in some instances may be connected such that they act compositely. In these cases, the assembly may be tested as either a beam or a floor assembly in which the applied loading is adjusted to account for the overall stiffness of the structure.

Where an evaluation of performance in terms of integrity and thermal insulation is required, a separate test shall be carried out as specified in TCVN 9311-5: 2012.

The evaluation of fire resistance of a beam needs to take into account the effects of fire attack on the underside, sides and possibly the top of the beam and the thermal losses at the beam ends.

The test procedures described above are specified for beams subject to flexural stresses, but the principles may be applied for the testing of members in tension.

A.2 Specimen construction

Beams as defined above are normally free of connections except for those at their upstanding supports. There are some forms of beam construction which may incorporate connections such as finger joints in glued laminated timber elements. Where such connections exist a representative number shall be incorporated into the test specimen.

Particular care is needed where beams project out of the test furnace to ensure that there is no interaction with any deflection which may occur.

The density of concrete used in the test has a direct relationship to the thermal inertia. Concrete of low density has lower thermal conductivity than concrete of high density. This is particularly important where tests are conducted on the protection to steel beams with dense concrete used in the associating element. Higher heat transfer may occur between the steel and dense concrete and tends to lower the temperature in the specimen. This phenomenon affects the direct field of application of test results obtained under such conditions.

The width of the actual floor (7.1.2) or standardized cover slab (7.1.3) shall be sufficient to deflect any gas currents which may pass in the gap from the loading frame. This shall not restrict any lateral deflection of the beam during the test.

A.3 Support and loading conditions

A.3.1 Installation of test specimen on test furnace

Where specimens need to be fixed against rotation or translation at their supports, this may be achieved by extending the beam over the supports and fixing in position. The degree of fixity can be determined from the extent of this projection and the forces recorded by load cells resisting the moment. The position of this projection is invariable. Hence the force recorded by a load cell on the projection is modified as thermal attack progresses into the specimen.

A.3.2 Loading

When a beam is tested at a span shorter than the span used in practice the loading applied to produce the effect of this could result in stresses in the specimen of a different pattern and magnitude from those in the full size element.

Care shall be taken when testing a beam of correct cross-section but over a short span to ensure that limiting stresses generated in the specimen are of the same type as those in the full size member and that excessively large shear stresses are not generated due to large loads over a short span.

For this assessment to be applicable to beams as flexural members, it is important that flexural stresses in the simply supported construction are the same as those intended in practice. This doesn’t affect the choice of other forms of self-generated test, therefore flexural stresses are not diminished because of requirements associated with rotational fixity.

A.4 Effect of restraint and loading conditions

The restraint to thermal expansion, thrust, or rotation, can be applied in a number of ways.

In the case of simple arrangements, the specimen is installed in a restraint frame of dimensions such that it can resist the thrust from the specimen members without appreciable deflection. In some instances, the axial thrust is measured by calibration of the restraint frame. In other cases, the degree of restraint is provided by allowance of expansion gaps between the ends of the structural members and the restraint frame. Such an arrangement also generates rotational forces since contact and fixity of the structural member end is maintained over the full depth of the member and of the restraint frame.

In the case of more complex arrangements, the restraint and measurement of the degree of restraint are provided by the use of hydraulic jacks arranged axially and transverse to the member.

In circumstances where restraint to thermal expansion occurs, heating during the course of a fire resistance test causes the axial compressive forces in the members concerned to increase. In most cases, this force acts at the neutral axis position of the member generating a bending moment which tends to counteract the bending moment due to loading. This may enhance the load-bearing capacity and fire resistance unless spalling or instability failure occurs which overrides this beneficial effect.

A.5 Temperature measurement

The positioning of thermocouples within the test specimen shall be such as to provide the most useful information from the temperature curves.

Where composite construction is used (e.g. H-section steel beams infilled with concrete between the flanges), identification of the temperatures of individual components as well as temperature gradients across the assembly is useful and permits a more exact technical assessment of the data.

Thermocouples may be used to measure temperatures between beams and fire protection materials. Information thus obtained may by extrapolation determine the fire protection, using the same protection material, of beams with different limiting temperatures.

A.6 Characteristics of test specimens

The cold strength of a simple element, such as a beam, is one of the primary structural characteristics which may be widely applied from a test if the test loading relates to the actual strength of the materials used rather than to typical values obtainable for that material.

With completely homogeneous materials such information may be obtained from cut samples and usually tested cold under ambient temperature. Prior to the fire test, the actual stress/strain relationship at ambient can thus be determined. However, testing at ambient should not exceed the elastic limit of the material as this affects subsequent yield strength. Other factors having a significant influence on fire resistance include:

a) Variation in cross-sectional area along the length of the beam (checked at a number of positions);

b) The density of the beam material, of any components, protective boards or coatings;

c) The average thickness and variation of any protective material;

d) The moisture content of hygroscopic materials used in the beam construction, coating or protection.

Annex B (Informative) Field of direct application of test results

The results of a fire resistance test may be applied to similar horizontally load-bearing separating elements not tested, provided that all of the following are true:

a) The span is not increased;

b) The loading is not increased and its distribution with respect to position is not changed;

c) The rotational fixity and longitudinal restraint are not reduced;

d) The cross-sectional dimensions are not reduced;

e) The characteristic strengths and density of any substantive materials are not reduced;

f) The number of heat-exposed faces is not changed;

g) The lengths of non-fire-exposed portions of the structural members are not reduced;

h) There is no change in the cross-sectional design (such as reinforcing bars within the cross-section).

For specimens which have been tested unprotected, failure of these non-load-bearing protective elements may induce failure of the individual load-bearing structural member. The protective elements normally fail at some limiting condition dependent upon the temperature-deflection interaction. Since this interaction may be changed for a given element by the support conditions, a caution shall be expressed against the use of limiting temperature for such an element, transposed from one condition of support to another which is more critical from a deflection point of view; e.g. the use of a limiting temperature, arrived at for a restrained element, for a simply supported element, other things being equal.

1) These TCVN standards are to be published soon.