SIST EN 15037-2:2009+A1:2011
(Main)Precast concrete products - Beam-and-block floor systems - Part 2: Concrete blocks
Precast concrete products - Beam-and-block floor systems - Part 2: Concrete blocks
This European Standard deals with the requirements and the basic performance criteria for blocks made in normal or lightweight aggregate concrete, used in conjunction with precast concrete beams in compliance with EN 15037-1, with or without cast-in-situ concrete for the construction of beam-and-block floor and roof systems.
Examples of typology of floor and roof systems are given in Annex B of EN 15037-1:2008.
Betonfertigteile - Balkendecken mit Zwischenbauteilen - Teil 2: Zwischenbauteile aus Beton
Diese Europäische Norm legt die Anforderungen und die grundlegenden Leistungskriterien für Zwischen-bauteile aus Normal oder Leichtbeton fest, die zusammen mit vorgefertigten Betonbalken nach EN 15037 1 mit oder ohne Ortbeton zur Herstellung von Balkendecken mit Zwischenbauteilen sowie von Dachsystemen verwendet werden.
EN 15037 1:2008, Anhang B enthält eine Übersicht über die verschiedenen Typen von Decken und Dachsystemen.
Produits préfabriqués en béton - Systèmes de planchers à poutrelles et entrevous - Partie 2: Entrevous en béton
La présente Norme européenne spécifie les prescriptions et les critères de performance de base des entrevous fabriqués en béton de granulats courants ou légers, utilisés conjointement avec des poutrelles préfabriquées en béton conformes à l’EN 15037 1, avec ou sans béton coulé en place, pour la construction de systèmes de planchers et de toitures à poutrelles et entrevous.
Des exemples de typologie de systèmes de planchers et de toitures sont donnés dans l’Annexe B de l’EN 15037-1:2008.
Montažni betonski izdelki - Stropni sistemi iz nosilcev in polnil - 2. del: Polnila
Ta evropski standard obravnava zahteve in osnovna merila za zmogljivost za polnila iz betona iz lahkega agregata, ki se uporabljajo v povezavi z montažnimi betonskimi nosilci v skladu z EN 15037-1, z ulivanjem betona in-situ za gradnjo stropnih sistemov iz nosilcev in polnil ali brez njega.
Primeri tipologije talnih in stropnih sistemov so podani v dodatku A EN 15037-1:2008.
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Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Montažni betonski izdelki - Stropni sistemi iz nosilcev in polnil - 2. del: PolnilaBetonfertigteile - Balkendecken mit Zwischenbauteilen - Teil 2: Zwischenbauteile aus BetonProduits préfabriqués en béton - Systèmes de planchers à poutrelles et entrevous - Partie 2: Entrevous en bétonPrecast concrete products - Beam-and-block floor systems - Part 2: Concrete blocks91.100.30Beton in betonski izdelkiConcrete and concrete products91.060.30Stropi. Tla. StopniceCeilings. Floors. StairsICS:Ta slovenski standard je istoveten z:EN 15037-2:2009+A1:2011SIST EN 15037-2:2009+A1:2011en,fr,de01-junij-2011SIST EN 15037-2:2009+A1:2011SLOVENSKI
STANDARDSIST EN 15037-2:2009/kFprA1:2010SIST EN 15037-2:20091DGRPHãþD
SIST EN 15037-2:2009+A1:2011
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 15037-2:2009+A1
February 2011 ICS 91.100.30 Supersedes EN 15037-2:2009English Version
Precast concrete products - Beam-and-block floor systems - Part 2: Concrete blocks
Produits préfabriqués en béton - Systèmes de planchers à poutrelles et entrevous - Partie 2: Entrevous en béton
Betonfertigteile - Balkendecken mit Zwischenbauteilen - Teil 2: Zwischenbauteile aus Beton This European Standard was approved by CEN on 25 January 2009 and includes Amendment 1 approved by CEN on 10 January 2011.
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-CENELEC 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-CENELEC Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, 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 © 2011 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 15037-2:2009+A1:2011: ESIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 2 Contents The numbering of clauses is strictly related to EN 13369:2004 Common rules for precast concrete products, at least for the first three digits. When a clause of EN 13369:2004 is not relevant or included in a more general reference of this standard, its number is omitted and this may result in a gap on numbering.
Page Foreword .4Introduction .61 Scope .72 Normative references .73 Terms and definitions .74 Requirements . 104.1 Material requirements . 104.2 Production requirements . 114.3 Finished product requirements . 114.3.1 Geometrical properties . 114.3.2 Surface characteristics . 134.3.3 Mechanical resistance . 134.3.4 Resistance and reaction to fire . 144.3.5 Acoustic properties . 154.3.6 Thermal properties . 154.3.7 Durability . 154.3.8 Other requirements. 155 Test methods . 155.1 Measuring of dimensions and surface characteristics . 155.1.1 Block dimensions . 155.1.2 Surface characteristics . 175.2 Mechanical strength . 185.2.1 Resistance to concentrated loads . 185.2.2 Bending strength . 205.2.3 Longitudinal compression test for resisting and semi-resisting concrete blocks . 215.2.4 Transverse testing of resisting and semi-resisting concrete blocks . 235.3 Gross dry density of block . 245.4 Drying shrinkage of lightweight concrete . 246 Evaluation of conformity . 246.1 General . 246.2 Type testing . 246.3 Factory production control . 247 Marking . 258 Technical documentation . 25Annex A (normative)
Sampling for initial type testing and for independent testing of consignments . 26A.1 General . 26A.2 Sampling procedure . 26A.2.1 Random sampling . 26A.2.2 Representative sampling . 26SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 3 A.2.3 Dividing the sample . 27A.2.4 Number of blocks required per test . 27Annex B (normative)
Inspection schemes for concrete blocks . 28Annex ZA (informative)
Clauses of this European Standard addressing the provisions of the EU Construction Products Directive . 29ZA.1 Scope and relevant characteristics . 29ZA.2 Procedure for attestation of conformity of concrete blocks for beam-and-block floor systems . 31ZA.2.1 System of attestation of conformity . 31ZA.2.2 EC Certificate and Declaration of conformity . 31ZA.3 CE marking and labelling. 32ZA.3.1 General . 32ZA.3.2 Simplified label . 33ZA.3.3 Information to be provided with the CE marking . 34Bibliography . 36 SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 4 Foreword This document (EN 15037-2:2009+A1:2011) has been prepared by Technical Committee CEN/TC 229 “Precast concrete products”, the secretariat of which is held by AFNOR. This document was examined by and agreed with a joint working party appointed by the Liaison Group CEN/TC 229 – CEN/TC 250, particularly for its compatibility with structural Eurocodes. 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 August 2011, and conflicting national standards shall be withdrawn at the latest by August 2011. 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 includes Amendment 1, approved by CEN on 2011-01-10. This document supersedes EN 15037-2:2009. The start and finish of text introduced or altered by amendment is indicated in the text by tags ! ". 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 EU Directive(s). For relationship with EU Directive(s), see informative Annex ZA, which is an integral part of this document. The European Standard for beam-and-block floor system is made of 5 parts: EN 15037-1, Precast concrete products — Beam-and-block floor systems — Part 1: Beams; EN 15037-2, Precast concrete products — Beam-and-block floor systems — Part 2: Concrete blocks; EN 15037-3, Precast concrete products — Beam-and-block floor systems — Part 3: Clay blocks; !EN 15037-4", Precast concrete products — Beam-and-block floor systems — Part 4: Polystyrene blocks; prEN 15037-5, Precast concrete products — Beam-and-block floor systems — Part 5: Lightweight blocks. This European Standard is one of a series of product standards for precast concrete products. For common aspects of concrete products, reference is made to EN 13369, from which also the relevant requirements of the EN 206-1 are taken. The references to EN 13369 by CEN/TC 229 product standards are intended to make them homogeneous and to avoid repetitions of similar requirements. !Eurocodes are taken as a common reference for design aspects. The installation of some structural precast concrete products is dealt with by EN 13670:2009 Execution of concrete structures. In all countries it can be accompanied by alternatives for national application." The program of standards for structural precast concrete products comprises the following standards, in some cases consisting on several parts: SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 5 EN 1168, Precast concrete products — Hollow core slabs EN 12794, Precast concrete products — Foundation piles EN 12843, Precast concrete products — Masts and poles EN 13224, Precast concrete products — Ribbed floor elements EN 13225, Precast concrete products — Linear structural elements EN 13693, Precast concrete products — Special roof elements EN 13747, Precast concrete products — Floor plates for floor systems EN 13978, Precast concrete products — Precast concrete garages EN 14843, Precast concrete products — Stairs EN 14844, Precast concrete products — Box culverts EN 14991, Precast concrete products — Foundation elements EN 14992, Precast concrete products — Wall elements EN 15037-1, Precast concrete products — Beam-and-block floor systems — Part 1: Beams EN 15050, Precast concrete products — Bridge elements EN 15258, Precast concrete products — Retaining wall elements This European Standard defines in Annex ZA the application methods of CE marking to products designed using the relevant EN Eurocodes (EN 1992-1-1:2004 and EN 1992-1-2:2004). Where, in default of applicability conditions of EN Eurocodes to the works of destination, design provisions other than EN Eurocodes are used for mechanical strength and/or fire resistance, the conditions to affix CE marking to the product are described in ZA.3. 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, Croatia, 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.
SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 6 Introduction The evaluation of conformity refers to the completed precast elements which are supplied to the market and covers all the production operations carried out in the factory. For design rules reference is made to EN 1992-1-1:2004. Additional complementary rules are provided where necessary. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 7 1 Scope This European Standard deals with the requirements and the basic performance criteria for blocks made in normal or lightweight aggregate concrete, used in conjunction with precast concrete beams in compliance with EN 15037-1, with or without cast-in-situ concrete for the construction of beam-and-block floor and roof systems. Examples of typology of floor and roof systems are given in Annex B of EN 15037-1:2008. 2 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 771-3:2003", Specification for masonry units — Part 3: Aggregate concrete masonry units (Dense and light-weight aggregates) EN 772-13, Methods of test for masonry units — Part 13: Determination of net and gross dry density of masonry units (except for natural stone) EN 772-14, Methods of test for masonry units — Part 14: Determination of moisture movement of aggregate concrete and manufactured stone masonry units EN 1992-1-1:2004, Eurocode 2: Design of concrete structures — Part 1-1: General rules and rules for buildings EN 12390-4:2000, Testing hardened concrete — Part 4: Compressive strength — Specification for testing machines EN 13369:2004, Common rules for precast concrete products EN 15037-1:2008, Precast concrete products — Beam-and-block floor systems — Part 1: Beams 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 non-resisting block block with no mechanical function in the final floor system (designated NR) NOTE Its only mechanical function is that of formwork during the construction of the floor system. Floor systems built with non-resisting blocks always have a cast-in-situ structural topping. 3.2 semi-resisting block block participating in the transfer of loads to the beams (designated SR) NOTE In conjunction with a cast-in-situ concrete, it may contribute to the final strength of the system. However, its top flange alone cannot act as a compression slab in the final floor system. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 8
Figure 1 — Example of semi-resisting concrete blocks 3.3 resisting block block with the same functions as semi-resisting blocks but whose top flange can, under certain conditions, play a role as compression slab in the final floor system (designated RR)
Figure 2 — Examples of resisting concrete blocks 3.4 nib shape given to the block so that it will bear on the beam 3.5 longitudinal block block where the axis of the perforations are parallel to the beam NOTE An example of a longitudinal block is given in Figure 3. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 9
Key 1 web 6 depth of the nib 2 overall length 7 overall width 3 flange (top part of the block) 8 overall height 4 height to nib 9 direction of the beam 5 width of the nib
Figure 3 — Example of a longitudinal block 3.6 transverse block block where the axis of the perforations are perpendicular to the beam NOTE An example of a transverse block is given in Figure 4. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 10
Key 1 web 6 depth of the nib 2 overall length 7 overall width 3 flange (top part of the block) 8 overall height 4 height to nib 9 direction of the beam 5 width of the nib
Figure 4 — Example of a transverse block 3.7 open block non-resisting or semi-resisting block without top flange NOTE An example of an open block is given in Figure 5.
Figure 5 — Example of open block 3.8 gross dry density mass per unit gross volume after drying to constant mass 4 Requirements 4.1 Material requirements 4.1.1 and 4.1.2 of EN 13369:2004 or 4.1 of !EN 771-3:2003" shall apply. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 11 Normal and lightweight concrete blocks should be considered as non-resisting blocks, or semi-resisting blocks or resisting blocks (with a minimum class and density for lightweight concrete). In addition, in the case of resisting blocks lightweight concrete shall be at least Class LC 12/13, with a minimum density of 800 kg/m3 according to 11.3.1 of EN 1992-1-1:2004. 4.2 Production requirements Where applicable, 4.2.1.1 of EN 13369:2004 shall apply. Only the materials described in 4.1 shall be used in the production of concrete blocks. 4.3 Finished product requirements 4.3.1 Geometrical properties 4.3.1.1 General For Technical documentation see Clause 8. 4.3.1.2 Production tolerances 4.3.1.2.1 Dimensional tolerances The manufacturer shall declare which of the following tolerance classes the concrete blocks fulfil: ! a) General for the two classes: ± 3 mm for the width of the nib; ± 5 mm for all other dimensions (excepted length, width and height); b) Class T1: ± 10 mm for the length L, the width l and the height h (see Figure 7);" c) Class T2: length L: ± 5 mm; width l and height h: (0 ; + 5) mm !(see Figure 7)". NOTE Lower tolerance may be declared by the manufacturer. 4.3.1.3 Minimum dimensions 4.3.1.3.1 General Complementary to 4.3.1.2 of EN 13369:2004, the next subparagraphs shall apply. The dimensions shall be verified according to 5.1.1. The effective width of concrete block nib shall be greater than: minimum dimensions Class N1: 20 mm; SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 12 minimum dimensions Class N2: 25 mm. 4.3.1.3.2 Top flange of resisting blocks The top flange of concrete resisting blocks shall be at least: top flange Class TF1: 30 mm thick; top flange Class TF2: 35 mm thick. Moreover, one of the top edges shall have a nib to !the minimum dimensions" given in Figure 6 so that the joint between abutting blocks can be grouted. !Minimum dimensions in millimetres"
Figure 6 — Minimum thickness of top of concrete resisting blocks These blocks shall not be used as compressive flange if the grouting is not effective. When the top flange of concrete resisting blocks has a slope, the dimensions shall be declared by the manufacturer. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 13 4.3.1.4 Nominal configuration of blocks The nominal values shown on Figure 7 shall be declared.
Key 1 width (l) 5 width of the nib 2 height (h) 6 depth of the nib 3 length (L) 7 horizontal part of the slope 4 height to nib
Figure 7 — Nominal dimensions declared for blocks When relevant to the use for which the blocks are put on the market also the minimum thickness of webs and shells shall be declared. 4.3.2 Surface characteristics Blocks shall not have cracks or spalling which may be detrimental to their mechanical performance. For test method see 5.1.2. 4.3.3 Mechanical resistance 4.3.3.1 General 4.3.3.4 of EN 13369:2004 shall apply. The manufacturer shall declare which class of mechanical resistance the blocks fulfil. 4.3.3.2 Class R1 The following properties shall be verified: the resistance to concentrated loads, for each type of block (see a)); the characteristic longitudinal compressive strength, for resisting and semi-resisting blocks (see b)). a) Resistance to concentrated loads SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 14 The blocks shall be classified in type defined by the characteristic failure load obtained in the resistance to concentrated loads test described in 5.2.1. Table 1 gives the minimum characteristic resistance (5 % fractile) for each type of block. Table 1 — Concrete block types Type of block Minimum characteristic resistance to concentrated loads (5 % fractile) in kN Non-resisting (NR) 1,5 Semi-resisting (SR) 2,0 Resisting (RR) 2,5 NOTE Greater values may be declared by the manufacturer. For resisting and semi-resisting blocks used in conjunction with self-bearing beams, without topping, the resistance to concentrated loads test may be replaced by the transverse testing described in 5.2.4. b) Longitudinal compressive strength of resisting and semi-resisting blocks When the manufacturer declares a longitudinal compressive strength of at least 20 MPa with a view to take it into account in the design of the finished floor system, the longitudinal compressive strength of the resisting and semi-resisting blocks shall be tested according to 5.2.3. 4.3.3.3 Class R2 The following properties shall be verified: the bending strength (see a)); the longitudinal compressive strength of resisting and semi-resisting blocks (see b)). a) Bending strength Concrete blocks of all types shall withstand a strip load of not less than P = 12 L (kN) acting at the centre or at the least favourable point, with L being the length expressed in m and tested in accordance with 5.2.2. The strip load shall be not less than 2,0 kN. b) Longitudinal compressive strength of resisting and semi-resisting blocks When the manufacturer declares a longitudinal compressive strength of at least 16 MPa with a view to take it into account in the design of the finished floor system, the longitudinal compressive strength of the resisting and semi-resisting blocks shall be tested according to 5.2.3. 4.3.4 Resistance and reaction to fire 4.3.4.1 Resistance to fire 4.3.4.1 to 4.3.4.3 of EN 13369:2004 shall apply. For the verification of fire resistance of the floor system by testing, EN 1365-2:1999 may apply. NOTE A method is given in Annex K of EN 15037-1:2008 to determine the fire resistance of the floor system. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 15 For blocks intended to be used in elements subject to fire requirements the manufacturer shall alternatively declare the gross dry density and the configuration of the blocks, when relevant. 4.3.4.2 Reaction to fire 4.3.4.4 of EN 13369:2004 shall apply. 4.3.5 Acoustic properties 4.3.5 of EN 13369:2004 shall apply. For design purposes of the finished floor system and in the absence of measurement results, the method given in Annex L of EN 15037-1:2008 may be used (airborne and impact sound insulation). When relevant to the uses alternatively the gross dry density and the configuration of the blocks shall be declared. 4.3.6 Thermal properties 4.3.6 of EN 13369:2004 shall apply. When relevant to the uses alternatively the gross dry density and the configuration of the blocks shall be declared. 4.3.7 Durability 4.3.7 of EN 13369:2004 shall apply. The durability of blocks shall be consistent with the durability of beams used for the construction of the floor system. 4.3.8 Other requirements 4.3.8 of EN 13369:2004 shall apply. The gross dry density of the blocks shall be declared by the manufacturer and tested in accordance with 5.3. The density of lightweight-aggregate concrete shall not deviate from the manufacturer's declared value (see EN 1992-1-1) by more than ± 10 %. For lightweight concrete blocks, the drying shrinkage shall be no more than the declared value. It shall be measured in accordance with 5.4. 5 Test methods 5.1 Measuring of dimensions and surface characteristics 5.1.1 Block dimensions 5.1.1.1 General Measure the following dimensions with an accuracy of 1,0 mm, after removal of any burr that could induce measurement errors. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 16 5.1.1.2 Procedure a) Width Take the two measurements as shown in Figure 8 (the larger width).
Figure 8 — Measurement of width b) Length Take the two measurements as shown in Figure 9.
Figure 9 — Measurement of length c) Height Take the two measurements as shown in Figure 10.
Figure 10 — Measurement of height SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 17 d) Thickness of the top flange Take two measurements: one in the middle of each end.
Figure 11 — Measurement of thickness of the top flange e) Thickness of the web Take four measurements: two at each end of both sides.
Figure 12 — Measurement of thickness of the web f) Width and depth of the nib Place the block on a level horizontal surface (the flatness shall be controlled). Take two measurements: one in the middle of each side. Repeat the operation to measure depth. 5.1.1.3 Interpretation of results !Calculate the length, width and height for the sample as the mean of the values of the individual specimens. All results shall comply with the requirements of 4.3.1 and the values declared by the manufacturer. NOTE Measurements shown in Figures 8 to 12 may refer to different nominal values." 5.1.1.4 Test report !All the measured dimensions and the calculated dimensions such as length, width and height of the sample shall be recorded." 5.1.2 Surface characteristics The surface appearance of blocks shall be inspected visually. SIST EN 15037-2:2009+A1:2011
EN 15037-2:2009+A1:2011 (E) 18 Defects likely to be detrimental to their mechanical performance shall be rejected. 5.2 Mechanical strength 5.2.1 Resistance to concentrated loads 5.2.1.1 General The test method given in the next paragraph shall apply to determine the resistance to concentrated loads of concrete blocks. 5.2.1.2 Equipment Class 3 testing machine for applying forces according to EN 12390-4:2000; rule accurate to within 1,0 mm for positioning of the load; piece of wood or steel for applying the load (the load shall be transmitted by means of an articulation). 5.2.1.3 Procedure The test piece shall be the block simply resting on supports representing the beams. The position of the blocks relative to the supports shall be adjusted so that forces are transmitted solely through the normal surfaces of transmission. The load, P, shall be applied gradually until the sample fails, with a rate of loading equal to 10 daN/s ± 3 daN/s. It shall be applied by means a rigid piece (e.g. wood, steel) of 50 mm × 50 mm, centred in the longitudinal direction and placed transversally in the most critical position for the strength of the block (see F
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