[0001] The present invention generally relates to a fire resistant steel-concrete floor structure.

Background Art:

[0002] Steel-concrete floor structures are well-known in the art. They comprise a concrete slab supported by horizontal structural (i.e. load bearing) steel members, as for example rolled steel beams, welded steel beams, cellular steel beams or steel girders, steel joists or steel trusses. (For the sake of simplicity, the term "steel beam" is used herein as an equivalent for the term "horizontal structural steel member".)

[0003] Today, most steel-concrete floor structures are so-called composite steel- concrete floor structures. In such composite structures, the concrete slab and the steel beams structurally cooperate to provide the load bearing capacity of the structure. This structural cooperation is generally achieved by welding shear connectors, generally so-called shear-studs, onto the top flange of the steel beams. These shear connectors penetrate into the concrete slab, wherein they usually cooperate with steel reinforcement meshes or steel bars placed below the head of the studs to transmit shear forces between the top flange of the steel beam and the concrete slab. Consequently, when dimensioning a steel beam in a composite steel- concrete floor, a slab portion with a certain width located above the top flange of the steel beam and connected to the latter by means of the shear connectors, may be considered as being part of a composite steel-concrete section, wherein the steel beam takes all the tension stresses, but the concrete section takes most of the compression stresses due to the bending moment.

[0004] While massive steel is inherently a non-combustible material, it is well known in the art that the strength and stiffness of structural steel members are significantly reduced, when such structural steel members are heated to temperatures experienced in a standard fire scenario. For example, it is generally admitted that for temperatures above 500°C, the strength and stiffness of structural steel members is reduced to such an extent, that they are no longer capable of performing the bearing function for which they have been designed. Consequently, in order to achieve the fire resistance prescribed in building codes, structural steel members have to be protected with a fireproof heat insulation or intumescent paints or coatings slowing down temperature rise in the structural steel members during a fire scenario.

[0005] It is also known to provide structural steel members with a partial or full concrete encasement, thus substantially increasing their mechanical resistance in a fire scenario. However, it substantially increases the weight of these steel beams. Hence, using steel beams with a concrete encasement in a composite steel- concrete floor structure, would substantially reduce the weight advantage with regard to a full concrete floor structure.

[0006] It is well known in the art that the necessity to equip steel beams in steel- concrete floor structures with a fire protection as described above, substantially increases the building costs. Consequently, it would be of advantage, if in a steel- concrete floor structure, one could dispense with expensive fire protection measures, at least for the horizontal structural steel members.

[0007] In this context, a research project financed by the European Commission entitled "Membrane Action of Composite Structures in Case of Fire" has come to the conclusion that when designing composite steel-concrete floor structure at elevated temperatures, one may take into account the enhancing effects of the membrane action in a reinforced slab. Taking advantage of this membrane action, it becomes e.g. possible to dispense with a fire protection for secondary beams, but the primary beams must still be protected.

[0008] US2012/0066990 discloses a fire resistant composite steel-concrete floor structure, in which the steel beams supporting the concrete slab are equipped with fire-resistant tension members. The latter have their ends anchored outside the steel beam in the support structure and are arranged in relation to the steel beam in such a way that, when a steel beam is overheated and yields under its vertical load in case of a fire scenario, the overheated beam rests on the fire-resistant tension members and is vertically supported by the latter. With this solution it is possible to dispense with any fire protection measure for the steel beams themselves, but the tensions members as such must either be made of a steel (or any other material) still having a high tensile strength at high temperatures (i.e. temperatures above 500°C), or they must be protected with a fireproof heat insulation, or an intumescent material slowing down temperature rise in the tensions members during a fire scenario. In practice, both solutions have not yet been applied on the market and their competiveness still has to be proven.

[0009] The object underlying the present invention is to propose a steel-concrete floor structure that has an outstanding behaviour in a fire scenario without necessitating any expensive fire protection measures at all.

[0010] This object is achieved with a fire resistant composite steel-concrete floor structure as claimed in claim 1.

Summary of invention:

[0011] Within the context of the definition of the present invention, the following terms shall have the meanings set forth below:

- "Prescribed fire resistance" shall mean the ability of a structure, a part of a structure or a member thereof to fulfil its required function(s) (load bearing function and/or fire separating function) for a specified load level, for a prescribed fire exposure, and for a prescribed period of time (as prescribed in standards, building codes or by local authorities);

- "Prescribed fire scenario" shall mean a fire scenario as prescribed in standards, building codes or by local authorities for proving compliance of buildings, engineering works with regard to safety requirements in case of fire. Such a prescribed fire scenario is generally characterized by a standard temperature time curve as defined e.g. in EN model of a fully developed fire in a compartment and allows, amongst others, to compute the time-dependant temperature evolution in structural members during the fire scenario;

- "Fully developed fire" shall mean the state of full involvement of all combustible surfaces; - "Fire compartment" shall mean a space within a building that is enclosed by separating elements such that fire spread beyond the compartment is prevented during the relevant fire exposure;

- "Structural member" shall mean a load bearing member of a structure including bracings;

- "Primary beam" a horizontal structural member directly supported by a vertical load bearing structure and designed as a main load carrying member of a floor structure;

- "Secondary beam" a horizontal structural member designed as auxiliary load carrying member of a floor structure (generally smaller steel sections arranged between primary beams to provide intermediary support for the slab);

- "Normal operation" shall mean the state in absence of a fire, i.e. at ambient temperatures;

- "External bearing structure" shall mean a bearing structure located outside the fire compartment that is considered to be exposed to a fire scenario;

- "Fire resistant bearing structure" shall mean a bearing structure having a prescribed fire resistance with regard to its bearing function;

[0012] A steel-concrete floor structure in accordance with the present invention is a steel-concrete floor structure designed for having a prescribed fire resistance when exposed to a prescribed fire scenario in a fire compartment immediately below the steel-concrete floor structure. It comprises horizontally extending structural steel members and a concrete slab. In normal operation, the horizontally extending structural steel members have to take the main part of the tensile stresses generated by the bending moment due to the load for which the steel-concrete floor structure is designed. According to an important aspect of the invention, the floor structure further comprises a plurality of tension members, each of the tension members horizontally extending through the concrete slab between two anchor points. These anchor points are arranged in one or more fire resistant bearing structures within the fire compartment, and/or in one or more external bearing structures, so that, when the strength and stiffness of the horizontal structural steel members substantially diminish in the prescribed fire scenario, the tension members extending through the concrete slab, wherein they are heating up more slowly than the structural steel members, are capable of supporting the loaded slab by catenary effect and of limiting the deflection thereof. It will be appreciated that such a steel- concrete floor has an outstanding behaviour in a fire scenario without necessitating any expensive fire protection measures at all. In particular, with the proposed floor structure, deflections of the slab may be limited to an extent that is sometimes difficult to achieve with horizontal structural steel members protected with a fireproof heat insulation, intumescent paints or coatings or concrete encasements.

[0013] If the steel-concrete floor structure is moreover a composite structure in which the horizontally extending structural steel members and a concrete slab are interconnected by shear connectors, the slab will be able to support the horizontally extending structural steel members, if the latter lose most of their strength and stiffness during the prescribed fire scenario.

[0014] To control deflection of the floor structure in case of a prescribed fire scenario, it is recommended to pre-tension the tension members. The tension efforts are hereby taken by the fire resistant bearing structures and/or the external bearing structures in which the tension members are anchored, as well during the pre- tensioning operation, as during normal operation and during the prescribed fire scenario. A compression of the concrete slab is hereby principally not desirable. Indeed, due to the arrangement of the tension members in the upper half of the concrete slab, where they are best protected against a rapid temperature rise in a fire scenario, such a compression would result in a supplementary load for the floor structure.

[0015] In a first embodiment, the tension members extend through horizontal ducts embedded in the concrete slab. This embodiment allows to pre-tension the tension members after casting the concrete slab. The horizontal ducts are preferably arranged in the upper half of the concrete slab, where they are best protected against a rapid temperature rise in a fire scenario.

[0016] Within these ducts, the tension members are advantageously embedded in a material injected into the ducts after pre-tensioning the tension members. This injections provides a more direct interaction between the tension members and the concrete slab surrounding the ducts, protects the tension members against corrosion by water penetrating into the ducts and, in case of a fire scenario, prevents hot combustion gases from penetrating into the ducts.

[0017] In an alternative embodiment, the tension members are tensioned prior to casting the concrete slab and are directly embedded in the cast concrete. This embodiment has the advantage to provide a very direct interaction between the tension members and the concrete slab, to efficiently protect the tension members against corrosion and against direct contact with hot combustion gases in a fire scenario. Compared to the solution in the previous paragraph, it has the disadvantage that the tension members must be pretensioned prior to casting the concrete slab, when the fire resistant bearing structures and/or the external bearing structures for the anchor points may not yet be available, which may cause delay of a building project.

[0018] In normal operation, the tension in a tension member amounts to less than 70%, preferably to less than 60%, of its nominal tensile strength; and is preferably in the range of 20% to 60%, and more preferably in the range of 40% to 60%, of its nominal tensile strength. A minimum pre-tension is recommended in order to warrant that in case of a fire scenario, the tension members rapidly start developing a catenary support function. Indeed, the higher the pre-tension in normal operation, the sooner the tension members will start developing said catenary support function, and the lower the deflection of the slab after a certain time of fire exposure will be. However, the pre-tension in normal operation shall also be limited upward to a certain percentage of the nominal tensile strength. Indeed, during the first minutes of the fire scenario, the tension members must be able to follow the increasing deflection of the horizontally extending structural steel members without plastically deforming. It will in this context be appreciated that— surprisingly— up to 60% or even 70% of the tensile strength of the tensile members may be used for pre- tensioning them, while still warranting their proper operation in a fire scenario. Indeed, as will be explained hereinafter, during a fire scenario, after an initial increase of the tension in the tensile members during the first 20 minutes, this tension diminishes again. [0019] The tension members are advantageously dimensioned so that during the prescribed fire-scenario, the tension in a tension member increases to a maximum that is close to its nominal tensile strength.

[0020] The tension members are advantageously protected along their whole length on their bottom side by at least 6 cm, preferably at least 9 cm concrete. It is the thickness of the concrete below tension members that is mainly responsible for a slow temperature increase in the tension members during a fire scenario. A thickness of 9 cm usually warrants e.g. that the temperature in the tension members remains— in a fire scenario characterized by a standard temperature-time curve according to EN 1991-1 -2: 2002, section 3.2.1— for at least 90 minutes below 200°C, whereby the temperature induced decrease of Young's modulus and the tensile strength of the tension members, as well as their thermal elongation remain within acceptable ranges.

[0021] In a preferred embodiment of the floor structure, the horizontally extending structural steel members comprise primary beams; and the tension members are arranged parallel to the primary beams and are regrouped in slab bands centred over the primary beams, the slab bands having a width of less than 2 m, preferably of less than 1.6 m. The tension members are indeed most efficient when they are located close to the primary beams, i.e. in or close to the concrete in which, in case of a composite floor structure, the primary beams are connected by means of their shear-connectors.

[0022] The tension members are advantageously high tensile strength wire- strands.

[0023] In normal operation, the contribution of the tension members to the support of the loaded slab is negligible. The catenary curve of the tensile members is indeed too flat to be capable of contributing to a signification extent to the support of the loaded slab.

[0024] The fire resistant bearing structures for the anchoring points of the tension members normally have a fire resistance at least equal to the fire resistance prescribed for the steel-concrete floor structure and comprise at least one of the following elements: a concrete column; a composite concrete-steel column; a steel column protected with a fireproof heat insulation or an intumescent material; a fire resistant steel bracing; a concrete wall; or a concrete core of a building.

[0025] The external bearing structures for the anchoring points of the tension members may have a fire resistance lower than the fire resistance prescribed for the steel-concrete floor structure. This is for example the case, if the standards, building codes or local authorities allow to consider that the fire scenario is limited to one fire compartment or that a less severe fire scenario is considered for external bearing structures. In this case it is even possible to have an external bearing structure that has no fire resistance at all, as e.g. an unprotected steel structure. This may also be the case, if the external bearing structure is located outside of the building and may therefore not be affected by a fire scenario inside of the building.

[0026] In a very advantageous embodiment, the anchor points for tension members are on one side arranged in a central fire resistant core of the building, in particular in central core made of reinforced concrete, and on the other side, arranged in an external bearing structure surrounding the central core, in particular an external bearing structure made of structural steel members. In this embodiment, none of the vertical or horizontal structural steel members has to receive a fire protection for warranting its stability in a fire scenario. The fire resistance of such a building is mainly warranted by its central fire resistant core and the tension members in accordance with the present invention.

Brief description of drawings:

[0027] The afore-described and other features, aspects and advantages of the invention will be better understood with regard to the following description of several embodiments of the invention and upon reference to the attached drawings, wherein:

FIG. 1 : is a schematic plane view from above of a portion of a steel-concrete floor structure in accordance with the invention, wherein the concrete and reinforcement meshes in the concrete are not shown;

FIG. 2: is a schematic vertical sectional view as indicated by arrows 2-2' in FIG. 1 ;

FIG. 3: is a schematic vertical sectional view as indicated by arrows 3-3' in FIG. 1 ; FIG. 4: is an enlarged detail of FIG. 3, showing tension members in a duct;

FIG. 5: is a schematic vertical sectional view illustrating an anchor point for a tension member in a fire resistant support column;

FIG. 6: is a schematic plane view as FIG. 1 , wherein anchor points for tension members are, on the left side, arranged in a bearing wall and, on the right side, in a central core of the building;

FIG. 7: is a very schematic plane view as FIG. 1 , wherein anchor points for tension members are on one side arranged in a central fire resistant core of the building, and on the other side arranged in a bearing structure surrounding the building;

FIG. 8: is a diagram showing the evolution in time of the deflection of the steel- concrete floor structure during a standard fire scenario, wherein:

- curve (a) shows the deflection without tension members;

- curve (b) shows the deflection with tension members without any significant pre-tension in normal operation;

- curve (c) shows the deflection with tension members pre-tensioned in normal operation at 58% of their tensile strength;

FIG. 9: is a diagram showing the evolution in time of other parameters during a standard fire scenario, wherein:

- curve (d) shows the evolution of the bending moment taken by a steel beam in the steel-concrete floor structure as a percentage of the moment taken by the same steel beam in normal operation;

- curve (e) shows the evolution of the temperature of this steel beam;

- curve (f) shows the evolution of the tensile force in a tension member as a percentage of its tensile strength; and

- curve (g) shows the evolution of the temperature of this tension member.

Detailed description of several embodiments of the invention

[0028] It will be understood that the following description and the drawings to which it refers describe by way of example embodiments of the invention for illustration purposes. They shall not limit the scope, nature or spirit of the claimed subject matter.

[0029] FIG. 1 illustrates a portion of a composite steel-concrete floor structure 10 in accordance with the invention. This steel-concrete floor structure 10 is designed for having a prescribed fire resistance when exposed to a prescribed fire scenario in a fire compartment immediately below (i.e. in the prescribed fire scenario, the floor structure 10 is heated from below). Depending for example on the occupation of the building and the number of storeys, the prescribed fire resistance may for example be 60 minutes, 90 minutes, 120 minutes. In exceptional cases, the prescribed fire resistance may even be more than 120 minutes, or be lower than 60 minutes, e.g. be only 30 minutes.

[0030] The composite floor structure 10 comprises a horizontal support structure comprising horizontally extending structural steel members 12, also called "primary beams 12" hereinafter, and a concrete slab 14 arranged above the primary beams 12. In FIG. 1 , this concrete slab 14 is shown as a transparent body for being able to see the support structure located below or within the slab 14.

[0031] The concrete slab 14 may for example be: a full concrete slab cast in situ, using e.g. removable framework between the primary beams 12; a composite concrete steel deck slab, wherein the slab is cast in situ on a profiled steel deck supported by the primary beams 12 and possibly by secondary beams 28; or a concrete slab cast in situ on precast concrete planks, which are supported by the primary beams 12. The concrete slab 14 may further comprise steel reinforcement (shown in FIG. 2 and 3, but not in FIG. 1 ) under the form of steel meshes 32, 44 or steel bars embedded into the concrete, and/or steel fibres mixed into the concrete before casting it. The object of this steel reinforcement is mainly to prevent cracking of the concrete and to provide reinforcement in the event of fire, relying on the so- called membrane action of the reinforced concrete slab 14 to warrant its integrity in case of major deflections.

[0032] The primary beams 12 may be rolled steel beams, welded steel beams, cellular steel beams or steel girders, steel joists or steel trusses. The primary beams 12 shown in FIG. 2 and 3 are e.g. I-shaped sections (also called double-T sections), such as e.g. IPE sections, having an upper flange 16, a lower flange 16' and a web 18. It will be appreciated that the structural steel members 12 are not provided with a passive fire protection, at least not with a passive fire protection substantially increasing their load bearing capacity under fire exposure. Consequently, when the structural steel members 12 are exposed to a fire resulting in temperatures of the structural steel members 12 above 500°C, their strength and stiffness are reduced to such an extent, that they are no longer capable of performing the bearing function for which they have been designed.

[0033] The primary beams 12 are supported by vertically extending structural members 20. In distinctive contrast to the primary beams 12, the vertically extending structural members 20 must either warrant as such the prescribed fire resistance (this may e.g. be the case if these vertically extending structural members 20 are concrete structural members) or must be protected with a fireproof heat insulation or intumescent paints or coatings or provided with a concrete filling or encasement to warrant the prescribed fire resistance. In the embodiment shown in FIG. 1 , 2, 5 & 6, these vertically extending structural members 20 are e.g. H-shaped steel sections with a reinforced concrete filling 22 between the flanges 24, 24' (such vertical structural members are generally called "composite columns" or "partially encased composite columns"). The reinforced concrete filling 22 warrants that the composite columns 20 maintain their load bearing function during the required time of fire exposure. Alternatively, the vertically extending structural members 20 may be steel sections, e.g. H-shaped sections, protected with a fireproof casing made e.g. of silicate or gypsum plates with or without mineral fibre insulations, with spray applied fireproofing materials, intumescent paints or coatings, respectively steel sections completely encased in concrete or closed steel sections completely filled with concrete. The vertically extending structural members 20 may further be steel reinforced concrete columns or wooden columns or they may be replaced by other suitable support elements for the horizontally extending structural steel members 12, as e.g. a concrete wall or a brick wall, which have already an inherent fire resistance.

[0034] As seen in FIG. 2, the primary beam 12 is supported between the columns 20, in such a way that it may axially expand when heated in a fire scenario, thereby avoiding that the expanding horizontal beam 12 exerts excessive horizontal forces onto the columns 20. Such a free expansion can easily be implemented by providing interconnection means allowing a free thermal expansion of the horizontal beam 12, when the latter is heated in a fire scenario. Such interconnection means may e.g. be a double web cleated connection (as identified e.g. with reference number 26 in FIG. 2) or a fin plate connection (not shown) between the web 18 of the steel beam 12 and a flange 24 of the column 20, wherein fixing bolts on the side of the web 18 may e.g. horizontally slide within oblong bolt holes when the steel beam 12 expands. These interconnection means 26 need not be fire protected. To the contrary, it may even be of advantage if the interconnection means 26 get less rigid under fire conditions, whereby the deflection of the unprotected primary beam will exert smaller forces onto the protected columns 20.

[0035] It will be noted that the length "L" of the primary beams 12 is generally greater than the distance "D" between two parallel rows of primary beams 12. For example, with a concrete slab cast in situ on precast concrete planks, the distance "D" would normally be less than 5 m, whereas the length of the primary beams 12 may be bigger than 10 m, wherein a length of 16 m for the primary beams 12 is not unusual. If necessary, horizontal secondary beams may be arranged transversally to the primary beams 12. In FIG. 1 , a single secondary beam, identified with reference 28, is shown for illustration purposes. Normally, such a secondary beam would be arranged every 2 to 4 m. It will of course be understood that the distance "D" between two primary beams 12, and the number and spacing of secondary beams 28, will be conditioned by the type of concrete slab 14 to be installed, the procedure for installing it, the design load of the floor structure 10 and space or architectural requirements (e.g. the spacing of the columns 20 and/or free space for technical equipment, e.g. HVAC equipment, within the horizontal support structure of the floor). For example: when working with a concrete slab cast in situ on precast concrete planks, it will normally not be necessary to have secondary beams 28, as such concrete planks may easily span distances up to 5 m without necessitating any intermediary support during casting of the slab 14 (if they are moreover pre- stressed, they may even span distances of more than 5 m). When working however with a concrete slab cast in situ on a profiled steel deck, it will normally be necessary to have secondary beams 28 for supporting profiled steel deck. Such a secondary beams 28 would be arranged between the primary beams 12 and be spaced e.g. between 2 m and 4 m (sometimes even 5 m) from each other.

[0036] As seen in FIG. 2 and 3, the concrete slab 14 bears on the upper flanges 16 of the horizontally extending structural steel members 12. It will be noted that the floor structure 10 shown in FIG. 3 is conceived as a composite steel-concrete structure, i.e. the concrete slab 14 and the steel beams 12 structurally cooperate to provide the load bearing capacity of the floor structure 10. This structural cooperation is achieved by shear-studs 30 (or equivalent shear connectors) welded onto the upper flange 16 of the steel beams 12. The shear-studs 30 penetrate into the concrete slab 14, wherein they advantageously cooperate with reinforcement steel 32, e.g. under the form of meshes and/or bars, placed below the head of the shear-studs 30, to transmit shear forces between the upper flange 16 of the steel beam 12 and the concrete slab 14. Consequently, when dimensioning a steel beam 12 in a composite steel-concrete floor 10, a slab portion with a certain width located above the top flange of the steel beam and connected to the latter by means of the shear-studs 30, may be considered as being part of a composite steel-concrete section, wherein the steel beam 12 usually takes all the tension stresses, but the concrete section takes most of the compression stresses due to the bending moment.

[0037] In accordance with the present invention (see in particular FIG. 1 and 3), the floor structure 10 further comprises a plurality of tension members 34. Each of these tension members 34 horizontally extends through the concrete slab 14 between two anchor points 36. These anchor points 36 are arranged in one or more fire resistant bearing structures 38, so that, when the strength and stiffness of the horizontal structural steel members 12 substantially diminishes in the prescribed fire scenario, the tension members 34 extending through the concrete slab 14 (wherein they heat up more slowly than the unprotected structural steel members 12) are capable of supporting the loaded slab 14 by catenary effect and of limiting the deflection thereof.

[0038] These tension members 34 are normally high tensile strength wire-strands (i.e. wire strands having a tensile strength of at least 1000 MPa, preferably of at least 1500 MPa). They extend through the concrete slab in parallel to the primary beams 12 and are grouped in slab bands 40, which are centred with regard to the primary beams 12 and have a width "W" less than 2 m. If the primary beams 12 are supported by columns 20 aligned with the primary beams 12 (as it is the case in FIG. 1), there are preferably no tension members 34 arranged directly over the primary beams 12, because the columns 20 would have to be pierced for providing a passage for the tension members 34. Such piercing would weaken the columns 20 and the development of a catenary curve over more than beam length would be prevented. However, if it is possible to arrange tension members 34 centrally over the primary beams 12, without having to pierce bearing columns 20 for providing horizontal passages for the tension members 34, such central arrangement is most often preferred. It will further be noted that no tension members 34 are arranged at a distance of more than 1 m from the vertical central plane of the primary beams 12. Indeed, the tension members 34 are most efficient when they are located close to the upper flange 16 of the primary beam 12, i.e. in or close to the concrete in which, in case of a composite floor structure, the upper flange 16 of the primary beam 12 is anchored by means of its shear-studs 30.

[0039] The tension members 34 are advantageously pre-tensioned. In normal operation, the tension in a tension member amounts to less than 70%, preferably to less than 60%, of its nominal tensile strength; and is preferably in the range of 20% to 60%, and more preferably in the range of 40% to 60%, of its nominal tensile strength. Pre-tensioning the tension members 34 warrants indeed that, when the strength and stiffness of the horizontal structural steel members diminishes in the prescribed fire scenario, the tension members 34 begin to work more rapidly as a catenary support means for the loaded slab 14 and to limit the deflection thereof. The higher the "cold" pre-tension of in the tension members 34, the sooner the tension members 34 begin to work in case of a fire, the slower the deflection of the floor structure 10 increases in case of a fire, and the smaller the final deflection of the floor structure 10 for the prescribed fire resistance will be. However, the tension in a tension member in normal operation shall amount to less than 70%, preferably to less than 60%, of its nominal tensile strength, because the initially horizontal tension members 34 must first be able to elastically yield to the initial deflection of the slab 14, in order to develop a catenary type support means, when the strength and stiffness of the primary beams 12 substantially diminishes in the prescribed fire scenario. Without the possibility to elastically yield to the initial deflection of primary beams 12 in a fire scenario, the tension members 34 would risk to plastically deform and even rupture under the initial deflection of the heated primary beams 12.

[0040] The tension members 34 can be directly embedded in the concrete slab 14. If they are to be pre-tensioned, the pre-tensioning has to be carried out prior to casting the concrete slab 14, i.e. concrete has to be cast over the pre-tensioned tension members 34. As shown in FIG. 3, tension members 34 can also extend through horizontal ducts 42 embedded in the concrete slab 14. In this case, the tension members can be pre-tensioned after the concrete slab 14 has been cast, which is for example required, if the anchor points for the tension members 34 only get available after casting of the slab 14. To warrant a more direct interaction between the tension members 34 in the duct 42 and the concrete of the slab 14 surrounding the duct 42, the tension members 34 are advantageously embedded within the duct 42 in a material 43 (as e.g. a cement-based grout) injected into the duct 42 after pre-tensioning the tension members 34. Another advantage of filling the ducts 42 with an injected material is that in case of a fire, when cracks begin to appear in the slab, hot combustion gases may not penetrate into the ducts 42 heat the tension members 34 extending there through.

[0041] Still referring to FIG. 3, it will be understood that the tension members 34 are over their whole length preferably arranged in the upper half of the section of the concrete slab 14. The tension members 34 are hereby protected on their bottom side by a certain thickness of concrete. This concrete forms, at no extra costs, an excellent and very reliable heat insulation, capable of substantially slowing down a temperature rise in the tensions members 34 during a fire scenario. For example, a thickness of 9 cm concrete below the tension members 34 usually warrants that the temperature in the tension members remains— in a fire scenario characterized by a standard temperature-time curve according to EN 1991 -1 -2: 2002, section 3.2.1— for at least 90 minutes below 200°C, whereby the temperature induced decrease of Young's modulus and the tensile strength of the tension members, as well as their thermal elongation remain within acceptable ranges.

[0042] It will further be noted that the tension members 34 are ideally located just below upper reinforcement steel 44, e.g. under the form of meshes or bars, placed at some centimetres from the upper side of the concrete slab. This reinforcement steel 44, whose task in normal operation is to avoid cracks in the slab 14 and above the tension members 34, assists during a fire scenario the tension members 34 in supporting the loaded slab 14 when the strength and stiffness of the primary beams 12 substantially diminishes. The lower reinforcement steel 32, whose task in normal operation is to transfer shear-stresses from the upper flange 16 of the primary beam 12 onto the concrete slab 14, helps during a fire scenario the concrete slab 14 to support the primary beam 12, when the latter has lost most of its strength and stiffness.

[0043] In FIG. 1 , the anchor points 36 of the tension members 34 are arranged in the bearing structures 38, which are in this case e.g. bearing walls laterally delimiting the fire compartment of which the floor structure 10 forms the upper floor. The anchor points 36 may be arranged in the bearing walls 38 themselves (as shown in FIG. 1) or in an external bearing structure located behind the bearing wall. If the bearing wall is to be considered as a fire wall having a fire resistance at least equal to the prescribed fire resistance for the floor structure, or as an outer wall, one could even dispense with a fire protection for the anchor points 36.

[0044] In FIG. 5, the anchor point 36' is arranged in a fire resistant bearing structure 38' within the same fire compartment as the floor structure 10. In this case the fire resistant bearing structure 38' is more particularly an H-shaped steel section with a reinforced concrete filling 22 between the flanges 24, 24', i.e. a composite column as described hereinbefore. As this composite column 38' also supports one of the primary beams 12, it must necessarily have a fire resistance at least equal to the floor structure 10 it supports. This means that the fact that an anchor point 36 is arranged in the composite column 38' does generally not required that the composite column 38' receives an additional fire protection.

[0045] In FIG. 6, the anchor points 36 are arranged within a bearing wall 38, just as described with reference to FIG. 1. The anchor points 36" are arranged within a central core 38" of the building, housing e.g. the lifts and/or the emergency stairways. Such a central core 38" generally has the highest fire resistance of the whole building and is consequently an ideal bearing structure for arranging anchor points 36" of the tension members 34 therein. [0046] In FIG. 7, the anchor points 36" are arranged within a central core 38" of the building, just as described with reference to FIG. 6. The anchor points 36"' are arranged within a bearing structure 38"' surrounding the building. This bearing structure 38"' is advantageously an external bearing cage constructed with structural steel members. The unprotected primary beams 12 are supported on one side by the central core 38" and on the other side by the bearing structure 38"'. The bearing structure 38"' further supports a curtain-wall facade with internal air space, arranged so that the bearing structure 38"' itself is not to be affected by a fire scenario within the building. Consequently, none of the horizontal or vertical structural steel members of such a building has to receive a fire protection for warranting its stability in a fire scenario.

[0047] The following description of the diagrams of FIG. 8 and 9 will help, if necessary, to still better understand the claimed invention.

[0048] Curve (a) in FIG. 8 shows the computed evolution of the deflection of a composite steel-concrete floor structure comprising unprotected steel beams 12 and no tension members 34 in the slab 14, when this floor structure is exposed to a design fire resulting in an evolution of the temperature of the steel beams as shown in FIG. 9 with curve (e) (this temperature evolution has e.g. been computed using a standard temperature-time curve according to EN 1991 -1-2: 2002, section 3.2.1 ). The deflection is measured in the middle of a steel beam 12 with a length L = 16.8 m supported between two fire resistant columns 20 as shown in FIG. 2. The steel beam is an unprotected IPE600 section. The slab 14 is a full concrete slab with a thickness of 14 cm, and steel reinforcements under the form of steel meshes arranged as illustrated in FIG. 3 (reinforcing wires with a diameter of 7 mm every 15 cm, i.e. a steel section of 1.4 cm ² /m). The live load is 2,5 kN/m ² .

[0049] After 10 minutes of fire exposure, the temperature of the steel beam 12 has already reached about 500°C and the deflection is already 0.56 m, i.e. L/30. After 13 minutes of fire exposure, the temperature of the steel beam 12 is above 600°C, and the deflection has increased to 0.84 m, i.e. to L/20.

[0050] Curve (c) in FIG. 8 shows the computed evolution of the deflection of a composite steel-concrete floor structure as described above with regard to curve (a), with the sole difference that the slab now comprises per primary beam 12 twelve tension members 34 arranged as shown in FIG. 3 in a concrete band of a width W = 1.6 m located above the beam 12. These tension members 34 are high tensile wire strands each having for example a section of 100 mm ² and a tensile strength of 1860 MPa. Each of the tension members 34 is pre-tensioned so that in normal operation the tension in a tension member 34 is about 58% of the tensile strength of the tension member 34. The tension members 34 extending horizontally through the slab 14 are protected at their underside over their whole length by 9 cm of concrete, so that the temperature in the tension members increases, as shown by curve (g) in FIG. 9, by far more slowly in the tension members 34 than in the unprotected primary beams 12 (after two hours, the temperature in the tension members is still below 200°C, whereas the primary beams 12 have reached already a temperature above 1000°C). Considering again curve (c) in FIG. 8, one notices that up to about 5 minutes, the tension members 34 do not produce a significant amelioration of the situation. The deflection increases indeed from 0.05 m (in the cold state) to 0.20 m, which is about the same deflection as for the floor structure without the tension members 34 (see curve (a)). This is due to the fact that the catenary curve of the tension members 34 is yet too flat to enable them to take an important vertical load (in other words, the vertical component of the tension in the tension members is still relatively small). The unprotected primary beams 12 rapidly heat up and achieve a temperature of 500°C in about 10 minutes. During this first time interval the moment taken by the beam decreases by about 30% (see curve (e) in FIG. 9). During the same time interval, the tension in the tension members 34 increases from 58% to 70%, but for the reason explained above (catenary curve too flat) this is not sufficient to slow down the deflection of the floor structure exposed to the fire.

[0051] According to curve (c), a deflection of L/30 is reached after about 20 minutes. The moment taken by the primary beams 12 has in the meantime decreased by about 80%, and the effect of the tension members 34 now gets more pronounced. The more pronounced catenary curve of the tension members 34 allows them to take a bigger vertical load, thereby slowing down the deflection increase (see curve (c)). After about 30 minutes, the tension in the tension members 34 reaches its maximum, wherein it is about equal to (or only slightly lower than) the nominal tensile strength of the tension members 34. In curve (c) of FIG. 8, the maximum tensile strength in the tension member 34 is e.g. about 95% of its tensile strength (=100%). Thereafter the tensile strength in the tension members 34 decreases again, because their Young's modulus decreases as the temperature increases, and because they experience a non-negligible thermal expansion. During the same time interval, the deflection of the floor structure continues to increase, but much more slowly (see curve (c)). It requires about 100 minutes to reach a deflection of L/20, i.e. 0.84 m.

[0052] Curve (b) in FIG. 8 shows the computed evolution of the deflection of a composite steel-concrete floor structure as described above for curve (c), with the sole difference that each of the tension members 34 is pre-tensioned so that in normal operation the tension in a tension member 34 is only about 10% of the tensile strength of the tension member 34. (When wire-strands are used as tension members 34, a minimum pre-tension is always required in order to warrant that wire- strands extend horizontally through the slab 14.) Comparing curve (b) and curve (c), one notices that it takes more time for the tension members 34 to produce a significant amelioration of the deflection situation. Already after about 14 minutes, the deflection has reached a value of L/30 (according to curve (c) this value is reached only after 20 minutes). The deflection value L/20 is reached according to curve (b) in about 40 minutes and according to curve (c) in about 100 minutes. This clearly shows the advantage of pre-stressing the tension members 34.

[0053] Last but not least, in order to avoid any misunderstanding, it will be noted that the proposed solution is not to be confused with the concept of "pretensioned concrete", of "bonded post-tensioned concrete" and of "unbonded post-tensioned concrete". According to the concept of "pretensioned concrete", the concrete is cast around already-tensioned tendons. After the concrete has cured, the tension in the tension means is released and thereby transferred to the concrete as compression by static friction. According to the concept of "bonded post-tensioned concrete", the concrete is cast around curved ducts that are placed in the area where tension would occur in the concrete element. A set of tendons is arranged in these ducts before the concrete is poured. Once the concrete hardens, the tendons are tensioned by hydraulic jacks that react against the concrete slab. When the tendons have stretched sufficiently, they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete slab. The duct openings are then grouted to protect the tendons from corrosion. The tension in the tendons compresses the concrete and, in case of a slab, the curved tendons further produce an upward directed load, which reduces the deflection of the slab. The concept of "unbonded post-tensioned concrete" differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. The transfer of tension to the concrete is solely achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab.

[0054] It is recalled that according to the present invention, a plurality of tension members horizontally extends through the concrete slab between two anchor points, wherein the anchor points are arranged in one or more fire resistant bearing structures within the fire compartment, and/or in one or more external bearing structures, so that when the strength and stiffness of the horizontal structural steel members (which in normal operation are responsible for taking the main part of the tensile stresses generated by the bending moment) substantially diminish in the prescribed fire scenario, the tension members are capable of supporting the loaded slab by catenary effect and of limiting the deflection thereof. In case of (recommended) pre-tension of the tension members, the tension efforts are taken by the fire resistant bearing structures and/or the external bearing structures in which the tension members are anchored, as well during the pre-tensioning operation, as during normal operation and during the prescribed fire scenario. A compression of the concrete slab is hereby not desirable. Indeed, due to the arrangement of the tension members in the upper half of the concrete slab, where they are best protected against a rapid temperature rise in a fire scenario, such a compression would just result in a supplementary load for the floor structure. List of reference signs:

10 composite steel-concrete floor structure

12 horizontally extending structural steel members (primary beams)

14 concrete slab

16 upper flange of 12

16' lower flange of 12

18 web of 12

20 vertically extending structural members (columns)

22 reinforced concrete filling of 20

24, 24' flanges of 20

26 connection between 12 & 20

28 secondary beam

30 shear connectors (shear-studs)

32 lower reinforcement steel in 14

34 tension members

36, 36', anchor points of 34

36", 36"'

38, 38', fire resistant bearing structures

38", 38"'

40 slab band over 12

42 horizontal duct

43 filling material in 42

44 upper reinforcement steel in 14