EP2427604B1

EP2427604B1 - Fire resistant steel structure

Info

Publication number: EP2427604B1
Application number: EP10721733.3A
Authority: EP
Inventors: Louis-Guy Cajot; Riccardo Zanon
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2009-05-04
Filing date: 2010-05-04
Publication date: 2017-02-22
Anticipated expiration: 2030-05-04
Also published as: US20120066990A1; EP2427604A1; WO2010128046A1; CA2760671A1

Description

Technical field

The present invention generally relates to a fire resistant steel structure.

Background Art

Unprotected structural steel members like columns, girders, beams etc. lose most of their load bearing capacity when they are exposed to temperatures above 400 C. For warranting the required fire resistance rating in multistorey steel structures, it is well known in the art to use a fireproof heat insulation slowing down temperature rise in load bearing structural steel components. Known heat insulation measures comprise e.g.: fireproof encasements with slab-type materials made e.g. from calcium silicate or gypsum; mineral fiber insulations; spray applied fireproofing materials and intumescent paints or coatings. These fireproof insulations must generally be applied in-situ to all load bearing structural steel components, which is a costly and time-consuming operation.
It is also known to use composite profiles, i.e. steel profiles with a partial or full concrete encasement or, alternatively, concrete filled steel tubes. Such composite profiles have a substantially higher mechanical resistance in case of fire than bare steel profiles, i.e. they maintain their load bearing function much longer. However, they are also much heavier than bare steel profiles, which is a substantial disadvantage, in particular for horizontal load bearing structural steel members, as e.g. beams and girders. (In the following, the term "beam" will be used for designating beams as well as girders.)
A composite profile is e.g. described in EP 1 405 961 , which is used to support pre-fabricated floor elements. The composite profile comprises a closed trapezoidal steel section, the inner volume of which is filled with concrete. The inner volume further comprises a couple of tendons anchored within the bar and arranged to provide pre-tensioning in such a way as to cause a bending moment opposed to that caused by the external load.
FR 1 544 207 relates to a pre-stressed composite metallic beam. It comprises a steel beam having a vertical web extending between two horizontal flanges. A pair of tie members from a steel with high limit of elasticity are anchored, at both ends of the beam, on the beam web, on both sides thereof, and in the vicinity of the neutral line. The tie members are bonded to the web by elements imposing them a curve, the lower point of which is located in the vicinity of the bottom flange, so that the efforts exerted on these elements and resulting from the tension of the tie members produce deformation stresses in the beam in the direction opposed to that of the deformation due to the load.
Document US 2007/0028551 describes a beam attachment system comprising two posts and a beam horizontally supported by the two posts. A beam tie is provided to compensate for the stress exerted by the beam on the posts. Accordingly, the beam tie is supported at the head of the posts and engaged in a passage inside the beam to support it. This system is designed so that the beam tie compensates, at least partially, for the moment exerted by the beam on the posts and hence retain the stability of the system. FR 1018618 relates to a composite structure comprising steel beams supporting a concrete slab. Tension members provide an initial upward bending of the unloaded beam, so as to compensate for a downward deflection due to the load.

Technical problem

It is a first object of the present invention to provide a fire resistant steel structure in which a load bearing beam maintains its load bearing function during the required time of fire exposure without necessarily necessitating costly and time consuming insulation measures or a heavy concrete encasement or filling.
This object is achieved by a fire resistant steel structure as claimed in claim 1.
It is a further object of the present invention to provide a fire resistant steel-concrete floor structure having a good fire resistance even without expensive and time consuming insulation measures on the load bearing beams.
This object is achieved by a fire resistant steel-concrete floor structure as claimed in claim 14.

General Description of the Invention

A fire resistant steel structure in accordance with the present invention comprises a steel beam for receiving a vertical load and a support structure for supporting the steel beam at two horizontally spaced locations (generally but not necessarily at both ends of the steel beam). At least one fire-resistant tension member, which has its ends anchored outside the steel beam in the support structure, is arranged in relation to the steel beam in such a way that when the steel beam is overheated and yields under its vertical load in case of severe fire conditions, the overheated beam rests on the at least one fire-resistant tension member and is vertically supported by the latter.
It will be appreciated that such an emergency backup support system will-by providing an external, collapse retarding catenary support mechanism for the overheated steel beam-substantially increase the time during which a bare steel beam maintains its load bearing function when it is overheated in case of a fire. It follows that a costly and time consuming application of a fireproof insulation onto the steel beam is not necessary, and that a bare steel beam (i.e. a steel beam without fireproof insulation or concrete encasement) may maintain its load bearing function in case of a fire at least as long as a heavy composite steel beam (i.e. a steel beam with a partial or full concrete encasement).
For this purpose, the fire-resistant tension member is advantageously designed in such a way as to be able to take at least 70% of the load of the beam that yields during the fire. In other words, the fire-resistant tension member(s) is/are designed to be able to take, under the severe fire conditions, essentially all of the load that should be taken by the steel beam (i.e. the load taken by the beam without fire - as in the cold state).
Preferably, the tension member shall be able to take during the fire at least 80% and more preferaby essentially all of the load taken by the beam in the cold state.
It is to be noted that, as will be explained in more detail below, the fire-resistant tension member may be a tension member having appropriate mechanical performance (in particular an appropriate tensile strength) that is protected against the fire, thus forming a fire-protected tension member. Alternatively the fire-resistant tension member may be a tension member having appropriate mechanical performance (an appropriate tensile strength) and having an inherent good fire resistance, i.e. it keeps a good tensile strength even at high temperatures (of major interest is the range above 600°C, more specifically 600 to 1100°C).
It will further be appreciated that efficiently using an inherently fire resistant tension member or protecting a slender tension member with a fireproof heat insulation is by far easier, less costly and less time-consuming than applying such a fireproof heat insulation to the steel beam itself. Furthermore, such fire-resistant tension members result in a smaller surcharge of the support structure than a partially encased composite steel beam (with reinforced concrete between the flanges).
In a preferred embodiment, the at least one fire-resistant tension member extends along the steel beam, e.g. parallel to a beam web. In this embodiment, at least one intermediate support member is advantageously arranged on the steel beam, in such a way that when the yielding overheated steel beam rests via the at least one intermediate support member on the at least one fire-resistant tension member and is vertically supported by the latter. However, the overheated steel beam may also rest directly with a lower flange (or any other beam element) directly on the at least one fire-resistant tension member when it yields under its vertical load.
The at least one intermediate support member arranged on the steel beam is advantageously integrated in a transversal web-stiffener, which is e.g. equipped with a through hole or a cut-out for the at least one fire-resistant tension member. Alternative embodiments of intermediate support members comprise e.g. studs or hooks fixed to the beam or cut-outs or holes in an element of the beam itself (as e.g. a flange or web).
In an optimized embodiment for force transmission between the steel beam and its emergency backup support system, a series of intermediate support members are arranged on the steel beam so that the at least one fire-resistant tension member has a polygonal shape approximating a parabola. The more intermediate support members are foreseen, the better the fire-resistant tension member approximates the optimal parabola shape and the better force transmission between the steel beam and its emergency backup support system is. However, for reasons of economy, the steel beam will most often comprise not more than three intermediate support members, which are generally sufficient to warrant the required ISO fire resistance for the steel beam.
In an alternative embodiment, the emergency backup support system for the steel beam includes at least one fire-resistant tension member arranged transversally to the steel beam. When the overheated steel beam yields in this embodiment, it rests on the at least one transverse fire-resistant tension member, e.g. directly with its lower flange or by means of an intermediate support member. Such a solution with at least one fire-resistant tension member arranged transversally to the steel beam may be of particular advantage in combination with a cellular steel beam having apertures in its web. This is because a transverse fire-resistant tension member does not impede the passage of conduits through the apertures in its web of the cellular steel beam.
Preferably, the at least one fire-resistant tension member is only slightly pre-stressed when the steel beam is cold, so as to have a sufficient reserve for supporting the overheated steel beam. Under the maximum load of the cold beam, the prestress tension in the at least one fire-resistant tension member should preferably not exceed 25 %, more preferably not more than 15% of the tensile strength of the tension member. The slight prestress tension shall e.g. warrant that there is no substantial play in the anchoring of the ends of the fire-resistant tension member and that the tension member is already in close contact with beam when a fire breaks out, i.e. that the at least one fire-resistant tension member is capable of developing a catenary support mechanism for the overheated beam as soon as the latter begins to yield in case of a fire. It is however to be noted that the fire-resistant tension member does not need to play a structural role in the cold state so that it does not need to be pre-stressed. This greatly facilitates the installation of such tension members.
In order to increase the fire resistance of existing steel structures, this system is very suitable to be applied for two reasons. First, the installation is easy since it requires neither complicate erection phases nor pre-stressing technology. Second, since it is active only in fire condition, it does not require changing the statical functionality of the structure in cold condition.
In a preferred embodiment, a double-T shaped steel beam with an upper flange, a lower flange and a web connecting the upper flange to the lower flange, comprises on each side of the web, at least one fire-resistant tension member that is anchored in the support structure and extends along the web between the upper flange and the lower flange. Intermediate support members are arranged on both sides of the web, symmetrically in relation to the latter. It follows that when the overheated steel beam yields under the vertical load, it rests via the intermediate support members on the fire- resistant tension members and is vertically supported by the latter symmetrically in relation to the web.
The steel beam is preferably supported by the support structure in such a way that it may axially expand when heating up under severe fire conditions, whereby excessive compressive axial forces in the beam, which may cause a buckling of the latter, are avoided.
The support structure may comprise a H-shaped steel column with two flanges and a concrete filling between the flanges, wherein a first end of the steel beam is fixed to a first of the two column flanges, and one end of the fire-resistant tension member passes through a through-hole in this first column flange and is provided with an anchoring element that is embedded in the concrete filling between the column flanges.
A first embodiment of the at least one fire-resistant tension member, it may advantageously comprise a high strength steel strand (or any equivalent tension member) provided with an envelope filled with a fireproof mortar or grout.
The at least one fire-resistant tension member may alternatively comprise a high strength steel strand (or any equivalent tension member) provided with an intumescent coating, an intumescent paint, an intumescent sleeve, a sprayed fire insulating material or a fireproof insulation sleeve.
Besides, as previously mentioned, the fire-resistant tension member may be a tension member having appropriate tensile strength and having an inherent good fire resistance. In this case, one may use any appropriate material, presently existing or to be developed, having a tensile strength which does not severely drop even at high temperatures, namely above 600°C; the material may be in the form of tendons, wires, strand or fibers that may be assembled to form a tension member of larger section.
Those skilled in the art may identify suitable non-metallic materials, namely synthetic materials having a high elastic limit and showing good fire resistance, and typically materials allowing the manufacture of a tension member with a tensile strength of at least 500 MPa at high temperatures (above 600°C).
The present invention also provides a fire resistant steel-concrete floor structure comprising a concrete slab and a support structure for the concrete slab including at least one steel beam. At least one fire-resistant tension member having its ends anchored outside the steel beam in the slab is arranged relative to the steel beam in such a way that when said steel beam is overheated and yields under its load in case of severe fire conditions, the overheated beam rests on the at least one fire-resistant tension member and is vertically supported by the latter, the fire-resistant tension member being designed to be able to take essentially all of its load.
The at least one fire-resistant tension member has its ends anchored in the concrete slab advantageously in direct vicinity of a support column or an other vertical support member. This warrants that the tensile force in the tension member exerts no significant bending moment onto the steel beam or the slab.

Brief Description of the Drawings

Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Fig. 1: is a cross-sectional view of a fire resistant steel structure with a steel beam equipped with an emergency support system comprising a fire-resistant tension member;
Fig. 2: is diagrammatic view of a fire resistant steel structure as shown in Fig. 1, showing-in the absence of fire-the bending moment in the steel beam and the axial force in the fire-resistant tension member;
Fig. 3: is diagrammatic view as in Fig. 2, showing-under fire conditions-the bending moment in the overheated steel beam and the axial force in the fire-resistant tension member;
Fig. 4: is a diagram illustrating, during an ISO fire exposure, the load bearing mechanism of a steel beam equipped with an emergency support; the numbers on the x-coordinate represent the time of ISO fire exposure in seconds (s) and the numbers on the y-coordinate represent the portion of the load taken by the steel beam and the tension member in percent (%) ;
Fig. 5: is a diagram comparing the deflection of an of an unprotected steel beam without fire-resistant tension members, an unprotected steel beam with fire-resistant tension members interacting with two intermediate support members and an unprotected steel beam with fire-resistant tension members interacting with three intermediate support members; the numbers on the x-coordinate represent the time of ISO fire exposure in seconds (s), and the numbers on the y-coordinate represent the deflection of the steel beam in meters (m);
Fig. 6: is a cross-sectional view illustrating first anchoring of a fire-resistant tension member on a column;
Fig. 7: is a cross-sectional view illustrating an anchoring of a fire-resistant tension member in a slab;
Fig.8: is a cross-sectional view illustrating an alternative anchoring of a fire-resistant tension member on a column; and
Fig. 9: is a cross-sectional view of an embodiment of a fire-protected tension member.

Description of Preferred Embodiments

Fig. 1 shows a fire resistant steel structure 10 in accordance with the invention. This steel structure 10 comprises two columns 12, 12' (i.e. vertical structural members) forming a support for a steel beam 14 (i.e. a horizontal structural member) at two horizontally spaced locations. The steel beam 14 serves as support element for a slab 16, in this case e.g. a concrete slab with profiled steel sheets.
The columns 12, 12' shown in Fig. 1 are H-shaped steel beams with a reinforced concrete filling 18 between the flanges (such columns are generally called composite columns). The reinforced concrete filling 18 warrants that the columns 12, 12' maintain their load bearing function during the required time of fire exposure. Alternatively, the columns 12, 12' might also be steel profiles protected with a fireproof casing made e.g. of silicate or gypsum plates with or without mineral fiber insulations, with spray applied fireproofing materials, intumescent paints or coatings, respectively steel profiles completely encased in concrete or closed steel profiles completely filled with concrete. The columns 12, 12' may also be steel reinforced concrete columns or wooden columns or they may be replaced by other suitable support elements for the steel beam, as e.g. a concrete wall or a brick wall.
The steel beam 14 shown in Fig. 1 is a double-T-shaped steel beam (or an I-beam), i.e. a steel beam having an upper flange 20, a lower flange 22 and a vertical web 24. It will be noted that the steel beam 14 as such is not provided with a passive fire protection, at least not with a passive fire protection substantially increasing its load bearing capacity under fire exposure. Consequently, when the steel beam 14 as such is exposed to a severe fire (i.e. a fire resulting in temperatures of the steel beam above 400°C), it will generally not resist more than half an hour before collapsing under its load.
It will be noted that the steel beam 14 is supported between the columns 12, 12' in such a way that it may axially expand when heating up under severe fire conditions, thereby avoiding excessive compressive axial forces in the steel beam 14. Such a free expansion can easily be implemented by providing e.g. a double web cleated connection (as identified e.g. with reference number 26 in Fig. 1) or a fin plate connection (not shown) between the web 24 of the steel beam 14 and a flange 28 of the column 12, wherein fixing bolts on the side of the web 24 may horizontally slide within oblong bolt holes when the steel beam 14 expands.
Reference number 30 identifies a fire-resistant tension member, which forms an emergency backup support system for the steel beam 14, when the latter yields under severe fire conditions. This fire-resistant tension member 30 has its ends anchored outside said steel beam 14. The first end 32 of the fire-protected tension member 30 is e.g. equipped with an anchor 34 cooperating with the flange 28 of the column 12 for anchoring it on the column 12, and the second end 32' is e.g. equipped with an anchor 34' cooperating with the flange 28' of the column 12' for anchoring it on the column 12'. In accordance with a general principle underlying the present invention, the fire-resistant tension member 30 is arranged in such a way that when the overheated steel beam 14 yields under its vertical load in case of severe fire conditions, it rests on the fire-resistant tension member and is vertically supported by the latter. Due to its fire resistance, the tension member 30 keeps its load bearing capacity longer than the unprotect beam 14. The tension member 30 is thus advantageously designed to be able to take essentially all of the load of the beam under the severe fire conditions.
It will be appreciated that using a fire-resistant tension member is by far less costly and time-consuming than providing a passive fire protection to the steel beam 14 itself.
Hence, under fire conditions, the tension member or members will progressively take up the load that is no longer taken by the yielding beam, and the design of the tension member is made so as to be able to support essentially all of the weight of the beam together with the vertical load received by the beam, and this during the fire. This is possible, firstly, since the tension member 30, respectively the group of tension members, is/are: (a) dimensioned to be able to take essentially all of the load taken by the beam (preferably at least 70%, more preferably at least 80%, or possibly up to nearly 100%). And secondly because the tension member(s) is/are fire resistant, either by the help of a protective coating or due to inherent fire resistance of the material from which the tension member is made. As it is known, the tensile strength (rupture point) of a metallic material is temperature dependent. What matters here is that the fire-resistant tension members be able to withstand the load supported by the beam (and of the beam itself) during a certain time of fire exposure. The materials for the tension members, and the possible amount of fire protection, is thus to be selected keeping this aspect in mind. It is however clear that the when exposed to fire, the tensile strength of the tension members may however decrease, but still remain at a level sufficient to bear the load of the beam.
In summary, a tension member, whether protected or inherently fire resistant, shall advantageously be designed so as to be able to keep a bearing capacity at room temperatures above 600°C, more preferably in the range of 600°C to 1100°C, sufficient to take essentially all of the load of the yielding beam. Room temperatures between 600°C and 1100°C are in Civil Engineering typically the consequence of a severe fire.
At the design stage of the tension member, as first simplified approach, one may determine the section of the tension member based on the load borne by the beam, and therefore use the following, well known formulae: $S = \frac{F}{T_{S}} and F = \frac{Q \cdot l^{2}}{d \cdot 8}$

where F is the force in the tension member; Q is the load on the beam (the load of the beam itself is actually negligible but can be taken into account in Q); l is the span of the beam; d is the distance between the top and low points of the tension member along the beam (function of the initial given shape and of the beam deflection); S is the section of the tension member and Ts is the tensile strength (rupture point). Of course, one shall use a coefficient of security in these calculations.
In the preferred embodiment shown in Fig. 1, the fire-resistant tension member 30 extends parallel to the steel beam 14 between the upper flange 20 and the lower flange 22. When the overheated steel beam 14 yields under its vertical load in case of overheating in a fire, it rests by means of intermediate supports 38₁ and 38₂ on the fire-resistant tension member 30. Such intermediate supports 38₁ and 38₂ are advantageously transversal web stiffeners as shown in Fig. 1, each of them having a through hole or cut-out therein through which the fire-resistant tension member 30 passes. Other possible embodiments of such intermediate supports 38₁, 38₂ are e.g. studs or hooks (welded or bolted directly to the web 24 or to the lower or upper flange 22) or are formed by cut-outs or holes in the lower flange 22 of the steel beam 14. It will be appreciated that intermediate supports 38₁, 38₂ incorporated in robust transversal web stiffeners (as shown in Fig. 1) have the advantage of being less exposed to the risk of becoming prematurely ineffective due to a local buckling of the overheated steel beam 14. The intermediate supports shall preferably be fire-resistant as well and may hence advantageously be provided with a fireproof heat insulation as e.g. a fireproof casing, a spray applied fireproofing material or an intumescent paint. In order to even further increase the efficiency of the emergency backup support system for the steel beam 14 with relatively low additional costs, the transversal web stiffeners may be provided with a fireproof heat insulation too.
It will be appreciated that force transmission in the emergency backup support system for the overheated steel beam 14 may be optimized by providing a series of such intermediate supports on the steel beam 14, wherein these intermediate supports are advantageously arranged so that the fire-resistant tension member 30 has a polygonal shape approximating more or less a flat parabola, with its minimum near the lower flange in the middle of the steel beam 14. Furthermore, if the steel beam 14 has a web 24 and a vertical plane of symmetry (such as e.g. a double-T beam as shown in Fig. 1), the emergency backup support system for the steel beam 14 preferably comprises at least one fire-resistant tension member 30 and at least one intermediate support arranged on each side of the web 24, so as to be symmetric in relation to the latter. When the overheated steel beam 14 yields under its vertical load, it is vertically supported by the fire-protected tension members on both side of the web 24 and this symmetrically in relation to the latter, thereby reducing the risk of an asymmetrical deformation of the overheated steel beam, which could result in a premature collapse.
Instead of having fire-resistant tension members 30 extending between the upper flange 20 and the lower flange 22 parallel to the web of the steel beam 14, the emergency backup support system for the overheated steel beam 14 could also include one or more fire-resistant tension members (not shown) arranged transversally to the steel beam 14, wherein the overheated steel beam 14 could e.g. rest directly with its lower flange on the fire-resistant tension member. Such an arrangement of transverse fire-resistant tension members could support more than one steel beam. It may be of particular advantage when used in combination with cellular steel beams.
For computing the diagrams of Fig. 2, 3, 4 and 5, finite element calculations have been performed with the software SAFIR developed by the University of Liege (Belgium). The steel beam 14 is an unprotected IPE 500 beam with a span of 12 m. At each end of the steel beam 14, vertical and lateral displacements are blocked. The right end of the steel beam 14 is however free to expand horizontally. There are two fire-resistant tension members 30 arranged symmetrically in relation to the web 24 and actually taking each the form of a fire-protected steel cable. Each of these tension members 30 is fully restrained at both ends in an independent support structure, i.e. the tension in the tension member 30 does not affect (i.e. compress) the steel beam 14. Consequently, in contrast to known external unbonded tendons used in concrete beams or steel/concrete composite beams, the tension members 30 do not significantly compress the steel beam 14, neither in the cold state nor in the hot state. The tension members 30 are considered to remain cold during the whole fire, and the intermediate supports are supposed to be designed in such a way that they are not affected by premature local collapse or buckling of the steel beam 14. For the calculation it has been assumed that the steel beam 14 supports a distributed loading of 30 kN/m, that the yield strength of the steel beam 14 is 355 MPa, and the tensile strength of each tension member 30 is 1860 MPa.
Referring now to Fig. 2 and 3, the working principle of the steel beam 14 with its emergency backup support system will be described. It will first be noted that Fig. 2 shows the steel beam 14 under a uniformly distributed load in the absence of fire (i.e. in a cold state), and Fig. 3 shows the same steel beam 14 when it has already substantially yielded under its uniformly distributed load due to overheating in case of a fire (i.e. when the steel beam 14 has e.g. reached a temperature above 600°C). The two anchoring points of the fire-protected tension member 30 are identified with reference numbers 36 and 36'. For each tension member 30 there are two intermediate supports 38₁ and 38₂, the first one located at 4 m from the first beam support the second one located at 4 m from the first beam support. It remains to be noted that the bending moment and tensile force diagrams in Fig. 2 and 3 are drawn in the same scale.
In Fig. 2, reference number 40 identifies the bending moment diagram for the steel beam 14 in the cold state, i.e. when it still has its whole load bearing capacity. This diagram 40 is a well-known parabolic bending moment diagram for a uniformly charged beam having at one end a pin-type support 42 and at the other end a roller-type support 44. Reference number 46 identifies the tensile force diagram for the fire-protected tension member 30. It will be noted that in the cold state of the steel beam 14, the fire-protected tension member 30 is subjected to a small prestress, which is sufficient to warrant that there is no substantial play in the anchoring points 36 and 36', and that the tension member 30 is in close contact with the intermediate supports 38₁ and 38₂. This prestress tension is only a small percentage (i.e. generally less than 15%) of the tensile strength of the tension member 30 at cold state.
In Fig. 3, reference number 40' identifies the bending moment diagram for the overheated steel beam 14, that is when it has itself only a small remaining load bearing capacity. The overheated steel beam 14, which has yielded under its load, rests now with its intermediate supports 38₁ and 38₂ on the fire-protected tension members 30, which-due to their fire protection-have conserved all their tensile force and bearing capacity. The bending moment diagram 40' for the steel beam 14 is now a typical diagram for a uniformly charged beam having at one end a pin-type support 42 and at the other end a roller-type support 44 and, between these two end supports, two intermediate supports 38₁ and 38₂, which rest on the fire-protected tension members 30. Due to the intermediate supports 38₁ and 38₂, the maximum moment to which the steel beam 14 is exposed is considerably reduced (roughly by a factor 10), so that the steel beam 14, which is considerably weekend by overheating, may still support the reduced moment to which it is exposed. The tensile force diagram 42' for the fire protected tension member 30 shows that the tensile force in the tension member 30 has substantially increased. This increase causes no problem because the fire-protected tension member 30 is still relatively cold, so that it has nearly its full tensile force bearing capacity for which it has been designed. It will be noted in this context that a high-strength steel strand, as e.g. a seven wires-strand with an equivalent diameter of 15.7 mm, may easily have a rupture limit above 1800 MPa. In conclusion, with a small diameter commercial steel strand equipped with a suitable fire protection, it will be possible to prevent the unprotected beam 14 from a premature collapse.
Fig. 4 is a diagram illustrating the load bearing mechanisms of a steel beam equipped with an emergency backup support system during an ISO fire exposure, i.e. using the time/temperature curve ISO 834 for simulating the fire. The numbers on the x-coordinate represent the time of ISO fire exposure in seconds (s), and the numbers on the y-coordinate represent the portion of the bearing capacity taken by the steel beam 14 and the tension members 30 in percent (%). The curve 48 shows how the load taken by the steel beam 14 decreases and the curve 50 how the load taken by the fire-protected tension members 30 increases with fire exposure time. Initially, the steel beam 14 takes nearly 100% of the load. After 15 minutes (900 s) of ISO fire exposure, the steel beam 14 takes 70% of the load, and the fire-protected tension member takes the complementary load no longer taken by the beam (30%). After about 32 minutes (1920 s) of ISO fire exposure, the situation is reversed, i.e. the steel beam 14 takes now 30% of the load and the fire-protected tension member 70%. Arrow 52 identifies an initial time sector of about 15 minutes during which the bending resistance of the steel beam 14 prevails. Arrow 53 identifies a transient phase during which the bending resistance of the steel beam 14 becomes less important than the resistance of the fire-protected tension member 30, and arrow 54 a phase during which a catenary load carrying mechanism prevents a collapse of the unprotected beam until 60 minutes (3600 s) of ISO fire exposure. It will be appreciated that this catenary load carrying mechanism optimally uses strength reserves of the steel in the steel beam 14 and in the fire-protected tension members 30.
Fig. 5 is a diagram comparing, for an ISO 834 time/temperature curve, the deflection: (1) of an unprotected steel beam without fire-protected tension members (see curve 56); (2) of an unprotected steel beam with fire-protected tension members interacting with two intermediate support members (see curve 57); and (3) of an unprotected steel beam with fire-protected tension members interacting with three intermediate support members (see curve 58). The numbers on the x-coordinate represent the time of ISO fire exposure in seconds (s), and the numbers on the y-coordinate represent the deflection of the steel beam in meters (m). The unprotected steel beam without fire-protected tension members loses its bearing capacity in less than 10 minutes (see curve 56). The unprotected steel beam with fire-protected tension members interacting with two intermediate support members maintains its bearing capacity for one hour (see curve 57). The unprotected steel beam with fire-protected tension members interacting with three intermediate support members maintains its bearing capacity even for more than two hours (see curve 58).
In summary, it has been seen that the tension member, or group of tension members, are designed and arranged in such a way to be able to support the beam and its load under severe fire conditions (typically at high temperatures above 600°C and preferably in the range of 600°C to 1000°C), for a desired exposure time.
The required load bearing capacity for the tension member(s) is determined from the load to be supported by the beam and the beam weight in cold conditions. And the tension member(s) are thus able to withstand this load under severe fire conditions, due to the fact that they are fire protected or made from a material having inherently good fire resistance. In other words, the tensile strength of the tension members, during fire exposition, is still sufficient to support at least 70%, preferably at least 80% of the load constituted by the beam and the load supported by the latter without fire.
In addition, the number of intermediate supports has an incidence on the resistance of the structure over time during ISO testing. The number of intermediate supports is advantageously designed in such a way to reach the desired fire resistance of the system. As a simplified approach, the beam 14 with the tension members 30 can be considered as a continuous girder over x supports, where x-2 is the number of intermediate supports provided by the tension member deviation device. In pratical applications, two or three deviators will be sufficient for most of the cases.
Fig. 6, 7 and 8 illustrate three embodiments of the anchoring of the fire-protected tension member 30. In the embodiment of Fig. 6, the end 32 of the tension member 30 passes through a hole in the flange 28 of the column 12 and is secured by means of an anchor 34 on the inside of the flange 28. Here, the anchor 34 and the end 32 of the tension member 30 are embedded in a concrete filling 60 put in place in-situ in between two transversal stiffener plates 62, 64 of the column 12. This concrete filling 60 slows down heating-up of the anchoring of the fire-protected tension member 30 in case of a fire. In the embodiment of Fig. 7, the end 32 of the tension member 30 passes through a hole in the upper flange 20 of the steel beam 14 and is anchored within the slab 16, preferably within a slab reinforcement 66. Here, the concrete of the slab 16 slows down heating-up of the anchoring of the fire-protected tension member 30 in case of a fire. It will be noted that the anchoring in the slab 16 is located close to the supporting column 12, so that-in case of fire-the considerable tensile force in the tension member 30 exerts no significant bending moment onto the steel beam 14 or the slab 16. In the embodiment of Fig. 8, the end 32 of the tension member 30 passes through a hole in flange 28 and a hole in flange 29 of the column 12. Here, this end 32 is secured by means of an anchor 34 on the outside of the flange 29. The anchor 34 and the end 32 of the tension member 30 are embedded within the slab 16, which slows down their heating-up in case of fire.
It may be noted that a tension member may be associated with several aligned beams, in which case it may be supported in the two columns directly neighboring the beam, but the tension member may still be extended and pass through one column to support the next beam and so on. In such case, the tension member may be anchored only in the extremity columns.
Fig. 9 is a cross-section of a first embodiment of a fire-protected tension member, i.e. a tension member provided with a fire protection to ensure fire resistance. The tension member 70 itself is advantageously a steel strand, e.g. a seven-wire, uncoated steel strand, such as used e.g. in pre-tensioned and post-tensioned prestressed concrete constructions. Reference number 72 points to an envelop, made e.g. of a fireproof material, delimiting an annular space around the tension member 70. This annular space is filled with highpressure fireproof grout or mortar 74, which should have good heat insulation qualities in order to obtain the required ISO fire rating for the fire-protected tension member without having an isolation that is too big. An alternative embodiment of a fire-protected tension member consists e.g. of a high strength steel strand provided with an intumescent coating or paint or an intumescent or fire insulating sleeve or a sprayed fire insulating material. Instead of using a high strength steel strand as tension member, one might also use traction cables or slender traction bars. However, with their high tension strength, commercial steel strands are probably the most suitable product for the present use.
The skilled person may select for the tension members other materials having an appropriate tensile strength to take the load of the beam and having a better fire resistance, to be used with or without fire-protective coating.
For example, fire resistant steel may be used. Fire-resistant steels have been widely developed in Japan or in Germany and their specifity is to keep a significant percentage of their tensile strength even at high temperatures. For example, they can have still 93% of the initial tensile strength until 600°C. For increased safety, a fire-resistive coating may however still be used. Stainless steel may e.g also be used, preferably with a fire-resistive coating.
Those skilled in the art may alternatively identify suitable non-metallic materials, namely synthetic materials having a high tensile strength and showing an intrinsic good fire resistance, capable without fire protection of taking the load under severe fire conditions.

Legend:

10	steel structure	54	arrow in Fig. 4
12,	columns	56	curve in Fig. 5
12'		57	curve in Fig. 5
14	steel beam	58	curve in Fig. 5
16	slab	60	concrete filling
18	concrete filling of 12, 12'	62,	stiffener plates of 12
20	upper flange of 14	64
22	lower flange of 14	66	reinforcing steels of 16
24	vertical web of 14	70	high strength steel strand
26	double web cleated connection	72	envelop of 70
28	flange of 12	74	grout or mortar
29	flange of 12
30	fire-protected tension member
32	first end of 30
32'	second end of 30
34	anchor on 32
34'	anchor on 32'
36,	anchoring points of 30
36'
38₁,	intermediate supports
38₂
40	bending moment diagram for 14 (cold state)
40'	bending moment diagram for 14 (hot state)
42	pin-type support
44	roller-type support
46	tensile force diagram for 30 (cold state)
46'	tensile force diagram for 30 (hot state)
48	curve in Fig. 4
50	curve in Fig. 4
52	arrow in Fig. 4
53	arrow in Fig. 4

Claims

A fire resistant steel structure comprising:
a horizontal steel beam (14) for receiving a vertical load;

a support structure for supporting said steel beam (14) at two horizontally spaced locations; and

at least one fire-resistant tension member (30) having two opposite ends (32, 32') anchored outside said steel beam (14) in said support structure
characterised in that
said at least one fire-resistant tension member (30) is arranged in relation to said steel beam (14) in such a way that when said steel beam (14) is overheated and yields under said vertical load in case of severe fire conditions, said overheated beam (14) rests on said at least one fire- resistant tension member (30) and is vertically supported by the latter, said at least one fire-resistant tension member (30) being designed in such a way as to be able to take at least 70% of the load of said beam (14) in a temperature range of 600°C to 1000°C for a desired exposure time.
The fire resistant steel structure as claimed in claim 1, wherein said at least one fire-resistant tension member (30) is dimensioned in such a way as to be able to take at least 80% of the load.
The fire resistant steel structure as claimed in claim 1 or 2, wherein:
said at least one fire-resistant tension member (30) is slightly pre-tensioned when said steel beam (14) is not exposed to a fire;

the pre-tension in said at least one tension member being preferably not more than 25%, more preferably not more than 15% of its tensile strength.
The fire resistant steel structure as claimed in any one of the preceding claims, wherein:
said steel beam (14) has a lower flange (22) with which said overheated beam (14) rests on said at least one fire-resistant tension member (30) and is vertically supported by the latter.
The fire resistant steel structure as claimed in any one of the preceding claims, wherein:
said steel beam is a cellular beam with a web having apertures therein; and

said at least one fire-resistant tension member is arranged transversally to said steel beam.
The fire resistant steel structure as claimed in any one of claims 1 to 4, wherein:
at least one fire-resistant tension member (30) extends along said steel beam (14); and

at least one intermediate support member (38₁, 38₂) is arranged on said steel beam (14), in such a way that said overheated steel beam (14) rests via said at least one intermediate support member (38₁, 38₂) on said at least one fire- resistant tension member (30) and is vertically supported by the latter.
The fire resistant steel structure as claimed in claim 6, wherein:
said at least one intermediate support member (38₁, 38₂) arranged on said steel beam (14) is integrated in a transversal web-stiffener; or

said at least one intermediate support member (38₁, 38₂) arranged on said steel beam (14) is a stud, hook or eye arranged on said steel beam (14).
The fire resistant steel structure as claimed in any one of claims 6 or 7, wherein:
a series of intermediate support members (38₁, 38₂) are arranged on said steel beam (14) so that said at least one fire-resistant tension member (30) has a polygonal shape approximating a parabola.
The fire resistant steel structure as claimed in any one of claims 6 to 8, wherein:
said steel beam (14) is a double-T shaped steel beam (14) with an upper flange (20), a lower flange (22) and a web (24) connecting said upper flange (20) to said lower flange (24);

on each side of said web (24), at least one fire-resistant tension member (30) is anchored in said support structure and extends along said web (24) between said upper flange (20) and said lower flange (24); and

intermediate support members (38₁, 38₂) are arranged on said steel beam (14), symmetrically in relation to said web (24), in such a way that said overheated steel beam (14) rests via said intermediate support members (38₁, 38₂) on said fire-resistant tension members (30) and is vertically supported by the latter symmetrically in relation to said web (24).
The fire resistant steel structure as claimed in any one of the previous claims, wherein:
said steel beam (14) is supported by said support structure in such a way that it may axially expand when heating up under severe fire conditions, thereby avoiding excessive compressive axial forces therein.
The fire resistant steel structure as claimed in any one of the previous claims, wherein:
said support structure comprises a H-shaped steel column (12) with two flanges (28, 29) and a concrete filling (18, 60) between said flanges, wherein a first end of said steel beam (14) is fixed to a first of said two flanges (28, 29); and

one end of said fire-resistant tension member (30) passes through a through- hole in said first flange (28) and is provided with an anchor (34) that is embedded in the concrete (60) between said column flanges (28, 29).
The fire resistant steel structure as claimed in any one of the previous claims, wherein said at least one fire-resistant tension member (30) is a fire-protected tension member, comprising:
a high strength steel strand (70) provided with an envelope (72) filled with a fire insulating mortar or grout (74); or

a high strength steel strand provided with an intumescent coating or paint or an intumescent sleeve or a sprayed fire insulating material.
The fire resistant steel structure as claimed in any one of claims 1 to 11, wherein said at least one fire-resistant tension member (30) comprises one or more strands of material having an appropriate tensile strength under severe fire conditions, without fire-protective coating.
A fire resistant steel -concrete floor structure comprising:
a concrete slab (16);

a support structure for said concrete slab including at least one horizontal steel beam (14); and

at least one fire-resistant tension member (30) having two opposite ends (32, 32') anchored outside said steel beam (14) in said slab (16) characterised in that said at least one fire-resistant tension member (30) is arranged relative to said steel beam (14) in such a way that when said steel beam (14) is overheated and yields under its load in case of severe fire conditions, said overheated beam (14) rests on said at least one fire-resistant tension member (30) and is vertically supported by the latter, said at least one fire-resistant tension member (30) being designed in such a way as to be able to take at least 70% of the load of said beam (14) in a temperature range of 600°C to 1000°C for a desired exposure time.
The fire resistant steel-concrete floor structure as claimed in claim 14, wherein:
said support structure for said concrete slab (16) comprises support columns (12); and

said at least one fire-resistant tension member (30) has its ends (32, 32') anchored in said concrete slab (16) in direct vicinity of said support columns (12).