FIRE RESISTANT STEEL BEAM COACTING WITH CONCRETE

Abstract: The fire resistant, pre-stressed structural flooring carrying beam of steel includes two vertical webs (2) and a bearing plate (4) along the bottom of the beam extending out­ side the webs (2) for bearing the floor structure, the upper side of which is above the upper side of the steel beam after finished concrete pouring. The webs (2) are those of two channels (1) fixed to the bearing plate (4) with their toes in opposing, mutual, spaced relationship. The upper flanges (5) of the channels are provided with projecting means (6, 6′) fixed at pre-determined mutual spacing along the beam. An open space between the channels (1) is thus pro­ vided between the toes of the channels for filling with con­ crete (10) when concrete is poured, such as to achieve static co-action between the steel beam and concrete via the means (6).

Description: The present invention relates to a prefabricated steel beam intended as a load-bearing structure for structural flooring and where such flooring is preferably made from fabricated concrete elements. The beam has a bottom flange projecting out on either side and is formed such as to come within the depth of the structural flooring, and such that static co-action is obtained between the steel beam and concrete suitably cast on site in connection with the normal pouring of topping concrete on the flooring elements. Such a beam is known from, for example SE-A-448 897. Due its special implementation the beam has great rigidity in relation to steel weight and structural height as well as its obtaining great fire resistance. Steel beams are becoming more and more usual as supporting structures to flooring structures in multistorey buildings. Depending on the number of storeys and the activity which is to be carried out in the building, there is usually a requirement of between one and two hours fire resistance for the beam and flooring structure. A conventional implementation is to use rolled steel joists of H or I section with the floor structure bearing on the upper flange. A disadvantage here is that the total flooring structure depth will be large. The downwardly projecting supporting beams also make ventilation and cable laying more difficult. The beams must also be suitably protected to obtain necessary fire resistance. This must often be done by cladding the entire beams by such as plasterboard. A method of reducing some of the above mentioned disadvanages is to have the flooring structure bearing on the bottom flanges of the supporting beams. The total flooring structure depth is thus reduced and ventilation and cable laying is simplified. Fire protection of the beams is also simplified since only the bottom flange will be exposed to fire, while the inner parts of the beam remain cool, due to the surrounding concrete, against the effect of fire for a long time. The required fire protection is normally accomplished by the bottom flanges of the beams being painted with fire protective (infumescent) paint. A disadvantage with this implementation is that the structural height of the beam is limited by the depth of the floor structure. In certain cases this can mean difficulties in achieving sufficient rigidity for long spans. The greatest disadvantage is, however, the difficulty of erecting the flooring elements, since their ends must be thrust in between the flanges of the beam. In order primarily to obviate the latter problem, beams supporting flooring structures have been developed with a closed hollow section and laterally projecting bottom flanges. The beams are fabricated by four plates being welded together with continuous welds. A disadvantage with these beams is that they are more expensive in fabrication than rolled sections with the same weight or with the same load bearing capacity, inter alia due to fabrication requiring expensive special machines for welding. A further development of this kind of beam is the one illustrated and described in the Swedish published specification SE-A-448 897. This beam is fabricated by plates being welded to the toes of a rolled H-section beam. The flanges of the section will now be "webs" in the new box beam, and the original web of the section will be an intermediate "flange". This beam can be fabricated using ordinary automatic welding equipment and is therefore cheaper in fabrication than the above mentioned beam with a hollow box section. On the other hand, economoy of material will generally be worse, since the comparatively thick "webs" and intermediate "flange" are not optimum from the aspect of stress analysis. Beam weight for a given rigidity or carrying capacity would therefore generally be greater. Fire resistance is possibly somewhat better than for the ordinary hollow beam, due to the comparatively heavy "webs", where temperature increase in a fire is limited, answering for a greater proportion of the carrying capacity. On the other hand, the underside of the beam must be painted with fire protective paint if fire resistance times of an hour are to be achieved, the alternative being that the beam is not statically utilized up to permitted values, which then causes material economy to deteriorate even further. In all the above mentioned embodiments it is difficult to achieve any great degree of static co-action between beam and concrete topping, since the latter will be very restricted round the beam. Static co-action here could increase fire resistance as well as carrying capacity and rigidity. The latter is very important, since particularly rigidity can be a problem for beams which are to be kept within the depth of the flooring structure due to the limited structural height available. The present invention has the object of achieving a prefabricated steel beam of the kind mentioned in the introduction, which dispenses with the above mentioned problems by the implementation of the beam resulting in that substantial static co-action with concrete is obtained, and that the implementation otherwise is such that the bottom flange does not need to be coated with fire protective paint, not even for very high fire resistance requirements. The invention will now be described in more detail below with reference to the accompanying drawings, where Figure 1 is a cross section of a prefabricated beam in accordance with the present invention Figure 2 is a side view of the beam in Figure 1 Figure 3 is a cross section of the beam according to Figures 1 and 2, when it is erected on site and carries two floor elements Figure 4 is a cross section of a second embodiment of the beam in accordance with the invention carrying two floor elements Figure 5 is a cross section of a third embodiment of the beam in accordance with the invention carrying two floor elements Figure 6 is a schematic longitudinal section through a fourth embodiment of a beam in accordance with the invention and Figure 7 is an end view, partly in section of the beam illustrated in Figure 6. As will be seen in cross section from Figure 1, a beam in accordance with the invention includes two mutually spaced rolled steel channels 1 with their toes facing each other. The webs 2 of the channels 1 form webs in this new beam section and from the aspect of material economy this achieves advantages compared with the beam according to SE-A-448 897, since the channel webs 2 are thinner than the flanges of the appropriate H-section. The bottom flanges 3 of the channels 1 are welded to a bearing plate 4. At a pre-determined spacing there are means 6 welded at right angles to the longitudinal extension of the channels 1 for the transmission of shear force and static co-action between the beam and concrete, which is poured between the channels later. The means 6 may be such as small angled sections or reinforcement bars, the spacing between them being determined by the loads to be transmitted. Alternatively, the means 6 may be screws 6′, so-called studs projecting from or in the vicinity of the flanges 5 and fastened thereto e.g. by welding, the screws 6′ being schematically shown by dashed lines in Fig. 2. A plate 7 is welded to either end of the beam, for transmitting the beam load to such as a column support. The beam in accordance with the invention can be fabricated very simply, since ordinary automatic welding equipment can be used. As distinct from the other beams of hollow beam type described above, only two longitudinal welds are required instead of four. Welding the means 6 to the top flanges 5 only requires short fillet welds. The beam can be prefabricated in the shops and erected on site. After erecting the floor elements 8 (Figure 3) on the beam bearing plate 4, topping concrete 9 is poured over the floor elements 8. The beam cavity and the space between the elements 8 and beam is filled with concrete 10 at the same time. Effective static co-action with the aid of the means 6 is obtained between the beam and the cast concrete, which considerably increases the carrying capacity and rigidity of the structure, compared with beams, of the kind mentioned in the introduction, which have the same steel weight or structural height. The embodiment of the beam in accordance with the present invention is also favourable with regard to the effect of fire. A substantial part of the carrying capacity is contributed by the bottom flanges 3 of the channels 1, and these flanges are protected by the bearing plate 4. Even though steel has high heat conductivity, the bearing plate 4 is still a direct radiation protector for the flanges 3, and in combination with the cast concrete, which has great heat capacity, the temperature rise in the flanges 3 is slower than for the bottom flanges in conventional beams of the hollow box type. This condition, and that the cast concrete will take over an increasing part of the carrying capacity if there is a fire, ensures that the beam is given substantial fire resistance without the bearing plate 4 needing to be fire insulated at all. An embodiment of the present invention for further increasing the carrying capacity and fire resistance is to arrange one or more reinforcement bars 11 in the beam cavity, as illustrated in Figure 4. The bars 11 are fastened to the beam before the means 6 are welded in place and can suitably be suspended in these means. The bars 11 are placed at a spacing from the bottom of the beam such that they are kept sufficiently cool for the required fire resistance time, simultaneously as the distance to the upper side of the beam is selected as being sufficiently large for giving an effective moment arm. Reinforcement bars 12 can also be arranged in the vicinity of the upper flanges 5 of the channels 1, thereby to increase the steel area in the upper zone of the beam. A still further embodiment of the beam in accordance with the present invention is illustrated in Figure 5 and can give extremely long fire resistance times. A thin heat insulation layer 13 is arranged between the bearing plate 4 and the bottom flanges 3 of the channels 1. Although this insulation is thin, together with the high heat capacity of the cast concrete it gives a radical delay of the temperature rise in the bottom flanges 3 of the channels 1 when there is a fire, which has great importance for the carrying capacity of the beam and thereby its fire resistance. Another embodiment of the beam in accordance with the present invention is illustrated in Figures 6 and 7, where the reinforcement 11 is utilized for pre-stressing the beam, thus giving the beam camber. Pre-stressing can be adjusted, for example, so that it compensates the deflection caused by the own weight of the floor structure. The pre-tension can be achieved, e.g. by a reinforcement bar 11 being made somewhat longer than the distance between the end plates 7 of the beam, the bar 11 being threaded at either end for nuts 14, the bar then being tensioned by the nuts between the end plates 7. To provide easy placing of the bar 11 between the end plates 7 the latter can be provided with elongate slots 15.

Claims: 1. Fire resistant, prefabricated steel floor structure supporting beam in co-action with concrete and including two vertical webs (2), a horizontal bearing plate (4) projecting outside the webs (2) for supporting floor structures, the upper side of which after finished topping casting is above the upper side of the steel beam, characterized in that the webs (2) are those of two channels (1) which are fixed to the bearing plate (4) with their toes spaced and facing each other and in that the upper flanges (5) of the channels (1) are provided with means (6, 6′) projecting from them and fixed at pre-determined mutual spacing along the beam, said spacing between the toes of the channels providing an open space between the channels (1) for filling the beam with concrete (10) during concrete casting, for achieving static co-action via the means (6, 6′) between the steel beam and the concrete. 2. Beam as claimed in claim 1, characterized in that at least one reinforcement bar (12) is fastenable to it close to the upper flanges (5) of the channels (1). 3. Beam as claimed in claim 1 or 2, characterized in that at least one longitudinal reinforcement bar 11 is fastenable to it in the space between the channels (1) above the bearing plate (4) but under the centre of gravity of the composite section. 4. Beam as claimed in any one of the preceding claims, characterized in that the beam includes a thin heat insulation layer (13) between the bearing plate (4) and the bottom flanges (3) of the channels (1). 5. Beam as claimed in any one of the preceding claims, characterized in that the reinforcement bar (11) is extended a distance such that its total length is somewhat greater than the distance between the end plates (7), whereby it can be pre-tensioned between the end plates (7) with the aid of nuts (14) against the outsides of the end plates (7), the nuts co-acting with threads formed in the end portions of the bar (11). 6. Beam as claimed in any of the preceding claims, characterized by an elongate slot (15) in each end plate (7) for facilitating the insertion of reinforcement bars for pre-tensioning the beam between the end plates (7). 7. Beam as claimed in claim 1, characterized in that the means (6) projecting out from the upper flanges (5) connect said flanges preferably at right angles to the longitudinal direction of the beam at pre-determined spacing and/or comprise screws (6′) projecting from the upper flanges (5) or the area adjacent these flanges and welded thereto.

Inventors: THOR, JOERGEN, Thor, Jörgen
Applicants: THOR, JOERGEN
Application year: 1988
Main classification: E04C3
IPC codes: E04C3, E04B5, E04B1