Method for manufacturing a composite girder and so manufactured girder

Abstract: A method for manufacturing a composite girder (11) comprising one or more vertical cores of steel plate, associated and cooperating with one or more horizontal slabs (13,14) or flanges of concrete. At least one of said horizontal slabs (13,14) or flanges is made from high-performance concrete in order to allow mutual forces or co-actions to be imposed between the several components of the girder which are much higher/stronger than the mutual forces/co-actions which can be imposed by means of other methods and materials. If the composite girder is at least partially manufactured at the prefabrication factory, according to the present method, the starting element will be an I girder (12) preflexed or inflexed with welded connecting means (15); the I girder (12) will be positioned on the prefabricating bench imposing such constraints as to straighten the girder, then in a suitable position steel cables (16) are installed prestressed between external anchoring points (17), and the concrete casting is carried out in a flange (13) in which said cables (16) are associated with high-performance concrete. According to the method of the present invention, after setting is complete, the auxiliary constraints are cut and removed. In such a way, the by now set concrete, by being integral with the iron girder (12) thanks to the iron reinforcer elements and welded connecting means (15) and with the cables (16) by adhesion, causes the several components to mechanically cooperate and the composite beam to be endowed with much better characteristics than of the girders known from the prior art.

Description: The present invention relates to a method for manufacturing a composite girder and to the so manufactured girder (see e.g. BE-A-861.444). Above all in the field of the constructions for public works (such as, e.g., bridges, viaducts, roofings for large premises for social activities, and so forth), intense engineering efforts have been done in the past, and are still being done, aiming at manufacturing structures capable of spanning long distances without intermediate supporting structures, with the overall dimensions of the structure being kept small and without sacrifices as regards performance and costs. A very thin girder has a pleasant appearance and allows either the available room to be better used, or savings to be accomplished in connecting works (if the viaducts of a road/motorway turn-off are thin, the delivery road ramps will be shorter; if railway bridges are thin, they make it possible the whole railway line level to be lowered with the volumes of embankments being reduced); or also its environmental impact will be less striking. The present state of the art suffers from technical and financial limits. For example, in order to demonstrate this topic, it will be enough to observe that in a girder which must withstand bending stresses, the most suitable commercial materials for withstanding tensile or shear stresses are steel and, above all, that steel with high elastic limit from which cables are made. Unfortunately, while they are of light weight and very strong, both of these materials display an excessively high deformability, which is an unacceptable characteristics when applied loads are highly variable during time. In that case, they cannot be used to their full capabilities, also because the fatigue phenomena must be controlled. Further problems have to be faced also in the case of the by now classic prestressed concrete girders. In fact, one should note that the application of prestressing to girders entirely made from concrete presupposes that the concrete portion has undergone a tensional history which strongly limits the possibility of having girders which are simultaneously light-weight, thin, and the concrete of which has not been excessively tortured during the manufacturing and shipping steps. The above problems will in any case stimulate research work aiming at developing technical solutions alternative to the however generally valuable technical structural solutions used until now. Further problems are associated with the respect of the environmental conditions, in particular when manufacturing bridges and viaducts in which the structures known from the prior art, owing to their considerably large dimensions and the auxiliary services associated with their construction, have a negative impact on the whole surrounding environment and sometimes cannot be realized, beacuse they do not meet the environmental constraints imposed by the cognizant Authorities. In view of the above, a purpose of the present invention is of providing a girder structure which combines the favourable features of the solutions known from the prior art, eliminating, as far as possible, the technical problems and drawbacks associated with them and which, of course, have derived from them until now. A further purpose of the invention is of providing a girder structure which overcomes all of the limitations of use and strength which affected the girders used in the structures known from the prior art. Another purpose is of providing a structure which is able to meet the environmental constraints and which, while invading the territory, alters and damages the surrounding environment to an as small extent as possible. These purposes according to the present invention are achieved by providing a method according to the appended claims. Also advantageously, a composite girder is provided in which the bottom slab of the girder is made from high-performance concrete. According to the present invention, by cooperatively associating the high-performance concrete slab or flange, which performs the function of bonding agent which binds the cables and the steel girder and submitting said slab or flange to a suitable co-action during the manufacturing process, the purpose is achieved of stiffening the girder in such a way as to have simultaneously the necessary stiffness to bending and the maximal use of steel which, obviously, is no longer subject to fatigue phenomena. Actually, using steel for manufacturing a portion of the girder, besides making it possible a lighter weight structure to be obtained, introduces, in the laws which control the girder design, the possibility of varying the material additionally to the possibility of varying the geometry. The above benefits are then advantageously enhanced by the use of high-performance concrete, the compression strength characteristics of which are more similar to steel than to conventional concrete. The characteristics and advantages of a composite girder and of the method of manufacturing it realized according to the present invention will be more evident from the following exemplifying, non-limitative disclosure, referred to the accompanying schematic drawings in which: Figure 1 shows a cross sectional view of a composite girder manufactured according to the present invention; Figure 2 is a chart illustrating the behaviour of a girder of the prior art not submitted to co-action (shown in chain line) and of a composite girder submitted to co-action according to the present invention (solid line); Figure 3 shows a longitudinal elevation view of a steel girder preflexed or inflexed and fitted with connecting means during a first step of the method according to the present invention; Figure 4 shows a longitudinal elevation view of the girder of Figure 3, when on it forces are imposed by means of auxiliary constraints and with positioned cables prestressed by means of external constraints and in Figure 4a a chart is reported which displays the torques impressed on the girder; Figure 5 shows the longitudinal elevation view of the girder of Figure 4 on which the bottom concrete flange has been realized; Figure 6 illustrates the girder of Figure 5 from which the external constraints of cables prestressing, the forces and the auxiliary constraints have been removed, and in Figure 6a a chart is reported which displays the tensions imposed on the lower edge of such a girder; Figure 7 is a further illustration of the girder of Figure 6, to which the upper concrete flange is applied and in Figure 7a the chart is reported which displays the tensions existing at the lower edge of said girder; Figure 8 illustrates a further step in which the concrete of the upper flange has set and the construction of the upperworks takes place and in Figure 8a the chart is reported which shows the tensions existing at the lower edge of said girder; Figure 9 displays the application of moving loads on the finished girder according to the present invention, and in Figure 9a the chart is reported which shows the tensions existing at the lower edge of said girder. Referring to Figure 1, a composite girder manufactured according to the present invention, generally indicated with (11), has a "T"-shaped cross-section and comprises an "I"-shaped steel girder (12) the core of which is vertically arranged Associated with the two flanges (12a, 12b) of the steel girder (12) there are two slabs (13) or (14), i.e., the lower (or bottom) slab (13), and the upper (or top) slab (14), fastened by means of steel connecting means (15). The slabs (13) and (14) are made from concrete, and the bottom slab (13) is made from high-performance concrete. High-performance concrete is characterized by a high compression strength associated with a high elastic modulus, which is constant over time. In particular, under "high-performace concrete", a high- or very high-strength concrete is understood, which displays a compression strength comprised within the range of from 70 MPa to 200 MPa, preferably of 100 MPa, and an elastic modulus comprised within the range of from 30 GPa to 60 GPa, preferably of 40 GPa. In order to obtain such concrete grades, cements can be used which at least meet the requirements of class 42.5 according to European Standard ENW 197.1, selected aggregates with high physical-mechanical characteristics, for example granite, limestone, quartz and/or basalt, high dispersing superfluidizers, in order to obtain water/cement ratios of less than 0.45, preferably of less than 0.30, still more preferably of less than 0.25. The use is furthermore possible of fumed silicas with an average particle size of approximately 0.2 micron and a specific surface area of approximately 18 m 2 /g. Then, also the use of metal and/or polymeric fibres can be advantageous in order to obtain high-performance concretes displaying high characteristics of ductility and resistance to combined compressive and bending stresses. Using high-performance concrete improves the durability of the material. Inside the interior of the bottom slab (13) a plurality, or bundle, of prestressing cables (16) are then installed, e.g., arranged as two mutually superimposed rows, laid under a base flange (12a). These cables (16) realize the co-action of axial-eccentric type -- after that the external prestressing constraints are removed -- because they adhere to the surrounding concrete. According to an alternative embodiment, the cables can be housed inside cable ducts and can then be prestressed after the occurred concrete ageing. The bottom slab (13) is usually manufactured at the prefabrication factory, whilst the top slab (14) can be alternatively manufactured also in place, besides being manufactured in the prefabrication factory, according to the actual requirements. A girder constituted by these materials shall withstand bending stresses applied on its middle vertical plane, which would tend to stretch the bottom fibres. The specific characteristics of the materials used, on the other hand, would not allow the girder to operate correctly when submitted to the operating loads, if the materials were not installed while simultaneously submitting them to co-action according to the present invention, so as to obtain that the high initial mechanical characteristics (under low stress levels) are retained also at the stress levels originated by the operating loads, as it results from the chart of Figure 2. In this chart, the behaviour is depicted of a girder known from the prior art not submitted to co-action (indicated in chain line) and of a girder submitted to co-action according to the present invention (shown in solid line). As mentioned hereinabove, the co-actions can be induced partially at the manufacturing factory and partially in place at installation time. The practical availability of high-performance concrete makes it possible two co-action types to be combined, i.e.: 1.The co-action of flexural type, obtained by inflexing the steel girder (12) so as to prestress the bottom fibres and "freezing" this prestressed state by applying the bottom slab (13) made from high-performance concrete. The high value of the elastic modulus of the latter, and the constance of this value over time, enables the so obtained co-action state to be retained at a high level and without any significant creep phenomena. 2.The co-action of axial type, obtained by prestressing the steel cables (16) before manufacturing the slab (13), so that, when the prestressing constraints are removed, the whole girder results to be in a state characterized by the presence of a combined compressive and bending stress, i.e., inversely loaded relatively to the loads generated by the use stresses. The characteristic of high compression strength displayed by the high-performance concrete is the feature which allows the bending co-action to be combined with the axial co-action, with a large increase in ultimate strength performance being consequently obtained without increasing the steel amount of girder (12). Then, as the value and the distribution of the bending co-action can be regulated relatively to the axial co-action, one can obtain the result that, under operating conditions, the state of compression of the slab (13) is nearly constant throughout the length of the girder. As a consequence, the prestressing cables (16) can be kept adhering throughout the girder length, with benefits of product compactness and durability. The presence of the steel girder (12), electrically connected with the cables (16), causes said cables to be protected from phenomena of galvanic corrosion. The state of tensile stress impressed on an upper flange (12b) made from steel allows the girder to be installed for casting the upper concrete slab (14) without either tension or elastic stability problems. A composite girder according to the present invention is manufactured according to a method which can be schematically disclosed as follows. In fact, the necessary steps sequence for impressing the co-actions and consequently producing the composite girder can be isolated and represented as displayed in Figures 3-9, with some of which the relevant stress charts are associated. In a first step of the method, a steel girder (12) is selected and to the flanges (12a) and (12b) of it, a plurality of connecting means (15) made from steel are fastened. Furthermore, the girder is constructed in a preflexed or inflexed configuration. In a second step, the bundle of cables are installed as one or two series of adhering cables (16) which are prestressed and blocked by means of auxiliary external constraints (17), with furthermore forces (F) being applied by means of auxiliary constraints on the workbench. Said arrangement can be observed from Figure 4, which also displays the chart of the torques impressed on the steel girder by the forces (F) and the presence of the constraints (12). When such a condition is reached, the bottom slab (13) is manufactured by suitably casting the high-performance concrete from which it is made in such a way as to abut against the flanges (12a, 12b) of the steel girder 12), and causing said concrete to set after that it took its required shape, catching the purposely provided connecting means (15) (Figure 5). After that, the constraints acting on the cables (16) (which get released), the auxiliary constraints (17) and the forces (F) are all removed, as shown in Figure 6. The partially realized girder (11) is now submitted to tensile stresses, as displayed in the scheme (6a) associated with Figure 6. Now, the basic importance is demonstrated of using high-performance concrete, which is able to withstand the stresses induced by the bending co-action simultaneously with the stresses induced by the prestressing -- and, in such a way, allows the process to be implemented. All this series of steps are carried out at the prestressing factory. Contrarily to the above, the next steps can be performed either at the prefabrication factory, or directly in place, as briefly mentioned hereinabove. In fact, the upper concrete slab must be manufactured. For the sake of a better understanding, in the following the construction is hypothesized for exemplifying, non-limitative, purposes, of a viaduct with an upper slab made from concrete (which may be either a normal, or a high-performance concrete) realized in place, and cooperating. After that the girder manufactured at the prefabrication factory is installed in its end position, a further step of concrete casting is carried out with application of its own weight and of the weight of the slab, both schematically shown in q d , (Figure 7). The chart of the tensile stresses is consequently changed as shown in Figure 7a. When this first step in place is complete, concrete (14) is allowed to set and then the upperworks are manufactured, with the relative weight q d' , being applied (Figure 8). The chart consequently changes as illustrated in Figure 8a. By means of the application of mobile loads, indicated with q e , the end behaviour of the girder according to the present invention is recorded. As already said several times hereinabove, the end composition of the girder of the present invention can be reached as well at the prefabrication factory, by means of the reproduction of the application points. One will observe that the tensile stresses undergone by the (high-performance) concrete of the slab (13) reach high levels during the initial construction steps, during which they perform the function of blocking the deformability of steel of cables (16) and of girder (12), and then decrease to extremely low values during use, with small changes deriving from the application of moving loads. This feature secures that the initial qualities will be retained over time. Also the stresses deriving from mutual connection of slab (13) and girder (12) decrease considerably when the girder is finished. This feature is a consequence of the matter of fact that the chart of tensions impressed on the slab (13) is very similar to the tensions arising from the loads (chain line). A composite girder as realized according to the method of the present invention displays advantageous characteristics of light weight, low cost, durability associated with high-performance of strength, stiffness and thinness, thus resulting, as already said, to be particularly suitable for building railway and road viaducts thanks to its limited dimensions in height as compared to the girders known from the prior art, as mentioned and discussed hereinabove. The effectiveness and functionality of the girders according to the present invention is furthermore due to the use of high-performance concrete which is characterized by a high compression strength associated with a high value of elastic modulus, essentially not much variable with time. In fact, this type of concrete performs the task of stiffening the girder, reducing its bending deformability consequent to the application of moving loads. All the above features cause a large improvement in the limit state of use, with reduction in the fatigue phenomenon the girder is submitted to. A further feature is the use of the steel strand in the bottom slab according to the steps of the method according to the present invention. The installation of the steel cables or strands results to be still more beneficial thanks to the use and presence of high-performance concrete which is able to be prestressed not only by the bending co-action developed by the steel girder which constitutes the core of the structure, but, above all, by the prestressing of the rather large number of strands installed inside it. It should be furthermore observed that the steel girder of the central core is cheap, while having good characteristics. The same is true for the cables which are very cheap with the developed strength being the same. Furthermore, according to the present invention, the advantageous structure of the girder of the invention is accomplished thanks to the prestressing of the cables and to their being blocked against the concrete either by adhesion, or by means of anchoring heads, to the consolidation of said concrete and to the subsequent release of the external auxiliary constraints. The cables are prevented from getting loose by the stiffness of the steel girder coupled with the intrados slab. The possibility of partially manufacturing at the prefabrication factory facilitates the shipping thereof to the installation place and the installation thereof.

Claims: Method for manufacturing a composite girder (11) comprising a steel girder (12) with a vertical core of steel, with which a slab (13, 14) of concrete is associated with longitudinal arrangement, with steel connecting means (15) being provided for said concrete, said method providing a first step consisting in positioning said steel girder preflexed or inflexed, a second step consisting in arranging at least one series of cables (16) adhering to a flange of said steel girder and in submitting said at least one series of cables to prestressing with simultaneously forces and auxiliary constraints being applied on said steel girder (12), a third step of concrete mixing, casting and setting in order to produce a high-performance concrete slab embedding said at least one series of cables, and a fourth step consisting in removing the external prestressing of the cables, the forces and the auxiliary constraints, characterized in that the third step includes positioning a flange (12a or 12b) of the steel girder (12) and casting the concrete such that this flange (12a or 12b) abuts against the respective concrete slab (13, 14). Method according to claim 1, characterized in that it provides a step of producing a second slab (13, 14), such that the other flange (12b or 12a) of the steel girder (12) also abuts against the respective concrete slab (13, 14). Method according to claim 1 or 2, characterized in that said high-performance concrete slab is the bottom slab. Method according to claim 1, characterized in that it is performed at the manufacturing factory. Method according to claim 2, characterized in that said second slab is manufactured in place. Method according to claim 1, characterized in that said steel girder is electrically connected with said series of cables. Composite girder manufactured according to the method of the preceding claims, comprising at least one steel girder (12) with vertical core with which at least one slab (13, 14) of concrete is associated with longitudinal arrangement, with steel connecting means (15) being provided for said concrete, with said steel girder(12) being preflexed or inflexed and integral with it at least one series of cables (16) being installed adhering to a flange of said steel girder, which cables (16) are submitted to prestressing with simultaneously forces and auxiliary constraints being applied on said steel girder, with a slab made from high-performance concrete and embedding said at least one series of cables (16) being provided, after that the external prestressing of the cables, the forces and the auxiliary constraints are removed, characterized in that at least one flange (12a, 12b) of the steel girder (12) abuts against the respective concrete slab (13, 14). Composite concrete girder according to claim 7, characterized in that said girder is equipped with a second concrete slab, which abuts against the other flange (12a, 12b) of the steel girder (12). Composite concrete girder according to claim 7, characterized in that said high-performance concrete has a compression strength comprised within the range of from 70 MPa to 200 MPa, preferably of 100 MPa, and an elastic modulus comprised within the range of from 30 GPa to 60 GPa, preferably of 40 GPa. Composite concrete girder according to claim 7, characterized in that said high-performance concrete uses superfluidizers with a high dispersing effect in order to obtain water/cement ratios lower than 0.45, preferably lower than 0.3, more preferably lower than 0.25. Composite concrete girder according to claim 7, characterized in that said high-performance concrete uses fumed silicas having an average particle size of about 0.2 micron and a specific surface area of at least 18 m 2 /g.

Inventors: CASSAR LUIGI, COLLINA VINCENZO, GUERRINI GIAN LUCA, MIGLIACCI ANTONIO, CASSAR, LUIGI, COLLINA, VINCENZO, GUERRINI, GIAN LUCA, MIGLIACCI, ANTONIO
Applicants: GIPIEFFE ARCHITETTURA STUDIO A, ITALCEMENTI SPA, GIPIEFFE ARCHITETTURA STUDIO ASSOCIATO, ITALCEMENTI S.P.A.
Application year: 1997
Main classification: B28B23
IPC codes: E04C3, B28B23