METHOD FOR THE CONSTRUCTION OF A PRESTRESSED STRUCTURE AND PRESTRESSED STRUCTURE THUS OBTAINED

Abstract: The invention falls within the sector relating to techniques for the construction of prestressed structures and may be applied within the area of civil engineering. With the method according to the present invention a prestressed structure (1) is made, which structure comprises a primary structure (5) and one or more tendons (7). The primary structure (5) comprises two or more glued laminated timber beams (5a), two terminal elements (5b) and one or more deviating elements (5c, 5d, 5e). The one or more tendons (7) apply to the primary structure (5) forces which are due to the tensioning of the one or more tendons (7), performed during the construction of the prestressed structure (1), and to the loads (4) applied to the said prestressed structure (1); the abovementioned forces produce bending in the primary structure (5), inducing in the said primary structure (5) a compressive normal force.

Description: METHOD FOR THE CONSTRUCTION OF A PRESTRESSED STRUCTURE AND PRESTRESSED STRUCTURE THUS OBTAINED TECHNICAL FIELD AND BACKGROUND ART. The present invention relates to a method for the construction of a prestressed structure and the prestressed structure thus obtained. In the art various types of prestressed structures are known. In a prestressed beam, as it is known, steel reinforcements are used, said reinforcements, suitably tensioned, inducing in the said beam a compressive normal force, bending moments and, in some cases, shear forces. Beams made of prestressed concrete are very widespread. DISCLOSURE OF INVENTION. The object of the present invention is that of providing prestressed structures which comprise glued laminated timber beams and tendons of the "external" type and which have considerable transverse stability. These and other objects are achieved by the method for the construction of a prestressed structure according to the present invention and by the prestressed structure thus obtained, also according to the present invention. The abovementioned method for the construction of a prestressed structure is characterized by the fact that it comprises the following steps: - first step: two or more glued laminated timber beams, two terminal elements, one or more deviating elements and one or more tendons are made; said glued laminated timber beams, said terminal elements, said one or more deviating elements and said one or more tendons, from the start, are designed and made so as to produce the prestressed structure; - second step: a primary structure is assembled, applying said terminal elements and said one or more deviating elements to said glued laminated timber beams; said terminal elements are applied to the ends of said glued laminated timber beams; at the end of said second step of the method, said glued laminated timber beams are spaced from one another and are connected together by means of said terminal elements and said one or more deviating elements; - third step: said one or more tendons are arranged in position, in contact with said one or more deviating elements and with said terminal elements; at the location of said one or more deviating elements, the path followed by said one or more tendons changes direction; each of said one or more tendons is located outside the cross-section of said glued laminated timber beams and is positioned, at least partly, between said glued laminated timber beams; - fourth step; tensioning of said one or more tendons is performed, said tendons being then anchored, at their ends, to said terminal elements; said tensioning is performed using means which act against said glued laminated timber beams through the terminal element with which said means are in contact; each of said one or more tendons, during said tensioning, being subject to a tensile normal force, lengthens, sliding longitudinally with respect to said primary structure; following said tensioning, said one or more tendons, at the location of said terminal elements and of said one or more deviating elements, apply to said primary structure a system of forces which produces bending in said primary structure, inducing in said primary structure a compressive normal force; said fourth step of the method is completed before the prestressed structure is put into service. The prestressed structure obtained in accordance with the method described above (prestressed structure which also forms the subject of the present invention) is characterized by the fact that it comprises in combination: - a primary structure comprising two or more glued laminated timber beams, two terminal elements applied to the ends of said glued laminated timber beams and one or more deviating elements applied to said glued laminated timber beams; said glued laminated timber beams are spaced from one another and are connected together by means of said terminal elements and said one or more deviating elements; - one or more tendons, each of which has its ends anchored to said terminal elements and is in contact with said one or more deviating elements; at the location of said one or more deviating elements the path followed by said one or more tendons changes direction; each of said one or more tendons is located outside the cross-section of said glued laminated timber beams and is positioned, at least partly, between said glued laminated timber beams; said one or more tendons, at the location of said terminal elements and of said one or more deviating elements, apply to said primary structure forces which produce bending in said primary structure, inducing in said primary structure a compressive normal force; said forces are due to the tensioning of said one or more tendons, performed during the construction of the prestressed structure, and to the loads applied to said prestressed structure; in particular, as a result of said tensioning, said one or more tendons, at the location of said terminal elements and of said one or more deviating elements, apply to said primary structure a system of forces which produces bending in said primary structure, inducing in said primary structure a compressive normal force. BRIEF DESCRIPTION OF DRAWINGS. The characteristic features and advantages of the present invention will emerge more clearly from the following description of two embodiments illustrated purely by way of a non-limiting example in the accompanying plates of drawings, in which: - Figure 1 shows a partly longitudinal view and partly longitudinally sectioned view of a prestressed structure obtained according to the present invention; - Figure 2 shows, on a scale larger than that of Figure 1, the cross-section along the line I-I of Figure 1 ; - Figure 3 shows, on the same scale as Figure 2, a detail of Figure 1 ; - Figure 4 shows, on the same scale as Figure 2, the view, from one of the two ends, of the prestressed structure according to Figure 1 ; - Figure 5 shows a partly longitudinal view and partly longitudinally sectioned view of a prestressed structure obtained in accordance with the present invention, according to a further embodiment; - Figure 6 shows, on a larger scale than that of Figure 5, the cross-section along the line II-II of Figure 5. For the sake of simplicity of the illustration, welding, bolting, nailing or other joining systems have not been shown in the abovementioned figures. BEST MODE FOR CARRYING OUT THE INVENTION. Below, in order to facilitate the understanding of the steps of the method according to the present invention, first of all, with reference to Figures 1, 2, 3 and 4, a prestressed structure 1 obtained according to the present invention is described. The prestressed structure 1 which below is described when in use, is supported at its ends; more precisely, it is constrained with two external constraints 2, each of which is schematically represented as a hinge, and with two external constraints 3, each of which is schematically represented as a horizontally rolling support. The prestressed structure 1 which (during use) is subject to vertical concentrated loads 4 comprises in combination: - a primary structure 5 comprising two glued laminated timber beams (glulam beams) 5a, two terminal elements 5b applied to the ends of the two glued laminated timber beams 5a and five deviating elements 5c, 5d, 5e applied to the two glued laminated timber beams 5 a; the glued laminated timber beams 5 a are spaced from each other and are connected together by means of the terminal elements 5b and of the deviating elements 5c, 5d, 5e; the glued laminated timber beams 5a, which have a straight and horizontal axis, have a rectangular cross-section; the glued laminated timber beams 5a are identical and parallel to each other; each of the two glued laminated timber beams 5a consists of the set of two glued laminated timber beams 6 which are identical to each other and have a rectangular cross-section and which are placed in contact with each other and joined together; - eight tendons 7, each of which has its ends anchored to the two terminal elements 5b and is in contact with the five deviating elements 5c, 5d, 5e; at the location of the deviating elements 5c, 5d, 5e, the path followed by the tendons 7 changes direction; each tendon 7 is located outside the cross-section of the two glued laminated timber beams 5a and is positioned, partly (i.e. along the portions close to its ends), between the two said glued laminated timber beams 5a; it is pointed out that the expression "positioned partly between the two glued laminated timber beams 5a" is understood as meaning "positioned partly in the space between the two glued laminated timber beams 5a"; the tendons 7, at the location of the terminal elements 5b and of the deviating elements 5c, 5d, 5e, apply to the primary structure 5 forces which produce bending in the primary structure 5, inducing in the said primary structure 5 a compressive normal force; the abovementioned forces are due to the tensioning of the tendons 7, performed during the construction of the prestressed structure 1 , and to the loads 4 which are applied, during use, to the prestressed structure 1 itself; in particular, as a result of the abovementioned tensioning, the tendons 7, at the location of the terminal elements 5b and of the deviating elements 5c, 5d, 5e, apply to the primary structure 5 a system of forces which produces (upwards) bending in the primary structure 5, inducing in the said primary structure 5 a compressive normal force. It is pointed out that the intensity of the forces applied by the tendons 7 to the primary structure 5 is influenced by the decrease in tension due (among other things) to the relaxation of the steel and to the long-term deformations (creep) of the glued laminated timber. Each tendon 7 consists of a high-strength steel strand (steel, that is, having for example a tensile strength greater than 1750 N/mm 2 ) contained in its own sheath of synthetic material inside which the strand itself is able to slide. Each of the two terminal elements 5b consists of a set of metal elements which comprises two end-pieces 8, each of which is in contact with one of the two glued laminated timber beams 5a, two cross-pieces 9a, 9b which are joined to the two end-pieces 8 and an anchoring element 10 which is joined to the cross-piece 9a; the tendons 7 are anchored at the location of the anchoring elements 10 by means of wedges. It is pointed out that, according to a variation, not shown in the figures, with reference to each of the two terminal elements, the end-pieces, as well as the end cross-sections of the glued laminated timber beams, are substantially perpendicular not to the axes of the said glued laminated timber beams, but to the axes of the tendons which are anchored at the location of the terminal element itself. It is pointed out that, according to another embodiment, not shown in the figures, the end- pieces may have dimensions which are smaller than the height of the cross-section of the glued laminated timber beams. Each of the five deviating elements 5c, 5d, 5e consists of a set of metal elements which comprises elements placed in contact with the glued laminated timber beams 5a and suitably shaped elements which, placed in contact with the tendons 7, deviate the path of the said tendons 7. It is pointed out that, in the prestressed structure 1, the axes of the deviating elements 5c, 5d, 5e are vertical and are perpendicular to the axes of the two glued laminated timber beams 5a; according to a variation, not shown in the figures, the axis of each deviating element is directed along the bisector of the angle formed between the two sections of each tendon located respectively on either side of the said deviating element. The method, according to the present invention, for the construction of the prestressed structure 1 is described below, said method comprising the following steps: - first step: the two glued laminated timber beams 5a, the two terminal elements 5b, the five deviating elements 5c, 5d, 5e and the eight tendons 7 are made; the two glued laminated timber beams 5a, the terminal elements 5b, the deviating elements 5c, 5d, 5e and the tendons 7, from the start, are designed and made so as to produce the prestressed structure 1 ; - second step: the primary structure 5 is assembled on-site, applying the terminal elements 5b and the deviating elements 5c, 5d, 5e to the two glued laminated timber beams 5a; the terminal elements 5b are applied to the ends of the glued laminated timber beams 5a; at the end of the second step of the method, the two glued laminated timber beams 5a are spaced from each other and are connected together by means of the terminal elements 5b and the deviating elements 5c, 5d, 5e; - third step: the tendons 7 are arranged in position, in contact with the deviating elements 5c, 5d, 5e and with the terminal elements 5b; at the location of the deviating elements 5c, 5d, 5e the path followed by the tendons 7 changes direction; each tendon 7 is located outside the cross-section of the two glued laminated timber beams 5a and is positioned, partly, between the said two glued laminated timber beams 5a; it is pointed out that the tendons 7 are inserted inside the deviating elements 5c, 5d, 5e so as to be in contact with the suitably formed metal elements referred to above; then the primary structure 5, with the tendons 7 already arranged in position (as described above), is installed, positioning it on the external constraints 2 and the external constraints 3; - fourth step: tensioning of the tendons 7 is performed, said tendons being then anchored, at their ends, to the terminal elements 5b; the abovementioned tensioning is performed using a hydraulic jack (not shown in the figures) which acts against the two glued laminated timber beams 5 a through the terminal element 5b with which said hydraulic jack is in contact; each tendon 7, during the abovementioned tensioning operation, being subject to a tensile normal force, lengthens, sliding longitudinally with respect to the primary structure 5; following the abovementioned tensioning, the tendons 7, at the location of the terminal elements 5b and of the deviating elements 5c, 5d, 5e, apply to the primary structure 5 a system of forces which produces bending in the primary structure 5, inducing in the said primary structure 5 a compressive normal force; the fourth step of the method is completed before the prestressed structure 1 is put into service. With respect to the fourth step of the method the following is pointed out: the hydraulic jack, mentioned above, which is positioned at the location of the terminal element 5b close to the external constraints 3, tensions, in accordance with a suitable tensioning sequence, the tendons 7, acting, by means of the said terminal element 5b (close to the external constraints 3), against the two glued laminated timber beams 5a; the ends of the tendons 7 close to the external constraints 2 have been previously anchored, by means of the associated wedges, to the terminal element 5b close to the said external constraints 2; during the tensioning operations each tendon 7 slides, except for its end close to the external constraints 2, with respect to the primary structure 5, substantially (the term "substantially" is used since frictional forces are present) free to slide along its whole length inside its sheath. Then, after the tendons 7 have been anchored by means of the associated wedges to the terminal element 5b close to the external constraints 3, in order to obtain (as far as possible) a symmetrical distribution of the stresses, by means of the abovementioned jack, which is now positioned at the location of the terminal element 5b close to the external constraints 2, the operations of tensioning of the said tendons 7 are completed. The hydraulic jack is then deactivated and definitively removed. The result is that, at the end of the fourth step of the method, the ends of the tendons 7 are definitively anchored to the terminal elements 5b. The fourth step of the method terminates with the completion of the operations described above. The method for the construction of the prestressed structure 1 is concluded with termination of the fourth step. Then, once the method has been terminated, the prestressed structure 1 is put into service and the loads 4 are applied to it. Figures 5 and 6 illustrate a prestressed structure 20 obtained according to the present invention, which is supported at its ends on two external constraints 21, each of which is schematically represented by a hinge, and on two external constraints 22, each of which is schematically represented by a horizontally rolling support. The prestressed structure 20, which is described below during use, is subject to vertical concentrated loads 23 and comprises in combination: - a primary structure 24 comprising two glued laminated timber beams 24a with a rectangular cross-section, two metal terminal elements 24b which are applied to the ends of the two glued laminated timber beams 24a, two metal deviating elements 24c applied to the two glued laminated timber beams 24a, and metal connection elements 24d applied to the two glued laminated timber beams 24a; the two glued laminated timber beams 24a are spaced from each other and are connected together by means of the terminal elements 24b, deviating elements 24c and connection elements 24d; the two glued laminated timber beams 24a have a straight and horizontal axis; - six tendons 25, each of which has its ends anchored to the terminal elements 24b and is in contact with the deviating elements 24c; each tendon 25 consists of a high-strength steel strand contained inside a sheath of synthetic material inside which the strand itself is able to slide. It is pointed out that, for the sake of simplicity of the illustration, structural steel elements, which are arranged in an X shape and located in the horizontal plane which contains the axes of the connection elements 24d, have not been shown in Figures 5 and 6: the abovementioned structural elements arranged in an X shape complete the formation of a bracing structural system situated in the plane in which said elements lie. According to another embodiment, not shown in the figures, the connection elements (mentioned above) may be made of glued laminated timber. The behaviour of the prestressed structure 20 and the method for the construction thereof are technically equivalent to those already described for the prestressed structure 1. It is pointed out that, during the first step of the method, the connection elements 24d are also made, which elements, during the second step of the method, are applied to the two glued laminated timber beams 24a. In general, the method according to the present invention may sometimes comprise a fifth step, following the first four steps, during which relative displacements, permitted during the fourth step of the method, between the one or more tendons and the primary structure are permanently prevented; the abovementioned possible fifth step of the method is completed before the prestressed structure is put into service. In general the possible fifth step of the method may be useful for optimizing the behaviour in the "ultimate state" of the prestressed structure. With reference to the method according to the present invention for the construction of a prestressed structure, generally, some of the permanent loads or all the permanent loads may be applied to the said structure before the start of the fourth step of the method. In a prestressed structure obtained according to the present invention each of the two or more glued laminated timber beams comprised in the primary structure may consist of the set of two (as in the case of each of the glued laminated timber beams 5a) or more glued laminated timber beams, with a rectangular cross-section, which are arranged in contact with one another and joined together so as to form a single beam. In a prestressed structure obtained according to the present invention each of the two or more glued laminated timber beams included in the primary structure may have a straight axis or curved axis or axis formed as a broken line (with possible connecting radii). In a prestressed structure obtained according to the present invention the two or more glued laminated timber beams (included in the primary structure) may be of the solid-wall type or truss type or Vierendeel type or other types; in the case where the glued laminated timber beams are of the solid-wall type, each of them may have (for example) a rectangular cross-section or T-shaped cross-section or I-shaped cross-section with the upper flange the same as the lower flange or different from the lower flange. In a prestressed structure obtained according to the present invention each of the two or more glued laminated timber beams (included in the primary structure) may have a constant cross-section or a variable cross-section. In a prestressed structure obtained according to the present invention each of the two or more glued laminated timber beams (included in the primary structure) may comprise two or more segments joined together and located, in the longitudinal direction, one after another; these segments are assembled on-site before implementing the third step of the method; it is pointed out that, generally, the stress states at the location of the joints between the abovementioned segments may be improved as a result of the prestressing. It is possible to have a prestressed structure obtained according to the present invention (this embodiment is not illustrated in the figures) in which each tendon is entirely positioned between the glued laminated timber beams (i.e. is entirely positioned in the space between the glued laminated timber beams). It is therefore possible to have a prestressed structure obtained according to the present invention in which only one tendon is present. It is possible to have a prestressed structure obtained according to the present invention in which the tendons consist of high- strength steel strands grouped together in one or more groups of strands; each of the abovementioned groups of strands is contained in a tubular housing element which is made of synthetic material and inside which a filling material has also been introduced; it is pointed out that each strand may be greased and contained inside its own sheath made of synthetic material such that sliding of each strand inside its sheath is permitted. It is possible to have a prestressed structure obtained according to the present invention in which a single deviating element is present. It is possible to have a prestressed structure obtained according to the present invention in which more than two glued laminated timber beams are present; these glued laminated timber beams are spaced from each other and are connected together by means of the terminal elements and the one or more deviating elements; it is pointed out that connection elements applied to the abovementioned glued laminated timber beams may also be present. In a prestressed structure obtained according to the present invention the tendons consist, for example, of strands or bars or wires; it is pointed out that the tendons may be made of steel or may be made using synthetic materials or composite materials or also other suitable materials. It is possible to have a prestressed structure obtained according to the present invention (not shown in the figures) in which the primary structure also comprises retaining elements which connect the primary structure and the tendons at points where the path followed by the tendons does not change direction. According to the present invention it is possible to produce, in addition to prestressed structures externally constrained in an isostatic manner, also prestressed structure externally constrained in a hyperstatic manner. An advantage of the present invention consists in the fact that, using a prestressed structure obtained according to the present invention instead of an "equivalent" traditional glued laminated timber bam, it is possible to reduce the quantity of glued laminated timber to be used.

Claims: CLAIMS 1. Method for the construction of a prestressed structure, characterized in that it comprises the following steps: - first step: two or more glued laminated timber beams (5a, 24a), two terminal elements (5b, 24b), one or more deviating elements (5c, 5d, 5e, 24c) and one or more tendons (7, 25) are made; the glued laminated timber beams (5a, 24a), the terminal elements (5b 24b), the one or more deviating elements (5c, 5d, 5e, 24c) and the one or more tendons (7, 25), from the start, are designed and made so as to produce the prestressed structure (1, 20); - second step: a primary structure (5, 24) is assembled, applying the terminal elements (5b, 24b) and the one or more deviating elements (5c, 5d, 5e, 24c) to the glued laminated timber beams (5a, 24a); the terminal elements (5b, 24b) are applied to the ends of the glued laminated timber beams (5a, 24a); at the end of said second step of the method, the glued laminated timber beams (5a, 24a) are spaced from one another and are connected together by means of the terminal elements (5b, 24b) and the one or more deviating elements (5c, 5d, 5e, 24c); - third step: the one or more tendons (7, 25) are arranged in position, in contact with the one or more deviating elements (5c, 5d, 5e, 24c) and with the terminal elements (5b, 24b); at the location of the one or more deviating elements (5c, 5d, 5e, 24c) the path followed by the one or more tendons (7, 25) changes direction; each of the one or more tendons (7, 25) is located outside the cross-section of the glued laminated timber beams (5a, 24a) and is positioned, at least partly, between said glued laminated timber beams; - fourth step: tensioning of the one or more tendons (7, 25) is performed, said tendons being then anchored, at their ends, to the terminal elements (5b, 24b); said tensioning is performed using means which act against the glued laminated timber beams (5a, 24a) through the terminal element (5b, 24b) with which said means are in contact; each of the one or more tendons (7, 25), during said tensioning, being subject to a tensile normal force, lengthens, sliding longitudinally with respect to the primary structure (5, 24); following said tensioning, the one or more tendons (7, 25), at the location of the terminal elements (5b, 24b) and of the one or more deviating elements (5c, 5d, 5e, 24c), apply to the primary structure (5, 24) a system of forces which produces bending in said primary structure, inducing in said primary structure a compressive normal force; said fourth step of the method is completed before the prestressed structure (1, 20) is put into service. 2. Method according to Claim 1 , characterized in that it comprises a fifth step, following the first four steps, during which relative displacements, permitted during the fourth step of the method, between one or more tendons and the primary structure are permanently prevented; said fifth step of the method is completed before the prestressed structure is put into service. 3. Method according to Claim 1, characterized in that, during the first step of said method, connection elements (24d) are made, which elements, during the second step of said method, are applied to the two or more glued laminated timber beams (24a). 4. Prestressed structure obtained according to the method according to Claim 1 , characterized in that it comprises in combination: - a primary structure (5, 24) comprising two or more glued laminated timber beams (5a, 24a), two terminal elements (5b, 24b) applied to the ends of said glued laminated timber beams and one or more deviating elements (5c, 5d, 5e, 24c) applied to said glued laminated timber beams; the glued laminated timber beams (5a, 24a) are spaced from one another and are connected together by means of the terminal elements (5b, 24b) and the one or more deviating elements (5c, 5d, 5e, 24c); - one or more tendons (7, 25), each of which has its ends anchored to the terminal elements (5b, 24b) and is in contact with the one or more deviating elements (5c, 5d, 5e, 24c); at the location of the one or more deviating elements (5c, 5d, 5e, 24c) the path followed by the one or more tendons (7, 25) changes direction; each of the one or more tendons (7, 25) is located outside the cross-section of the glued laminated timber beams (5a, 24a) and is positioned, at least partly, between said glued laminated timber beams; the one or more tendons (7, 25), at the location of the terminal elements (5b, 24b) and of the one or more deviating elements (5c, 5d, 5e, 24c), apply to the primary structure (5, 24) forces which produce bending in said primary structure, inducing in said primary structure a compressive normal force; said forces are due to the tensioning of the one or more tendons (7, 25), performed during the construction of the prestressed structure (1, 20), and to the loads (4, 23) applied to said prestressed structure; in particular, as a result of said tensioning, the one or more tendons (7, 25), at the location of the terminal elements (5b, 24b) and of the one or more deviating elements (5c, 5d, 5e, 24c), apply to the primary structure (5, 24) a system of forces which produces bending in said primary structure, inducing in said primary structure a compressive normal force. 5. Prestressed structure according to Claim 4, characterized in that the primary structure (24) comprises connection elements (24d) applied to the two or more glued laminated timber beams (24a); the glued laminated timber beams (24a) are spaced from one another and are connected together also by means of the connection elements (24d). 6. Prestressed structure according to Claim 4, characterized in that each of the two or more glued laminated timber beams (5 a) consists of the set of two or more glued laminated timber beams (6), with a rectangular cross-section, which are arranged in contact with one another and joined together so as to form a single beam. 7. Prestressed structure according to Claim 4, characterized in that the two or more glued laminated timber beams (5 a, 24a) have a straight axis and have a rectangular cross-section. 8. Prestressed structure according to Claim 4, characterized in that each of the two or more glued laminated timber has a curved axis or axis formed as a broken line. 9. Prestressed structure according to Claim 4, characterized in that each of the two or more glued laminated timber beams has a T-shaped cross-section or I-shaped cross- section with the upper flange the same as the lower flange or different from the lower flange. 10. Prestressed structure according to Claim 4, characterized in that each of the two or more glued laminated timber beams comprises two or more segments joined together and located, in the longitudinal direction, one after another.

Inventors: GASPERI ANTONELLO, GASPERI, ANTONELLO
Applicants: GASPERI ANTONELLO, GASPERI, ANTONELLO
Application year: 1999
Main classification: E04C3
IPC codes: E04C3