OBJECT OF THE INVENTION

The object of the invention is a prefabricated roof joist, made of prestressed concrete, constituting an element of the solution for construction industry as a prefabricated roof system. The invention can be used individually or can constitute a bearing element of a joist and hollow core block ribbed flooring system including prefabricated elements, monolithized on site. The joist according to the invention, thanks to specific parameters and structural solutions is intended for constructing buildings, single- and multi-family, public utility and historic buildings.

STATE OF THE ART

In the existing state of the art, prefabricated ribbed prestressed concrete floors are known. The most popular group of solutions is based on the use of prefabricated hollow core blocks provided with longitudinal openings or chambers. This type of elements is connected on site with lengthwise bar reinforcement in the form of a spatial truss laid between the prefabricated elements. A second group of structural floor solutions is based on the use, as a bearing element, of shaped joists, usually with T or double-T cross-section cast with concrete on site. Solutions, in which the main bearing element is a prefabricated reinforced joist made of prestressed concrete are also known.

In the publication of a French patent FR2907704, a prefabricated concrete joist, optionally longitudinally reinforced with stressing tendons and with an upside-down "T" letter cross-section is known. The patent generally discloses also the use recesses on sides of the web.

A disadvantage of the known solutions is the limitations related to the length of bearing elements and the weight of the element. The joist according to the invention significantly reduces the [imitations by increasing the bearing capacity of this construction element and allowing its extension.

OBJECTIVE OF THE INVENTION

The objective of the invention is to solve technical problems related to the bearing capacity and weight of the floor in building construction, especially at larger span of the bearing element. With relation to the economical aspect, the solution is an alternative to cast concrete floor technologies and other ribbed systems that is less expensive and easier to install on a construction site. The joist according to the invention is characterized by a very advantageous weight to bearing capacity and span ratio.

ESSENCE OF THE SOLUTION

The essence of the solution is based on the use of an innovative joist profile and innovative configuration of reinforcement of that element, particularly through an innovative method of arrangement of stressing tendons. The joist according to the invention can be used especially as a bearing element of a prefabricated bearing ribbed floor system. Usability qualities of the invention have been confirmed with construction calculations, which have confirmed its advantages as an element of building construction.

A prefabricated floor joist made of prestressed concrete according to this invention has a cross-section resembling an upside-down "T" letter. On the installation site, the joist is placed on a base and the web (vertical element) of the joist is directed upwards like an interlocking block and it is cast with concrete after possible connection with hollow core blocks. The joist web has a varying width, taking an isosceles trapezoid shape in the cross-section. There is a lower ledge of the joist between the web and the base. The upper surface of the lower joist ledge is slightly bent downwards. The optimum inclination is between 5-10°. The upper surface of the web is shaped as concrete interlocking blocks with the length between 40 - 140 mm and height between 4-14 mm. A part of the side surface of the web, in the middle of its height, has been also formed like an interlocking block by regularly repeated, symmetrically arranged narrow sections. Surfaces of the joist formed in this way ensure very good conditions for connection of the joist with additional concrete that monilithizes the floor. In the joist according to the invention the disadvantage of the necessary use of lateral clamping reinforcement on the interface of the prefabricated joist and the additional cast concrete known from the previous designs has been eliminated. Elimination of this disadvantage significantly improves the process of joist installation on the construction site.

The joist according to the invention is provided with longitudinally arranged prestressing reinforcement. Stressing tendons are wire lines that are strands of high-quality Y2060 grade steel. The joist according to the invention is differentiated by its cross-section having two characteristic forms (A) and (B) with different height but the same width of the base. Simultaneously, geometry of the joist has a characteristic specific proportion, which is based on the fact that the maximum height of the web equals the width of the base of the joist. Preferably from the point of view of the manufacturing process, the floor joist according to the invention is provided with a transition section located between the sections of the joist having the characteristic lateral cross-section (A) and (B). The intermediate section with height varying in a continuous way.

The joist is provided with at least two longitudinal reinforcing elements in the form of stressing tendons arranged on the base symmetrically to the axis of symmetry of the joist.

Optionally, if higher bearing capacity is required, the prefabricated floor joist according to the invention can be provided with additional stressing tendons. One or more additional stressing tendons are arranged vertically, one over the other, along the joist, on its vertical axis of symmetry.

Another option to obtain higher bearing capacity of the joist is to use in its n ear-to-sup port zones, being generally approx. 0.15 - 0.3 of the total joist length, elements of lateral reinforcement in the form of a spatial truss. For those joists, trusses with the length of a single small truss between 0.15 to 0.3 of the total joist length should be used.

ILLUSTRATIONS

The object of the invention has been presented drawings where: - Fig. lA presents the lower characteristic form of the cross-section

(A) in the basic version;

- Fig. l B presents the higher characteristic form of the cross-section

(B) in the basic version;

- Fig.2 illustrates the vertical projection of the joist (view from side);

- Fig.3 illustrates the horizontal projection of the joist (view from top);

- Fig.4A illustrates the lower characteristic form of the cross-section (A) with an additional stressing tendon and visible outline of the lower form of the cross-section (B) marked with a dotted line;

- Fig.4B illustrates the lower characteristic form of the cross-section (A) with an additional stressing tendon and visible outline of the lower form of the cross-section (B) marked with a dotted line;

- Fig.4C illustrates the lower characteristic form of the cross-section (A) with an additional stressing tendon and lateral reinforcement in the form of a truss and a visible outline of the lower form of the cross-section (B) marked with a dotted line;

- Ftg.4D illustrates the lower characteristic form of the cross-section (A) with an additional stressing tendon and lateral reinforcement in the form of a truss and a visible outline of the lower form of the cross-section (B) marked with a dotted line;

- Fig.5 presents the lateral reinforcement in the form of a spatial truss.

EMBODIMENT

Example 1

Prefabricated floor joist in the first embodiment consists of a base (1 ) and a web (2) with a cross-section in the isosceles trapezoid shape. In the embodiment, the web (2) of the joist has varying width reduced upwards in an interlocking block form. The top surface of the base (1 ) of the joist is made with inclination of 7°. The top surface of the web (2) has been formed as a concrete interlocking block with the length of 70 mm and height of 7 mm. A part of the side surface of the web (2) is also shaped like an interlocking block at the height of 48 mm from the top edge. Side surfaces of the web (2) are provided with regularly repeated symmetrically arranged narrow sections (3). Joist surfaces formed in this way ensure very good conditions for connection of the joist with the additional concrete. The height of the joist changes in gradual steps, while sections (A) and (B) with a characteristic cross-section are joined by an intermediate section (C) with gradually varying height. The joist includes two sections repeated alternatively: the lower (A) and higher section (B), between which there is a connecting intermediate section (C) with its height varying continuously. The value of the maximum height present on sections (B) equals the width of the base. Sections (B) are characterized with a symmetrical narrow section (3) of both sides of the web (2). The joist in one of the embodiments is provided with two stressing tendons (4) made of high-strength steel wire strands. The joist in the discussed embodiment is made of C50/60 grade concrete, based on cement class of CEM I 52.5 R, with broken granite concrete with the grain size not exceeding 8 mm. The tendons are made of 7-wire strands with a diameter of 6.85 mm, consisting of an arrangement of spirals of six wires with 0 _O ut = 2.25 mm around the wire center wire with 0 _m = 2.35 mm, with the total surface area of the strand of Ap = 28.2 mm ² . Optionally, in order to increase the bearing capacity, the joist of the embodiment is provided on the end sections adjacent to the supports (5) with lateral reinforcement made of a spatial truss (6) shown in the spatial projection on fig. 5 and in the cross-section on fig. 4C and fig. 4D. In the discussed embodiment, lateral reinforcement in joist zones adjacent to the supports on the length of 0.25 of the total length of the joist, counting from its both ends, has been used. Application of the spatial truss (6) may but does not have to be connected with the use of additional reinforcement in the form of additional stressing tendons (7) arranged in the axis of symmetry (8) of the joist. In the embodiment, trusses (6) made of AillN class, ST-500b grade steel, have been used. While the number of diagonals (9) used has been 20 rods per 1 running meters (nodes every 200 mm).

Example II

In the second embodiment illustrated on fig. 4A and fig. 4B, the joist according to the invention has been provided with additional stressing tendons (7) running on the whole length of the joist and arranged vertically, on its axis of symmetry (8). In a variant of the embodiment with more than one additional stressing tendon (7), the additional stressing tendons (7) have been arranged in one vertical line, one over the other.

Example Ml

in the third embodiment illustrated on fig. 4C and fig. 4D, the joist according to the invention has been provided with additional stressing tendons (7) running on the whole length of the joist and arranged vertically, on its axis of symmetry (9) and additionally with lateral reinforcement in the form of a spatial truss (6). The truss has been placed on sections with their length being 0.25 of the joist length from each end of the joist.

ADVANTAGES

The invention is classified as a construction material with improved parameters, particularly as the only one in the market that allows to obtain floor span on a single joist of more than 9.4 m. The joist according to the invention has high parameters of resistance to aging and fire, thanks to the reduced use of steel and concrete the installed energy per 1 m ² of the floor area is reduced as compared with the existing solutions. The design and shaping of the joist surface classify it as a material with the highest parameters of constructional connection with concrete over- layer available in the market and full compliance with the standards related to the conditions specified by the bearing capacity ultimate limit states (ULS). Dimensional diversity and reduced weight of the joist allow precise selection of the solution for specific requirements, e.g. ensuring lower loads, proper floor dimensions, etc. Therefore, it is an element that is perfectly adapted to the needs of building restorations, including historic buildings, the age and technology of which does not allow using other, heavier solutions.