EP3911809A1

EP3911809A1 - Structural section

Info

Publication number: EP3911809A1
Application number: EP20741827.8A
Authority: EP
Inventors: Marek NARLOCH; Lukasz Supel; Krzysztof NARLOCH
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-01-19
Filing date: 2020-01-16
Publication date: 2021-11-24
Also published as: PL243278B1; WO2020149752A1; EP3911809A4; PL428630A1

Abstract

A section according to the invention with a dual-plane bend of flanges provides formation of a dilatation slit in part of a sheathing supporting area against the flange, which necessitates an advantageous transfer of location of the forces transmitted at the contact point beyond the dilatation slit area, towards the centre of the flange, eliminating beam torsion. The Z-section is formed of a web (1) with two flanges (2 and 3) with an edge stiffener (4). At least one of the Z-section flanges has a dual-plane bend in a form of an off+set (5) or fold (11), dividing thereof into a lower portion (6, 12) located at the web (1) and an upper portion (7) at the edge stiffener.

Description

Structural section

The subject of the present invention is a Z-shaped, cold-formed structural section, so called Z-section, to be used in the construction industry in connection with various types of sheathing for lightweight building cladding available on the market.

It is known a Z-shaped structural element, so called Z-section, commonly used in purlin roof construction systems, as a beam support structure of a building sheathing. Most of the construction serviceability time, a roof Z-beam carries gravity load. At the gravity loads, the location of a transmission point of the forces from a sheathing onto a beam flange is disadvantageous due to the fact that the beam is not only bent in the load plane but also twisted and laterally bent. A beam restrained from sheathing, two-way bent and twisted, reaches the ultimate state at a lower load than a Z-beam restrained from sheathing, one-way bent without torsion and without lateral bend. The beam bending resistance at gravity load is reduced due to additional torsion and lateral bending due to the force application disadvantageous eccentric from the sheathing to the flange.

A Z-section is uplift loaded only at strong winds, when uplift pressure prevails over the weight of a sheathing structure, that is rather rare. Then, however, it reaches higher resistance due to the point of application of the load transmitted from the sheathing to the flange has changed. A Z-section is loaded by nearly elementary one axis bending in the load plane, almost without additional torsion and reaches the ultimate state at a higher load. Thus, a standard Z-section, most of the time, is loaded in a disadvantageous manner and by interactions of a leading value when it reaches lower resistance. Only in rare situations, at strong winds, a Z-section is loaded advantageously and reaches higher resistance, whereas the uplift interaction is most often lower than the gravity interaction.

At uplift load, a beam is loaded eccentrically advantageously while at gravity loads eccentrically disadvantageously. Differences in the beam resistance in both cases are quite significant and result solely from the change of the point of transmission of gravity and uplift interactions from the sheathing to the beam.

There are known Z-sections which have longitudinal folded groove at flanges and a web, or those with shallow web bends whose purpose is to stiffen the section structural walls classified as slender according to Eurocode 3 standard. Whereas, the longitudinal grooves of the strip flanges of said sections do not provide a step of the planes as a result of the wall embossment.

There are known Z-sections that have the web bend of a greater depth such that the web secant is diagonal and the flanges partially overlap.

There is known from Polish protection description of utility model PL65210 a structural member of a shape similar to a Z-shaped section that has one shorter side and a longer side parallel thereto, whereas there is a perpendicularly located therebetween longer end side that is parallel to the attached to the other end of the longer side by the shorter end side of the upper portion forming a profile of a hook ended with a claw. The opening has a curvilinear cutting surface. It is known from patent description PL 410265 a Z-section whose key feature is a very deep groove of a equilateral triangle shape located symmetrically on a flange and taking 1/3 of the width thereof. The portions of the flange at the web and at the edge stiffening are in one plane (a single-plane flange). The groove is bent inwards the cross-section and provides an intermediate stiffening of the wall. The flange is symmetrical. The solution has been protected in three variants of the section manufacture. In the first of three variants of the solution, the web is flat and a Z-section modification consists of adding large intermediate stiffening on the flange in a form of the described groove. In the second variant, there is an additional intermediate stiffening on the web, placed symmetrically, in a form of bends. In the third variant, the edge stiffening is additionally folded into two planes. The advantage of the solution is improved local stability of the flange, or even also of the web. The groove of that size additionally slightly increases the amount of material in the flanges. Bending resistance of a cross-section is increased is only in the case of thin sheets.

The portion of the flange at the web is not lowered in relation to the portion at the edge stiffening and it is not provided with an dilatation slit. Transverse compression from the sheathing is transmitted like in a standard Z-section, on the edge of the flange and web. The section is cold-formed from thin sheets, thus a finite transverse bending stiffness (contour stiffness) and deflection of the flange is the cause of disadvantageous resting of the sheathing on the edge of the flange with the web and causes torsion of the beam. If the sheathing rests on the edge of the flange and web, then the state of loading of the patented Z-section is the same as in a standard Z-section, i.e. ultimate beam load is decreased due to torsion of the Z-section caused by lateral reactions from the sheathing. Moreover, in said solution a wide groove in the intermediate portion of the flange prevents correct fitting with screws of the sheathing with the section. It is a commonly known principle to place the screw in the middle of the flange, which is structurally justified (proper stability of the beam). For the screw to be properly secured in two portions of the construction being attached both portions need to be directly in contact. If they are apart, the bar member, instead of being only sheared or pulled out, is disadvantageously broken out due to a non- technological seating.

There is known from patent description PL 220411 a C-section intended for partition racks. The C-section according to the invention is intended exclusively for cooperation with a two-sided plasterboard lining. The C-section according to the invention is characterized in that the flanges on the portions at the web are diagonally bent out inwards the cross-section and a raised portion of the flange is maintained at the width that is required to correctly support the plasterboard. A key objective of the flange modification is to improve acoustic insulation (acoustic resistance) of the partition by 5÷7 dB. In typical solutions, sound is transmitted directly via a web. In the solution according to the invention, the sound route via the section is prolonged by two additional diagonal portions of the flange. The problem with the patented C-section with a dual-plane flange is double torsion. Double torsion is precisely a consequence of the dual-plane flanges used in the C- section. Standard C-sections and Z-sections are twisted by torques of opposite signs. The dual-plane flange in modified C-sections and Z-sections brings an additional torsional eccentric. In a Z-section said eccentric advantageously eliminates torsion to zero (eccentrics are opposite and cancel each other out) while in a C- section in contrary, disadvantageous^ doubles it (the torques are compatible and they double). The use of a dual-plane flange in connection with a C-section is thus a particularly disadvantageous construction modification.

It is known from the invention application AU2008101288 a Z-section whose key feature is a plurality of embossments in a form of folds on flanges, a web and edge stiffening. The flange is symmetrical, consists of three portions: two very narrow, located symmetrically at the web and edge stiffening and a leading wide portion also located symmetrically. The wide central portion is located higher than both narrow extreme portions. The flanges are of different widths to allow attachment of the cross-sections by support overlaps. The web is divided, similar to the flange, into three portions, whereas of equal width. The fold of the central portion is towards the side of the narrower flange to allow attachment of the cross-sections by support overlaps. There is provided one additional embossment on the edge stiffening. All additional cross-section wall embossments are intermediate stiffenings and they are considered as such by the authors of the patent, which is evident from the patent description. There is information included in the description of the invention that a key objective of the Z-section modification is to improve local and distortion stability of the cross-section, which is in agreement with the state-of-the-art included in Eurocode 3, part 1-3 standard. Thus, a key objective is to increase bending resistance of a cross-section in the case of thin sheet sections. In the description of the invention it is indicated that in the case of an edge stiffening used on a flange general stability of a beam is increased, at flexural-torsional buckling, whereas it is increased only in relation to Z-sections without edge stiffening, which nowadays are in principle not in use.

The shape of the modification explicitly shows that the authors of the invention mainly aimed to improve construction parameters of a Z-section in the case of uplift reactions, which is justified in parts of the world where said reaction is dominant in relation to gravity reactions, e.g. Australia (no snowfalls or scarce atmospheric precipitation). At gravity reactions, a shape of the flange is a key (forces transmitted via transverse compression), at uplift reactions a shape of the flange is of lesser significance (forces transmitted via mechanical fasteners).

The flange is symmetrical, consists of three portions of different length. Two external portions of the flange at the web and at the edge stiffening are lowered in relation to the third, uplifted central portion of the flange. The external portions at the web and at the edge stiffening are possibly short, such that the central portion of the flange is possibly wide. The key for the solution is that the width of the middle portion of the flange is only slightly reduced in relation to the width of the entire flange, which improves the local stability level. The reduction of the middle portion is to be possibly little to allow the sheathing to be rested onto the flange at nearly the same width as in the case of a standard Z-section. A disadvantage of the solution may be the flange symmetry, little width of the portion lowered at the web, lowering of the flange at the edge stiffening, where the cross-section should support itself against the sheathing. When the sheathing rests in the proximity of the web, then the state of loading of the patented Z-section is nearly the same as in a standard Z-section, i.e. ultimate load of the beam is reduced due to torsion of the Z-section resulting from the lateral reactions from the sheathing. Torsion is not eliminated. A key object of the solution is to increase bending resistance of a cross-section of thin sheets sections. It is not to eliminate torsion. The ultimate load capacity of thin walled sections is increased by the increase of the bending resistance of the cross-section.

There are known from invention application description US 4461134 eight solutions for construction beams from the earlier stage of knowledge development in relation to bent sections (1984). The said eight solutions may be arranged in three groups of solutions. Within a particular group, the solutions are similar and they differ in production details. The solution groups vary rather significantly.

The first group may be assigned to a bent section (Fig. 5) that may be used directly, as a section, and as a base for prefabricated elements attributed to the second and third group. The remaining seven solutions refer to multi-portion prefabricated elements, i.e. complex construction members manufactured from several sections or several portions combined into a single whole. The second group contains two composite precast, i.e. systems where a basic section (Fig. 5) has been combined with attachments on flanges (Fig. 1 and 2). The third group contains five complex prefabricated elements comprising a basic section (Fig. 5), where densely and periodically located members stiffening both flanges are attached thereto, of five shape options (Fig. 3, 6, 7, 9 and 11).

The section according to Fig. 5 (group 1) is a Z-section whose web is bent diagonally into three portions such that the flanges are located one above the other. The centre of the flange is above the middle portion of the web. The Z-section is manufactured by traditional cold-forming. When reversed, the section may be nested into each other to allow section coupling by support overlap.

A key object of said solution was to approximate the Z-section behaviour to the behaviour of a rolled I-section. The diagonal break of the web and flange nesting into each other, partially or entirely, reduces the influence of one of the faults of standard Z-sections, i.e. diagonal inclination of the principal coordinate system of the cross-section, by approximately 15-20°. The rotation of the principal coordinate system to an orthogonal setting (intermediate object) was erroneously identified as achieving the symmetry of the cross-section load (key object). The key object (loading symmetry) was not achieved despite achieving the intermediate object (advantageous rotation of the principal coordinate system). The problem was a significant deformation of the bent section contour, which in rolled sections was entirely negligible. The torsion, preliminarily eliminated by orthogonal inclination of the principal coordinate system, was anyway replaced by the torsion resulting from the disadvantageous eccentric of the vertical forces, transmitted on the flange at web edges.

At an early stage of development of cold-formed sections (30-40 years ago), there were many solutions suggested, which were an urgent attempt to transfer the principles for forming rolled section to cold-formed structures. Many of said solutions failed at verification resulting from the development of knowledge in relation to cold-formed sections and the increased awareness that the principle of forming of those two different structures are entirely different.

The symmetrical location of flanges according to Fig. 5 was to ensure symmetrical force transmission from sheathing to a flange, in line with the principle known for rolled sections (at the centre of the flange), however, unfortunately, not in compliance with the principles developed later for cold-formed sections (at the edge of the flange).

The prefabricated elements according to Fig. 1 and 2 are a follow-up of the basic solution, such as according to Fig.5. At the precast stage, there are sheet attachments added to the basic section. The prefabricated element is combined. An object of the solution is to thicken the area of the section flanges, with maintenance of the thickness of the web. Certain limitation of constant thickness sheet-formed sections is that the thickness of all portions of the cross-section must also be identical. In the solution according to Fig. 1 and 2, the flanges have been thickened, analogically to rolled I-sections whose flange is thicker than the web. The prefabricated elements according to Fig. 1 and 2 vary in the depth of the flange overlap. When reversed, the precast may be nested into each other allowing coupling of the sections by support overlaps. Whereas, then the total thickness of the flange sheets and attachments thereto in the area of the support overlap is very significant and in the case of the solution according to Fig. 2 is probably entirely not technological.

The prefabricated elements according to Fig. 3, 6, 7, 9 and 11 are the follow-up of the basic solution, such as according to Fig. 5. At the precast stage, there are periodically located members supporting the flanges against the web attached to the basic section, of five shape options. The periodical flange supports allow stiffening of the cross-section contour of the cold-formed section. It may be considered that the flange deflection is reduced enough for the sheathing to transmit the force onto the flange symmetrically. At least in some of said solutions, depending on a web deformation. Unfortunately, the disadvantage of the solution is that the periodically located flange supports are conflicting and prevent coupling of the precast by support overlaps. Even though the precast is manufactured on the basis of a Z-section, following the modification according to the patent, it loses the possibility to be used in typical solutions for Z-sections due to not being able to be coupled in the most effective manner for Z-sections, i.e. by support overlap. In the precast solution (group 3), the load symmetry has probably been achieved, however at the expense of significant extension of the technological process, and what is worse, with entire loss of the key feature for Z-sections, i.e. ability to be coupled by support overlaps. The precast according to Fig. 3, 6, 7, 9 and 11 should thus be considered more as I-sections precast in quite a non-typical and complex manner, intended solely for single-span solutions (marginal part of applications).

In the solution according to Fig. 1, 2 and 5, the portion of the flange at the web is not lowered in relation to the portion at the edge stiffening. The compression from the sheathing is transmitted as in a standard Z-section, at the flange and web edge. The compression on said edge results in torsional eccentric. The flanges are flat and combined from several sheets. The member is a combined precast and after the basic section is formed it requires further, other preparatory processes. The sheathing rests on the flange and web edge and the load state of the patented Z-section is the same as in a standard Z- section, i.e. a ultimate load of the beam is reduced due to a torsion of the Z-section. Whereas, in a standard Z-section torsion results from lateral reactions from the sheathing while in Z-sections with diagonal webs a torsion is provoked by the eccentric of vertical forces. The section is cold-formed from thin sheets, thus finite transversal bending stiffness (contour stiffness) and deflection of the flange is the cause for disadvantageous resting of the sheathing on the flange edge with the web and provokes torsion of the beam. Thickening of the flange without thickening of the web results in the flange remaining very susceptible. The deflection of the flange results from distortion of the web, which is not reinforced in said solution.

In the solution according to Fig. 3, 6, 7, 9 and 1 1, the portion of the flange at the web is not lowered in relation to the portion at the edge stiffening. The flange is flat, but densely and periodically supported by additional fastening members which are conflicting and prevent coupling of precast by support overlaps. The advantage has been achieved (load symmetry) at the expense of the introduction of a very significant disadvantage (lack of possibility for coupling by support overlap). The member is a multi-portion precast and after the basic section is formed it requires further, other preparatory processes. In the solution according to Fig. 3, 6, 7, 9 and 11 a key objective, i.e. load symmetry, is achieved in a technologically complicated manner. The key objective of the solution according to Fig. 3, 6, 7, 9 and 1 1 is to link two features: advantageous rotation of the middle main system and complex reinforcements of the contour, which said two features together provide the load symmetry. Torsion is not generated at orthogonal inclination of the main system and load symmetry of the system. Thus, the torsion is not eliminated. Unfortunately, the advantageous change in load is entirely lost as after the modification the precast is not able to be coupled by overlap and the intended use thereof becomes unclear. It certainly is not suitable for use in typical solutions for structures with Z-sections.

Thus, the solutions presented above significantly vary in relation to the invention presented below mainly by the method to improve section structural parameters, purpose and use of sections, type, kind of structure, geometry or complexity of technological manufacturing process thereof.

The key object of the present invention to develop such a structure of a Z-section that at gravity load it will enable an intended shift of the location of an application plane of the loads transmitted from a sheathing to a flange, from a web edge to the centre of a strip.

The aim of the present invention is also to maintain the ability to interconnect Z-beams using support overlaps, whether the edge stiffening is made as single or double folding.

The key aim for the use of dual-plane flanges and an dilatation slit at a web is an advantageous modification of the method of how a Z-section is being loaded by building roof sheathing interaction therewith. The loads referred to herein are the loads resulting from gravity and loads resulting from pressure sourced from the (roof) sheathing weight, snow, ice or wind pressure. Current Z-sections with single-plane flanges, flat or with additional longitudinal intermediate stiffeners or other, are being bent and twisted by lateral reactions generated in the plane of sheathing. Tension causes additional material stress, which significantly decreases the beam ultimate load capacity and the material is used inefficiently.

The object of the present solution is to advantageously modify construction parameters at gravity loads (pressing onto a sheathing), in relation to current solutions: standard Z- beams and modified versions thereof. The load distribution of the beam at uplift load (pulling off the sheathing in result of wind suction) does not change in relation to current solutions. The construction parameter being changed is a ultimate beam load.

It is an object of the present solution to increase the ultimate beam load capacity (limit state loading), however not by an increase of bending resistance of a cross-section. Currently known modifications of Z-sections aim at increasing the ultimate beam load by indeed an increase of bending resistance of a cross-section.

It is another key object of the present invention to maintain the ability to couple sections by support overlaps, following reversal thereof. In optimal Z-beams, the sections of adjacent spans are interconnected by support overlap, by a simple insertion from above of one section into the other, necessarily transversal in relation to the bar and not along the bar. The Z-beam reaches the highest ultimate load capacity in a multi-span system consisting of single-span precast, interconnected by support overlaps specific for Z- sections. After the components have been assembled, a quasi-continuous multi-span system is achieved, with strengthening of the support areas. All other static schemes of structure with the use of Z-sections are particularly unadvisable. Due to that, an ability for interconnection by support overlap is the most significant feature of the Z-section shape and modification thereof. Lack of ability for interconnection by overlap excludes the reason for a Z-section to be used. To be able to interconnect the members by support overlap, a Z-section should be provided with flanges of varied width and a flat, vertical web. The said web, not only enables interconnection by overlaps but also enables the use of support holds of a simple structure. The solution according to the present invention has been extended by a variant of the embodiment where only one flange is dual-plane and the other is classical. The said variant is by assumption to be used in exceptional cases when it is only possible to use a beam of solely a single-span scheme. It is quite a rare case. The said variant of a cross-section will not be by assumption used in multi-span solutions.

Finally, an objective of the present invention is to maintain the Z-section standard manufacturing methods. The section is cold-formed, similarly to standard sections, and the shape thereof is obtained by folding of a flat sheet into a section. It is possible to form the section using pressing breaks or roll formers. No other or additional method of section forming is needed (hot rolling, welding, soldering, brazing, bonding, coupling by mechanical fasteners, adding other members or portions). Thus, the said type of the section may be defined as„cold-formed”. It is not a„composed” or„combined” element, i.e. prefabricate.

The crucial property of a Z-shaped construction section with strips consisting flanges and edge stiffening thereto is that at least one of the flanges has a dual-plane bend in a form of an off-set of a height D equal 0.5÷20mm measured on the plane of the web, dividing the flange into the lower portions with the width of 0.05÷0.55 of the flange width, located at the web and converging with the web at the angle a = 8 C 20° measured at the inner side of the flange and into a higher portion to support the sheathing.

It is advantageous when the deviation angles of the edge stiffening with a double-fold in relation to the flanges thereof vary in relation to each other.

According to the alternative Z-section construction with strips consisting flanges and edge stiffening thereto, is characterized in that at least one of the flanges thereof has a fold which divides the flange into a diagonal portion of a width equal 0.05÷0.55 of the width of the flange, lowering towards the web and a higher portion to support the sheathing, whereas the angle b of junction of both portions of the flange is 130÷179° and it is measured from the inner side of the flange.

It is advantageous when the deviation angles of the edge stiffening with the doublefold in relation to the flanges thereof vary in relation to each other.

The use of the dual-plane bend of the flanges results in forming an dilatation slit in part of the supporting area of sheathing against the flange, which causes a shift of the plane of transverse compression transmitted from the sheathing onto the section flange from the web edge, outside of the dilatation area, into the off-set edge of the flange.

Z-sections with dual-plane flanges have higher resistance than standard Z-sections at gravity load, without the need to increase the mass thereof. The eccentric created as a result of the dilatation slit, at the junction of the sheathing with the Z-section flange, reduces the influence of torsion and lateral bending of the beam increasing the bending resistance thereof at the load plane.

At gravity load, the beams with dual-plane flanges have an increased resistance in relation to standard beams as a result of nearly entire elimination of lateral bending and torsion of the beam restrained by the sheathing. In the case of standard Z-beams, the influence of said forces result in significant increase of the normal stress distribution of the material due to which the bending resistance of the beam in the load plane decreases. The beam according to the invention is loaded nearly exclusively by bending in the load plane. The material of a Z-beam with dual-plane flanges is entirely used to add resistance and bending stiffness in said plane. In the case of the Z-beams available on the market, the material is used only in approximately 80% for said purpose and the remaining part of the used material is lost due to torsion and lateral bending.

The beams with dual-plane flanges with double-folded edge stiffening of varied angles of deviations in relations to each other may be interconnected by support overlaps, in contrary to standard Z-beams with double edge folds, by a simple insertion of one section into another, transversal in relation to the longitudinal axis thereof, such as in the case of beams with a single fold.

In conclusion, the key feature of the Z-section that is the subject of the present invention are dual-plane and asymmetrical flanges, with the first portion preferably lowered at the web and the second portion advantageously flat and horizontal and advantageously elevated in relation to the first portion, at the edge stiffening. The lowering of the flange portion at the web aims at creating an dilatation slit between the section flange and sheathing, which by intension is to be rested thereon. The dilatation slit in the area of the lowered portions of the flange necessitates an advantageous shift of the vertical forces transmitted from the building sheathing onto the flange, outside of the dilatation slit area, onto the edge of the additional flange fold, in the proximity of the load plane. The elevated portion of the flange at the side of the additional fold in the middle portion takes over reactions from the sheathing and the flat and horizontal portion located at the edge stiffening enables bracing the section against the sheathing at the attempt of rotation, specific for the Z-sections under such types of loads.

In the section being the subject of the present invention, torsion of the Z-section from lateral forces is eliminated. The torque generated by lateral reactions from the sheathing is reduced to zero by the opposite torque resulting from the flange being loaded by vertical forces, at an advantageous eccentric resulting from the dilatation slit. Following elimination of torsion from the beam load state, the beam reaches higher ultimate loading than current Z-sections.

Elimination of torsion is possible after the use of dual-plane flanges exclusively in a Z- section. In the case of a C-section, the torque from horizontal reactions from the sheathing is opposite than in a Z-section. The dual-plane in the C-section would disadvantageously increase torsion, what would directly lead to significant reduction of the ultimate load of the beam. It would also disable bracing of the C-section against the sheathing at rotation. That is why a key feature of the solution being submitted is interconnection of the dualflanges with a Z-section and not with a C-section. An advantageous change is achieved in the beam loaded with external forces, and as a result thereof, an advantageous change in the beam loaded with internal forces. The dual-plane flange of relevant proportions provides reduction to zero of the torque specific for current Z-sections. The beam load state is advantageously reduced to shear bending and the material is effectively used. The above result is achieved despite significant deformation sections bent at transversal bending of the sheet thereof (in other words: distortion; contour deformation). This is the problem of all thin sheet formed sections, also those modified. A flange of a Z-section under load deflects veiy strongly, sometimes several millimetres, and the deformation is the sum of the flange deformation and web deformation. Due to that, current Z-sections with single-flanges are always disadvantageously loaded by the sheathing at the contact edge of the flange and web as only at that point the flange deflection is little and only at that area the sheathing may be supported. Then, the Z-section is under torsion and reaches lower ultimate load capacity.

In the invented solution, there is a dilatation slit at the contact area of the flange with the web. The dilatation slit prevents the sheathing from resting on the section on the web edge, despite the flange being bent. In the solution of the invention, the flange obviously bends as well, whereas said bending is always off-set by properly selected thickness of the dilatation slit (by the difference in the location of two planes of the flange), described in the application. Then, regardless the flange bending, the force is always transmitted at the desired area, at the edge of the additional fold of the flange, and torsion is eliminated. The key objective of the solution is achieved despite the disadvantage of all cold-formed sections, i.e. contour susceptibility, i.e. significant deformation of the section shape under transverse load. The advantageous change in the ultimate load of the beam results directly from the key object having been achieved, i.e. elimination of the beam torsion due to the use of dualplane flanges in a Z-section cross-section and shift of the load interaction line outside of the dilatation slit area, i.e. outside the lowered portion of the flange.

The presented solution increases the ultimate load capacity of the beams both made from a thin and thick sheet. A standard Z-section torsion is a problem that arises regardless the thickness of the section sheet. It results from the Z-shape and single-sided interconnection of the beam with the sheathing. In the section applied for protection, torsion is always eliminated. Thus, said solution is the only modification of a Z-section that improves structural parameters not only formed from thin sheets but also from thick sheets, maintaining standard technologies of section forming.

Improvement of the structure features of the sections from thicker sheets is one of the objects of the present invention. Nowadays, designers and manufactures are increasingly aware that the sections from thicker sheets are more effective than the sections from thin sheets due to higher resistance relatively to the mass thereof. The sections from thicker sheets become more commonly used due to which current modifications that improve resistance only of the thin sheet sections become insufficient.

The subject of the present invention is illustrated in the preferred embodiment in the drawing, where Fig. 1 shows a cross-section of the invented structural section, with two double-plane flanges with a step and single edge stiffening, Fig. 2: a cross-section of invented structural section, with two double-plan flanges with a step and with double- folded edge stiffening of varied angles of deviation in relation to each other, Fig. 3: a construction section in a cross-section, with a dual-plane flange and a standard flange, Fig.4: a construction section in a cross-section, flanges with a fold and with a single-folded edge stiffening, and Fig. 5: a construction section in a cross-section with flanges with a fold and double-folded edge stiffening with varied angles of deviations in relations to each other.

An exemplary Z-shaped construction section shown in Fig. 1 consisting a web 1 with two double-plan flanges 2 and 3 with edge stiffening 4. Each flange 2 and 3 has a dualplane bend in a form of a step 5 dividing thereof into a lower portion 6 whose width bi is 0.46 of the width b of the flange 2, located at the web 1 and converging therewith at the angle a = 95°, and a higher portion 7, such that a load plane 8 is shifted towards a higher portion 7 of the flange, onto the edge of the step 5.

In Fig. 2 it is presented a Z-shaped structural section with the dual-plane bend of the flanges as in Fig 1, whereas the edge stiffening 4 thereto are double-folded 10, and the deviation angles in relation to the flanges thereof vary in relation to each other.

In Fig. 3 it is presented a Z-shaped structural section whose one flange 2 has the dualplane bend in a form of the step 5, and the second flange 9 is standard.

In Fig. 4 it is presented a Z-shaped structural section consisting the web 1 with two flanges 2 and 3 with edge stiffening 4. Each flange 2 and 3 has a bend 1 1 dividing thereof into a diagonal portion 12 being lowered towards the web 1 and the higher portion 7 to support sheathing. The junction angle b of both portions of the flange 2 and 3 is 167 °. In Fig. 5 it is presented a Z-shaped construction section whose flanges 2 and 3 consist the bend 11 dividing thereof into the diagonal portion 12 being lowered towards the web 1 and the higher portion 7 to support the sheathing. The edge stiffening is double-folded and of varied angles of deviation in relation to the strip thereof. The junction angle b of both portions of the flange 2 is 167°.

It is specific for a Z-shaped construction section with a strip consisting flanges and edge stiffening thereof according to the present invention that at least one of flanges (2, 3) has a dual-plane bend in a form of a step of height D equal 0.5÷20mm measured on the plane of a web (1), dividing thereof into a lower portion (6) whose width /bi/ is 0.05÷0.55 of the width /b/ of the flange (2, 3), located at the web (1) and converging therewith at angle a = 80÷120° measured from the inner side of the flange (2, 3) and into a higher portion (7) to support sheathing. The deviation angle of edge stiffening (4) with a double fold (10) in relation to the flanges (2, 3) thereof vary in relation to each other.

According to the alternative construction of a Z-shaped section with a strip consisting flanges and edge stiffening thereof is characterized in that at least one of the flanges (2, 3) has a fold (11) dividing thereof into a diagonal portion (12) of width lb_\l 0.05÷0.55 of the width /b/ of the flange (2, 3), being lowered towards the web 1 and the higher portion (7) to support the sheathing whereas the junction angle b of both members (7,12) of the flange (2, 3) is 130÷179°, measured from the inner side of the flange.

Preferably, the deviation angles of the edge stiffenings (4) with the double-fold (10) in relations to the flanges (2, 3) thereof vary in relation to each other.

It was not an object to change the bending resistance of a cross-section, and particularly it was not to improve the local stability or dilatation slit distortional stability. As an explanation, the local stability is understood as a critical load of a slender, compressed wall that when exceeded it causes flexural buckling of the plate. The loaded wall buckles, thereby does not reach the resistance such as at plastic yielding of the material. The phenomenon that arises in cross-section class 4, according to Eurocode 3, part 1-1 classification. Buckling of an post-critical type. Whereas, dilatation slit distortional stability is understood as critical load of compressed portion of the cross- section which when exceeded will cause flexural transversal deformation of the contour cross-section (distortion). The bent member does not achieve the resistance resulting from yielding of the material. The phenomenon that arises exclusively in cold-formed sections (t<4.0 mm). Buckling of a critical mode.

The thin-walled cold-formed sections are prone to a local instability (cross-section of class 4 according to Eurocode 3 part 1-1 classification ). In the case of Z-sections to be used as roof structural members, made from the thinnest sheets (1.5 mm; 2.0 mm), it is only possible to slightly increase the bending resistance by adding intermediate wall stiffening in a form of longitudinal folded grooves or bends. The intermediate stiffening slightly increases the level of the critical stress of local instability (problem described in Eurocode 3, part 1-5). The same solution for sections made from thicker sheets (>2.0 mm; cross-sections class 3 according to Eurocode 3, part 1-1) does not lead to the increase in resistance due to the fact that the local instability in said cross-sections does not occur below the steel yield stress, thus it is not decisive at cross-section failure. The improvement of the Z-shaped and other cold-formed sections available on the market is mainly provided by strengthening of slender walls with longitudinal folds and it aims at reducing the influence of local instability, if it occurs.

In a Z-shaped construction section, the overall stability of a beam has not be decreased in relation to current solutions (model described in Eurocode 3, part 1-3). Whereas, it was quite significant not to, together with the suggested improvement, in contrary, to decrease rotational restrained stiffness of the beams against sheathing, in comparison with standard Z-sections. The Z-section rotation is restrained against the sheathing on a leverage principle, i.e. pairs of forces: fasteners tensile and flange transverse compression at the edge stiffening. That is why it is important that the Z-section flange be flat and horizontal in a direct proximity of the edge stiffening. A Z-section that is the subject of the present invention has a flat portion of the dual-plane flange at the edge stiffening and may hamper rotation by bracing against the sheathing, according to the description of Eurocode 3, part 1-3 standard.

If the flange, at the edge stiffening, is not flat, then it is unable to restrain rotation against the sheathing. Even though, as a result thereof, the beam does not become unstable, the general stability level thereof is significantly reduced and the beam has a lower ultimate load. Then, it is justified to provide lateral stability to the beam in another way: either bracing bars (anti-sag), or continuous in-plane restraints, e.g. lower sheathing.

Thus, a Z-section enables elimination of the anti-sag bracing bar, commonly used in the case of Z-sections and justified when the torsional participation in the beam load state is particularly disadvantageous (description of the problem according to Eurocode 3, part 1-3 standard). In a Z-section, torsion is eliminated, and thus it is not needed to eliminate the effects thereof, e.g. by using a lateral restraining system.

Claims

1. A structural section of a Z-profile shape with flanges comprising flanges and edge strengthening thereto, characterized in that at least one of flanges (2, 3) thereof has a dualplane bend in a form of an offset (5) of a height D equal 0.5÷20mm measured on a plane of a web (1), dividing thereof into a lower portion (6), whose width Ibj is 0.05÷0.55 of a width Ibl of the flanges (2, 3), located at the web (1) and converging therewith at an angle a = 80÷120° measured from the inner side of the flange (2, 3) and a lower portion (7) to support sheathing.

2. A section according to claim 1, characterized in that a deflection angle of the edge stiffener (4) with a double fold (10) with relation to the flanges (2, 3) are diverse in relation to each other.

3. A structural section of a Z-profile shape with flange comprising flanges and edge stiffener thereof, characterized in that at least one of the flanges (2, 3) has a fold (11) dividing thereof into a diagonal portion (12) of a width /bi/ 0,05÷0,55 of a width Ibl of the flange (2, 3), lowering towards the web III and the upper portion (7) to support sheathing, whereas junction angle b of both portions (7, 12) of the flange /2, 3/ is 130÷179°, measured from the inner side of the flange.

4. A section according to claim 3, characterized in that a deflection angle of the edge stiffener (4) with the double fold (10) in relation to the flanges (2, 3) thereof vary in relation to each other.