FLAT SOFFIT, DOUBLY PRESTRESSED, COMPOSITE, ROOF-CEILING CONSTRUCTION FOR LARGE SPAN INDUSTRIAL BUILDINGS

Abstract: The roof-ceiling construction comprises a wide and thin concrete plate (1) and a two-part upper steel construction (2), interconnected by means of vertical elements (3). The construction is twice prestressed by two independent methods. The concrete plate (1) is centrally prestressed in the mould (86) and after the plate (1) concrete has hardened, the upper steel construction (2) is prestressed by pushing apart, at the midspan, the steel separated halves (2) which are then connected. Prestressing of the concrete plate (1) is applied to eliminate or reduce cracks in its concrete while prestressing of the upper construction by pushing apart the steel halves (2) is used to control the deflections.

Description: FLAT SOFFIT, DOUBLY PRESTRESSED, COMPOSITE, ROOF-CEILING CONSTRUCTION FOR LARGE SPAN INDUSTRIAL BUILDINGS TECHNICAL FIELD According to the international patent classification, the present invention relates to the field signed by E04B1/00 that generally relates to constructions and to building elements E04C3/00 or more particulary to the group E04C3/00 and 3/294. TECHNICAL PROBLEM The double prestressed, composite, roof-ceiling constructions with flat-soffit ceilings are plane- space bearing pre-fabricated elements for constructing industrial large-span buildings that solve several partial technical problems intending to achieve following: to construct the flat-soffit in large-span buildings eliminating generally an unaesthethic view to the roof construction from the interior of the building, eliminating the unuseful space between sloping roof girders and reducing the unnecessary heated volume of the interior, to form naturally ventilated space between ceiling and roof that saves the heating energy and enables instalations to be guided unvisibly through the shallow loft space, to solve the safety of works on height and to increase the speed of large-span roofs-ceilings constructing by use of large-panel but relative light elements. The solution of above mentioned technical problems is focused to the solution of the constructive technical problem to ensure bearing cappability, the proper serviceability characteristics and durability of the construction preventing too large deflections and width of cracks of the slender soffit concrete plate. The use of the ordinary reinforced-concrete soffit-plate would reduce the span of these slender constructions and would make the long-term servieability characteristics of the construction to become unreliable. Too large deflections of the reinforced concrete soffit-plate could be decreased by applying stiffer upper construction or to be compensated by the counter-deflection in form but that would be only uneconomical and unreliable manner to reduce deflections whereby the problem of cracks would remine unsolved. The reinforced-concrete soffit-plate applied to a large span undergoes a great amount of tension that causes cracks and their progress due to concrete creep and schrinkage whereby the magnitude of deflection increases interactively as the witdh of cracks increase. The initial cracs in soffit-plate due to combination of the large tension axial force and a small-amount local bending moments concentrated locally at points where the upper construction is connected to the soffit plate, growing wider in time, instead to distribute along the whole length of the soffit- plate, what would be more desired in reinforced concrete behavior. The problem is therefore focused to the proper prestressing method that can reliably and durable counteract the large deflection and eliminate or reduce concrete cracking in the high- tensioned soffit plate, the prestressing method that causes the upward deflection of the concrete soffit-plate and introduces the compression force in it. This problem can not be solved by the customary concrete-prestressing method because of the specificity of these constructions whereby the centric prestressing force applied to the soffit- plate gravity center because of its small eccentricity to the gravity center of oweral cross-section can only influe cracks in soffit-plate and practically does not influe deflections. The usual prestressing techniques introduce the compressive force into a beam or a concrete- truss construction below the concrete cross-section gravity center that due to specific geometry causes upward deflection of the element solving simultaneously the problem of deflections and the problem of concrete cracking. The specific composite, roof-ceiling, flat-soffit construction, becouse its oweral cross-section gravity center is placed at negliglibly small eccentricity from the soffit-plate can not be prestressed by the usual prestressing method introducing the compressive force into concrete body to obtain the the counter-deflection of the soffit plate upwards and to close its cracks simultaneously. Introducing of such a prestressing force at the eccentricity below the cross-section gravity center would require positioning of the tendon gravity center below the soffit-plate level that would ruin the flat soffit. The apply of centric prestressing that would introduce compressive force into the soffit-plate gravity center because of the small eccentricity influe only cracks but it does not influe deflections at all. The additional technical problem at large spans is stabilisating upper slender construction against lateral buckling ower the entire its length that can cause its instability and colapse of entire construction. BACKGROUND OF THE ART The present invention concerns to specific composite, roof-ceiling constructions whereby no simillar solution I know. All the adventages given by the present inovation are enabled owing to solution of the prestressing method that makes them aplicable to large spans suitable for constructing of industrial buildings. All custom concrete-prestressing methods are adapted to concrete specificities with adapted cross-section shapes whereby indroducing of the prestressing force in lower zone of the beams, trusses or plates, due to compressive force acting on eccentricity below the gravity center of the cross section problem of deflections and cracks is solved simultaneously. Several ways of prestressing are custom in constructing steel buildings whereby some elements of trusses are forced mechanicaly or thermaly to introduce prestressing effects. Above mentioned prestressing methods are well known and are applied to one-material constructions, adapted thereby to its specific characteristics. These constructions, because of their specificities that they have as composite, made of concrete and steel parts, can not be compared, under the criterion of prestressing effects, to usual ones whereby several technical solutions are applied in the same sense, to introduce the prestressing force below the gravity center of the cross-section. DISCLOSURE OF THE INVENTION The present inovation solves prestressing of specific, composite, roof-ceiling, flat-soffit constructions for constructing industrial large-span buildings with some advantages such as: The presence of the flat-soffit in large-span buildings eliminates generally an unaesthethic view to the roof construction from the interior of the building, these constructions, except generally used for hard industries and warehouses, become suitable for fine industries, shops and likely. Pre-fabricated soffit is finished and need not additional work in site. Eliminated unuseful space between sloping roof girders reduces the heated volume of the interior and saves the heating energy. The naturally ventilated loft that is simply thermo insulated by rollig balls improves the insulation of the roof whereby it is enabled all instalations to be guided invisibly through the shallow loft space, with ensured acces for their maintenance instead of being usually guided visible across the walls and other interior parts. The safety of works on height during assembly, roof covering works is improved because all the works are carried out on the flat surface of soffit plates whereby working in the natural, standing position is enabled. Use of the plate-like, large-panel elements that cover the big portion of the roof at once has many advantages compared to many custom constructing methods where primary and secondary girders are used. To achieve above mentioned advantages of these constructions at large spans the problem is focused to the constructive technical solution how to ensure bearing cappability, the proper serviceability characteristics and durability of the construction. The problem is solved by double prestressing by by the combination of two undependent prestressing methods whereby one reduces deflections of the concrete soffit-plate of the construction and the other one eliminates or reduces its cracks due to high tension. For better understanding of the technical problem that is solved by this invention, on the simplefyed model shown in Fig 1 and Fig 2 the custom prestressing method is compared to presstressing applied to composite flat-soffit roof-ceiling constructions. By usual methods of prestressing beams or trusses as shown on Fig 1 the compression force (Po) is introduced below the gravity center of the concrete gravity center (T), at eccentricity (e), in the tension zone or out of it, pushing the beam ends towards the middspan whereby produces the negative bending moment (M=e x Po) that causes upward beam deflection (u). By such a prestressing the the upward deflection reduces the downward deflection of applied external load whereby simultaneously, the applied compressive force (Nt) closes cracs in tension zone of the beam. This method is not applicable to specific, composite, roof-ceiling constructions which comprise the wide soffit-plate with low positioned gravity center of the overal cross section. The application of the weighty concrete soffit plate for lower part of the construction with lighty upper steel part seems to be unlogical because steel that often has stability problems undergoes high compression and concrete that can bear only slight amount of tension is exposed to the considerable tension. Nevertheles, this choice is the price that must be paid for achieving the flat soffit and its advantages. Becouse of such load-bearing unlogical choice this prestressing will require more expences then usual prestressing of concrete. Introducing of the prestressing force (Po) below the gravity center of the cross-section would require descending of the tendon below the soffit plate that would ruin the flatt soffit effect. The prestressing principle of the present invention shown in Fig 2 presents a kind of inversion to the usual one. The upward-deflection (u) effect is obtained by pushing the upper construction separated in the middle, from middle span towards its ends whereby the compressive prestressing force (Po) acts at the eccentricity (e) over the concrete gravity center of the cross-section (T). In both compared methods, the negative bending moment (M=e x Po) was achieved that produces the upward deflection (u) of the soffit plate. But since by usual prestressing the applied desirable compressive force (Nt) is introduced in the soffit plate, in other case, by pushing the upper construction towards its ends, the undesirable tension force (Nv) was introduced that must be reduced or eliminated by an additional prestressing and this is the price to be paid to achie the flat soffit. Fig 3 shows at the same model this second, additional, centric prestressing that introduce the compression force (Nt1) into the soffit-plate by which eliminates tension, due to both external load and first prestressing, shown at Fig 2. This second prestressing produces no bending moments because it acts on the negligible eccentricity from concrete gravity center and does not match the deflections achieved by prior prestressing. Thus, the technical problem of controlling cracks and deflections in the construction is solved by two independent prestressing methods. On the real model, on Fig 4, the practical execution both prestressing methods is illustrated. The upper steel construction comprises two symmetrical, in the middle of the span disconnected halves (2) and vertical connecting elements (3). At the break point in the middle span, there is the detail with vertical wedge by which the upper construction is presstresed and then interconnected. Both halves of upper construction are first positioned to the form (6) for casting the soffit plate. The steel tendons are prestressed at the mould (4), being previously conducted through holes (5) at the ends of bars (3) to connect steel parts (3) to the concrete soffit plate (1) and the plate (1) is then concreted. After the concrete is hardened the prestressed tendons are released from the form (6) so the soffit plate becomes subjected to the compressive force. The construction is now prestressed by the first step. The upper construction (2) is now incorporated to the concrete soffit plate (1). The concrete plate is now under the compressive stresses, as shown on Fig 1 , but the soffit plate doesn't undergo upward deflection. Now the additional prestressing is to be applied, by the principle shown in Fig 2. At the interrupt of the upper construction (2), the steel wedge (7) is positioned into the connecting channels incorporated in both ends of the separated parts and the driving device (8) that pushes the wedge is prepared. Driving the steel wedge inside of the detail (7) causes both separated parts of upper construction (2) to push towards ends of the soffit plate (1) introducing the tension force in it, but the soffit plate is already subjected to previous compression due to first prestressing. The compressive force introduced by the first prestressing must be of such an amount that after subtraction of the tension due to second prestressing still remains the sufficient compression reserve whereby after subtracting the tension due to applied external load in concrete soffit plate remains tension below the allowed limit or is eliminated to zero. DESCRIPTION OF DRAWINGS Fig 1 ilustrates on the simplefied modell the principle of the usual prestressing method by introducing compressive prestressing force below the cross-section gravity center and shows developed internal forces. Fig 2 ilustrates on the simplefied modell the principle of the prestressing method by introducing compressive prestressing force by pushing appart of the upper construction, above the the cross-section gravity center and shows developed internal forces. Fig 3 ilustrates on the simplefied modell additional centric prestressing into construction soffit plate and shows developed internal forces. Fig 4 is the lateral view of a real model showing necessary to ilustrate prestressing methods and the constitutional parts. Fig 5 is the cross-section of the construction with its constitutive parts. Fig 6 is the detail of the disconected upper construction where the prestressing force is applied. Fig 7 presents the manner how the upper construction is prevented against buckling. DESCRIPTION OF THE PREFERRED EMBODIMENT The upper steel construction (2), separated at middle span simmetrically at two equal parts, is placed to the mould (6) for concreting the soffit plate (1) to stand on vertical element (3). The steel tendons are prestressed at the mould (4), being previously conducted through holes (5) at the ends of bars (3) and the soffit plate (1) is then concreted. After concrete hardening, fastened by the steam curing process, tendons (4) are released from the mould (6). Thus, the first prestressing step is over. At the interrupt of the steel construction (2) into the prepared detail, that lesses the stress concentration, the steel wedge (7) is positioned and the driving device (8) that pushes the wedge is prepared. Driving the wedge inside of the detail (7), both separated parts of upper construction (2) are prestressed whereby the introduced force is controlled by measuring upward deflection of the soffit plate (1) at the middle span and measuring the wedge driving force by manometer pressure on the driving device (8). From results of these two measures, the introduced force can be calculated reliably. The double prestressed, composite, roof-ceiling constructions with flat-soffit are intended for constructing large-span industrial buildings and similar large span buildings. Due to their specific solutions there are many advantages when compared to some custom constructing systems such as: the plate-like, large elements solve at once both roof and the ceiling with finished soffit. An aesthethic soffit closes the unuseful space between sloping roof girders and reduces the heated volume of the interior that saves the heating energy. The naturally ventilated space between ceiling and roof is formed that enables all kinds of installations to be guided invisibly through the shallow loft space, instead of being guided through the interferes interior of the building and is more expencive. Use of the plate-like, large-panel elements that cover the big portion of the roof at once has many advantages compared to many custom constructing methods where primary and secondary girders are used. An aesthethic soffit closes the unuseful space between sloping roof girders and reduces the heated volume of the interior that saves the heating energy. The safety of works on height during constructing is ensured after the soffit plates are assembled whereby the thermo insulation can be placed on the wide flat plane, working in stending position is enabled without need to climbe the girders. The low costs of these constructions is due to fact that the roof-ceiling plates that comprise finally finished soffit are the bearing construction simultaneously, with low material spend. The prestressing pushing-apart method is cheep, the large-panel roof-ceiling construction that is quickly assembled covers big portion of the roof at once and the surface to volume ratio of thise elements is suitable for quick concrete hardening by steam that enables rapid production. Due to above mentioned adventages of the flat sofitt on which an arbitrary deep thermoinsulation can be placed closed to the shallow, naturally ventilated loft space these constructions are suitable for buildings with fine, climatized interiors such as fine industries, big markets, sport and similar buildings.

Claims: CLAIMS 1. The double prestressed, composite, roof-ceiling construction with flat-soffit construction for constructing industrial large-span buildings characterized in that comprises distinkted wide and thin, finihed concrete plate (1) and two-part upper steel construction (2), sloped or arch shaped, connected to soffit-plate (1) by vertical elements (3), that is prestressed centric, by adhesion prestressing on mould (6) whereby the upper steel construction (2) is prestressed by pushing-apart with the wedge (7) in the middle span and separated steel parts are then connected. 2. The prestressed, composite, roof-ceiling construction with flat soffit as claimed in claim 1 characterized in that the connection between concrete plate (1) and the steel construction is realised by incorporated to concrete vertical elements (3) whereby through holes (5) at bottom ends of vertical elements (3) tendons (4) were conducted serving the same time to hold reinforcing welded meshes at the mould-distance during concreting. 3. The prestressed, composite, roof-ceiling construction with flat soffit as claimed in claim 1 , characterized in that is prestressed by two independent methods whereby the deflection of the concrete soffit plate (1) is controled by prestressing the upper beam (2) and the wide of cracks in concrete soffit-plate (1) is controled by the centrical prestressing. 4. The prestressed, composite, roof-ceiling construction with flat soffit as claimed in claim 1 , characterized in that the upper beam (2) is prevented against buckling by lateral elements (9) being anchored in concrete of the soffit-plate (1). 5. The prestressed, composite, roof-ceiling construction with flat soffit as claimed in claim 1 , characterized in that the prestressing force (Po) that is introduced to the construction by pushing-apart, according to Fig.2, acts overthe gravity center of the overal cross-section (T) of the composite construction at the eccentricity (e).

Inventors: SKENDZIC MILOVAN, SMRCEK BRANKO, SKENDZIC, MILOVAN, Smrcek, Branko
Applicants: MARA D O O TVORNICA KUCA I HAL, SKENDZIC MILOVAN, SMRCEK BRANKO, MARA D.O.O., TVORNICA KUCA I HALA, SKENDZIC, MILOVAN, Smrcek, Branko
Application year: 2001
Main classification: E04C3
IPC codes: E04B1, E04B7, E04C3