WO2003074810A1

WO2003074810A1 - Building beam

Info

Publication number: WO2003074810A1
Application number: PCT/SE2002/000367
Authority: WO
Inventors: Nils-Gustav Svensson
Original assignee: Nils-Gustav Svensson
Priority date: 2002-03-01
Filing date: 2002-03-01
Publication date: 2003-09-12
Also published as: AU2002235076A1

Abstract

The beam is intended for use in framed building constructions as a support for floor structures of prefabricated concrete. During the erection of the frame, the beam is self-supporting and when incorporated in the final construction, interacts with the floor and can thereby support heavy loads. The beam can be used in the construction of office buildings, apartment houses and industrial buildings. It has a large load-bearing capacity and excellent fire-resistance characteristics.

Description

BUILDING BEAM

Range of application, conventional technology

The structure of an office building, industrial building or apartment house consists normally of concrete poured in situ, or of a prefabricated frame or wall slabs of concrete or steel or in certain cases of timber.

The beams/columns and/or wall slabs support in turn a floor construction. The most usual type of floor construction is of concrete poured in situ, hollow-core concrete planks, Filigran floor planks ( formwork planks of thin, reinforced concrete), timber joists or concrete beams with a concrete topping. The different floor constructions are suitable for different spans and satisfy different economic requirements.

When building with prefabricated concrete planks, a bearing structure of steel columns and beams or of concrete columns and beams or a combination of these is often used. The beams can be constructed in different ways, the most usual are described below.

Welded steel beams of many different types have been used increasingly during recent years. These make possible floors with a low overall construction height and are of many types. The most usual is a beam with a wide bottom flange. Two parallel vertical web plates are welded to this. A top flange is welded between the tops of the webs. Vertical end plates are welded to the ends of the beam to accommodate fixings to the columns and to stiffen the beam against vertical loading forces. The projections of the bottom flange provide support for the floor construction. There are different versions of this type of beam. The web may be in the form of an inverted channel of bent steel plate to which the top flange is welded. Holes may be formed in the web plates to permit poured concrete to fill the cavity in the beam to obtain joint structural action of the steel and concrete and resistance to fire. The web can also be provided with longitudinal stiffeners to permit the use of thinner steel plate. These beams are variations of the same type of welded beam consisting of a wide bottom flange, two vertical webs and a top flange. The simplest version of a beam is the standard rolled steel joist (I-beam) and different versions of this. Welded beams mentioned above are manufactured by specialist firms and always in plants with special welding equipment. This consists of machines for gas-cutting the bottom flange, the web plates and the top flange to the required shape. As an alternative, these elements can be purchased directly from a steel mill or a steel merchant. When cut to the required width and length, the steel is welded together at an advanced stationary welding machine which can often weld the bottom and top flanges to the webs in one continuous operation. To obtain a good result, it is important that the webs and flanges are located accurately in the welding machine before welding begins. This requires great care and takes a relatively long time.

After the beam is welded together, it may require straightening because of distortion caused by the high temperatures of the welding. This is often required to obtain the required straightness, or the beam must be bent to obtain a degree of upward deflection. The production methods vary from firm to firm but are the same in principle. An advanced welding machine, considerable peripheral equipment and suitably large fabrication halls are required. After the endplates are welded to the beams, the beams are delivered to the construction site for installation on the columns already in place. When the beams are in place, the floor construction elements can be installed. The spaces between the elements and the beams are then filled with concrete. In certain cases, the beams can also be filled with concrete to obtain joint action of the beam and the concrete and for improved resistance to fire.

The manufacture of welded beams of this type is time and labour consuming and they are relatively expensive to purchase. Production is only possible by specialist firms which limits the degree of competition. More than 50% of such beams used in Sweden are imported.

The purpose of the present invention and its most important characteristics

The purpose of the invention is to provide an alternative to the technology described above which is an improvement with respect to manufacturing methods and is, as a result, more economically competitive.

In the invention, the form of the supporting beam is a further development to improve the technology and the final product. The invention has a much simplified and improved fabrication technology and an increased structural interaction between the supporting beam and the floor it supports. Descriptions of the beam, how it is fabricated, how it is used and how it functions are given in the following.

The fabrication of the beam requires a relatively simple plant and equipment. In principle, it can be manufactured in any workshop of a suitable size with a paved floor suitable for truck traffic, normal heating and electric power and an overhead travelling crane. This type of premises can be expected to be readily available at a low rental, requiring only minor further investment. As the beams can be manufactured locally, transport costs to the building site will be minimized.

The beam is based on a bottom flange of standard steel plate which can be ordered from steel merchants, with the dimensions required and with short delivery times which are reflected in short delivery times of the final product. The bottom flange can be obtained sand-blasted and prime-coated on delivery from the steel merchant, ready for the final painting of the finished product. The bottom flange can have plain or deformed checker-plate surfaces which increase the structural interaction between the steel and the concrete.

So-called welded bolts are welded in longitudinal rows to the upper surface of the bottom flange, using a special welding pistol with an associated mobile power unit. The pistol is loaded with a bolt with a ceramic ring at the bottom. The welding unit is charged to give a high amperage current which generates very high temperatures during the 5 seconds it takes to weld the bolt to the bottom flange. Large numbers of bolts can be welded in place very quickly using this technique, which is well-proven in bridge-building projects.

Longitudinalo rows of bolts are welded to the bottom flange which is supported on a horizontal level bed or a bed with a certain degree of upward curvature to give a finished beam with the required longitudinal profile. A prefabricated form of suitable height and width is fixed in place around the welded bolts and filled with concrete of high quality, mixed at the beam factory or purchased ready-mixed.

The Swedish concrete industry has improved its products over recent years in response to an extensive programme of bridge building. Concrete with very high compressive strength has been developed and research in this field continues. It is intended that the concrete to be poured around the welded bolts should be of this high compressive strength type. When the concrete formed to the required width and depth hardens, the concrete, the welded bolts and, via its deformed upper surface, the bottom flange itself, interact structurally. During the pouring of the concrete, pressure plates are embedded in the concrete at the ends of the beam and, in the case of a beam continuous over a mid-support, at its midpoint. Short lengths of box section tubing are welded to the pressure plates to provide fixing guides for columns to be welded in place in. The column loads are transmitted through the pressure plates to the concrete of the beam this making unnecessary the stiffening plates normally required here in comparable traditional steel beams. Longitudinal steel strips are embedded in the concrete between the pressure plates for fixing concrete elements to prevent structural collapse.

The concrete is hardened for a suitable time under controlled conditions and when the beam is incorporated in a building structure, mounted on its columns and loaded with the floor it supports, the upper zone of the high compressive strength concrete of the beam is subjected to compressive forces which are transmitted diagonally downward in the concrete and vertically in the welded bolts.

The bolts transmit shearing forces between the concrete and the bottom flange, thereby contributing to the interaction of the concrete and the flange. The bolts also function as anchorages in the concrete for vertical forces when the bottom flange is loaded with the floor construction elements. The anchorage around the head of the welded bolts is particularly effective because the heads of the welded bolts are encased in the compression zone of the beam. The number of bolts required to be welded to the bottom flange depends on the span of the beam and the dead and live loads.

The concrete incorporated in the beam also has a fire-resisting function. The concrete functions as a heat-sink if the beam is exposed to fire and the bottom flange needs only supplementary fire protection which can be site-applied. There is no need for concrete to be applied at the site which saves time. The form of the beam can also be varied. For long span or heavily loaded beams, the welded bolts can be of such a length that they project above the top surface of the concrete poured during manufacture. When concrete planks supported on the bottom flange are used as floor elements, a concrete topping is often laid as a finish, this then encasing the projecting bolts. Considerable compressive forces can then be distributed to the topping concrete and transmitted down to the tension zone of the beam in the bottom flange via the welded bolts and the high compression strength concrete in the beam. This gives a particularly strong and economically competitive construction.

Concrete floor planks can also be supported on the top of the beam with a topping encasing the projecting welded bolts. The interaction of the topping and the beam is then of the same nature as that described above.

The design principle of the beam ensures a powerful tensile zone with the bottom flange dimensioned for the design load. The same tensile capacity cannot be achieved so easily with traditional concrete reinforcement because of the coverage requirement. In addition, the absolute maximum static design height is obtained with an external web plate reinforcement.

If the compressive forces are too large during the installation of the floor elements, a suitable number of longitudinal reinforcing rods can be located at the level of the upper part of the welded bolts. When the concrete encases the welded bolts, the reinforcement interacts with the concrete in resisting compressive forces.

It should be noted that the beam is particularly simple to fabricate and has the same function as traditional supporting beams. Its manufacture is possible in any suitable premises close to a supply of concrete, or, in extreme cases, even outdoors. Because of this, the beam is suitable for use in the construction of apartment houses and office buildings in areas subject to earthquakes as it is simple to manufacture with local labour. The costs of manufacturing the beam are a fraction of those of traditional steel beams. The equipment required is simple and many medium size contracting firms with experience with concrete can perform the work. This means that the fabrication of the beam can be the subject of keen competition and lower prices.

Illustrations

Figure 1 A perspective drawing of the bottom flange with welded bolts.

Figure 2 A perspective drawing of part of the beam with concrete encasing the welded bolts and the pressure plates for columns.

Figure 3 A cross section of the beam with embedded longitudinal reinforcement.

Figure 4 A cross section of the beam with projecting welded bolts for joint action of beam and floor construction.

Figur 5 A cross section of the beam with superimposed floor construction

Description of beam

Figure 1 shows the bottom flange of a beam, of plain steel plate (1) or of checker-plate or similar with a deformed surface (12) to which are welded bolts (2). The number required and the spacing of the bolts depends on the span and the loading of the beam. Concrete is poured around the welded bolts as shown in Figure 2. Pressure plates (10) with column guides (17) to ensure accurate location of columns (11) are embedded in the concrete during the pouring. Longitudinal steel plates (16) can also be embedded in the concrete. When the concrete has hardened sufficiently, the beam is mounted on columns and welded in position.

Figure 3 shows how floor elements (5) are supported on the bottom flange (3) of the beam. The design of the beam can be varied to satisfy different requirements. For increased resistance to compressive loads during the erection of the building, longitudinal reinforcement rods (14) can be embedded in the concrete (4).

Figure 4 shows how welded bolts (2) can project above the top surface of the poured concrete (4). When floor elements are supported on the bottom flange of the beam, a concrete topping (7) is poured on the elements (6). This concrete (7), interacting with the welded bolts (2) and the transverse reinforcement constitutes an extremely effective compressive zone in the floor construction. This gives a very strong composite construction.

Figure 5 shows a beam with a superimposed floor construction. The welded bolts project above the top surface of the beam between the edges of the floor elements or the forms (13). Concrete (8) is then poured on the floor elements or forms (13), encasing the projecting bolts. The topping then contributes to the strength of the compressive zone of the floor construction.

The following suggestions with respect to materials and dimensions should not be considered to define specific limits. The bottom flange can be of plain steel plate (1) or plate with a deformed active surface (12). The steel can be of normal or high-strength steel. The width of the bottom flange can vary between 200 and 700 mm and its thickness between 8 and 40 mm. The welded bolts (2) can be of diameter 5 to 22 mm and their length from 50 to 600 mm. The concrete (4) can be of width 100 to 700 mm with a height from 100 to 600 mm and its quality can be from K40 to K80. The embedded longitudinal reinforcing rods can have diameters from 12 to 25 mm. Their number is only restricted by the relevant codes for concrete construction. The welded bolts can be located in 2 to 6 longitudinal rows. The beam can vary in length between 4 and 15 metres and its width between 300 and 900 mm.

Claims

1. A beam of steel and concrete for building structures with, in principle, a steel bottom flange (1) anchored in the poured concrete (4) via a suitable number of longitudinal rows of vertical bolts (2) of varying height and number, welded to the bottom flange to constitute a beam with a projecting bottom flange providing support for floor constructions (5) and (6), characterized by the finished beam forming a strong and stiff construction with limited deflection by the concrete (4) in the compressive zone of the beam

interacting with the bottom flange (1) by a static interaction between the bolts welded to the flange whereby diagonal compressive forces develop through the concrete (4) and shear forces between the welded bolts (2) and the steel bottom flange (1) and vertical compressive forces through the welded bolts. (2).

2. The beam as claimed above is characterized by the poured concrete being of high-compressive strength type.

3. The beam as claimed above is characterized by the welded bolts (2) encased in the concrete (4) and welded to the bottom flange (2) constituting anchorages for floor construction elements supported on the bottom flange (1)

4. The beam as claimed above is characterized by the compression zone of the beam during erection being optionally strengthened with longitudinal reinforcement rods (14).

5. The beam as claimed above is characterized by the compression zone of the beam being strengthened by the concrete topping (7) or (8) acting in combination with the concrete of the beam (4) when loaded. The beam as claimed above is characterized by the pressure plates (10) and (15) embedded in the concrete (4) ensuring accurate location of columns (11) during the erection phase.

The beam as claimed above is characterized by it being possible to provide the beam with a straight bottom flange or a bottom flange with a certain upward deflection.

The beam as claimed above is characterized by the possibility of using it to support floor elements (5) or (6) and a concrete topping on the bottom flange (3) or on the top of the concrete (4).

The beam as claimed above is characterized by compressive zone (4) interacting with a steel plate bottom flange (1 )