Classifications

• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04B—GENERAL BUILDING CONSTRUCTIONS; WALLS, e.g. PARTITIONS; ROOFS; FLOORS; CEILINGS; INSULATION OR OTHER PROTECTION OF BUILDINGS
  • E04B5/00—Floors; Floor construction with regard to insulation; Connections specially adapted therefor
  • E04B5/02—Load-carrying floor structures formed substantially of prefabricated units
  • E04B5/04—Load-carrying floor structures formed substantially of prefabricated units with beams or slabs of concrete or other stone-like material, e.g. asbestos cement
  • E04B5/043—Load-carrying floor structures formed substantially of prefabricated units with beams or slabs of concrete or other stone-like material, e.g. asbestos cement having elongated hollow cores
• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04B—GENERAL BUILDING CONSTRUCTIONS; WALLS, e.g. PARTITIONS; ROOFS; FLOORS; CEILINGS; INSULATION OR OTHER PROTECTION OF BUILDINGS
  • E04B5/00—Floors; Floor construction with regard to insulation; Connections specially adapted therefor
  • E04B5/16—Load-carrying floor structures wholly or partly cast or similarly formed in situ
  • E04B5/17—Floor structures partly formed in situ
  • E04B5/23—Floor structures partly formed in situ with stiffening ribs or other beam-like formations wholly or partly prefabricated
  • E04B5/26—Floor structures partly formed in situ with stiffening ribs or other beam-like formations wholly or partly prefabricated with filling members between the beams
  • E04B5/261—Monolithic filling members
  • E04B5/265—Monolithic filling members with one or more hollow cores
• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04B—GENERAL BUILDING CONSTRUCTIONS; WALLS, e.g. PARTITIONS; ROOFS; FLOORS; CEILINGS; INSULATION OR OTHER PROTECTION OF BUILDINGS
  • E04B5/00—Floors; Floor construction with regard to insulation; Connections specially adapted therefor
  • E04B5/16—Load-carrying floor structures wholly or partly cast or similarly formed in situ
  • E04B5/32—Floor structures wholly cast in situ with or without form units or reinforcements
  • E04B5/36—Floor structures wholly cast in situ with or without form units or reinforcements with form units as part of the floor
  • E04B5/38—Floor structures wholly cast in situ with or without form units or reinforcements with form units as part of the floor with slab-shaped form units acting simultaneously as reinforcement; Form slabs with reinforcements extending laterally outside the element
• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
  • E04C2/00—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels
  • E04C2/02—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by specified materials
  • E04C2/04—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by specified materials of concrete or other stone-like material; of asbestos cement; of cement and other mineral fibres
  • E04C2/06—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by specified materials of concrete or other stone-like material; of asbestos cement; of cement and other mineral fibres reinforced

Definitions

• the invention refers to a hollow core slab, defined as a precast slab of prestressed concrete, including, in span direction, a plurality of mainly parallel voids and concrete webs between said voids.
• Hollow core slabs have been used widely throughout the past three decades for flooring, roofing and occasionally for walls. They have been applied in buildings like hotels, offices, cinemas, car parks and shopping centres.
• Hollow core slabs are used in favour of conventional reinforced concrete slabs because their weight is significantly lower and considerably larger spans can be achieved. This results in significant overall savings in construction costs, as the load bearing structure can be constructed lighter. This again leads to reduced dimensions of the foundations. Another advantage of hollow core slabs with respect to conventional reinforced concrete slabs is that the construction time is shorter.
• Hollow core slabs may be precast in a production plant according to standards set for the industry.
• the European standards for hollow core slabs are established by the European Committee for Standardization (CEN) as standard EN 1168 (Precast concrete products - Hollow core slabs).
• CEN European Committee for Standardization
• standard hollow core slabs are monolithic prestressed or reinforced elements with a constant overall depth divided into an upper and a lower flange, linked by vertical webs, so constituting cores as longitudinal voids the cross section of which is constant and presents one vertical symmetrical axis.
• Prestressed hollow core slabs may comprise prestressing steel in the form of prestressing wires up to a maximum of 11 mm in diameter or prestressing strands up to a maximum of 16 mm in diameter, according to the EN 1168 standard.
• prestressing wires or strands may be put under tension over production beds that can be a hundred meters or more in length. Tension may be on the order of 1100 N/mm 2 .
• an extruder is used to extrude concrete from a moving mould to form the hollow core slabs, incorporating the prestressing wires or strands.
• a so called slipformer pours concrete in several stages to form a hollow core slab.
• Other production processes are possible.
• the external tension on the wires or strands may be released and hollow core slabs of the desired lengths are cut from the long slab formed. The external tension from the prestressing wires or strands is at least partly transferred to the individual concrete slabs causing a stress in the concrete slabs.
• the hollow core slabs can bridge lengths of 15 m or more, depending on the specifications.
• the residual tension in the wires or strands and the resulting stress in a prestressed hollow core slab may be at least 100 N/mm 2 , but may be 900 N/mm 2 or more.
• the prestressing may cause the resulting hollow core slab to have a slight curvature.
• the upper flange may be convex
• the bottom flange may be concave.
• Prestressed hollow core slabs are produced in certain thicknesses, typically between 100 and 500 mm, and standard widths. Typical widths for hollow core slabs are 1200 and 2400 mm, but other widths may also be produced. The cores in conventional hollow core slabs are evenly distributed in the width direction of the slab, so that inner webs of equal width are formed. Factory production provides the advantages of reduced time, labor and training.
• a floor In order to construct a floor, concrete is poured in the joints between the hollow core slabs, which can be positioned beside and/or behind each other, and are supported by a load bearing structure.
• the joints are often filled with concrete that has a lower modulus of elasticity than the concrete that is used for the hollow core slabs.
• Hollow core slabs may be strengthened by additional concrete that is poured in the hollow cores or against one or more of the external surfaces.
• a concrete compression layer may e.g. be poured in-situ on top of the hollow core slab. This layer may contain reinforcement.
• special connection means for example anchors, may be used to connect the hollow core slab with the supporting or surrounding load bearing structure. These anchors are often partly incorporated in the concrete that is poured in the hollow cores.
• FIG. 1 A typical cross-section of a hollow core slab is presented in figure 1.
• the prestressing strands are represented by the black dots.
• the orientation of the concrete webs between the continuous voids of existing concrete hollow core slabs is mainly vertical. Only the orientation of the two outer webs is not mainly vertical because space is needed for a product specific clamp to lift the slabs and to facilitate in-situ making of a joint between the slabs.
• the shape of the voids of existing concrete hollow core slabs is for example circular, rectangular or elliptical.
• the slabs are significantly rounded off at their bottom corners. At least one of the reasons for doing so is to improve the visual appearance of a floor made up of hollow core slabs in which the vertical deformation of at least one of the slabs is larger than the deformation of the other slabs.
• Figure 1 is only illustrative.
• a precast prestressed hollow core slab is meant to comprise at least a monolithic prestressed element divided into an upper and a lower flange, linked by vertical webs, so
• cores constituting cores as longitudinal voids.
• the hollow core slabs are installed as a floor in (mainly) horizontal position, i.e. the top flange and the bottom flange will extend mainly
• the bottom flange will be the “hot flange” and the top flange will be the “cold flange”.
• the "hot flange” may be formed by one of both sides and the “cold flange” by the other side.
• the notions "hot flange” and “bottom flange” will be considered, in this patent application, as having the same meaning and implication and those notions are mutually interchangeable. The same applies, mutatis mutandis, for the notions "cold flange” and "top flange”.
• a novel hollow core slab having improved fire resistance which is achieved by providing the hollow core slab, i.e. the precast slab of prestressed concrete, including, in span direction, a plurality of mainly parallel voids and concrete webs between said voids, with one or more constructive elements, including means, dimensions, shapes, cross-sections and/or further arrangements for reducing the load on the webs and/or for increasing the resistance of the webs of the hollow core slab in case of fire, i.e. exposure to heat of the external surface of the hot flange of the slab.
• the hollow core slab i.e. the precast slab of prestressed concrete, including, in span direction, a plurality of mainly parallel voids and concrete webs between said voids, with one or more constructive elements, including means, dimensions, shapes, cross-sections and/or further arrangements for reducing the load on the webs and/or for increasing the resistance of the webs of the hollow core slab in case of fire, i.e. exposure to heat of the external surface of the
• one or more constructive elements of the novel hollow core slab according to the invention include means, dimensions, shapes, cross- sections and/or further arrangements and/or configurations for reducing the shear and/or bending force in said webs and/or for increasing the shear and/or bending resistance or capacity of said webs.
• one or more constructive elements of the novel hollow core slab according to the invention include at least one of the following elements:
• non- square webs preferably two or more mainly non- square webs, where non- square is defined as non-perpendicular to the planes of the top and bottom flanges;
• Figure 1 shows a prior art hollow core slab in cross-sectional view
• Figure 2 shows an exemplary embodiment of the novel hollow core slab in cross-sectional view.
• Figure 3 shows other embodiments of a hollow core slab according to the invention in cross- sectional view.
• Figure 1 shows the cross-section of a prior art hollow core slab, wherein the concrete slab 1 includes prestressing strands 2, as well as a number of voids 3 and concrete webs 4 or walls between the voids.
• stress may be at least 100 N/mm 2 , but may be as high 900 N/mm 2 or more.
• the main direction of the webs 4 is mainly vertical, i.e. perpendicular to the (horizontal) top and bottom layers, called top flange 5 and bottom flange 6 respectively.
• top flange 5 and bottom flange 6 In these vertical webs horizontal cracks may originate when exposed to a fire (in an area below the hollow core slab). These horizontal cracks will occur, already in an early stage of the fire, due to the occurrence of shear and/or bending forces in said webs which exceed the shear and/or bending resistance (capacity) of these webs.
• the invention includes a novel hollow core slab configuration which -in order to counteract or prevent that, like in the prior art configuration, the shear and/or bending forces in the webs occurring during fire, will exceed the shear and/or bending resistance of those webs in a (too) early stage- include means, dimensions, shapes, cross- sections and/or further arrangements or configurations for reducing the shear and/or bending force in said webs and/or for increasing the shear and/or bending resistance or capacity of said webs.
• Figure 2 shows schematically a number of exemplary embodiments of such means, dimensions, shapes, cross-sections and/or further arrangements or configurations for reducing the shear and/or bending force in said webs and/or for increasing the shear and/or bending resistance or capacity of said webs, which will be discussed in the following point by point.
• a first preferred elaboration of the present invention concerns a cross- sectional shape consisting of a bottom and a top flange that are connected by at least two webs that have a more or less opposite, non-vertical orientation, see item 11 in figure 2, which possibly could be completed with some mainly vertically oriented webs.
• the cross-section contains two or more webs that have a non- vertical orientation and together with at least a part of the bottom or the top flange form the shape of at least one triangle.
• the hollow cores are triangular or diamond-shaped.
• the corners of the hollow cores are rounded off.
• the cross- sectional shape concerns a truss comprising one or more triangular units.
• This first preferred elaboration is advantageous depending on the specific design of the hollow core slab, for example the thickness of the "hot” flange and the webs, the height and orientation of the webs and the length of the "hot” flange between the webs.
• This first preferred elaboration is advantageous when the increase of the capacity (i.e. the resistance) of the webs to resist horizontal forces, due to for example the fact that the non- vertical webs are loaded less on shear and bending and more on normal force than the vertical webs, is larger than the increase of the horizontal forces due to the fact that the non- vertical webs behave stiffer than the vertical webs.
• a second preferred elaboration of the present invention concerns expansion joints in the "hot” or “cold” flange of the hollow core slabs or in the compression layer if present, see item 12 in figure 2.
• the expansion joints may be positioned anywhere in the "hot” or “cold” flanges and may be applied partially or fully (not drawn) through the thickness of the flanges.
• the expansion joints that are applied partially through the thickness of the flanges may be applied from the exterior surface of the hollow core slab or from the hollow cores.
• the expansion joints can be continuous over the length of the hollow core slab. Discontinuous expansion joints are not excluded. Expansion joints can be applied to hollow core slabs that have already been applied in existing buildings and to hollow core slabs that still have to be produced.
• Expansion joints may be filled with a material that has a low modulus of elasticity compared to that of concrete. The mechanical stress in this material is therefore negligible during fire.
• the expansion joints may also be filled with a material whose modulus of elasticity is comparable to that of concrete in case the material is cold and whose modulus of elasticity decreases significantly when it is heated during fire. The mechanical stress in this material is therefore negligible during fire.
• This second preferred elaboration is especially advantageous because it reduces the compressive force in the "hot” flange and the tensile force in the "cold” flange, and in the compression layer if present, thereby reducing the forces in the webs. Expansion joints can even force harmless vertical cracks in the flanges and the compression layer thereby preventing harmful horizontal cracks in the webs.
• Non-homogeneous material distribution in cross- section A third preferred elaboration of the present invention concerns a non-homogeneous material distribution in the cross-section of the hollow core slab, two or more types of concrete are used anywhere in the cross- section, see item 13 in figure 2 where the cross-section is for example made up of four types of concrete with different properties.
• the "hot" flange is made of a concrete that has a low modulus of elasticity and/or tensile and/or compressive strength and/or thermal expansion coefficient. This is especially advantageous because it reduces the load on the webs when the "hot" flange is heated by a fire.
• a different concrete for the outer webs is for example advantageous when the bottom corners are significantly rounded off, the outer edges are not vertical and the outer cores have a normal size, which is often the case for prior art hollow core slabs and causes a relatively high bending moment in the outer webs during fire.
• a concrete with a high modulus of elasticity is used when the disadvantages of a stiffer concrete, causing an increase of load in the outer webs, do not weigh up against the advantages of the higher tensile strength, causing an increase of capacity of the outer webs.
• a fifth preferred elaboration of the present invention concerns the use of very wide outer webs or the absence of the outer cores and is not presented in figure 2.
• An outer core is absent or an outer web is very wide when the minimum width of the outer web is three times the minimum width of the inner web with the smallest width.
• Using very wide webs or the absence of the outer cores is especially advantageous when the bottom corners of the hollow core slab are significantly rounded off and/or the outer edges are not vertical, which is the case for prior art hollow core slabs and causes a relatively high bending moment in the outer webs, potentially causing horizontal cracks
• This fifth preferred elaboration is advantageous when the increase of the capacity of outer webs to resist horizontal forces, due to the fact that the outer webs are wider, is larger than the increase of the horizontal forces due to the fact that the outer webs behave stiffer.
• Bottom corners not significantly rounded off A sixth preferred elaboration of the present invention concerns the use of bottom corners that are not significantly rounded off and is presented as item 16 in figure 2.
• the height of the side of the "hot" flange which is not to come into (butt) contact with other slabs or any other building structure, e.g. by rounding off or bevelling, preferably has a maximum of about 10 mm, more preferably of about 5 mm.
• a seventh preferred elaboration concerns the use of vertical outer edges and is presented as item 17 in figure 2.
• the top corners of the vertical outer edges may be rounded off and small spaces for product specific clamps to lift the slabs may be applied (not drawn). These spaces should be located in the upper half of the slab.
• the vertical outer edges of two adjacent slabs may be positioned directly against each other when a large contact surface can be achieved in practice.
• the hollow core slabs with vertical outer edges may also be placed at a distance that is preferably as small as possible. In this case the joint should be filled with a material that has mechanical properties comparable to the properties of the concrete used for the hollow core slabs.
• An eighth preferred elaboration of the present invention concerns the use of reinforcement in the "hot" flange and is presented as item 18 in figure 2.
• the reinforcement may consist of reinforcement bars or grids.
• Reinforcement bars or grids are preferably positioned on top of the
• the reinforcement bars or grids are positioned at a regular distance in span direction of the hollow core slab.
• the reinforcement bars or grids are positioned in the colder top part of the "hot" flange.
• Having reinforcement in the "hot” flange is advantageous because it reduces the horizontal forces in the webs due to the fact that the internal resistance against expansion of the "hot" flange increases.
• Having reinforcement in the "hot” flange is furthermore advantageous because this can prevent the occurrence of horizontal cracks in the webs, depending on the specific design of a hollow core slab, due to the fact that the webs remain supported by the "hot” flange due to the prevention of vertical cracks in the "hot” flange or due to the prevention of decomposition of the "hot” flange, by the reinforcement. i. Reinforcement in webs
• a ninth preferred elaboration of the present invention concerns the use of reinforcement in the webs and is presented as item 19 in figure 2.
• the reinforcement may consist of reinforcement bars or grids.
• Preferably the reinforcement extends into the "hot” and/or "cold” flanges.
• FIG 3 A tenth elaboration of the present invention is shown in figure 3.
• narrow outer webs are combined with at least one wider inner web.
• the hollow core slab is provided with narrow outer webs 20 and, as an example, three wider inner webs 21.
• the number of webs and cores can be different, as well as the number of wider inner webs.
• the groove 22 in the middle web is an optional feature, which is explained below under k).
• the terms narrow and wider are used as relative terms here.
• the width of the outer webs can be dimensioned narrower than the width of the inner webs, as long as their total capacity complies with industrial standards. By dimensioning the outer webs relatively narrow, the following advantages can be obtained.
• Item 22 can be seen as a partitioning groove 22 through the "hot" flange of a hollow core slab where the flange is supported by an inner web. More specifically the partitioning groove can extend through the "hot" flange and into the web. The depth of the partitioning grooves can be shallow so that they hardly extend into the webs, but they can also extend further into the web, almost reaching the cold flange, as long as it does not affect the integrity of the hollow core slab.
• the partitioning grooves can be continuous over the length of the hollow core slab. Discontinuous partitioning grooves are not excluded.
• the partitioning grooves can be provided in hollow core slabs that are already included in a building, but can also be provided in new hollow core slabs.
• the partitioning grooves may be, at least partially, filled with a material that has a low modulus of elasticity compared to that of concrete. The mechanical stress in this material is therefore negligible during fire.
• partitioning groove 22 functions as an expansion joint as item 12 in figure 2. Since the partitioning groove 22 extends into a web, the effectiveness of the groove 22 as an expansion joint increases the further the groove extends into the web. In that way horizontal forces in the webs due to expansion of the "hot" flange are reduced.
• partitioning groove 22 subdivides the hollow core slab in groups of cores.
• a partitioning groove 22 may be provided in other positions than in the middle of the hollow core slab. Also, more than one of such partitioning grooves may be provided. Therefore, by means of the partitioning grooves, over the width of a hollow core slab two or more groups of voids forming the cores in the slab can be formed.
• a group may consist of one or more cores. Due to the subdivision of the hollow core slab in the width direction, the width of a flange portion in which expansion forces can build up due to heating, is reduced, such that the maximum shear and bending forces that the webs have to resist in case of a fire become less. The risk of cracks occurring in the webs is reduced, increasing the fire resistance of the prefab hollow core slabs of prestressed concrete to which the invention relates.
• partitioning groove 22 is shown in a wide web as used in elaboration j).
• the partitioning grooves define groups of webs with voids there between, the inner webs being wider than the outer webs.
• such a partitioning groove can also be formed extending to the inside of webs of hollow core slabs according to elaborations a) to i), having one ore more of the measures disclosed there.

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• Engineering & Computer Science (AREA)
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• Structural Engineering (AREA)
• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
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• 2010
  • 2010-09-15 EP EP10760124A patent/EP2478165A1/de not_active Withdrawn
  • 2010-09-15 WO PCT/NL2010/050593 patent/WO2011034420A1/en active Application Filing

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