EP0744511B1 - Prestressed concrete beam and process for making the same - Google Patents

Abstract

The beam, e.g. for constructing floors, consists of a central section (A) and two end sections (B) which are of lower height than the central section and have a length of between some tens of cm and 1/3 of the beam's length. The end sections (B) have projecting steel reinforcing elements (4), forming a beam which have a greater resistance to flexure in its central section and a greater resistance to shearing forces in its end sections. The height of the projecting reinforcing elements is not greater than the height of the beam's central section, and the two end sections are formed by packing the mould with a powdered material, polystyrene blocks or cardboard. The blocks can also be, for example, of neoprene or polypropylene.

Description

The present invention relates generally to concrete beams and more particularly joists for the production of floors.

The technique of producing floors by installing prefabricated beams between which are arranged interjoists then covering by a concrete screed has been known for a long time and is widely used in construction.

The beams used for making floors are concrete beams prestressed, i.e. beams in which steel wires are embedded under voltage. This first type of beam provides a structure whose flexural strength is important.

It is also used in the construction of prefabricated reinforced concrete beams with seam steel such as that described in FR 1,589,789, i.e. beams in which non-tensioned reinforcing wires are embedded, said wires of reinforcement having portions which protrude from the beam to be embedded in the concrete used around said beam. This second type of beam allows the realization of structures better supporting the shearing forces, these being supported by the seam steels embedded in the surrounding concrete. This type of beams whose section is the same from one end to the other allows to support over the entire length of the beam a significant bending moment but does not allow to resume at ends of large shear forces.

However, the need is currently felt for beams allowing the realization of structures resistant both to bending and to shear forces.

It has also been known since 1960, from FR 1 270 049, a reinforced concrete beam having at each of its ends a notch through which the frame protrudes internal. Such an arrangement allows only a very small improvement in the mechanical capacity of the beam which does not meet current demand.

The invention therefore seeks to propose a beam meeting this demand in making better use of the mechanical strength of concrete than before prestressed and sewing steels.

To this end, the invention relates to a prestressed concrete beam with a heel, an upper part and prestressing steels embedded in the heel and at the bottom of the upper part, said heel being wider and less high than said part upper, characterized in that the end portions of the beam are tall e1 less than the height e2 of the central part of the beam and have a length between a few tens of centimeters and about 1/3 of the length of the beam and in that shear reinforcement is provided in said end parts, said shear reinforcement extending in the space left free above said end parts without exceeding the bulk of the part Central.

• la longueur des parties d'extrémités (B) est au moins de 20 centimètres.
• la résistance aux moments de flexion est, en cas de besoin, largement améliorée en noyant dans la partie centrale des aciers conformés de la même manière que ceux disposés dans les parties d'extrémités.

The beam according to the invention is also remarkable in that:

• the length of the end portions (B) is at least 20 centimeters.
• the resistance to bending moments is, if necessary, greatly improved by drowning in the central part of steels shaped in the same way as those disposed in the end parts.

The present invention also relates to a method of manufacturing such a beam characterized in that there is a stuffing in the end parts of a non-opening one-piece mold usually used for the manufacture of prestressed concrete beams.

• les figures 1a, 1b et 1c sont des schémas représentant le moment de flexion Ma, Mb et l'effort tranchant Ta, Tb, Tc supportés par une poutrelle disposée sur deux appuis pour différentes formes de charge,
• les figures 2a et 2b représentent schématiquement deux poutrelles selon l'invention,
• les figures 3a et 3b représentent schématiquement des montages avec table de compression incorporée respectivement avec une poutrelle en béton précontraint de type connu 3a et avec une poutrelle en béton précontraint suivant l'invention 3b,
• la figure 4 est une coupe réalisée dans la partie centrale d'une poutrelle suivant l'invention,
• les figures 5 à 9 sont des coupes réalisées dans une partie d'extrémité d'une poutrelle selon l'invention pour cinq variantes de réalisation,
• la figure 10 est une vue en perspective de la poutrelle de la figure 5,
• les figures 11 et 12 sont des vues partielles en perspective d'une poutrelle suivant l'invention, la poutrelle représentée à la figure 12 étant celle vue en coupe à la figure 7.

Said stuffing can be carried out in any suitable manner. For example, it can use a pulverulent material such as sand or be constituted by a block of polystyrene split in the middle. It can also consist of a profile made of a flexible material such as neoprene, polypropylene or the like, or of a cardboard profile.

• FIGS. 1a, 1b and 1c are diagrams representing the bending moment Ma, Mb and the shear force Ta, Tb, Tc supported by a beam placed on two supports for different forms of load,
• FIGS. 2a and 2b schematically represent two beams according to the invention,
• FIGS. 3a and 3b schematically represent assemblies with a compression table incorporated respectively with a prestressed concrete beam of known type 3a and with a prestressed concrete beam according to the invention 3b,
• FIG. 4 is a section made in the central part of a beam according to the invention,
• FIGS. 5 to 9 are sections made in an end part of a beam according to the invention for five alternative embodiments,
• FIG. 10 is a perspective view of the beam of FIG. 5,
• FIGS. 11 and 12 are partial perspective views of a beam according to the invention, the beam shown in FIG. 12 being that seen in section in FIG. 7.

A beam P arranged on two distant supports 2 and bearing loads is subject to constraints which we are used to evaluating from curves M of the bending moment and T of the shear forces.

Figures 1a to 1c show these curves M and T for different loading of said beam P.

These curves are the same regardless of the section of the beam considered, we will therefore consider that it is rectilinear and of constant section.

On examination of these sketches, we see that the curves Ma and Ta concerning the beam subjected to a load uniformly distributed do not have the same appearance as curves Mb, Mc and Tb, Tc concerning the loaded beam at two points.

Note however that the bending moment is still of great value in the central part of the beam while it is of lower value in the end portions of the beam.

We also note that, conversely, the efforts sharp are always of greater value in the end portions of the beam while they have a lower value in the central part of the beam.

The invention then proposes a beam such as the beams 1a and 1b shown in FIGS. 2a and 2b, that is to say a beam whose height varies between e2 in the central part A, A 'and e1 less than e2 in the end portions B, B '. The beam there, 1b is thus more resistant in its central part A, A 'of significant thickness e2 which is the area in which the bending moment takes its values the more important.

In a manner known per se, and as visible in the figure 4, a beam for the production of floors has a base or heel 10 and an upper part 11. This upper part 11 is usually of trapezoidal section with the small base at the top as shown in solid lines but can also be rectangular as shown in dotted lines or trapezoidal section with the small base at the bottom like shown in dashed lines. Heel 10 is wider and lower than the top 11. The interjoists 6, visible in FIGS. 3a and 3b, are in support by their spoiler on the shoulders 12 formed of on either side of the upper part 11.

Prestressing steels 3 are embedded in the beam, some being arranged in the heel 10 and others near the bottom of the top 11.

On the one hand, an increase is obtained in the center resistance to bending and secondly to ends an increase in resistance to sharp efforts.

It is also possible according to the present invention to have in the central part (A) of the shaped steels of the same way as those arranged in the parts ends (B).

Figure 4 is a section of the following beam the invention, in the central part A thereof, for which it is similar to beams in known prestressed concrete.

As shown in Figures 5 to 9 showing sections made in an end part B of a beam according to the invention, the height e1 of this end portion B is significantly less than the height e2 of the central part A.

In Figures 5 to 7, this height e1 is a little greater than half the height e2 while at Figures 8 and 9, said height e1 is approximately equal to two thirds of the height e2.

Other variants of the beam according to the invention may have values e1 and e2 different.

To provide a beam with resistance to significant cutting efforts in parts of ends B in which as visible to Figures 1a to 1c these cutting forces have values important, shear reinforcement 4 are provided in these end portions.

By using such frames, one can double resistance of the beam to shearing forces while allowing the seam between the beam and the concrete poured in place.

The beam according to the invention with parts central and extremities of heights e2 and e1 different has the great advantage of facilitate the implementation of effort steels sharp and sewing 4.

Indeed, although the skilled person knows of long-standing improvement in resistance to sharp efforts by the placement of steels, these are not very often used in beams in prestressed concrete due to the inconvenience they provide when implemented in usual prestressed concrete beams.

As shown in Figure 3a, when you equip a usual prestressed concrete beam 100, i.e. a beam of constant section, of steels 40 shear forces, these protrude above the upper level of the concrete forming the beam. During the installation of such beams, said steels can hinder construction by creating a beam far too large in height. These sewing steels are especially troublesome when so-called compression table assemblies with interjoists of low height for which they prevent to perform normal keying because they protrude from the upper side of the finished floor.

In beam 1 according to the invention shown in Figure 3b, the seam steels 4 extend in the space left free above the end parts B, without exceeding the overall height of the part central A.

This arrangement of steels of sharp forces and seam in the overall height of the part control unit A also makes it possible not to disturb the stack of beams according to the invention which are can perform in the usual way by interleaving simple separating wedges.

Another advantage that beams provide the invention when their end portions B are fitted with cutting force steels and east seam that of being able to be easily transported, these steels allowing both their manual setting and their attachment to lifting devices.

Sharp and seam steels 4 can be of any known form.

Figures 5 and 10 show variants of realization using effort steels sharp and stitching in the form of simple Greek that is to say in the form of a succession of U arranged alternately up and down.

In Figures 8, 9 and 11, shear steels and sewing in Greek form with welded bars 5 along the roundings of the U are used.

As shown in the figures, the stress steels cutting and sewing can be arranged on one side or the other of the steels prestressed.

Figures 6 and 12 show stress steels sharp and stitched in the form of stiffener and the figure 7 shows a beam with efforts steel sharp and sewn formed as a flat stiffener.

Any arrangement of these steels can of course be implementation, whatever the difference between heights e1 and e2, the attached drawings do not showing just a few of the possible layouts.

The length of the end zones of thickness e1 less than the thickness of the central part can vary depending on the use to which intended for the beam, it is usually understood between a few tens of centimeters and a third of the length of the beam and never less than 20 cms.

The lengths of the central part A and the parts end B of a beam are determined by the location of the loads to be borne by the beam.

When the beam is intended for a structure in which it must support loads arranged near supports, as shown schematically in Figure 1c, the ends B of height e1 have a reduced length, for example between 20 and 50 centimeters.

The use of cutting-edge steels and seam in the end parts B allows even in the case of end portions B of length reduced to interestingly increase the resistance to shearing forces of the beam according to the invention with respect to conventional beams.

According to an alternative embodiment not shown in drawing, the resistance to bending moments of the beam according to the invention is, if necessary, largely improved by drowning in the central part A of it steels shaped in the same way as the steels provided in the end parts B.

The beam according to the invention thus allows, in calculating the characteristics of steels embedded in the central part A and steels in place on the end parts B to modulate the resistance of the beam at bending moment and efforts sharp and multiply it by 1.5 to 2.

A beam with end portions each extending over a third of the total length meets the criteria imposed by the regulations earthquake-proof for zones II and III.

Beams with intermediate lengths compared to these extreme dimensions are as to they manufactured according to the loads to which they must resist.

The present invention also relates to a method of manufacture of the beams according to the invention.

The process that immediately comes to mind is the use of an opening mold inside which we place an added bottom, for example in rubber, retaining the seam steels for their allow to extend outside of the cavity of molding and keep them in place.

This provision is not entirely satisfactory because it does not allow sufficient precision of the molding.

You should know that even a small improvement of the precision of the molding has very important on the finished product.

A beam with a weight of the order of 20 kilograms per linear meter can, by improving the molding precision, weigh only 15 kilograms per linear meter.

This weight gain makes it possible to provide beams weighing a few kilograms less is very important and has significant consequences on the handling of these beams: we can load one more on trucks, they are more easy to handle on construction sites ...

According to the invention, the beams with parts ends of height less than the central part and fitted with cutting and sewing steels are made in molds in one piece not opening usually used for the manufacture of prestressed concrete beams.

A jam, for example in a powdery material such that sand is placed in the end parts of the mold so as to reduce the mold cavity and allow the sinking of the seam steels to ensure their positioning.

The stuffing can also be achieved by a block of polystyrene split in the middle or a profile in flexible material such as neoprene, polypropylene, ... or a cardboard profile, in the case steels extending in a vertical plane, by example those in the form of Greek.

Claims (8)

1. A prestressed concrete beam having a foot (10), an upper part (11) and prestressing rods (3) embedded in the foot (10) and in the bottom of the upper part (11), said foot (10) being wider and less high than said upper part (11), characterised in that the end parts (B) of the beam are of a height e1 less than the height e2 of the central part (A) of the beam and have a length of between several tens of centimetres and about 1/3 of the length of the beam and in that shear reinforcements (4) are provided in said end parts (B), said shear reinforcements (4) extending into the space left free above said end parts (B) without extending beyond the space taken up by the central part.
2. A prestressed concrete beam according to Claim 1, in which the length of the end parts (B) is at least 20 centimetres.
3. A beam according to Claim 1 or Claim 2, in which the resistance to bending moments, if required, is greatly improved by embedding in the central part (A) rods shaped in the same way as those arranged in the end parts (B).
4. A method for manufacturing a beam according to any one of Claims 1 to 3, characterised in that:

   a non-opening one-piece mould usually used for manufacturing prestressed concrete beams is selected,

   a filler is arranged in the end parts of the mould,

   the protruding rods (4) are sunk into said filler in order to position them,

   the beam is moulded.
5. A method according to Claim 4, characterised in that the filler is of powder material.
6. A method according to Claim 4, characterised in that the filler is formed by a polystyrene block slit in its centre.
7. A method according to Claim 4, characterised in that the filler is formed by a block of flexible material.
8. A method according to Claim 4, characterised in that the filler is made of cardboard.

Images