GB2201438A - Cranked prestressed concrete beam - Google Patents

Abstract

A cranked, precast, prestressed, concrete beam of double-tee section has webs (12) which are chamfered longitudinally at their lower edges whereby to reduce the depth of the beam locally at the crank. This may provide extra head room between floors in structures such as multi-storey car parks. A kinked beam is also disclosed in which the tendons (52) lie parallel to one lower edge (56) at one end and in the other part of the beam are spaced from the lower edge (58). All the tendons are rectilinear throughout their lengths. <IMAGE>

Description

PRECAST CONCRETE BEAMS This invention relates to precast. prestressed concrete beams.

It is conventional practice that precast, prestressed, concrete beams should be rectilinear and indeed in most buil di ng construction it is beams of this kind which are required. It has been proposed to incorporate in cranked prestressed concrete beams longitudinal reinforcing strands which are deflected at some point, generally at the centre of the length of such beams. In our co-pending patent application 86.26615 filed November 7 1986 a cranked, prestressed, precast concrete beam is described which involves two, closely spaced deflection planes in the beam itself and also in the reinforcement tendons, such construction facilitating manufacture and providing good stress distribution within the finished beam itself.

Such cranked beams are particularly useful in - the erection of multi-storey car parks as the construction overall is facilitated and provides benefits to users as fewer vertical columns are required at changes in slope of the floors.

It is an essential requirement for multi-storey car parks which are invariably built of prestressed concrete beams and columns that the cost should be kept to an absolute minimum consistent with strength and appropriate dimensioning. In this respect the distance between the floor of one storey and the lowermost portion of the beams of the next storey is important but unless special measures are taken the height clearance reduces below the acceptable minimum thus blocking the use of such car parks by relatively tall vehicles which still fall within the private car category.This becomes particularly apparent where two cranked, upwardly concave, beams are disposed one above the other with the ncranksn matched so that as a vehicle traverses the floor defined by the lower crank, the effective distance between the roof of the vehicle and the lowermost point of the upper crank structure is reduced owing to the fact that the point of contact of the forward and rear wheels tends to span or bridge the floor so that although the required minimum distance is present in the structure as such, the vehicle itself has the effect of reducing this clearance.

According to the present invention in one aspect there is provided a cranked precast, prestressed, concrete beam of double-tee section, wherein the webs or uprights of the double-tee are chamfered longitudinally at their lower edges whereby to reduce the depth of the beam locally at the crank.

According to the present invention in another aspect there is provided a kinked, precast, prestressed concrete beam wherein one group of tendons extend rectilinearly over the whole length of the beam and lie at least substantially parallel and adjacent to the lower edge of one part of the beam and diverge away from the lower edge of the other part of the beam and another group of tendons extend rectilinearly over the whole length of the beam and lie at least substantially parallel and adjacent to the lower edge of the other part of the beam and diverge away from the lower edge of the one part of the beam.

The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a side elevation of a cranked beam in accordance with the invention; Figures 2, 3, 4 and 5 are sections respectively on the lines II-II, III-III, IV-IV and V-V; Figure 6 is an enlarged side elevation showing details of a kink zone of the beam; Figure 7 is a side elevation of a cranked beam with the concave face upwards and in accordance with à possible solution to the distribution ofvthe tendons in such a beam; Figure 8 is a side elevation of an upwardly convex beam illustrating locations of tendons in the beam in accordance with a second aspect of the invention; Figure 9 is a cross-section on the line IX-IX of Figure 8; Figure 10 is a cross-section on the line X-X; and Figure 11 is a section on the line XI-XI of Figure 8.

Referring first to Figures 1 to 6, a cranked-TT beam is illustrated although, in the sections of Figures 2 to 5 only one of the uprights 12 of the double-tee is shown. The other upright will have an identical construction.

The beam has a relatively longer left hand section 14 (as shown) a short intermediate section 16 and a shorter right hand section 18. The intermediate section defines the crank angle which in detail is defined by two planes 20, 22 (broken lines) of deflection all these features being substantially as disclosed in our co-pending application 86.26615. So as to improve head room at the cranks in the completed structure of which the beam forms a part, both of the uprights are chamfered along their lower longitudinal edges so that the lower edge of each upright defines a rectilinear line 24 which is parallel to the upper surface 26 so that it extends beyond that intermediate short section, the beam thus effectively diverging outwardly away from the intermediate section to an extent depending upon the precise angle chosen.

At each end of the beam recesses 28 and corresponding shoulders 30 are provided, as is conventional, for mounting the beam on the appropriate supporting abutments, not shown.

Figures 2 to 5 illustrate the sections as hereinbefore indicated and call for no further comment.

Figure 6 illustrates to, an enlarged scale, details of the intermediate section 16 including the planes of deflection 20, 22 and the precise extent of the intermediate section of the beam so that the upper plane surface 26 lies precisely parallel to the lower edges 24 of each upright of the double-tee.

Turning now to Figures 7 to 11, it is from time to time a requirement for building construction that a beam shall be slightly cranked and for the sake of distinguishing this embodiment from the embodiment of Figures 1 to 6 reference will be made to a kinked beam. This embodiment is not to be considered as limited to double-tee beams, and single T's and other sections can also make use of the invention in as far as an upwardly convex kinked beam is required. In this embodiment the requirement for inventive mouldage as disclosed in our co-pending application 86.26615 is avoided, since the reinforcing tendons are rectilinear throughout their lengths. The problem is to ensure while maintaining the rectilinear reinforcements that adequate strength is provided where it is most needed, namely in the lowermost parts of the beam, more especially at the centre and parts adjacent to the centre where bending moments are highest.

One possibility is illustrated in Figure 7 in which rectilinearly extending tendons 40 lie close at the centre only to the lower surface 42 of the beam 44 whether in an upright leg or elsewhere whereas at the ends the tendons are attached for prestressing adjacent to the upper surface 46 of the beam considered as the whole.

Obviously, such a proposal suffers from the disadvantage that where tensile loads are at their highest, apart from the centre section, the tendons are absent.

In accordance with a second aspect of the invention Figure 8 illustrates a kinked beam 50 in which the tendons 52, 54 are again rectilinear but the distribution of the tendons 52, 54 is such that there is always an adequate degree of tensile strength adjacent the lowermost portion whether in an upright leg or in a solid part of the beam. Figures 9, 10 and 11 illustrate the distribution of the two groups of tendons 52, 54 at three sections and show that at all points along its length at least two tendons are present adjacent the lowermost- edge. At each end struts are employed to ensure correct location of the tendons and such struts will be incorporated into the concrete of the beam.

The first group of tendons 52. are shown in chain lines and extend parallel to the right hand section lowermost edge 56, but diverge upwardlyaway from the left-hand section lowermost edge 58. In contrast, the second group of tendons 54 shown in double chain lines extend parallel to the left-hand section lowermost edge 58 but diverge away from the right-hand section lowermost edge 56.

It will be clearly apparent from Figure 10, the centre of the beam, that the maximum number of tendons are present where the bending moment is a maximum, but at intermediate sections, adequate reinforcement is provided as in Figures 9 and 10.

The kinked beam of Figures 8 to ii has the advantage that when incorporated to form a floor which will drain any external elevations all at the same level.

Claims (10)

1. A cranked, pre-cast, prestressed, concrete beam of double-tee section, wherein the webs or uprights of the double-tee are chamfered longitudinally at their lower edges whereby to reduce the depth of the beam locally at the crank.

2. A beam according to claim 1 wherein the crank is formed by two, adjacent, changes in direction.

3. A beam according to claim 1 or claim 2 wherein the crank is offset from the centre of the beam.

4. A beam according to claim 2 or claim 3 wherein the chamfer extends parallel to the local upper surface of the beam intermediate the two changes in direction defining the crank.

5. A beam according to claim 4 wherein the chamfer extends beyond the section of the upper surface of the beam lying between the two changes in direction.

6. A kinked, pre-cast, prestressed, concrete beam wherein one group of tendons extend rectilinearly over the whole length of the beam and lie at least substantially parallel and adjacent to the lower edge of one part of the beam and diverge away from the lower edge of the other part of the beam and another group of tendons extend rectilinearly over the whole length of the beam and lie at least substantially parallel and adjacent to the lower edge of the other part of the beam and diverge away from the lower edge of the one part of the beam.

7. A beam accordingly to claim 6 wherein at least two tendons are present adjacent the lowermost edge of the beam throughout its length.

8. A beam according to claim 6 or claim 7 wherein at the change in direction of the kinked beam the number of tendons at the lowermost portion of the cross section is at a maximum.

9. A pre-cast, prestressed, concrete beam substantially as hereinbefore described with reference to Figures 1 - 5 of the accompanying drawings.

10. A pre-cast, prestressed, kinked, concrete beam substantially as hereinbefore described with reference to Figures 7 - 11 of the accompanying drawings.