GB2142668A - Beam - Google Patents

Abstract

A T-section beam has a web of dovetail cross-section (12) and a bottom flange (16) of substantially greater width, i.e. 3 to 5 times the web width, than in conventional T- beams. Longitudinal reinforcement is provided in both parts and stirrups (34) are provided at spaced intervals along the length of the beam, parts of the stirrups extending beyond the free-and surface of the web. Each stirrup comprises a central generally oval member and two mirror image portions which extend from the exposed portion of each stirrup down the web and into the bottom flange on the opposite side of the oval section to the path down the web. <IMAGE>

Description

SPECIFICATION Pre-stressed Concrete Beams This invention relates to pre-stressed concrete beams, for example for use as bridge decking.

Many bridges and other structures comprise abutments of massive reinforced concrete or frameworks and relatively lighter members which serve as decking, roofs and the like. These members are effectively primary load bearers and particularly in the case of bridges are required to withstand substantial rolling loads. It has already been proposed and is indeed common practice to use beams of T-section which, in use, will generally be in the inverted position.From the standpoint of convenience in transportation and rapidity of construction on site, the large number of beams for any given width conventionally required is a substantial disadvantage and useful advantages from the cost standpoint will arise from using a small number of beams for any given width because the time to manufacture a beam remains substantially constant irrespective of its width and with wider beams a shorter assembly time is required.

According to the present invention there is provided a pre-stressed concrete beam of generally Tsection in which the bottom flange of the 'T' has a maximum width of from 3.0 to 5.0 times, preferably about 4.0 times the minimum width of the web of the 'T'.

Preferably the beam has a web of dovetail section and in addition to pre-stressing tendons, stirrups are provided at spaced intervals along the length of the beam, such stirrups extending both into the bar of the 'T' and also in the web and a portion thereof is exposed at a location extending beyond the concrete of the base or free end surface of the web.

Further according to the present invention there is provided a pre-stressed concrete beam of generally T-section, the web of the 'T' having a cross-section of dovetail form with the wider end remote from the bottom flange.

Still further according to the present invention there is provided a pre-stressed concrete beam of generally T-section, the reinforcement comprising pre-stressing tendons and also stirrups spaced at invertals along the beam, each stirrup including a central, elongate, loop extending from the bottom flange to the free face of 25 the web, a first element extending from adjacent the free end face of the web, down the web along a vertical path outside the central loop, into the portion of the bottom flange at the opposite side of the loop and returning to a location on the same side of the central loop as said vertical path, a remaining element of the 5 stirrup having a configuration which is a mirror image of the said further element.

Yet still further according to the present invention there is provided a pre-stressed concrete beam of generally T-secion, the beam comprising a web of dovetail section tapering outwardly away from the bottom flange of the 'T', the web having a width of from 3.0 to 5.0 times, preferably about 4.0 times, the minimum width of the web, prestressing tendons distributed in the web and the bottom flange and stirrups spaced along the length of the beam and embedded in the concrete matrix both in the web and the bottom flange, a portion of each stirrup extending at the free end free of the web, beyond the matrix to provide keying in a finished concrete structure.

The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross-section of a typical beam in accordance with the present invention; Figure 2 is a plan view of an end portion of the beam of Figure 1 , to a reduced scale; Figure 3 is a section on the line 3-3 of Figure 2; Figures 4A and 4B are side elevations showing in each part certain features of a beam in accordance with the invention; Figure 5 is a fragmentary view illustrating in elevation a transverse hole through a beam in accordance with the invention; Figure 6 is a cross-section of the feature illustrated in Figure 5; and Figure 7 is a fragmentary section showing beams as described with reference to Figures 1 to 6 when incorporated in a building structure such as a bridge.

Referring now to the drawings, it will be apparent from Figures 1 and 3 that a beam 10 in accordance with the invention has a generally 'T-section' with a web 12 of dovetail section with a portion 14 having parallel sides adjacent to the bottom flange 1 6 of the 'T' to which flange the web is connected by chamfers 1 8. The flange 1 8 has a plane base surface 20 (as shown). Two upper surfaces 22 of the flange 16 are inclined slightly to the horizontal and edge surfaces 24 of the flange are inclined by a shallow angle to the vertical, the latter surfaces being connected to the plane base surface 20 by further chamfers 26. The angle of the dovetail as shown is 10.650 to the vertical, but, of course, this figure should not be considered as limiting. Preferably the angle of the dovetail lies in the range 70 to 14 .

Figure 1 illustrates a typical beam size but as 15 indicated by the reference numerals J1-J1 2 variations in size are possible both in the web depth and correspodingly in the web width. For each reduction in length of the web the web will be correspodingly reduced in width. At the 20 manufacture stage the depth of the web can be reduced very simply by filling the mould to a lesser extent.

The following tables illustrate the properties of the beams which can be produced.

Section properties

Height of Section moduli mm3x 106 centroid Approximate Sec- Depth Area above bottom Top fibre Bottom fibre self weight tion mm mm2 fibre mm Yb Zt Zb KN/m J1 260 160700 94 4.06 7.14 4.02 J2 300 170700 105 5.13 9.50 4.27 J3 340 181000 117 6.51 12.35 4.52 J4 380 191900 131 8.27 15.69 - 4.80 J5 420 203400 146 10.39 19.44 5.09 J6 460 215500 163 12.89 23.53 5.39 J7 500 228200 180 15.78 27.93 5.70 J8 540 241500 199 19.05 32.59 6.04 J9 580 255400 219 22.72 37.50 6.39 J10 620 269900 239 26.80 42.63 6.75 J11 660 285000 261 31.30 47.98 7.13 J12 700 300700 282 36.22 53.54 7.52 Figure 1 also shows by crosses possible locations of pre-stressing tendons 30 which will be stressed by conventional pre-stressing means but for small sizes of beams the number of possible locations for the tendons will be correspondingly reduced.As will be appreciated the possible locations for the tendons in the web will be diminished by one row for each reduction in size from J1 2 down to J1.

In the bottom flange 1 6 it will be noted that the tendons 30 are in three groups 32. The possible tendon locations of the groups 32 of the flange are staggered when considered vertically whereas the possible tendon locations of the web are aligned both vertically and horizontally.

As is illustrated in Figures 3 and 4 the tendons 30 are supplemented by stirrups 34 spaced as indicated in Figure 4A and configured as shown in Figure 3. As will be readily apparent, each stirrup 34 projects beyond the upper surface of the concrete matrix which provides an important keying function when installed in situ as illustrated in Figure 7. Each stirrup 34 is assembled from three separate pieces and to enable moulding of the beam without displacement these pieces will be tied to the tendons 30.

A central elongate loop 31 extends from a location adjacent the lower free surface 20 of the bottom flange to a location beyond the free end surfaces 33 of the web. A further piece or element consists of a horizontal length running parallel with the surface 33, a vertically extending length parallel to the right-hand, as shown, surface of portion 14, a further horizontal portion extending parallel to the lower (as shown) surface 20 of the flange 1 6 and finally a length extending substantially parallel to the left hand (as shown) surface 22 and terminating between the vertically extending length of the same piece and the right-hand vertical length of the central loop 31. The final piece of the stirrup is a mirror image of a further piece or element just described.

Figures 2 and 3 illustrate an end section 36 of a beam in which plane vertical edges 38 are provided in place in the inclined edge 24 and chamfer 26 as illustrated in Figure 1.

Figure 4B and Figures 5 and 6 illustrate transverse holes 38 of elongate cross-section through which transverse connecting rods 40 can be passed as illustrated in Figure 7 when a plurality of beams has 10 been assembled in a finished structure. As will be apparent the lower surfaces 20 of the beams are exposed but the remainder of the surfaces are fully embedded in the concrete matrix 42. Figure 7 shows a typical mid-section, but at the edges the outer side faces of the outermost beams will normally be exposed.

The apertures or holes 38 are tapered to a minimum section 44 (broken lines) which corresponds substantially to the section of the reinforcing transverse bars passing through the beams as illustrated in Figure 7, the draft or taper of the apertures facilitating insertion if alignment between adjacent beams is not perfect and also facilitating manufacture.

Figure 7 which illustrates a typical structure using beams in accordance with the invetnion, renders apparent that the number is reduced in comparison with conventional beams, where, for a similar transverse section of bridge decking either 6 or 7 conventonal T-section beams would be required. The dovetail section of the webs of each beam in accordance with the invention ensures good keying with the matrix 42 of concrete in which the beams are embedded. Also as apparent in Figure 7 a small space is provided between each beam; this will be filled with grout or concrete mix 46.

The width of a bottom flange of a beam in accordance with the invention may be as much as twice as great as a standard T-beam currently manufactured and the cost savings at the manufacturing and installation stage will be appreciable.

Claims (11)

1. A pre-stressed concrete beam of inverted, generally T-section in which the bottom flange or bar of the "T" has a maximum width of from 3.8 to 4.2 times the minimum width of the web of the "T", the beam being prestressed by longitudinal tendons.

2. A beam according to claim 1, wherein the maximum width of the bottom flange of the "T" is 4.0 times the minimum width of the "T".

3. A beam according to claim 1 or claim 2, wherein the web is of generally dovetail section with the wide edge remote from the bar of the "T".

4. A beam according to any one of the preceding claims further comprising, in addition to prestressing tendons, transverse reinforcing stirrups disposed at spaced intervals along the length of the beam, such stirrups extending both into the bar of the "T" and also into the web, each stirrup including a central, elongate element extending from the bottom flange to the free, upper surface of the web, a second element extending from adjacent the free, upper surface of the web, but outside the concrete matrix down the web along a substantially vertical path outwardly of the central, elongate loop element into that portion of the bottom flange disposed at the opposite side of the loop element and returning to a location on the same side of the central loop element as the said vertical path, and a third element of stirrup having a configuration which is a mirror image of the said second element.

5. A beam according to any one of the preceding claims wherein the tendons are arranged in the web in vertical, aligned rows and transverse, aligned rows.

6. A beam according to any one of the preceding claims wherein the bottom flange of the "T" has two horizontal rows, as viewed in cross-section, of tendons, the tendons of these rows being staggered in the vertical sense.

7. A beam according to any one of the preceding claims wherein the junction zone between the bottom flange and the web is free of reinforcing tendons.

8. A pre-stressed concrete beam of, inverted, generally T-section with the vertically extending web of generally dovetail section, the reinforcement comprising pre-stressing tendons and also stirrups spaced at intervals along the beam, each stirrup including a central, elongate loop element extending from the bottom flange to the free, upper surface of the web, a second element extending from adjacent the free upper surface of the web, but outside the concrete matrix, down the web along a substantially vertical path lying outwardly of the central loop element, to that portion of the bottom flange disposed at the opposite side of the loop element and returning to a location on the same side of the central loop element as the said vertical path, and a third element of the stirrup having a configuration which is a mirror image of the said second element.

9. A beam according to claim 8, wherein a major proportion of the web is of dovetail section, a minor proportion, adjoining the bottom flange is of rectangular section, and the latter is joined to the bottom flange at opposed chamfers.

10. A beam according to claim 8 or 9 wherein the pre-stressing tendons are distributed in the web in vertical and horizontal rows to form a grid pattern and the pre-stressing tendons of the bottom flange are arranged in two, parallel, horizontal rows, the tendons of these two rows, being staggered vertically.

11. Pre-stressed concrete beams of inverted T-section substantially as hereinbefore described with reference to the accompanying drawings.

1 2. A building structure incorporating inverted T-section beams according to any one of the preceding claims.