GB2190611A - Manufacture of pre-stressed steel beams - Google Patents

Description

SPECIFICATION Manufacture of pre-stressed steel beams The invention relates to pre-stressed steel beams.

Pre-stressed beams are designed fora purpose which is self-evident, i.e. to increase the load capacity when the beam is subjected to bending.

According to Belgian Patent495318a pre-stressed reinforced-concrete beam is manufactured by first subjecting the metal reinforcement, before embedding,to bending, by then embedding in concrete at least a part ofthe portion of the reinforcementsubjected to tension after bending, by keeping the embedded reinforcement bent while the concrete sets, and by finally removing the bending force afterthe concrete sets. The reinforcement comprises a rolled steel beam orjoist. Bending is brought about either by applying a force at a place on the reinforcement between the ends, which are held fixed, or by applying forces to the ends of the beam, which is held fixed at an intermediate place.

This method yields pre-stressed beams where the pre-stress is brought about by partial embedding in concrete. Such beams are known by the name "Pre- flex" when used in heavy construction, bridges, or viaducts.

Afirst inherent disadvantage in these embodiments is the extra weight due to the concrete. A second disadvantage is the fact that the embedding concrete has itself to be reinforced and the steel reinforcements must be secured by welding to the main reinforcement, i.e. the steel beam, which in volves fitting and welding work which is difficult and is impossible to automate. Finally, a third disadvant- age is that the steel is so thoroughly embedded that it is difficu It to drill the beam atthese places and welding is impossible.

What is desired is a method of manufacturing pre- stressed beams which is free from the described disadvantages.

One idea would be to make up a pre-stressed steel beam from two separate concrete-free sections, prestress being induced by acting on one of the two sections so as temporarily to expand it in the length direction, joining the two sections, and eliminating the cause of expansion.

Expansion could be achieved by exerting a tensile force on one section before joining itto the other section. Alternatively, one section could be heated so as to iengthen it temporarily. In both cases it would be possibleto make up a pre-stressed beam, but it would be necessary to use enormoustension or heating means if it is desired to make up beams which are largerthan mini-sections.

The present invention provides a method in which: a) two (or more) separate sections are prepared suitablefor assemblyto form the desired beam; b) the sections are juxtaposed or superposed so thatthebeam is virtually made up; c) the sections are simultaneously subjected to a bending force so as to obtain a bent, virtually made up beam; and d) the adjacent areas of the sections are joined.

The invention also provides a pre-stressed steel beam having a bent web, comprising two T-sections, each having a web and a flange, the webs of the Tsections being in the same plane and joined atthe edges of the webs which are remote from the flanges.

Optional and preferred features ofthe invention are described in claim 2 etseq.

The sections can be two T-sections which, after assembly, form an Section. Assembly may be brought about by enclosing the two T-sections in a numberof steel frames as explained hereinafter. The sections can be prepared e.g. by cutting an I-beam to obtain two T-sections. The sections could, instead, be l-sections.

Preferably, a bending force is applied to the assembly comprising the two sections, when juxta- posed and framed. Bending can be brought about by applying two equal forces in the same direction to the ends of one of the sections, whereas the other section is supported on bearingsataquarterorthird of its span. The frames are dimensioned so thatthe assemblyformed by the two T-sections, when juxtaposed to form an I, is held stationary so thatwhen bonding forces are applied the two sections behave substantially like a complete beam, i.e. onlythe adjacent areas can move relative to or slide over one another.

Su bsequent joining may advantageously be brought about by using an automated mobile electricwelding apparatus.

Two T-sections can be prepared where each has a different composition or is made of a different grade of steel, e.g. by cutting different kinds of I-beams.

An alternative method is to start with two sections in the form of l-beams. Such sections are very suit- able for bending and can be joined via theirflanges, whether by welding or by bolting or by any other suitable means.

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which: Figure lisa side view of two T-sections, framed to form a virtual beam acted upon by bending forces applied to its ends; Figure 1A is a cross-section through the assembly comprising the virtually made-up beam and a secur ingframe,ata place where a bending force is app- lied; Figure 1B is a cross-section through the assembly comprising the virtually made-up beam and a securing frame, at the site ofthe a support bearing; Figure2 is a cross-section ofthetwo sections for forming the beam, emphasis being placed on the adjacent areas;; Figure2A and Figure 2B, on an enlarged scale, show bevels formed at the ends of the adjacent areas before welding; Figure 3 is a cross-section through a beam made up of two sections joined by steel plates; and Figure4shows one possible application of a beam according to the invention.

Figure 1 shows two sections 1,2 held in position such that areas 11 and 21 are adjacent. In practice these areas can touch or be situated at a distance of about 2 to 5 mm from one another, to avoid difficulties during subsequent welding. The sections are held by frames; in the case of fou r frames for ex- ample, as illustrated, two frames 3,4 are disposed at the ends of the sections whereas two frames 5,6 are each disposed at a quarter of a span.

The bottom section 2 is shorterthanthe top section 1 to allowforchanges in length during bonding.

The appearance of the assemblywhen bent is shown in broken lines.

As shown by way of example in Figures 1, 1A, and 1 B, bending forces (F) are applied to the frames 3,4 are received at bearing points (A) by the frames 5,6.

The frames 3,4,5,6 are identical in construction but can be variously disposed depending on their function. For example, frames 3,4 are disposed so thattheir opening faces downward, whereas frames 5,6 are disposed in the opposite direction. The applied forces and the reactions at the bearing points are represented by arrows to illustrate the places where the frames are under stress.

The frames are designed so that welding is easy, using an automatic welding machine. As shown in Figures 1A and 1 B, a welding machine (S) moves along a U-section 7 which atthe place corresponding to Figure 1A bears on the bottom part ofthoframo and at the place corresponding to Figure 1 B bears on the flange of section 2, via a bar (hatched) which compensates forthe difference in level. Advantageously, plates 8,9 respectively are disposed between the frames and the jacks (not shown) and between the frames and the bearings, to avoid damage.

The areas for welding together (see Figure 1 ) are treated by bevelling, preferably at the end of the top section. Figure 1Ashows a bevel suitableforthick- webbed beams. In the case ofthin-webbed beams, the bevel shown in Figure 2B is preferable.

Instead of being welded, the sections forming the beam can be joined by adhesion (see Figure 3). Inthe case shown, use is advantageously made of steel plates 33 overlapping the webs, stuck all the way along the webs of two sections 31,32, e.g. by lap welding.

The invention has been illustrated by describing the use ofT-sections, but of course beams can also be made up from mixed sections, hollow section members, I-sections, or a combination thereof.

When the l-sections are of differentflange size, it is easy to secure the flanges together by welding.

The process yields a pre-stressed beam having a part under tension and a part undercompression. A beam ofthis kind can withstand a greater maximum load than a rolled beam of similarsize,sincethe part of the beam undertonsion takes up compressive stresses and the part under compression takes up tensile stresses.

One possible application is shown in Figure 4, where a pre-stressed welded beam 40 bears a reinforced concrete slab 41. The slab is joined to the beam 40 by bolts 42. The T-sections used to make up the beam 40 have difforonttransverse dimensions, the smaller section being in contact with the slab 41.

Claims (16)

1. A method of manufacturing a pre-stressed steel beam, comprising the sequential steps of pre paring longitudinal sections suitable for being assembled to form the desired beam, juxtaposing the sections to form an assembly, subjecting the assembly to external bending forces to obtain a bent virtually made-up beam, joining adjacent areas of the sections, and removing the external bending forces.

2. A method as claimed in claim 1, in which two sections of substantially identical geometrical shape are prepared.

3. A method as claimed in claim 1, in which two sections of different geometrical shape are prepared.

4. A method as claimed in any of claims 1 to 3, in which two T-sections are prepared by cutting an Ibeam.

5. A method as claimed in any of claims 1 to 3, in which the sections are I-sections.

6. A method as claimed in any of claims 1 to 5, in which the sections are prepared from substantially identical grades of steel.

7. A method as claimed in any of claims 1 to 5, in which the sections are prepared from differentgrades of steel and the sections are juxtaposed in accor dancewith the required direction of bending andthe mechanical characteristics desired in the beam.

8. A method as claimed in any of claims 1 to 7, including holding two facing sections, each having a web and at least one flange, in position relative to one another by substantially rectangularframes, one side of the frame having central opening receiving andguidingthewebofoneofthesections,whose flange bears on the outside oftheframe,whilethe opposite side of the frame bears the flange of the other section and has an internal length substantially equal to the width of the flange thereof, and the remaining two sides of the frame have lengths such that once the sections are in position, the facing parts of the sections are no more than afewmillimetres apart.

9. A method as claimed in claim 6, in which bending forces are applied to at least two said frames disposed at the ends of the sections and the reactions are received via at least one said frame intermediate the ends of the sections.

10. A method as claimed in any of claims 1 to 9, in which the sections are joined by welding, boiting, or sticking.

11. A pre-stressed steel beam having a bent web, comprising two longitudinal sections, each having a web and at least one flange, the webs of the sections being in the same plane, and the sections being joined together.

12. A beam as claimed in claim 11, in which the sections are T-sections joined at the edges other webs.

13. A beam as claimed in claim 11, in which the sections are I-sections.

14. A beam as claimed in any of claims 11 to 13, in which the steel in the sections are of different gra des of steel.

15. A beam as claimed in any of claims 11 to 14, in which the sections have different shapes.

16. A method of manufacturing a pre-stressed steel beam, substantially as described with reference to the accompanying drawings.