US2842786A - Bridges - Google Patents

Description

BRIDGES I 2 Sheets-Sheet 1 Filed June 22. 1953 mu mm R. DIGBY-SMITH ET AL BRIDGES July 15, 1958 Filed June 22. 1953 2 Sheets-Sheet 2 United States Patent BRIDGES Rodman Digby-Smith, South sWoodford,;and Ralph Alan Sefton-Jenlrins, London, England, assignors to Engineering & Industrial Exports Limited, :London, England, a British company Application June 22, 1953, Serial No. 363,17-2

4 Claims. -(Gl. -14'6) This invention relates to bridges, and is particularly concerned With an improved method of and means for increasing the load-carrying capacity of ,temporaryor semi-permanent bridges built up of readily assembled standardized sections or units forming a braced girder structure including articulated upper and lower chord members.

All bridges of the braced girder type have some degree of sag due to their own weight which, in:bridges of conventional type, is compensated in advanceby constructing the bridge of components so designed that the bridge will have an apparent upward camber equal to the anticipated sag. In bridges of the character referred to, the sag is relatively great .due to the :fact that .-the bridge is built up of sections or units loosely pinned together, and as usually such sections or units *have to be usable in any combination, ,andeither way up, it is not usually possible to compensate for sag 'by modification .in the design or construction of thecomponents.

The present invention has for its object to provide "a simple and effective method and means whereby a bridge structure of the character referred to maybe pre-stressed so as not only to compensate for natural sa-g of the structure, but to enable the whole structural strength thereof to be employed for the support .of useful loadginstead of part of the strength being absorbed in supporting the weight of the structure itself, thereby enablingthe loadcarrying capacity of the bridge to be increased. The weight of the structure will often-absorb from one quarter to one half of the strengthiof the bridge, so that by prestressing the structure the carrying capacity can be increased by from 33 /3% to 100% depending on the circumstances.

According to the present invention, the load-carrying capacity of a bridge of the character referred to is increased by a method which comprises providing auxiliary tension members arranged and pre-tensioned to effect pre-stressing of at least the main members of the bridge structure opposite to the stresses therein due to dead-load.

More specifically, according to the present invention, in a bridge of the character referred to, in which the top chord members of the bridge structure is normally stressed in compression and the bottom chord member in tension, the load-carrying capacity is increased by a method which comprises providing tension members, e. g. of high tensile steel, below the lowest chord member of the bridge, these members being tensioned so that the bridge is pre-stressed in such manner that the stresses in the upper chord members of the bridge, due to the dead weight of the bridge itself, are exactly or approximately cancelled out, whilst the lower chord members are placed in compression. On the application of the live load to the bridge, the upper chord members can be stressed to their full capacity by the load alone, whilst 'load stresses and the strength of the structure. purpose the struts may be adapted for spacing the ten- 2,842,786 Patented July 15, 1958 .stressed by any suitable means, to pre-compress the lower chord members of the structure 'and at the same time exert an upward component accompanied by tensioning of the upper chord members of the structure. Preferably the tension members are arranged to extend below the bridge structure, said tension members being connected to anchorages adjacent the ends of the lower chord members and being spaced below said lower chord members by suitably arranged struts at a suitable distance from said anchorages. The tension members may be constructed in sections corresponding in length to that of the sections or units' of the bridge structure, and the said struts preferably are arranged at junctions between the sections or units near the ends of the bridge structure. It is also preferred that the ratio between the vertical height of said struts and the vertical height of the bridge structure should be equal to the ratio between the dea For this sion members at various distances below the lower chord members of the bridge structure, for example, the struts umay have a series of holes therethrough for the alternative reception of a slidable element or assemblyadapted for-the articulation of tension member sections.

.The invention is hereinafter described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Fig. l is a perspective view illustrating a bridge structure of the character referred to provided with pre-stressing means in accordance with the invention;

Fig. 2 is a detail View in side elevation of one of the anchorages for the tension members; and

Fig. 3 is a perspective exploded view of a strut assemblage.

In carrying the invention into effect according to a particularembodiment, and with reference to the accompanying diagrammatic drawings, the invention is shown as applied to a temporary or semi-permanent bridge built up of :braced girder sections 1 assembled end to end and pinned together. According to the invention, longitudinally arranged tension members 2, preferably in duplicate, are associated with the girder structure assemblage to extend from end to end of the bridge structure below the lower articulated chord member 3 thereof. The tension members 2 are spaced below the chord member 3 by struts 4 which preferably are associated with the junctions between end and adjacent girder sections 1 of the structure. The tension members 2 preferably are built up of parts in standardized lengths corresponding to the lengths of the section of which the bridge structure is composed and, asshown in Fig. 3, the duplicate tension members 2 may be connected by means of articulated coupling blocks 5, 6 connected by means of a pin (not shown), the parts being connected to the blocks by screwing so as to avoid welding of the parts. The terminal parts of the tension members 2 extend through holes in anchorages 8 provided at the lower parts of end posts 7 of the bridge structure, said anchorages having suitably inclined faces 9 against which are seated washers it? having concave outer faces for the reception of convex faces of outer washers 11 coacting with nuts 12 on screwthreaded end portions of the parts, the washers 10 having 3 enlarged clearance holes so as to allow for limited variation in the angular disposition of the washers 10, 11 according to the spacing of the tension members below the lower chord member 3 of the bridge structure. The tension members may be tensioned by any suitable means, such as hydraulic jacks adapted to engage the-projecting screw-threaded ends of the terminal rod sections, and when the desired tension indicated by conventional guage means is attained, the nuts 12 are screwed down so that the washers 10, 11 are engaged to retain the tension.

The struts 4 preferably are adapted to be readily connected to the bridge structure as by means of fittings 15 at the top, and preferably also are adapted to provide for spacing the tension members at various distances below the bottom chord member 3. For this purpose the struts 4 are of box-section, as shown in Fig. 3, and are provided with rectangular openings 14 therethrough, said openings 14 being spaced in the height of the struts 4 as shown. The openings 14 are adapted for the slidable reception of the block to allow for such relative movement between the parts as may be occasioned by changes in temperature and load conditions. The struts 4 are at the lower end provided with a projecting part 16 adapted to coact with a transom in the form of a girder 18. The transom 18 may be fixed against the part 16 by means of a clamp 19 having a hinged part 20 adapted to engage in a T slot 21 in the lower part of the strut 4 and having a clamping screw 22. The struts 4 are braced laterally by means of bars 24 or thelike connected to suitable fittings 23 of any suitable kind.

When the tension members 2 are tensioned to a suitable degree the resulting upward component exerted through the struts 4 takes up the natural sag of the bridge structure so that the lower chord member 3 of each braced girder is pre-compressed and the upper chord member 13 pre-tensioned, to an extent sufiicient to compensate approximately for the opposite stresses resulting from the dead-load. As a consequence of the pre-stressing of the structure, the whole structural strength of the bridge may be used to support useful load and as a result the load-carrying capacity may be increased substantially so that a higher load can be carried without the necessity of multiplication of sections or units and consequent increased demand on transport facilities. It is estimated that for a given load-carrying capacity, a saving of 15% may be effected in the total weight of bridging material required. Moreover, as the cost of the tension members is considerably less than that of additional girders, a very substantial saving estimated in the neighbourhood of 20% is obtainable.

Pre-stressing of a bridge structure in accordance with the invention may be etfected immediately after erection, or the bridge structure may be used normally and prestressed subsequently if it should be necessary to increase its load-carrying capacity. The advantage resulting from pre-stressing in the manner described increases as the proportion of dead-load to live-load increases and generally the greater the dead load the greater is the gain. Further increase in load-carrying capacity can be obtained if the bridge is stressed so that it has an initial upward camber under dead-load.

It will be understood that the invention is not limited to the particular embodiment hereinbefore described, and that the tension members may be arranged in any other 4 suitable manner to achieve pre-stressing of a bridge structure in the manner and for the purpose described. For example, in the case of continuous bridges, the tension members may extend for a part of their length above the top chord member of the bridge structure over the end posts or supports.

What we claim is: Y

1. In a bridge of the type formed of standardized sections assembled to constitute a braced girder structure including articulated upper and lower structural members vertically spaced from each other by web members and having fixed anchorages adjacent the ends of said lower chord members; the invention comprising auxiliary tension members extending below said lower chord members and connected intermediate said fixed anchorages, struts depending from said girder structure at junctions between adjacent sections and provided with a series of apertures selectively receiving the tension members at various distances below the lower chord members, and means tensioning said auxiliary tension members to exert an upward component on the girder structure to compress the lower chord member and to tension the upper chord member.

2. A bridge as defined in claim 1 wherein the tension members comprise a plurality of articulated sections substantially corresponding in length to the sections of the bridge structure, and further including means for connecting adjacent tension members comprising a first block secured to the end of one of the tension members and extending through an aperture in the associated strut, a second block connected to the end of the other tension member, and pin means connecting said first and second blocks.

3. A bridge as defined in claim 1 wherein said struts and said tension members are positioned on both sides of said bridge, and further including transverse transoms connected intermediate the lowermost portions of opposite struts below said tension means.

4. A bridge as defined in claim 1 wherein the face of each anchorage external with respect to the center of the grider structure is inclined inwardly, said anchorages having longitudinal bores therethrough perpendicular to the surface of said inclined face adapted to receive said auxiliary tension members, and the means for connecting each end of said tension member to said anchorages when the free end thereof projects from the associated anchorage inclined surface consists of a first washer seated against said inclined surface having a concave outer face, a second washer having a convex inner face adapted to cooperate with the concave face of the first washer, and nut means screw-threadably mounted on the end portions of said tension members.

References Cited in the file of this patent UNITED STATES PATENTS 530,425 Keating Dec. 4, 1894 534,032 Clymer Feb. 12, 1895 762,632 Headley June 14, 1904 922,885 Gourlay May 25, 1909 932,118 Gerhardt et al. Aug. 24, 1909 1,594,505 Frye Aug. 3, 1926 2,101,538 Faber Dec. 7, 1937 2,155,121 Finsterwalder Apr. 18, 1939 2,510,958 Coft' June 13, 1950

Images