US2712750A - Finsterwalder - Google Patents

Description

July 12, 1955 u. FINSTERWALDER 2,712,750

METHOD OF CONSTRUCTING REINFORCED CONCRETE BRIDGE Filed April ll, 1949 3 Shee'tS--SheelI l l' il l'neys July l2, 1955 u. FlNsTERwALDl-:R

METHOD OF CONSTRUCTING REINFORCED CONCRETE BRIDGE I 5 Sheets-Sheet 2 Filed April ll. 1949 /n ven for u, s n .fr

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July 12, 1955 u.F1NsTERwALDER 2,712,750

METHOD OF' CONSTRUCTING REINFORCED CONCRETE BRIDGE Filed April 1l, 1949 5 Sheets-Sheet 5 In vemor United States Patent 2,712,756 l THGD i323 CGNSRUCTING REINFQRCEB CNCRETE BRIDGE Ulrich Finsterwaldet', Munich, Germany Application April l1, i949, Serial No. 86,668 Claims priority, application France April 22, 1948 2 Claims. (Cl. 72-8) The present invention relates to a truss bridge of re-enforced concrete and to a method of its construction.

Prior to this invention it was the practice, as a rule, to construct bridges of re-enforced concrete extending over a plurality of spans as a continuous beam in form of a plate provided with webs. limited because of the extreme weight or such webs with large vertical dimensions. While it is true that the weight may be somewhat, reduced by a design of the beams as box-shaped elements better adapted to sustain bending torques, reduction of the thickness of the beams, structures of that kind are uneconomical for spans of l5() feet or more because of their excessive weight.

Moreover, steps have been taken to increase the Vertical dimension of the within the region of the middle pillars to thereby increase the zone of the negative bending torques and to decrease the zone of the positive bending torques intermediate the pillars. This development, however, did not result in a substantial increase of the spans of concrete bridges.

Some progress with respect to an increase of the Span and the reduction of the Weight was attained by the proposition to place the re-enforcing bars outside of the cross-section of the concrete beam proper, and to subject them by special means to a using a specially tough steel. Constructions of this kind involve the disadvantage that the concentrated tension set up in the steel bars must be guided around corners and taken up by anchoring means. Such tensional forces must be guided through points of the beam subject to but comparatively small bending torques, e. g. through the cross-section of zero torque. Therefore, the re-enforced concrete structure must be dimensioned at such points to take up much higher forces than would result from the load. The result is a high dead load limiting the spans for economical reasons.

Other designers consider it a drawback that, with the above stated method, the advantages resulting from the so-called bonding effect between the biassed steel bars and the concrete element is not attainable in each crosssection thus requiring a higher bias to compensate the effects of shrinking and creeping. Such designers use a bias combined with a bonding effect, that is to say, they subject the steel bars to tension in a so-called tensioning bed and then embed the biassed bars over their entire lengths within the concrete. Because of the extreme bias required, complicated and expensive implements must be used for producing it. Using such a method on bridges having several spans would require a sub-division of the structure into a plurality of elements and a subsequent assembly of such elements by means of a rather involved constructing process. It is the object of the present invention, to provide a pluralspan concrete bridge free from the above discussed shortcomings of the prior designs and having substantially increased spans permitting at the same time of a simple, economical and inexpensive way of construction.

According to the present invention this object is attained by the combination of the following features: The main supporting elements are designed as cantilevers freely extending from each of the pillars in both directions, the opposed ends of adjacent cantilevers being sep- The span of such bridges is and, therefore, permitting of a slight main beams in a brace-like manner f high tensional force ,coating the re-enforcing bars by a arated by a gap. The cantilevers are constructed as trusses of re-enforced concrete. Such concrete trusses are built up by progressively forming successive truss sections starting from the pillars without the use of any scaffold supported on the ground, the re-enforcing tension-bars being anchored by the use of elements as ordinarily employed in the steel building art.

In a preferred embodiment of the present invention the cantilever trusses are subject to negative bending torques preferably over their entire lengths as distinguished from a limited zone above the pillars. This effect is attained for instance with a brige extending over three spans by the provision of a separating gap in the middle of the central span, means being provided for ltransferring transverse forces excluding, however, the

transfer of any torques or longitudinal forces. In such a case the spans of a three-span bridge are preferably chosen at a ratio of 1:2:1 thus arriving at cantilever arms of equal lengths extending from each of the two pillars. With a larger number of spans a similar arrangement may be provided. This provision, however, is not apt by itself to solve the problem with respect to an economical method of construction. This object is attained by the design of the cantilever as a truss and by a special method of its construction.

The present invention relates more particularly to two alternative species of the novel method of constructing the trusses progressively. With the first method the truss beams subject to pressurehereinafter called thrust-beams-are formed by the usual process used in the art of concrete structures, whereas the truss beams subject to tension-hereinafter called tension-beamsare constructed by the methods used in the steel building art, the re-enforcing bars for the tension-beams and the thrust-beams being rst placed in position in a slack condition whereupon the thrust-beams are formed of concrete. Then the re-enforcing bars for the tensionbeams are subjected to tension by removal of the scaiolds and are then surrounded by and embedded in concrete.

ln the other species of the novel method of building up the cantilever trusses progressively section by section, both, the tension-beams as well as the thrust-beams of the frame-work are formed in accordance with standard principles used in re-enforced concrete construction, a pressure being set up, however, in the concrete element of the tension beams by means of a tensional bias produced in the re-cnforcing element, the specific pressure so set up in the concrete being substantially of the same magnitude as the specic pressure prevailing in the thrust-beams of the frame-work. To this end, the reenforcing bars consisting of a material having a high tensile strength are prevented from entering into a bonding relationship to the surrounding concrete either by plastic or by forming the concrete beam with longitudinal bores or cavities accommodating the reenforcing bars in spaced relationship to the concrete permitting of a free relative longitudinal displacement. After the concrete has set, the re-enforcing bars extending through the beam are subjected to a powerful bias by use of known implements, the reactionary force of the tension set up in the reenforcing bars serving to compress the surrounding concrete lengthwise. Subsequently, a bond maybe produced between the steel bars and the surrounding concrete element of the-thrust-beams by `lling up the bores or cavities with cement.

Important advantages in various regards are attained by the present invention.

Owing to the provision of a gap in the center, the bending torques approach zero with decreasing distance from the gap permitting of a particularly light design of the adjacent zones of the adjoining trusses as is desir.-

Vthe truss a' continuous one.

Y semifinished condition in which ableV on account of their large distance from the pillars, whereas a maximum bending torque resulting from the own weight of the bridge and largely fluctuating torques from the traic load would result within this zone, were The use of a truss of the described design permits of taking up the torque prevailing at any point with the largest vertical dimension of the trussavailable at such point.Y This again entails the admissibility of an extremely light design of the beams over a large range of the truss. While it is true that the bending torques prevailing in the truss within the Zone above the pillars will exceed those set up in a continuous beam of the customary design, 'this consequence is more than olf-set by the afore-stated advantage regarding the bending torques produced. The savings in weight of th'ela'rge middle span result in a surprising increase of the span possible with a given vertical dimension of the bridge above the pillars, such increase amounting 'to about 35%. With a given span the present invention makes it possible to reduce the dimensions of the bridge thus resulting in savings of concrete VandV steel enhancing the eliiciency. The reduction of the materials required is not purchased at the cost of complicated and expensive construction impiements and building methods for producing the beams. Regarding the dic'ulties involved, the new construction methods do not diiier from those ordinarily used in the normal re- 'enforced concrete construction ar The elimination of a scaffold supported by the ground offers great advantagesin the construction of bridges of large span, particularly in such cases in which such scafolds are hampering navigation on rivers or are endangered by lioods and ice. The novel method of constructing the trusses section by section progressively simplilies the process substantially resulting in arcost reduction. This holds true for both species of the novel .constructing method in which one and the same scalold and the same concrete forms supported by the finished truss sections are used requiring a mini um of capital investment for the constructing implements.

The second species of the constructing method as above described entails the advantageous possibility of adjusting the Vbias set up in re-enforcing bars, as the work proceeds, to meet the requirements arising in the course of the completion ofV the truss. Such'adjustrnents take care of the fact that the distribution of the forces yset up in the beams changes with the progress of the work, since in the iinished truss such distribution is quite diterent from that prevailing in the initial stages when the truss is in a semi-finished condition and has not yet reached its full length.

Another object of the present invention is a displaceable scaiold comprising steel beams imposed on the iinished joints of the upper chord and projecting therefrom a distance corresponding to one section, such scatold being to support the concrete forms so dimensioned as' required and the load t'o be taken up in section.

Further objects and features of the present invention and the manner in Vwhich the same may be performed as well as the advantages attained by the invention will appear from a detailed description ot two preferred embodiments of the invention described hereinafter with reference to the' accompanying drawings.

In the drawings: Y

Fig'. l is an elevation of the 'novel bridge comprising three spans;

Fig. V2 is a partial vertical cross-section through a cantilever ltruss forming part of the bridge said crosssection being take-n within the middle span at a distance from a pillar or" about one quarter of the span and shown on an enlarged scale;

Fig. 3 is a partial view of Fig. l on an enlarged scale showing the central joint of the two cantilever trusses;

Fig. 4 is an. elevation of one section of the truss in a the re-entorcing bars the assembly of one truss s shown in Figs. 4 and 5 are not visible in Fig.

section through a modisecond species of the placeable scaffold and concrete form used to successive-` ly construct one section after the other;

Fig. 9 is an end view from the right of Fig'. 8.

The bridge shown in its entirety in Fig. ll embodying the present invention extends over three spans A, B and C at a ratio of l:2l. lt comprises vtwo cantilever trusses carried by and projecting equal distances from the two pillars 2 and 3. The adjacent ends of the cantilever arms S and 6 are separated by a gap 1. The cantilever arms 4 and 7 constituting the spans A and C are supported on embankment rests or foundations 8 and 9 by customary supporting means such as roller bearings and counterweights or. other means not shown in detail adapted to transfer forces in a vertical direction.

Each of Sthe two cantilever trusses of identical design comprises an upper chord in form of a plate or reenforced concrete constituting the road bed, a lower chord of re-enfo'rced concrete preferably in form of a plate, vertical beams between the two chords and diagonal beams adapted to take up tensional forces.

taken through the truss shownv With this Varrangement the own weight of the structur'e produces negative bending torques only in each of the trusses over their entire lengths, whereas the traffic load produces negative torques only in the middle span, i. e. in the cantilever arms 5 and 6 and positive torques in the outer spans, i. e. in the cantilever arms 4 andV 7. Such positive turques, however, are small compared with the negative torque caused by the weight of the structure and may surpassV the same only in the immediate proximity tothe embankment rests and 9. In Figs. 2, 3, 4 and 5 details of the structure are shown to illustrate the cantilever truss composed of thrust-beams built up in accordance with the standard practice 'of reenforced concrete construction and of tension-beams built in accordance with the standard steel construction practice. Fig. 2 illustrating a cross-section through the middle span taken at a distance of about one quarter of the span from the pillar, shows the lower chord 10 constituted by a standard plate of re-enforced concrete and the upper chord 11 re-enforced by bundles of steel bars 12 in cross-section. These bundles take up a space of about half of the cross-section of the upper chord the latter forming the road bed. Moreover, Fig. 2`shows a vertical thrust beam which, in the present embodiment, is constituted by a plurality of posts or columns i3 of re-enforced concrete. The diagonal tension-beamsl 14 2 as they are hidden from View by the posts V13. Y

Fig. 3 shows the joint between the two cantilever trusses with a gap 1 provided therebetween in the middle of the spain B and with a vertical link 15 of known design for the transfer of vertical forces only.

Fig. 4 shows a section of the frame-work truss in. a semi-finished condition in which the vertical beams 13 and the lower chord section 1l), all subject to thrust, have been completely formed of re-enforced concrete, the scailolding and the concrete forms having beeny removed, whereas the re-entorcing bars. 12 of the upper chord 11 and the re-enforcing bars of the diagonal beams 14, all subject to tension under the weight of the semi-finished truss-sections, have not yet been provided with a mantle of. cement. It will be appreciated that owing to the i'es u moval of the scatiolding the thrust beams l!) and 11 as Well as the tension-bars 12 and i4 are placed under load.

Now the concrete forms are brought into position as will be described later for the purpose .Q mantling the tension- bars 12 and 14 with concrete. Fig. 5 shovf; the finished structure in section illustrating the re-enforcing bars embedded in concrete.

In a modified process the number of re-enforcing bars placed in position in a slack condition for the upper chord sections is limited to the figure required to the weight of the bridge. After assen 13 in the condition shown in Fig. 4, additional bars or cables sheathed with sheet metal or a plastic coat adapted to carry the load of the bridge in its finished condition are inserted in the concrete form for the upper chord and are then embedded in the concrete. After the latter has set` the additional re-enforcing bars or cables are subjected to a bias of such a degree in the surrounding concrete is suiiicient to prevent any tension from being exerted on the concrete of the upper chord section under the iniiuence of the biggest possible load of the finished bridge.

This process has the advantage that the road bed will have an increased density and that cracks are prevented from forming under load. Another advantage is the reduction of the number et sleeve joints and other connecting elements required to connect the tension-bars of adjoining upper chord sections. Each of the biassed tension-bars is anchored at its two ends only, that is to say at two points and not at any beam junction.

The tension-bars may be arranged and distributed in various manners. They may be arranged in a bundle located in a special zone or they may be distributed over the entire width of the upper chord section. The hollow space between each of the biassed tension-bars or cables and the surrounding sheathing may be filled up with cement.

In building up a bridge constituted by cantilever trusses as shown in Figs. l to 5 the following method may be employed:

First two sections of the truss above each of the two pillars 2 and 3 are constructed in accordance with the standard practice using the customary scaffolding and concrete forms except for the tension-beams which are left in semi-iinished condition as shown in Fig. 4. The i the auxiliary scatold shown in Figs. 8 and 9 is brought into position for the purpose of constructing the adjoining sections progressively. This scaffold consists of a plurality of beams 17 arranged parallel to each other at a suitable distance and distributed over the entire width tj the bridge resting on the last two beam-junctions 18 and projecting beyond the end section a distance corresponding to the length of one section as shown in Fig. 8. From such beams the concrete forms 19 are suspended by means of suspending elements 20.

With the aid of this scaffold the lower chord section 21 and the vertical posts 22 positioned thereon are formed of 1re-enforced concrete and the diagonal tension-bars 23 and the upper chord reenforcing bars 24 are assembled. The section '21 is formed adjoining f he previously finished section abutting against the latter at the joint 26. The tension- bars 23 and 24 are connected by means of connecting sleeves 27 and 28 with those of the preceding truss section. Care must be taken to postpone the mantling of the diagonal tension-bars 23 and of the bars 24 of upper chord with concrete until the entire cantilever arm has been built up in the described manner up to its end.

The other species of the novel method of constructing the cantilever trusses will now be described hereinafter with reference to Figs. 6 and 7.

Fig. 6 illustrates one section of the cantilever truss diftering from that illustrated in Figs. l to 5 by the provision of thrust-beams as diagonal braces 3? in lieu of the diagonal tension-beams shown in the first described embodiment. The road bed plate 32 constituting the upper chord that the resulting pressure set up titl 31 of the truss as w il as the vertical posts 33 are biassed in a special manner. Only part of the reenforcing bars of the upper chord are anchored at 34 at each beam junction, while the rest of the re-eniorcing bars 35 extend the whole length of the bridge. Whenever a truss section has been finished and its concrete has set, the short tension-bars of the upper chord are put under tension. In order to prevent the tension bars from entering into a bonding relationship with the surrounding concrete and to thus render a subsequent tensioning possible after the concrete set, the re-enforcing bars to be subjected to a bias are either coated with a plastic or inserted in hollows or cavities or bores provided in the concrete.

Fig. 7 shows clearly that the tension-bars 34 are anchored at the beam-joints whereas the tension-bars 35 are running clear through the entire length of the bridge. With this mode of construction the truss may be progressi ely built up section by section with the aid of the auxiliary scaffold shown in Figs. 8 and 9 with slight modiiications to suit the requirements.

The term re-enforcing bars used in the claims following hereinafter is to be construed as including wires, rods, solid bars, cables or the like. While my invention has been described hereinabove with reference to specic embodiments thereof 'i wish it to be clearly understood that my invention is in no way limited to the specific features of such embodiments but is capable of numerous modifications within the scope of the appended claims.

What I claim is:

l. A method or" constructing a reinforced concrete cantilever bridge comprising constructing a center pier, anchoring reinforcing rods for the tirst vertical center strut and first panel lower chords in said pier, these being compression members with the lower chord being started simultaneously vfrom opposite sides of said pier, supporting the outer ends of said lower chord members by reinforcing rods for forming a diagonal strut joined to the upper chord, said diagonal strut and upper chord being tension members, encasing the compression members with concrete and thereby adding load to pretension said tension members, progressively repeating in the same manner the erection of compression and tension members panel by panel in opposite directions from the pier until mid-span is reached, an-d then encasing said tension members with concrete.

2. A method of constructing a reinforced concrete bridge as in claim l, further comprising constructing said upper and lower chords as slabs extending the entire width of the bridge.

References Cited in the tiie of this patent UNiTED STATES PATENTS OTHER REFERENCES Civil Engineering Publication, March 1941, vol. 11, No. 3, pages 15G-153.

Engineering News Record, Prestressed Concrete Design, October 18, 1945, pp. 92 and 93.

Engineering News Record, June 12, 1947, pp 112, 113.

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