Long Span Prestressed Concrete Bridges in Europe

Transcription

1 PROCEEDINGS PAPER Long Span Prestressed Concrete Bridges in Europe by F. Leonhardt* INTRODUCTION Long span prestressed concrete bridges were built very early in Europe. Freyssinet built the first of his famous series of Marne bridges, the Luzancy bridge, in 1946, with a span of 245 ft. In 1949 the author began to build the first prestressed bridges with continuity over several spans including the bridge across the Neckar-Kanal in Heilbronn with a main span of 320 ft. In 1950 Finsterwalder built his first bridge by the free cantilevering method and in 1952 constructed the spectacular bridge over the Rhine at Worms with three spans of 330, 371 and 340 ft. Between 1950 and now, more than 300 bridges with spans over 250 ft. have been built in Europe and many different construction methods have been developed. The longest span of prestressed concrete is now 780 ft. and it was designed and constructed by European engineers for crossing Lake Maracaibo in Venezuela (Fig 1). Thus, prestressed concrete has proved its feasability for long span bridges, mainly by virtue of its great advantages: economy, durability, low maintenance costs, high *Consulting Engineer West Germany 62 fatigue strength and the possibility of achieving extreme slenderness. For long spans, only the posttensioning method is used. Different systems of prestressing tendons are utilized including high tensile bars with a prestressing force of 30 to 50 tons, wire cables in circular sheaths with forces between 50 and 180 tons and finally the so-called concentrated cables, mainly built up with seven-wire-strands, with prestressing forces between 1000 and 3000 tons per cable. METHODS OF CASTING IN PLACE From the very beginning, there have been different methods of construction. Freyssinet assembled his Marne bridges with precast elements having ducts through which the prestressing cables were pulled in and tensioned. The Heilbronn bridge was cast in place on a centering, and Finsterwalder's bridges were cast in place in short portions on cantilevering steel falsework. These three methods are still in use and the oldest method, casting the concrete in place, is still very popular in Europe, especially in Germany. Making forms on falsework or centering formerly required a large amount of skilled labor and expensive timber. However, this work has been greatly simplified and stand- PCI Journal

2 Fig. 1 Bridge Across Lake Maracaibo, Venezuela Fig. 2 Light Steel Centering for Casting in Place and parts have been developed which enable the forms to be assembled largely with unskilled labor and with less than half the manhours formerly needed. The main savings have been obtained by the use of special equipment for the centering (Fig. 2). This consists of lightweight steel-shoring, tripod shoring towers or lattice type horizontal shoring which can be altered in length by telescoping (Fig. 3) to adapt them to the necessary span and can also be modified in their load-carrying capacity by the insertion of additional chord-members. Usually, several shoring towers are combined by light lattice-work to increase the resistance to wind forces. The lengthwise spacing of these towers varies from 30 to 90 ft. depending upon the height and the load to be carried. The girders, bridging the gap, can be cambered to compensate for their deflection, thereby avoiding the troublesome timber-fillers formerly used on rolled steel-beams. Very large bridges have recently been built using such centering and forms and they have proved to be economical in keen competition with February

3 Fig. 3 Lattice Type Horizontal Shoring other methods such as pre-fabrication. MERITS OF CONTINUOUS BRIDGES Casting in place also helps make long bridges continuous over many spans, thereby eliminating expansion joints and saving steel. We do not fear differential settlement of piers because the slender prestressed concrete beams are not much affected by such settlement. The bending moments caused by the :settlement of a pier will be eliminated to a large extent by the creep of the concrete as was proved in Reference 1. Recently, we built a long beam-bridge, continuous over 12 spans, in a mining area in which ground settlements of about 15 ft. are expected during the next 25 years (Duisburg on Rhine, Fig. 4). The column footings of this bridge can be hydraulically adjusted in the vertical direction and rest on roller bearings to permit horizontal ground-movement. Even for such extreme cases, continuity of prestressed concrete beams proved advantageous over a steel bridge, and especially over statically determinate single beams with which it would have been almost impossible to keep the bridge under traffic during periods of settlement. The capacity for resilience following inelastic deformation is much greater with prestressed concrete beams than with steel beams. COUPLING OF TENDONS These continuous bridges are now often constructed span by span on centering with the tendons or cables being coupled at the 1/s-span section (Fig. 5). With this construction '._..-_. _:. I ^ 1 ^ axharyel wromnrreaors 3 Fig. 5 Coupling of Tendon at 1/5-span Section method, the centering and forms can quickly be reused. In some cases, the centering is taken down Fig. 4 Bridge Across Mining Area in Duisburg 64 PCI Journal

4 Fig. 6-4ypical Elevated Highway (Dusseldorf) and reerected, in other cases the whole centering is moved on rails. In this way, long elevated highways have been built (Fig. 6). The completed bridges are often curved and have total lengths of up to 2800 ft. with continuity over 34 spans (Hochstra j3e Dusseldorf). Some of our contractors have developed systems which are almost machines providing forms attached to steel-trusses which span from pier, to pier, and which can be moved span to span by rolling on other steel-girders (Fig. 7). The trusses carrying the forms are first placed in position for placing the concrete in the right span. As soon as the concrete has been partially prestressed, the hauling girders are connected to the cantilevering end of the concrete bridge and then the trusses can be moved to the next span. When the trusses between the main concrete girders are being moved to the next span, the outer trusses with the forms of the cantilevering portions of the roadway slab are still in the former span. The hauling girders and the a o a o F ^ o 0 o Q Q Fig. 7 Movable Steel Formwork (Strabag -Ko1n) February

5 trusses are carried by the final piers so that no extra foundations are needed. For one bridge with 120 ft. spans, it took two weeks to construct a span. Prestressing commenced four days after the concrete had been placed. A simpler but similar method is shown in Fig. 8. Hollow box steel girders placed below the concrete bridge to be built are supported by cross-beams resting on the bridge's pier columns. These box girders carry the forms. After the prestressing of the bridge, girders and forms are lowered hydraulically and moved to the next span using lattice-girder exensions to reach the next supports. This equipment proved very successful in difficult territory. For example, Fig. 9 shows it being used for a bridge along steep hill slopes in the Rhine valley. Casting in place into forms carried by steel trusses supported by auxiliary piers has been used also for the construction of the main spans of the Maracaibo Bridge (F'ig.10). Fig. 8 Simpler Movable Forms (Polensky u. Zollner, Koln) Fig. 9 Use of Equipment of Fig PCI Journal

6 Fig. 10 Steel Trusses Carrying Forms Maracaibo Bridge Fig. 11 Bendorf Bridge (Finsterwalder) Dywidag System FREE CANTILEVERING METHOD The free cantilevering method is, of course, known in the United States. Some of the latest achievements using this method to cast the concrete in place include the Medway Bridge near London with a 500 ft. span, and a bridge across the Rhine in Bendorf (Fig. 11) with a span of almost 680 ft. In the latter the equipment for cantilevering was much simplified by replacing the counterweight with vertical anchors into the finished part of the bridge. The deflections of the slender cantilevering beams are influenced greatly by the early creep of the concrete and, therefore, by the moisture and temperature of the air during the hardening period, both of which change with the weather. Therefore, much engineering skill is needed to make the method fully successful. In several cases, the free cantilevering method has been used to build parallel girders continuous over several spans. Of course, such bridges cannot be freely cantilevered, but must be supported temporarily by diagonal steel tie cables (Fig. 12) which run over the tower above the pier of the bridge. February

7 The auxiliary support can also be provided by steel-columns (Fig. 13) within the span of the bridge, as can be seen in bridges near Duisburg which have been built crossing over factories. With the Dywidag system, prestressing rods are used for this free cantilevering method and they are built in directly and coupled every twenty feet. These many couplers are costly and, since the prestressing force of such a bar is limited, a large number of such bars are needed for long spans. Large prestressing elements provided by wire cables are, of course, preferable. In fact, one of our firms has built a 470 ft. span across the River Main near Frankfurt with the cantilevering method using 100 ton prestressing tendons which are threaded through the ducts as they are needed for resisting the cantilevering moment. At this bridge, the joint in the middle of the main span between the ends of the cantilevering beams has been closed and full continuity has been achieved by careful arrangement of the tendons. Fig. 12 Free Cantilevering Method Using Tie Cables Fig. 13 Movable Cantilevering Equipment (Dywidag) 68 PCI Journal

8 There is no doubt that the free cantilevering method has great advantages, especially if large tendons can be used. The work cycle is repeated every three days step by step and the team of workmen can attain a very high standard of efficiency and workmanship by these repetitions. ADVANTAGES OF CASTING IN PLACE As a final reason for continuing to cast concrete for bridges in place, I would put forward the fact that the site-mixing equipment for concrete is today almost fully mechanized and can be erected anywhere on short notice. Usually two men are sufficient to operate it. Therefore, factory-made or readymixed concrete no longer has any advantage over site-made concrete if large quantities are needed. Some further advantages of casting concrete in place must not be overlooked: 1. There are no difficulties with joints and a fully homogenous structure is obtained. 2. The designing engineer has much more freedom to choose the most economical cross-section for the girder system and to consider all the irregular features which are so frequently encountered in crowded areas such as skew-crossings, curves, progressively widening or flaring portions of the bridges, etc. equal spans are constructed. The simplest type of bridge for prefabrication is that comprising simply supported beams with open joints over the piers. Especially in the United States, this type of bridge has been highly developed. However, the spans are normally limited to about 120 ft. because longer girders become too heavy for transporting and handling. For long bridges it pays to erect a plant for prefabrication close to the bridge. It also pays to use special equipment for hauling and placing heavy girders; for example, the bridge across the St. Lawrence River in Canada where 176 ft. long girders, each weighing 180 tons, were installed with a launching bridge made of steel trusses. Simpler means for moving such girders into the final position have been developed lately in Luxembourg and Germany. Nenning puts a simple steel bridge over two spans (Fig. 14) in PREFABRICATION Total Span Length Girders The subject of long span bridges for which prefabrication has been used will now be discussed. There is no doubt that in many cases precasting has great advantages over" casting in place, especially if many Fig. 14 Launching Precast Girders (Nanning. Luxemburg) February

9 gaps on the piers at an elevation just below the girders. He moves the girder on this steel-bridge with long hydraulic jacks with an engine of only 3.5 h.p. The girders are then moved transversely into the final position. The author has developed methods of establishing continuity in such girders after their erection and can thereby effect a saving of about 20% in the amount of prestressing steel in addition to a savings in weight. For spans of 190 ft. and a spacing of the main girders of 12 ft. the prefabricated part of the main girders (Fig. 15) weighed only 140 tons (normal weight concrete), and 10 prestressing cables of 72 tons each were sufficient. These 10 cables are prestressed in three different stages and coupled at the joint above the pier (Fig. 16) in such a manner that longitudinal compression is also obtained in the deck slab above the piers thereby providing resistance to negative moments without tension in this slab. This method might, however, be limited to spans of 200 ft. because longer girders for the full span length may become too heavy to be handled in one piece. If the bridge is to be built across water with sufficient depth for the use of floating cranes, some of which have carrying capacities up to 200 tons, such cranes might be used for placing even longer and heavier prefabricated girders. Fig. 15 Cross Section of Bridge with Precast Girders w:wa ema^.ap. Fig. 16 Tendons of Girders in Fig. 15 at Pier 70 PCI Journal

10 For the Maracaibo bridge, girders having a 154 ft. span and weighing 180 tons were placed in large numbers. The French engineer, Nicolas Esquillan, and his firm Boussiron have prefabricated the full span length and the total cross-section of a double deck bridge with a hollow box-section for a highway on top and a railway running through the box at Abidjan on the Ivory Coast in West Africa. The 140 ft. long superstructure weighing 800 tons was prefabricated on the shore and then floated to its final position on two barges. There is no doubt that even larger bridges can be prefabricated in this way and floated into their position. Good examples for these possibilities have been given in this country by the prefabrication and floating of large tunnel sections or sections of floating bridges. These examples relate to the prefabrication of girders for the full length of the span, and there are only longitudianl joints if separate units are used. It is, of course, also possible to prefabricate short lengths of the total cross-section or of parts making up the cross-section. The earliest example of this construction method was provided by Freyssinet for his five bridges over the Marne, which I have mentioned at the beginning. Another well known and more recent example is the construction of the Hammersmith flyover in London for which a centering was necessary which would carry the total weight of the bridge. Prefabrication with Transversely Cut Units A new method has been developed by the author with a view to using longer and heavier units for the prefabrication of large-span bridges (Fig. 17). This method is illustrated by the Ager Bridge in Austria, which has four spans, the largest being 314 ft. long. The cross section of the bridge shows two hollow box-girders, each carrying three lanes of the Autobahn, about 40 ft. wide. The depth of 14 ft. is constant. For prefabrication, these box-main-girders have been divided transversely in 30 ft. long portions and these portions with the full width, weighing about 180 tons each, were prefabricated using stationary forms. The reinforcement was pre-assembled and placed by a tower crane. Eleven men were able to make each section within four days. Since these pieces were too heavy to be moved by normal 5.SOi-i x i+5 20 in 1/2 in 1/4 29, zs0-7.5r 7rso lr--48 T5 50 n w i05 a ^' g ^ NaON S^nmrr oo ^ o_r RrtS^I FrngMl, ^ SO Fig. 17 Ager Bridge, Austria February

11 cranes, they were moved by sliding on two timber rails with lubricated surfaces (Fig. 18). These rails were supported between piers and temporary towers by a narrow centering along the valley so that all the sections could be placed one behind the other over the four spans to form the bridge. The concentrated tendons were then placed along the sides of the webs within the hollow box tunnel. A jeep, running on the bottom slab of the hollow boxes, was used for placing the strands. After the cables were installed, the gaps between the sections were concreted, including transverse frames with the cable-bearings for resisting the vertical forces due to the change of direction of the cables. The cables were then prestressed by means of movable anchor-blocks at the two ends of the bridge according to the Baur-Leonhardt method. Very low friction for the long and multicurved cables was secured by using teflon between the sliding plates at the cable bearings. A good bond (Fig. 19) between the cables and the webs is achieved by stirrups anchored in the webs and by the concrete cover which also protects the cables against corrosion. So far, this construction Fig ft Sections of Ager Bridge Sliding on Timber Rails method has been found to require the least amount of labor and a very small quantity of material considering the large spans. These large prefabricated units can also be used to build long span. bridges across waterways by means of the free cantilevering method if the bridge portions can be floated to the site (Fig. 20). Such a bridge has been fully designed for spans of 240 ft. and the prefabricated units shall be lifted with the equipment which has been developed in this country for the lift-slab method, hydraulic jacks resting on cantilevering steelgirders. After the portion of the hollow box-girder has been lifted to its final elevation, the prestressing cables will be pulled in through the ducts in the top-slab and tensioned. Then the next piece can be lifted. It is obvious that long and multispan bridges can be built rather economically and quickly in this way if there are a sufficient number of spans so that the forms for the large units can be reused often enough. Another variant of this method is being used to build a bridge across the River Caroni in Venezuela with six spans, four of 320 ft. and two of 160 ft. span length. Large rivers in 72 PCI Journal

12 j rd sec/ A A UL some ue - i ^ A y' finally COnrrelee rip 0r0 r Mf /Orce, slimup m8' benl back ^ smg/e-size grove% graulea /with Dro7ion/ a. rre ^,7, ; s^^ 6ie 'reinrcemenf' - ^ "` ' ar Nb 'e^, ^^^' f am rubber leakproof 5 ' 00 n B-B formwork sel ac-c Fig. 19 Detail of Traun Bridge, Austria Ekvafion O ss,o.ahox ^.vcvs DM 4 f Pb Free cantilevering method with prefab. hallow box units. Fig. 20 Precast Box Sections Set with Lift Slab Jacks-233 ft. Spans tropical countries are often liable to center of gravity and produce axial heavy flooding so that centering compression. In this way, the struccannot be used. For this reason the ture can resist variable positive or entire bridge with a total length of negative moments along its whole 1660 ft. is being prefabricated on length during the subsequent hanthe approach in the same way as dling of the bridge (Fig. 21). The the Ager Bridge. The large pre- prestressing is done from the far stressing cables along the inner end by moving one large anchor sides of the webs are, however, in- block. The elongation of the cable stalled straight in the first stage of is about 8 ft. construction at the elevation of the The bridge as a whole, weighing February

13 Fig. 21 Caroni River Bridge, Venezuela +uo..ssom.-.. rszom- vnm. wywemw.e.m^e^ Ovsf rx. _ 500 ft. span bridge for prefabrication $ and free cantilevering Fig. 22 Design for Indus Bridge by Author 8,400 tons, will be moved hydraulically across the river. A new type of sliding bearing will be used at spaces of 160 ft. along the approach and on top of the five permanent and four temporary auxiliary piers. Two hydraulic jacks of 250 tons capacity each will do this job. They are located at and act against the abutment. In order to decrease the cantilever bending moment of the head of the moving bridge, a light steel truss is fixed to the head. After the final position is reached, the cables will be moved vertically upwards at the supports and downwards in the spans, and fixed in their final curvature so that the bridge will be supported over the large spans by the forces due to the curvature of these cables. The auxiliary piers may then be removed. The full advantages of prefabrication are obtained by reusing the forms, placing fully pre-assembled reinforcement and concreting in one spot immediately beside the automatic concrete mixing plant. The number of joints and amount of concrete to be cast in place are thus reduced to a minimum. In these last-mentioned examples, the prestressing tendons have been installed after the assembly of the prefabricated parts by threading 74 PCI Journal

14 wire cables into ducts or by spinning strands so as to form cables over long lengths. Both these methods reduce to a minimum the amount of labor involved in making cables so that post-tensioning has become very economical, especially if large diameter wire such as 10 to 12 mm, and large cables developing over 100 tons of prestressing force are available. Long Spans with Tie Cables In tropical countries, it is often necessary to build bridges across large rivers where poor soil conditions make very deep foundations mandatory. In such cases, spans of 400 to 600 ft. are economical and, formerly, steel trusses were almost the only solution. For such cases, the author has developed a system using prestressed concrete in a very economical way (Fig. 22). On top of the well-foundations, an A-shaped tower is erected with the horizontal leg at the elevation of the roadway. This tower carries a long and slender beam, cantilevering to both sides, supported by inclined tie-cables every 60 ft. to 80 ft. The tie-cables are anchored directly to the beams, which are edge beams and which simultaneously form the railing. With this main-girder arrangement, the edge beams can be prefabricated in sections about 60 ft. to 80 ft. long, which can easily be erected by a crane or by a special transportation wagon which runs under the roadway and between the towers on a cheap trestle bridge. As soon as the edge beams are hung to the tiecables, the prefabricated elements of the road-slab are placed between the edge beams as can be seen from the cross-section in Fig. 22. The ends of the cantilevering maingirders between two towers are closed by a suspended span. The quantities of concrete and prestressing steel are remarkably low for this type of long span prestressed concrete bridge and only three different precast elements are needed for the superstructure. The towers can be built with slip forms, cast in place or with precast elements. The edge beams are prestressed longitudinally with two or three tendons which will be pulled into their ducts after the erection of the cantilevers is finished. The bending moments in these main-girders are relatively small if parallel wirecables are used for the inclined ties. CONCLUSION The author hopes that he has shown that the field of long span prestressed concrete bridges gives good opportunities for the civil engineer for using and proving intuition and creativeness and he is sure that further progress in this field is still ahead of us. REFERENCE 1. Leonhardt, F., Prestressed Concrete Design and Construction, English Edition, Wilhelm Ernst u. Sohn, Berlin, Presented at the Ninth Annual Convention of the Prestressed Concrete Institute, San Francisco, October, February