Pix4, Pix2 and Pix5
|
|
|
BRIDGES OF BUDAPEST – Megyeri híd
Remy P. Papp, PE
Bridges appear to have universal appeal. They evoke a sense of wonder, a sense of inner fulfillment, a bit of awe and sometimes of artistic appreciation.
Development of the art and science of building bridges did not come all at once, it took millennia to bring it to its present status. In the beginning, a bridge was nothing more than a log felled across a creek, by wind or later, by axe. The Romans built amazing bridges, but those were limited by the lack of materials strong enough to span greater lengths. Roman bridges were masonry arches developed for the construction of large edifices. Lack of tensile strength (resisting pull forces) limited spans to the compression resistance potential of stone or brick.
Introduction of iron was really the first innovation that made the construction of longer spans possible. Then, steel was introduced, which really advanced the art of bridge building. Not only were steel structures capable of extending the distances the structures were able to span, but combining steel with concrete, called reinforced concrete, also contributed.
Most people are unaware of the fact that bridge building advanced over time not only because of the development of technology, engineering and improvement of materials, but to a great extent due to innovation in the process of constructing bridges. Bridge contractors are a very special breed, something the author can attest to from personal experience. People engaged in bridge construction are dedicated and innovative professionals. The word “professional” in this group, however, is not restricted to university-trained engineers and specialists, but includes the workers as well: laborers, carpenters, ironworkers, etc. whose contributions not only to execution of the work proper, but its planning and methodology are vital and not sufficiently appreciated.
The Romans did have trying times in constructing those arched viaducts. The arches, put together stone by stone, brick by brick, did not stand up by themselves until completed, that is, until the arch reached across the span and the closure had been made, either by the two sides meeting in the middle or until the “keystone” had been inserted. Until then, enormous scaffolding was necessary, set on the bottom of the valley or founded in water that would hold up the weight of the masonry. This limited severely the location of a bridge. It was really only toward the end of the 19th century that material and methods could be developed to overcome the necessity for such temporary accessory structures for the construction of a bridge.
Let us start by describing the latest Budapest bridge, the “Megyeri híd”. Construction was started in 2006, and it opened to traffic on September 30th, 2008. It is one of the newest types of bridge, well represented in the world elsewhere, but a first in Hungary. It connects the northernmost shores of Budapest (Káposztásmegyer on the Pest side with Békásmegyer on the Buda side), and is part of a yet-to-be completed “beltway”, which will allow by-passing the City proper entirely.
The technical term describing this bridge in English is “Cable-Stayed”, in Hungarian “Ferde-kábeles” (“slant-cabled”). To the author’s knowledge, the first such bridge - the Severin Bridge - was erected in Köln, Germany, over the Rhine River, in 1958. That particular bridge was not only a pioneering one, but was also built asymmetrically, a feature that to our knowledge has not been replicated elsewhere. Although suspended by cables, it is not a cable bridge which would hang from a gracefully arching set of cables strung between two tower piers and be anchored within the abutments. A cable-stayed bridge is suspended entirely from the pier towers and, in effect, combines the functions of the “cable” itself and the “suspenders” hanging from the cable. In this type, no anchorage of the cables is required within the abutments (eliminating two very substantial structures in a bridge).
One of the main advantages of such construction is that a great deal – actually almost every part of the bridge except for the piers and abutments – can be prefabricated off-site and assembled on-site. Stringing a cable for a cable bridge (called “spinning” in the trade), on the other hand, is an elaborate process of pulling across the span, from one abutment to the other, each pair of “piano-wires” by means of a “spinning wheel” riding on its own separate cable. Considering that the famous Golden Gate Bridge’s cables each contain some 27,000 individual “piano wires”, you can easily see what a tedious and time-consuming task this phase alone becomes. Besides, spinning a cable involves a great deal of constant adjustment due to daily temperature changes, which lengthen or shorten each one of the “piano-wires” being spun and laid down.
On the other hand, in a cable-stayed bridge, once the towers have been
erected, pre-fabricated cables can be hung from those towers and used as the bridge sections are hung from them in sequence, starting from the towers and proceeding toward the center of the main span in one direction, and toward the abutments in the opposite direction. Again, the bridge sections will have been prefabricated off-site and brought to the bridge on barges; then they are lifted up from the bridge itself or, as in the Megyeri bridge, by floating cranes, as they are fabricated, and are attached to the previous section.
Once the bridge is complete, there remain the tasks of laying down an asphalt or concrete layer - a so-called “wearing course” (which can be replaced as traffic on the bridge wears away this surface with time); the construction of the railing and the median separating traffic in the two directions; perhaps sidewalks; installation of the light poles and signage; possible installation of utilities under the bridge, such as water-pipes, electrical conduits, telephone and other communication cables and cable trays, etc. In recent times, there is frequently also a travelling inspection bridge suspended under the bridge deck , to allow easy access to the underside of the deck for visual inspection, which is such an important part of the very necessary maintenance of the completed structure over the years of its useful life.
While such ease of construction is readily apparent, this type of construction has its own limitations. Each progressive cable marching away from the tower is flatter than the preceding one, in other words its angle to the vertical load each carries increases, and therefore each cable carries a bigger share of the vertical load of the bridge. At 45°, the vertical load translates into a force in the cable roughly one-and-half times the load carried. In order to keep down the load, the towers would have to go improbably higher and higher so as to keep within realistic limits the slant angle (in relation to the vertical) and the related angles. This limits the effective length of the span. By comparison, a cable bridge can support longer spans.
This new type of bridge can be constructed with one or two parallell sets of cables, with the cables either in a vertical plane, or, as in the case of the Megyeri híd, in a plane slanting toward the outside of the bridge. The individual bridge sections, perhaps 20 ft long and maybe 90 ft wide, can be constructed either with steel or prestressed concrete, and are correspondingly quite heavy. To reduce the number of times such a section must be handled and lifted, the sections are frequently built on the very barge on which they will be carried to the bridge location. The Megyeri híd sections were prefabricated in the Ganz factory on Csepel sziget, were towed up to the site, and lifted into place by two floating cranes. A website had been set up, and one could follow the progress of construction, as updated photos were posted every 15 minutes.
The important considerations in this type of bridge are the relative ease of construction, avoidance of having to build a very substantial anchorage block with the corollary number of heavy and specially fabricated anchor rods, saving time, etc. Nonetheless, in final appearance, the finished structure offers a pleasing and esthetically satisfying appearance, somehow reminding one of a giant harp suspended in the sky.
As indicated, such construction is quite widespread in the world, first and foremost in Asia, on the Pacific Rim. South Korea, Japan and China have built a number of them in the last two decades around the millennium.
On the Megyeri híd, only the eastern span is built in this fashion, over that branch of the Danube which at this location is split by Szentendre island (nagydunai ág). The bridge crosses that island and continues over the Szentendre branch (Kis Duna or szentendrei ág). This half of the bridge, however, consists more modestly of a structure consisting of steel girders over a number of piers. During construction, a temporary pontoon bridge had been set up all the way across, with at least two pontoons serving as “gates” to allow boat traffic to go through.
Crossing the Szentendre Island was a major headache, since a number of the fresh-water wells which supply the drinking water for the City are located in that area. Because of the wells, there is no connection to this island, which is some 30 miles long. The shorter bridge in the szentendrei branch skirts the southern end of Lupa Island, on which this author as a child spent a week or two camping, sometime in the mid-‘Thirties. There is no connection to Lupa Island either.
The bridge at present has two lanes plus a break-down lane in each direction, and may eventually be converted to three lanes in each direction. Ouside the A-frame pylons, there are also pedestrian/bicycle lanes on the bridge. It was built by a Joint Venture of the Austrian firm STRABAG and the Budapest firm of Hídépitő Kft.
Remy P. Papp, originally from Budapest, is a licensed Professional Engineer. Even before he set up his own consulting firm in 1986, he was involved in bridge design here and abroad. Connecticut contractors have called on his expertise for rehabilitation and reconstruction of spans, including the Charter Oak and Founders’ bridges in Hartford, the Baldwin Bridge in Old Saybrook, the Route 1 and I-95 bridges over the Saugatuck River, and many smaller bridges. In 2001, he was awarded the prestigious Benjamin Wright Award “in recognition for his outstanding practice and his significant contribution to the Civil Engineering profession.” (see the print version of Magyar News, October 2001)
PICTURE captions
Pix1
Aerial photo looking northeast shows the area and the bridge under construction. In the foreground is Buda, then the Lesser (szentendrei) Danube branch, Szentendre Island and the Big (Nagyduna) branch. The two pylons are just about finished and hanging the first bridge sections has begun. In the Lesser Branch the temporary pontoon bridge is visible just in front of the permanent bridge; further upstream is Lupa Island.
Pix2
Construction of the cablestayed main bridge is well under way and both the type of bridge and the construction method are well recognizable. Notice that the shoreward spans now continue to the abutments and that there are temporary initial supports in those spans to stabilize the balance of the partially completed main span. The floating cranes are clearly visible as are the two tower cranes used in the construction of the pylons. The view is to the southwest from upstream.
Pix3 (photo on header)
The finished bridge under traffic looking west across Szentendre Island toward Buda. Presently, the Buda end ties into Route 11 to allow local access to the bridge, later on Route M0 will continue toward the hills seen in the background and complete the roughly three-quarter finished beltway by-passing the City.
Pix4
Taken pretty much at the same time as Pix2, this time looking at the bridge from the water from downstream. Notice that the right (east ) span is ahead by one section, there is one more set of cables holding up one more section than on the western half.
Pix5
As if you were under the bow of an aircraft carrier, the unfinished deck of the bridge looms over you, seemingly suspended in the sky and that is not just an illusion! You can feel the mass of the bridge and begin to appreciate the forces those cables must withstand.
|
