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Foundation and Substructure - Учебное пособие для студентов II курса специальности


^ Foundation and Substructure

The new Acosta Bridge is supported on 1.5 m diameter drilled shafts with 7 to 8 shafts per back span support, and 31 and 22 shafts for each of the two main piers making a total of 82 shafts per bridge. An extensive test program utilizing 900 mm test shafts was used to check capacities. Testing involved the use of sister bar strain gauges, Osterburg cells, telltales and a wire line to measure stresses and movements. The use of the Osterburg hydraulic cell was innovative for this type of equipment, being applied in a silty clay marl to determine end bear­ing capacity.

Waterline footings were constructed on the drilled shafts using a ring sup­port bolted to each drilled shaft. With the rings in place, a 230 mm thick pre­cast seal slab was set. A top yoke sup­port system was then used to support the footing side forms. A 460 mm to 530 mm seal was then placed, enabling the removal of 1 m of water from the forms. The footing, column and cap were then constructed with conven­tional cast-in-place methods.

Due to physical restraints, and in an ef­fort to minimize the size of the founda­tions, the designer used pot bearings to support the bridge at all pier locations. The larger of the two cantilevers was erected on three 53.4 MN fixed pot bearings. The second antilever bear­ing system consists of three guided 38.7 MN pot bearings and are among the largest bearings of their type in North America. One of each of these large bearings was tested at the US National Testing Laboratory near Washington, DC.

The cantilevers were erected by bal­anced cantilever construction. The cantilever was limited to about one-half of a segment out of balance through the entire casting process to minimize the unbalanced moments.

A stability system was required to sup­port the out of balance moment. This system consisted of three 1.1m diameter concrete filled steel pipes with 15.9 mm walls. One support was located under each web of the box. Grout pads on the footing provide lower support for this system. These pipes served as compression posts on each side of the column 4.9 m from the center of rotation. At the top of each post, a sand jack with a concrete wedge was placed to provide the con­nection under the pier table bottom slab.


Superstructure


The river crossing is a five-span contin­uous structure composed of a 67 m back span, 110 m side span, 192 m main span, 83.8 m side span and a 48.8 m back span. The superstructure utilized cast-in-place segmental con­struction with a typical box section measuring 23.10 m wide at the deck (Fig. 24). The box girder out-to-out width of 14.94 m consists of two cells and three web walls. The depth of the boxes varies from 3.66 m at mid-span to 11.58 m at the main pier table. A typical segment pour was 4.9 m long. The casting cycle was five days per traveller.

Segments were post-tensioned once the concrete reached 24.13 MPa of the 37.92 MPa 28-day requirement. Three tendons made of fifteen 15 mm diame­ter strands were stressed to about 3.1 kN each after casting each cycle. Four-strand transverse tendons spaced ap­proximately 760 mm were stressed during the same cycle. Vertical web shear reinforcement was provided by 31.75 mm post-tensioning bars.

To speed up construction, the contrac­tor varied from the designer's erection procedures and built the back spans on falsework. This allowed the cantilever construction to go on independent of the back spans. The resulting time sav­ings exceeded 16 weeks.

Fig. 24


^ Vibration Limits

One of the greatest concerns in the project was the 60 year old railroad bridge located about 12 m to the west of the old Acosta Bridge. Historical levels for service vibrations were recorded by instrumentation placed on the old railroad bridge. Based on the recorded information and current blast literature, a blast limit was set for the railroad bridge at 101 mm/s peak particle velocity.

Throughout the blasting, monitors were placed on the railroad bridge and the new Acosta Bridge to measure the particle velocities. As the blasting pro­gressed, the blaster was able to use this information to set off more than 635 kg of explosive powder in one blast and remain within the peak particle velocity limits established for the pro­ject.


Owner-Florida Department of Transportation

Construction Engineering and

Inspection:

Steinman Boynton Gronquist and

Birdsall, Tallahassee, FL


Contractor:

Recchi America, Miami, FL

Engineers of Record:

DRC Consultants, Flushing, NY

Fred Wilson & Assoc, Jacksonville, FL

^ Service date: July 1994


The Normandie Bridge, France:

A New Record for Cable-Stayed Bridges


Michel Virlogeux

Prof., Ecole Nationale des Ponts et Chausees, Paris, France


^ Landmark Cable-Stayed Bridges


On August 8, 1994, the last steel plate was welded to close the main span of the Normandie Bridge, which, at 856 m, is the longest cable-stayed span in the world today (Fig. 25). The Normandie Bridge will begin its ser­vice life in January 1995. This is an ap­propriate occasion to analyse its de­sign and review the experience gained during the construction thus far.

I
Fig. 25
n the 1970s and '80s, it was generally considered that 500 m was a limit for cable-stayed bridges, and almost all projects were conceived with such a limit in mind. Consequently, the record span progressed slowly: 404 m in 1975 (Saint-Nazaire, France), 430 m in 1983 (Barrios de Luna Bridge, Spain), 465 m in 1986 (John Frazer Bridge to Anacis Island, Canada) and 490 m in 1991 (Ikuchi Bridge, Japan).

But engineers already had some indi­cations that cable-stayed bridges were very far from their limits: three major bridges had been built in Germany with a single pylon: the Koln Severin Bridge (302 m in 1959), the Diisseldorf Kniebrucke (320 m in 1969) and the Diisseldorf Flehe Bridge (368 m in 1979). For those who could foresee it, these three bridges proved that spans

of 600-700 m could be built from two pylons without major problems.

Some projects had been studied with long spans, but the bridges had not been erected at the time: a first design was done for the Normandie Bridge between 1976 and 1979, with a main span 510 in long; and a cable-stayed solution was proposed in 1978 for the Eastern Bridge of the Storebaelt, Den­mark, with a span of 780 m.

Fritz Leonhardt proposed a cable-stayed solution in 1968-1970 for cross­ing the Messina Straits with two pylons in the sea and a main span 1300 m long. He was followed by Rene Walther, who proposed that concrete cable-stayed bridges can be economi­cally built up to 600 m, and composite ones up to 800 m.


^ Recent Progress


The preliminary design of the Nor­mandie Bridge - called the Honfleur Bridge at the time - was developed be­tween September, 1986 and Spring, 1987. The project was presented in the first conference devoted to cable-stayed bridges, in Bangkok, in Novem­ber 1987.

Since that time, the world record pro­gressed with two bridges designed and built very quickly, probably helped by the Normandie Bridge project, which psychologically opened the way for very long spans: the Skarnsund Bridge in Norway (530 m in November, 1991), and the Yangpu Bridge in Shanghai, China (602 m in October, 1993). Two other projects were clearly inspired by « the Normandie Bridge: the Honshu Shikoku Bridge Authority decided, af­ter the Bangkok Conference, that the Tatara Bridge would not be a suspen­sion bridge, but a cable-stayed one. Its erection began in 1993, and it will be­come, in 1998 or 999, the new world record with its main span 890 m long. Danish engineers designed a new ca­ble-stayed solution for the East Bridge of the Storebaelt, extending the main span to 1200 m. All problems found appropriate solutions, illustrating the fantastic possibilities of cable-stayed bridges, but navigation requirements finally called for a 1624 m long main span, longer that the longest suspend­ed span in the world, and the cable-stayed solution was abandoned.

It is now interesting to compare the cable-stayed bridges which held the successive world records:

- Saint-Nazaire Bridge, 1975: steel orthotropic box-girder ...

- Barrios de Luna Bridge, 1983: prestressed concrete bridge

- Anacis Bridge, 1986:

composite deck with two I-shaped beams and a concrete sfab

- Ikuchi Bridge, 1991:

steel main span (and concrete ac­cess spans, like the Normandie Bridge) made of two parallel box-girders

- Skarnsund Bridge, 1991: prestressed concrete

- Yangpu Bridge, 1993: composite construction

- Normandie Bridge, 1994: steel orthotropic box-girder for its main span.

The cycle is closed, and a concrete ca­ble-stayed bridge with a main span of about 1000 m cannot be expected; nor probably a composite one due to high­er weight and increasing costs of ca­bles. But the competition which exist­ed during twenty years between con­crete, composite and steel decks is an­other indication that the limits have not have reached.


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