Grupo Triada, Mexico City, Mexico
ABSTRACT: The Baluarte Bridge has a total length of 1,120 m, a main span of 520 m, and a height from the bottom of the Baluate River of 402.57 m, for which it was awarded with the Guinness World Records Certificate to the highest cable-stayed bridge in the world. The Project required site’s topographic-hydraulic, geological, geophysical, geotechnical, seismic risk and wind incidence studies, in order to develop the structural and constructive design of the bridge.
1 BACKGROUND
The Mexican Communication and Transportation Ministry (Spanish acronym, SCT) is considering finishing and renewing 14 main roadways: longitudinal, North-South direction, from border to border; and transversal, from the Pacific Ocean to the Gulf of Mexico. The missing link to complete the Mazatlan-Matamoros Axis was the Durango-Mazatlan roadway, which has a 230 km length and closes the path between both cities, increasing travel safety and comfort. However, this segment was critical due to its high cost and technical difficulty, as it crosses the Sierra Madre Occidental over its sheerest section, known as “El Espinazo del Diablo” [The Devil’s Backbone]. It is precisely on kilometer 157 + 400 of the highway that the Baluarte Bridge (BB) is located, over the deep river canyon of the same name, that separates the States of Sinaloa and Durango. In 2003, the SCT contracted the professional services to develop the basic studies and the executive project; the latter was followed up during construction, since the beginning in 2007, until its end in 2012.
2 BASIC STUDIES 2.1 Topography
Defined by the geometrical project data and the SCT reference terms; a topographical survey was performed measuring 1140 m along the roadway trace and 60 m up and down current. A flight was carried out at a 1:6000 scale, to view the site’s physical characteristics and stereoscopic paired color photographs were obtained.
2.2 Hydraulic and hydrological
Semi-empirical methods, associated with a 100-year return period were used to determine the discharge of the river current, using rainfall information from the isohyets drawings for rainfall intensity-duration-return period. A discharge of 525 m3/s was recommended for design.
From the hydraulic section at the site the design water level (DWL) was obtained, associated with a 4.0 m/s speed, which was non-significant to the BB design, given the great height of the project level line over the riverbed.
2.3 Geological
It started with the gathering and analysis of the site’s existing information. Then a field geological survey was performed, helped by three site helicopter over flights, due to the difficult accessing
to the cliffs. Aerial color photographs were available to perform a photo-geological study of both river margins. The main conclusions included:
– Rocks on both river canyon slopes have the required resistance and quality for the structure support. However, the volcanic rocks in the cliffs presented pseudostratification with slight inclination towards the river.
– A recommendation was made to modify the bridge trace, away from the North shelf of the Cerro
de la Ollaon the right margin, to be centered between the faults on the left margin.
– Excavations to reach the finish grade for the footing must be performed with explosive control to avoid loosening of the rock near the canyon cliffs.
2.4 Geophysical
Consisted on a resistivity and seismic refraction study, in both river margins, focused in the founda- tion zone for the piers that would support the bridge. Several underlying rock units were identified by the seismic and electrical characteristic properties measured. This information was compared to the geological and geotechnical studies profiles, to improve interpretation and modeling of the rocky strata.
2.5 Geotechnical
Direct exploration and sampling works began once the working sites accesses were prepared, for drilling machinery and brigades. Six boreholes were performed on the river’s left margin, two on the main pylon of this margin and one for each remaining support. An additional eight borings were performed on the river’s right margin. Afterwards, another exploration campaign took place, in which nine borings were re-drilled, to reach convenient depths below the footings level grades, once they were determined.
The results from the field and laboratory works were integrated together with the other studies, to define the geotechnical characteristics of the subsoil rocky layers and their profile modeling. Geotechnical integration confirmed that the rocky massifs in the margins are formed by competent rocks, adequate for supporting the bridge’s foundations in their final positions.
The failure/service limit states were analyzed to determine the permissible load capacity between 45 and 110 t/m2, and expected settlements not greater than 5 centimeters.
Excavations to house the footings were projected with slopes 0:25:1 (horizontal:vertical), until reaching the final grade indicated in the project.
2.6 Wind incidence
Wind maximum velocity and turbulence were estimated for service and construction conditions, at a site with rugged and narrow topography.A sustained wind velocity of 130.5 kph was determined and wind gusts of up to 192.6 kph, for 200-year return periods.The following actions were recommended to have a better knowledge of the wind at the site:
– Physical measurements with installed instrumentation monitored during construction.
– Wind tunnel measurements of scale models of the topographical influences, were carried out in Nantes, France, resulting in recommendations for the installation of wind deflectors in the center of the main cable-stayed span.
2.7 Seismic risk According to this study:
– The BB is in the B risk seismic zone of Mexico (moderate seismicity), and foundations will be supported by Type I firm rocky terrain.
– Recommended spectrum for the elastic design was provided, corresponding to the failure limit state and service and construction conditions.
3 EXECUTIVE PROJECT
Once the basic studies were advanced or finished, the structural analysis and design stage proceeded, to generate the memories of the design calculations and executive project drawings.
The structural solution better suited for the site’s topographical and geotechnical conditions was chosen, once the bridge’s trace position was optimized. Alternatives studied included: suspension bridge, arch bridge and cable-stayed bridge. The latter was chosen due to cost advantages and construction ease, considering the impossibility of placing intermediate supports in the deep river canyon.
The resulting structure was complex, due to its dimensions and characteristics: a bridge with a cable-stayed main span of 520 m, total length of 1124 m and piers of up to 147.3 m in height, with a vertical distance of 402.57 m from the bottom of the river to the bridge deck, the highest in the world for its type.
The design was based on the SCT standards and the AASHTO Standard Specifications for
Highway Bridges, Edition XVI (1996); also, when applicable, in the AISC, AREA or European
Regulations Specifications.
For the structural analysis, combinations of gravitational loads (death load + live load), acciden- tal loads: seismic and wind were carried out, in accordance with corresponding regulations, and information resulting from the specific studies.
The superstructure, measuring 22.06 m of total width and 16.60 m of road width, was divided in 11 spans of 44, 68, 68, 70, 520, 54, 56, 72, 72, 60, and 40 m.
The bridge-deck structural solution is a combination of prestressed concrete beams in the spans 1–2 to 4–5, 6–7 to 11–12 and both ends of clearing 5–6, and metallic beams at the center of the latter, with a length of 410 m.
To support the 5–6 main span, a multiple cable-stayed system was used, with a total of 152 cables in two planes, the longest measuring 280 m. The bridge-deck was solved with a mixed superstructure, formed by steel longitudinal beams and cross-sectional bridge pieces spaced 4 m, on which the reinforced concrete road slab was casted. The metallic beams were mounted and attached together by screws and welding in 12 m lengths, supported by the cables to move forward over the canyon from piers 5 and 6, until they were joined at the center of the clearing.
Piers 2 to 4 and 7 to 11 and abutment No. 12 consisted of two reinforced concrete pillars casted on site, of rectangular section, joined by horizontal crossbeams of the same material. The pylons for supports 5 and 6 are hollowed concrete, with an inverted “Y” shaped geometrical configuration, thus the cable anchorings in the pylons are joined to provide torsional rigidity to the bridge-deck for wind and seismic forces.
Foundation for the abutments, piers and support pylons consisted of reinforced concrete footings of varying dimensions, supported on the rock at convenient depths.
4 FOLLOW-UP DURING CONSTRUCTION
During the development of a project of this kind, the contractors that will be in charge of construction are unknown, thus the designer has to make assumptions regarding the procedures and equipment for the works to be carried out:
– Installation process and trajectory of the different bridge and superstructure elements. – Weight and location of hoisting devices: fixed and mobile cranes.
– Weight and location of auxiliary equipment: compressors, energy plants, etcetera.
Given the structure complexity, it was required a high degree of precision and safety conditions for construction, thus leading to confirm or improve the former design criteria and hypothesis:
– Bridge and superstructure components real weight and geometry. – Materials measured properties for concrete, structural and cable steel, etc.
– Observed geological conditions in the excavations.
– Mechanical properties of resistance and deformability of rocks supporting foundations. – Thermal variation influence during the construction process.
– Dominant wind and seismic activity during construction.
Due to the particular characteristics of the bridge, SCT contracted services for advisory and follow-up to the project under construction, which include project modifications, clarifications and requirements during construction. The services rendered were:
• Structure
– Verification of geometry and constructive procedures for piers, pillars and superstructure, via precision topographical control.
– Detailed computer analysis to check and adjust the calculation hypothesis in accordance with the measurements performed.
– Defining the camber or countercamber level to be considered at each stage of deck construction.
– Determining the initial tension applied when installing each cable, and their final tension, to achieve the project expected theoretical level line.
– “As Built” Drawings. • Geology and Geotechnical
– Verifying that the exposed rock in the walls and at the bottom of the foundation excava- tions is within the determined geological and geotechnical basis established in previous studies.
– Checking that excavations are performed as specified, with adequate equipment and procedures, so that the rock conditions in the foundations and slopes are not altered. – Checking that foundation settlements are within the project foreseen levels. • Instrumentation
– Monitoring of lateral displacements of the top of the slope in the margin of the Durango side, using collimation lines and measurements with high precision topographical equipment.
– Monitoring of lateral displacements at depth from the Durango side, using three inclinometers installed down to 70 m of depth.
• Wind ocurrence during construction. Winds were monitored via the installation of 12 meteo- rological stations, in four 40 m height towers. At each tower, three stations were placed at 10, 20, and 40 m heights over the surrounding ground, to record values measured every 30 minutes. Measurements were important, as on October 21 2009, Hurricane Rick lashed over the site, with winds ranging from 80 to 100 kph and wind gusts of up to 149 kph. After analyzing the records, the following was concluded:
– Analyzed measurements cover close to a 4-year period, from February 2008 to February 2012.
– Rick’s winds analyzed and modeled were determined to have no consequences on the BB design, according to the results of the wind incidence study.
– From the turbulence analysis, hurricane wind parameters were determined and the recorded values were found to be within the recommended limits.
• Seismic activity during construction. Analysis of the earthquakes that occurred close to the BB site resulted in the following:
– 36 earthquakes were recorded with epicenters located at 300 km or less from the site. – Earthquakes had a minimum magnitude of 3.3 and maximum of 5.6 in the Richter Scale. – Neither of the two more important earthquakes that occurred during construction could be
detected, nor did they have impacts on structure.
– Results allowed for confirmation and validation of the seismic risk study recommendations.
Figure 1. Photography of Baluarte Bridge.
Figure 2. Elevation Baluarte Bridge.
5 TERMINATION
After four years from construction start, the last casting of the closing slab for the central span was performed on January 5, 2012. BB construction included that of 17 km of an internal roads net to access the different working sites.
Multi-Span Large Bridges – Pacheco & Magalhães (Eds.) © 2015 Taylor & Francis Group, London, ISBN 978-1-138-02757-2