e-mosty March 2019 Long Span and Multiple Span Bridges - Page 23

DESIGN REQUIREMENTS Subsequent geotechnical exploration conducted by CPC, the Contractor, confirmed the feasibility of a suspension bridge with two main spans and a center pylon founded on Roca Remolinos. MOP developed the Instructions to Builders (ITB) which contained the contract administrative and technical basis, as well as project specific requirements. In addition, the ITB referenced standard codes, including the Manual de Carreterras, the Chilean roads design manual; the AASHTO LRFD Bridge Design Specifications, the highway bridge design code for the United States; and the AASHTO Guide Specifications for Seismic Bridge Design. As a result of all of these studies, CPC's final design arrangement includes two spans suspended by continuous cables. Few continuous, multi-span suspension bridges exist in the world and Puente Chacao will be the first in the Western Hemisphere. The contract permitted application of other design standards in cases where the identified standards did not address a specific design issue, provided that the design build contractor proposed and received acceptance for use of portions of these standards. In addition, the Puente Chacao's suspended span lengths will be the longest in Latin America. Another unique feature of the Puente Chacao is the Central Pylon arrangement. While the North and South Pylons have relatively conventional arrangements of straight, vertical columns, the Central Pylon has an inverted wishbone frame arrangement with two longitudinally inclined lower columns intersecting above the deck elevation. The ITB also contained requirements for the design build contractor to conduct surveys, tests and studies related to site conditions and loadings necessary for the project design. These included Bathymetry and Topographic Surveys; Geotechnical Field Investigation Program; Geological Studies; Scour and Erosion Studies; Maritime Studies (including currents and wave heights); Seismic Studies including a Seismic Risk Study Assessment and a Seismic Monitoring Program; Wind Measurement Studies and a Wind Tunnel Testing Program; Temperature Studies; Vessel Traffic; and studies of other specific risks. CPC summarized the project design requirements in a Design Manual. Additionally the ITB mandated the structure to be designed considering access, ease of inspection & maintenance and with a 100 year durability requirement under normal maintenance practices. The intersecting lower columns are topped by variable-width pylon columns extending upward to the cable saddles. This arrangement is required by unbalanced loadings on the Central Pylon, as well as seismic and wind loads, which must be transferred to the foundations. A notable feature of the Puente Chacao results from the site topography. Steep banks between 30 and 50 meters tall allow the bridge abutments to be located close to the tops of both banks. Grade adjustments at either end of the bridge are accommodated by use of cuts and embankments, eliminating the need to construct long approach viaducts to connect with the approach roadways. Figure 2: General Bathymetry 1/2019