PuLSo Initiative

Introducing PuLSo—an initiative that goes beyond bridging geographical gaps, as we unveil the Santoña Bay Bridge. Spanning 890 meters, this cable-stayed marvel connects Laredo to Santoña with a 200m main span and innovative construction. A symbol of unity, PuLSo seamlessly integrates into the estuary, embodying progress, connectivity, and a shared future. Join us on this transformative journey.

Abstract

The new bridge over Santoña bay has a total length of 890 m and crosses from the city of Laredo to Santoña. The main stretch is a cable-stayed deck with a 200 m main span, 90 m side span and a vertical clearance for navigational purposes of 28 m. The simply supported steel deck is being built by segments incrementally launched from both sides. With that purpose, the launching process has been optimized so that its own cable stayed support system compensates the forces generated during construction. The conception of this new bridge aims at establishing a new valuable bond between Laredo and Santoña while at the same time achieving an adequate integration of the structure in the estuary.

Introduction

Laredo and Santoña, with populations of 11,347 and 11,004 inhabitants (INE), respectively, are, along with Castro-Urdiales, the two most populous municipalities in the eastern zone of the Province of Cantabria. Both coastal towns heavily rely on the fishing and canning industries for their economies. Tourism also plays a significant role, leading to a population surge of up to 100,000 during the summer.

The only existing land connection between the two towns and adjacent municipalities is the regional road CA-241, better known as the "Carretera de los Puentes" or Road of the Bridges. This 5.1 km road has an approximate daily traffic of 10,000 vehicles, with 6% being heavy vehicles. It connects to the Cantábrico Highway A-8 through National Highway N-634, passing through the Natural Park of the Marshes of Victoria and Joyel. Its unfavorable geometric characteristics (excessively narrow road, poor visibility in curves, inadequate lighting), and the degradation of the natural environment it is built upon, make it an unsuitable infrastructure from technical, social, and environmental perspectives. The CA-241 has been repeatedly listed as one of the most dangerous conventional road sections in the country, known as "black spots," according to the annual reports by the DGT.

Despite the two towns being within a maximum radius of 700 meters by sea through the San Martín Canal, the current shortest route following the A-8 – N-634 – CA-241 itinerary covers a distance of 18 km. This may result in a travel time of up to 35 minutes during peak hours, with associated risks when using the CA-241.

Location of Laredo in Spain.

Diagram of current land connections between the towns of Laredo and Santoña. The new bridge would be located in the Canal de San Martin.

Project goals

The current situation of land connections around the Bay highlights the need to propose new alternative links between Laredo, Santoña, and adjacent towns, aiming to partially or fully alleviate the technical, social, and environmental challenges mentioned earlier. In this regard, the construction of a new road bridge over the San Martín Canal, connecting the towns of Laredo and Santoña, is suggested. This bridge will facilitate not only the flow of vehicular traffic in both directions but also the passage of pedestrians and non-motorized vehicles. The objectives pursued with the construction of this infrastructure are as follows:

Reduction of the current route by 78%, from 18 to 3.96 km, from the urban center of Laredo to Santoña.
Decongestion and reduction of the Daily Traffic Intensity (IMD) on CA-241.
Decrease in the hazard at the intersection between N-634 and CA-241.
Mitigation of the environmental impact on the Marshes of Victoria and Joyel.

Finally, the intention is for the bridge to become a symbolic element of the bay, harmoniously blending into its beautiful natural surroundings without causing deterioration. Therefore, even though this is an academic project, given the importance of the aesthetic factor and its subsequent social impact in a real project of such magnitude, it is affectionately named "PuLSo," an acronym derived from the words: Puente entre Laredo y Santoña (Bridge between Laredo and Santoña). A logo has also been designed, incorporating colors representing the flags of the towns united by the bridge.

Mobility study

In order to assess the effectiveness of each proposed solution and its impact on the area's traffic, a transportation model has been developed for the study area. The four-step classical model based on trips has been employed, where the unit of analysis is a journey between an origin and a destination. The four steps include:

Trip Generation: Evaluation of each zone in the study area to determine the trips produced and attracted in each zone. A pre-calibrated production and attraction model based on a household survey was assumed due to academic and budgetary constraints.
Trip Distribution: Establishment of where each produced trip goes or from where each attracted trip comes. This step involves creating an origin-destination matrix using a gravity model, considering changes in the road network. The model uses an exponential friction function based on transport cost in minutes.
Modal Split: Determination of the proportion of trips for each mode of transportation, such as public transport, private vehicles, walking, and cycling.
Route Assignment: Determination of the routes that travelers follow within each mode of transportation, with a focus on private vehicles. The all-or-nothing assignment model has been used, assigning flows to the shortest route connecting each origin-destination pair.

The information was sourced from various institutional agencies, including traffic data from the Directorate General of Traffic (DGT), population and socioeconomic data from the National Institute of Statistics website, and demographic data at the census section level. The study area is divided into five zones for modeling purposes.

The transportation model reveals that the proposed "PuLSo" bridge would result in a substantial 78% reduction in travel distance from the urban center of Laredo to Santoña, leading to a more efficient and less congested traffic flow. Additionally, the model indicates potential improvements in safety, reduced congestion on the CA-241 route, and a notable decrease in environmental impact on the Marshes of Victoria and Joyel. Our mobility model predicts an average daily traffic intensity of 11000 cars per day using the bridge.

IMDs, which represent the average daily traffic intensities (vehicles per day) for each route according to the model proposed for the current situation (left) and the future scenario with the bridge (right)

Analysis of alternatives

Alignment alternatives

This section examines the viability of different bridge alignment alternatives, primarily focusing on access from the Santoña side due to constraints in space and urban conditions.

TR1 Alternative (Total Length: 2150 m):

Enters the sandy area through Av. Emilio Caprile Poloni in Laredo.
Crosses a water stretch of 630 m and connects to Santoña's Paseo Pereda near Fort San Martín.
Continues through a circumferential route along Monte Buciero, connecting to Av. Virgen del Puerto, with direct access to CA-241.
Moderate impact on Salvé Beach, but falls within the protection radius of Fort San Martín, a declared BIC (Bien de Interés Cultural) with conservation restrictions.

TR2 Alternative (Total Length: 900 m):

Quasi-straight alignment from Av. Caprile Poloni in Laredo to the intersection of Paseo Pereda and Calle O’Donnel in Santoña.
Shortest alignment, making it potentially the most cost-effective.
Minimal impact on historical heritage, avoiding elements listed in Santoña's heritage inventory.
Requires a study of foundation elements' influence on the interceptor collector.

TR3 Alternative (Total Length: 1139 m):

Originates from Av. Emilio Caprile Poloni in Laredo, connecting to Santoña's Calle Glez. Ahedo.
Extends deeper into the Marismas de Victoria and Joyel, impacting the natural park.
Longest alignment but avoids historical heritage impact.
Considered less favorable due to its significant intrusion into the Marismas de Victoria and Joyel Natural Park.

Based on a multicriteria analysis considering strengths and weaknesses, the TR2 alternative is selected as the preferred choice for the bridge alignment.

Alignment alternatives considered in the analysis.

Typology alternatives

Once the final alignment alternative is determined, various structural typologies are considered to span the water portion over the estuary. To facilitate the multicriteria analysis regarding structural typology, a set of requirements/conditions is imposed, providing boundary conditions that narrow down the search for the solution that best fits. These requirements are:

The maximum span is limited to 200 m to apply provisions outlined in IAP-11 regarding action definition. A structure with a main span greater than 200 m would require a rigorous aeroelastic study beyond the scope of a master's thesis project.
Approach spans will not be less than 40 m nor greater than 70 m to ensure an environmentally suitable number of supports on the Laredo sandy area.
Main structural elements (towers, pylons, arches, etc.) must be positioned outside any sandy area.
The bridge configuration will attempt to be symmetric to provide equal importance to both towns.
The minimum required clearance (20 m) must be met, preferably in the area of the estuary with the greatest depth.

Below we show the details of the different typology alternatives.

Typology alternatives considered.

Description of the proposed solution

The bridge has a total length of 717 meters, located between P.K. 0+058 and P.K. 0+775 according to this project. The reference point "P.K." stands for "punto kilométrico," representing kilometric points along the bridge. The main span, situated between P.K. 0+317 and P.K. 0+517, is solved with a cable-stayed system using a central semi-fan arrangement. This cable-stayed section has a symmetrical layout, consisting of a main span of la=200 m and two back spans of lm=90 m.

The approach viaducts are located between P.K. 0+58 and P.K. 0+233 from the Laredo side, and P.K. 0+6182 and P.K. 0+775 from the Santoña side. The span arrangement in these viaducts is asymmetrical, with sections of 55, 60, and 42 m in length. Both the main span and the approach spans share the same deck, providing maximum longitudinal continuity to the bridge, which is simply supported on the six piers forming the structure.

The entire bridge being supported on the piers results from the construction process, which involves pushing the bridge from both shores as a whole, utilizing the cable-stayed system as a launching mechanism. The torsional stiffness of the system is provided by the anchoring supports at the ends of the back spans, achieving a recommended ratio of la/lm < 0.5. The deck, with a closed section, also contributes to stiffening the structure against torsion.

The masts have a total height above the ground of 55 m, adding to the height of the piers they rest upon (30-40 m), resulting in a total of 85-95 meters for the towers nearest to Laredo and Santoña, respectively. The deck follows a parabolic profile with a 7% slope in elevation, reaching its maximum height at the bay's midpoint, facilitating the passage of fishing vessels and their entry into the ports of Laredo, Santoña, and Colindres. The highest point of the deck is 30 m above the design water level, considering the rise in sea level due to Astronomical and Meteorological Tide, as analyzed in the Annex on Extreme Sea Level Analysis in the bay.

Access to the bridge on the Laredo side starts at Avenida Emilio Caprile Poloni, with the abutment located in the current El Puntal parking area. On the Santoña side, the access embankment starts at Paseo Pereda and ends at zero level on Calle O’Donnel. Both closed abutments have a height of 7 m.

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