BRIDGES 1999

Bridges Article December 28, 2000
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More than 750,000 vehicles use the Metropolitan Transportation Authority (MTA), Bridges and Tunnels facilities each day, making it among the nation’s largest bridge and tunnel toll authorities in terms of traffic volume. The MTA, Bridges and Tunnels’ responsibilities consist of operating seven bridges and two tunnels in the New York metropolitan area and providing surplus toll revenues to help support public transit.

One of MTA, Bridges and Tunnel’s busiest bridges is the Bronx-Whitestone Bridge, which opened to traffic in 1939 in time for the opening day of the 1939 New York World’s Fair in Flushing Meadow Park. This six-lane cable suspension bridge spans the East River connecting Queens to the Bronx. Nearly 40 million vehicles cross it each year.

This combination of old age and heavy traffic makes it imperative that the bridge is properly maintained with minimal interruptions.

Evidence of surface corrosion

During regular bridge safety inspections, eyebars and other steel members housed in the bridge’s four anchorage chambers began to show signs of surface corrosion. If the corrosion were allowed to continue in the anchorages, the bridge’s stability would eventually be affected.

Corrosion is often catalyzed and accelerated by moisture and will turn metal to rust. Because bridges are typically placed over water, they are particularly susceptible to corrosion. Moreover, there is a direct correlation between the rate of corrosion and the level of relative humidity, which is increased by environmental pollutants such as car exhaust or proximity to sea air—factors the Bronx White-Stone Bridge faces daily.

To ensure maximum safety of the bridge, careful monitoring and comparisons were made of both the air inside and outside the bridge’s anchorage chambers. The monitoring confirmed that under certain weather conditions, such as humid days, each chamber’s air had a higher dew point than the surface temperature of the chamber’s surfaces, such as the eyebars, and that this was causing water condensation.

Adding to the condensation problem was the bridge’s exposure to water seepage, high relative humidity, pollution and salt laden air infiltration.

Moisture removal needed

To correct the problem, moisture needed to be removed from the air and surfaces inside the anchorage chambers.

Design consultant Weidlinger Associates Consulting Engineers, New York, under an agreement with the MTA, Bridges and Tunnels, retained BJLJ Engineers & Architects, P.C., to investigate and control moisture and humidity conditions in the four anchorage chambers of the bridge. BJLJ has more than 10 years experience in the analysis and design of dehumidification systems for the anchorage of suspension bridges and has worked on two other MTA bridges.

BJLJ designed a dehumidification system in each chamber that would maintain a 40%relative humidity year-round. Phil Ritola of Accuspec, a manufacturers representative specializing in engineered air systems, noted that, “when BJLJ contacted us about dehumidifying equipment for bridge application, we were able to pull information on several such projects from Munters Corp.’s Cargocaire Division.”

To help achieve this stable rate of relative humidity, BJLJ recommended that MTA, Bridges and Tunnels install eight HC-300EA Cargocaire desiccant dehumidifiers, equipped with electric heaters and titanium silica gel desiccant wheels. Each chamber would have two dehumidification units working alternately.

According to BJLJ, the units were preferred over conventional refrigeration equipment due to their:

• Dehumidification range and performance in cold conditions;

• Warm dry air discharge;

• Adaptability to site conditions;

• Environmentally friendly, chemical resistant and stable desiccant material;

• Relatively compact and light package; and

• Reliability.

How dehumidification works

Unlike refrigeration-type dehumidifiers that cool the air to condense its moisture and cannot effectively dry at or below a 40 degree dew point, desiccant dehumidifiers attract moisture from the air by creating an area of low vapor pressure at the surface of the desiccant. Because the pressure exerted by the water in the air is higher, the water molecules move from the air to the desiccant to make the air drier.

Desiccants can attract and hold from 100to more than 10,0000of their dry weight in water vapor and they are very effective in removing moisture from the air at low humidity levels.

When a desiccant has a low surface vapor pressure, cool and dry, it attracts moisture from the air. When a desiccant has a high surface vapor pressure, wet and hot, it gives off vapor to the surrounding air. As a result, vapor moves from the air to the desiccant and back again depending on the vapor pressure differences.

Desiccant dehumidifiers use the changing vapor pressures to dry air continuously in a repeating cycle.

The desiccant—silica gel, lithium chloride or molecular sieve—is infused into a corrugated composite material that resembles the honeycombs of a beehive, and this is formed into a wheel. The wheel rotates slowly between the process and reactivation air streams. Moisture extracted from the process air is removed from the desiccant by the heated exhaust air stream.

A cost-effective solution

There were cost saving benefits in addition to all of the performance attributes of the desiccant dehumidifiers. According to BJLJ, desiccant dehumidification provided a cost-effective solution to the anchorage’s corrosion issue. By creating a closed and controlled environment around a sensitive part of the structure, the amount of maintenance that would normally be required was diminished. Essentially, each anchorage chamber was in its own cocoon, no longer exposed to the damaging effects of condensation, pollution and salt laden air.

The only logistical issue of note was the need to temporarily close a lane of the bridge traffic in order to drop the equipment into the anchorage, stated BJLJ. Once installed, simple monitoring and operating instructions were provided to MTA’s Bronx-Whitestone Bridge personnel.

According to BJLJ, knowledge about the benefits of dehumidification technology in bridge operation is growing. Unheard of 30 years ago, it is now the norm in the design of new suspension bridges around the world.

About the author: 
Gregorski is Technology Editor for Roads & Bridges
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