Hydropneumatic support systems are composed of liquid (hydro) and gas (pneumatic). They can dynamically isolate a structure and dissipate energy by utilizing gas compressibility for flexibility and hydraulic flow for energy dissipation. The advantage of hydropneumatic systems is that they allow very long-span structures to be built because they can accommodate large displacements of well over 150 mm (6 inches). In the classical sense, long-distance crossings are accomplished by means of series of economically constructible discrete spans. The number of spans and supports is a function of the static, quasistatic and dynamic loads from environmental and operational phenomena, as the required energy dissipation must be distributed at a series of joints along the structure. Hydropneumatic support systems make it possible to respond to these effects by accommodating large displacements and providing energy dissipation at the ends of the structure, thus eliminating the need of a large number of roadway joints. This is in contrast to other expansion joints, which typically accommodate displacements of only up to 150 mm (6 inches), meaning that in a long distance crossing of, let’s say 3 km (2 miles), more than 100 expansion joints would be required.

Details of our two-year research effort are discussed in “Flexible Support Systems,” PB’s Research and Development Publication No 1, which can be obtained from the PB Corporate Library in New York (212-465-5474). Our effort brings PB to the forefront of these developments and provides us with a competitive edge in the design and retrofitting of long-span structures.

In general, service and environmental loads acting on transportation structures fall into two distinct categories:

• Quasistatic loads such as thermal and creep effects
• Dynamic loads such as live load, traction, and seismic forces. Dynamic loads from high-intensity seismic events can be treated as accident loading conditions.

New opportunities in design that create considerable savings are provided by replacing the traditional design philosophy for long-distance structures (resisting all loads by strength design) with an approach of modifying structural behavior in accordance to the nature and intensity of the acting loads. Adapting a structure to loads that are diverse in nature involves using mechanical force and displacement control devices designed to either:

• Alter the natural period of the structure to dissipate energy
• Act as rigid restraints to receive large loads.

The mechanical systems for such accomplishments are developed to isolate and to dissipate the energy by utilizing hydraulic flow as energy dissipation and gas compressibility as flexibility that can accommodate very large displacement associated with long structures.

Why Costs are Lowered

This support system will eliminate the conventional deck joints commonly used for long-distance crossings, resulting in significant savings in capital cost, maintenance and other life-cycle costs. Considering the global appeal of the build-operate-transfer type contracting, savings in life cycle costs can play a major role in the final project selection.