Underground caverns have been used for Compressed Air Energy Storage in power plants first in Germany Huntorf in 1978 (Crotogino et al 2001)1 and later in McIntosh power plant of Alabama, USA in 1991 (Linden 2003)2. The Energy Storage Unit of a power plant is to regulate the mismatching supply and demand of the grid power, such that the electricity would not be wasted when it is not needed. To store the equivalent energy of a power plant, a sizable container is needed. Surface mounted or buried steel pipes have been proposed for small energy storage units (Linden 2003)2. Suitable underground caverns are difficult to find. Bridge structures are plentiful in many cities and their body space (spacing between girders) is voluminous. If steel pipes are used to store energy, they can be used as load-carrying beams/girders. The key to the application is that the steel pipe when subject to internal air pressure, the pipe is under tension. The hoop stress is twice as much as the axial stress, see FIG. 15. If the design is to limit the hoop stress to the yield stress, the axial stress is only half of the yield stress, leaving another half of the yield stress to be mobilized to carry loads. As the air pressure is very high in order to store sufficient energy, the thickness of the steel pipe is sizable. The mobilized tension force in the pipe (half of the yield stress times sectional area) is large enough to resist the vehicle load. The mass energy storage scheme can be implemented in the sea-crossing bridges such as the Sunshine Bridge, whilst small energy storage schemes can be placed in many road bridges in cities. The invention presented here is to turn the bridge body space into energy storage container but at the same time maintaining its bridge function, i.e. carrying vehicle loads from A to B over a horizontal distance. 1 Crotogino, F., Mohemeyer, K-U and Scharf, R., “Huntorf CAES: More than 20 years of Successful Operation”, Spring 2001 Meeting, Orlando, Fla., USA.2 van der Linden, S., “The Commercial World of Energy Storage: A Review of Operating Facilities (under construction or planned)”, 1st Annual Conference of Energy Storage Council, Houston, Tex., March, 2003.