An arch system that combines concrete with modular steel sheets has been introduced recently for both earth-sheltered and cut-and-cover construction. The steel sheets function as both formwork and reinforcement for the concrete. This forming-and-reinforcing system has been used for highway bridge construction, transit system tunneling and mining. Several earth-sheltered homes have been built using this system, resulting in cost savings in labor and materials. However, lack of actual strength information has led to a conservative approach in structural design. This article reports new strength data for the system and discusses how they can be applied to achieve optimum structural design with minimum concrete thickness and adequate safety.
DESCRIPTION OF FORMING-AND-REINFORCING SYSTEM
Precurved interlocking steel sheets are linked together on site to form an arch. Sheets are approximately 3 x 4 feet, made of 14-gage steel and weigh 46 pounds each. They are manufactured with ribbed corrugations to contribute strength for forming. The continuous steel needed for development of flexural strength is provided by the V-ribs. Sheets are assembled so that the continuous V-ribs are toward the inner surface of the arch and run parallel to the arch perimeter. A temporary shoring system is used at center span. Once assembled, the steel form is covered with low-slump concrete or shotcrete. When the arch is completed, the temporary supports are removed leaving a clear span. A thin layer of shotcrete is usually applied to the underside of the arch for protection of the steel.
For determination of flexural capacity the effective steel area should be taken as the actual cross-sectional area of the V-ribs alone. Shear strength design can safely be taken as the shear strength provided by the concrete, ignoring any contribution by the steel.