Low-volume road abutment design standards.

Although several superstructure design methodologies have been developed for low-volume road bridges by the Iowa State University Bridge Engineering Center, no standard abutment designs had been developed. Thus, there was need for an easy-to-use design methodology, generic abutment construction drawings, and other design aids for the more common substructure systems used in Iowa. A survey of the Iowa county engineers determined that while most counties use similar types of abutments, only 17% use some type of standard abutment designs or plans. In consultation with the Project Advisory Committee, a design methodology was developed for single-span stub abutments supported on steel or timber piles for bridge spans ranging from 20 to 90 ft and roadway widths of 24 and 30 ft. Using the foundation design template provided, other roadway widths can also be designed. The backwall height was limited to between 6 and 12 ft, while both cohesive and cohesionless soil types were considered. Depending upon the combination of variables for a specific site, tiebacks may be required; the design of tiebacks is also included. Various design aids, for example charts for determining dead and live gravity loads based on the roadway width, span length, and superstructure type, were developed for the design of the stub abutments. A foundation design template was developed in which the engineer can check a substructure design by inputting basic bridge site information. Information for estimating pile friction and end bearing for different combinations of soils and pile types published by the Iowa DOT were also included. Generic standard abutment plans were developed to enable engineers to detail county bridge substructures more efficiently. In addition to briefly describing the substructure design methodology developed in this project, two example problems with different combinations of soil type, backwall height, and pile type, plus a construction drawing example, will be presented to show the versatility and applicability of the materials developed.