At locations where the bedrock is shallow enough to prevent adequate penetration from piling, square-post, frame bents are placed atop concrete pedestals, which are seated on bedrock. In situations where wood trestles are used in combination with steel bridges, short frame-bents are placed atop the bridge pier. It is common to see frame bents and pile bents used within the same trestle. There is a subtle difference between the “Type C” bent and the “Type D” bent. Instead of two, 18-foot sashes as used on the “Type C” bent, the “Type D” substitutes a single 18-foot sill, which is bolted to the posts below with ¾” by 24” drift bolts. The top posts are bolted to the sill with ¾” x 18” drift bolts. At locations, where more than one “Type C” bent is required, that standard plan requires that “Type C” and “Type D” bents are to be alternated. I suspect that the cost of long square posts may be expensive, so the shorter and perhaps less expensive “Type D” posts are used. The drift bolts may be less expensive than the Plain Grid Telco Connectors.