This article is the third part in a blog article series focused on the basic fundamentals related to selecting material type, structure type, structure shape and size selection for culverts and buried bridge structures. Part 1 of this series was intended to be a brief overview of the various key factors related to the process of logical structure selection; while Part 2 delved deeper into several areas – specifically the practical differences of shape type for a particular site location. In this third blog article in the series, the focus will be on arch shapes and footings, including attention on required site soil bearing strength. As with the previous articles, the primary focus here will be on corrugated metal and structural plate offerings.

Arch structures offer some distinct advantages over structures with inverts, such as:

• Spanning wetlands, streams, creeks, ditches, etc. where encroachment may not be permitted due to environmental regulations or wildlife considerations.
• Offering the ability to leave the stream bed undisturbed, which can be a critical factor and advantage in fishery management.
• Arches installed on concrete footings are often combined with a paved invert system for enhanced hydraulic performance and extended durability / service life.
• Arches may be installed with a paved roadway, walkway or perhaps a railroad bed installed inside.

Due to the fact that arches must be installed on some type of footing system, proper attention must be given to the required bearing capacity of the in situ soils. The sides of the arch apply reactions to the footings based on the size / span of the arch, the amount of surcharge over the arch and any applied live loads. Footings must be adequately designed and sized accordingly, taking into account the magnitude of the vertical and horizontal footing reactions and allowable bearing capacity of the underlying soil.

One aspect of the project’s site conditions is the allowable width within the span of the arch that can be encroached upon for construction and installation of the footings. Some sites do not allow equipment to operate within certain wetland environments. Such concerns must be taken into consideration.

Project engineers should be aware that concerns related to site soil bearing strength, concerns that may preclude selection of an arch installed on footings, may also present problems associated with full periphery structures (i.e., structures with a bottom or invert). The site soils must provide adequate support for the structure chosen as well as provide support for the adjacent side embankments. Keep in mind that the side embankments place significantly greater loads on the underlying soil than does the structure itself. Consequently, should those embankments adjacent to the structure be inadequately supported, excessive settlement could occur, leading to drag down loads and insufficient structural support.

In summary and conclusion, arches offer a number of advantages when selecting a structure to fit given site conditions. However, the project engineer must be aware of the various design and practical issues and considerations involved when selecting a shape type and size for that particular application.