By Jim Noll, P.E.
| Thursday, June 01, 2017 | 2395 Views |
A key element in the process of selecting a choice for culverts or buried bridge type structures is the determination of appropriate shapes and sizes of structure to fit a given site application. While this might seem a fairly innocuous task, it actually brings into play a number of practical and functional issues and concerns related to the decision making process. Project engineers and site developers should carefully consider these factors early on in the planning stages of a project. I will attempt to outline a number of appropriate questions necessary to the logical thought and decision-making processes related to proper structure selection to aid in such planning. This article is the first in a series and thus is intended to be a brief overview of structure type and shape selection decisions.
Culverts and buried bridges come in a variety of material types, structure shapes and sizes. When it comes to material choices, such structures can be corrugated metal – steel or aluminum; they can be reinforced concrete; they can be plastic. Additionally, corrugated metal culverts and buried bridges come in various corrugation profiles depending upon the size and shape of the structure. They also are available in a variety of coating options – galvanized steel, aluminum, aluminized steel, coated steel and aluminum, etc. Selection of the appropriate material type and coating type must factor in issues such as durability, anticipated service life, site environmental issues, corrosion factors, relative economics, etc. That aspect of structure selection is a separate topic entirely and left for another discussion.
Once a material type is chosen, the size, shape and number of structures must be determined for the given site. The possibilities at first seem a bit overwhelming – one could pick a single barrel wide span structure or a battery of much smaller structures to fit a given site’s geometric characteristics. However, if you begin by ferreting out the key functional and practical needs at your site, the process can be simplified and streamlined.
Let’s start with the functionality of the structure(s) in question.
- Is the structure to function as a culvert carrying stream flow and stormwater runoff?
- Is it intended to function as a grade separation structure carrying some fixed clearance requirement (diagram) under a roadway or railroad or similar surface transportation requirement?
- Are hydraulic factors the primary consideration at the site?
- What are the headwater and tailwater conditions and limitations for the site?
- Would multiple barrel, smaller size culverts work or is a wider single opening needed to pass flow and debris – or traffic (vehicular or boat perhaps)?
Next, let’s address the structural design of the structure(s).
- What is the total available headroom at the site - - meaning the distance measured vertically from the stream bed (or traffic surface through the structure) to the traveling surface / final grade at the ground surface above the structure?
- What is the live load situation at the site – what type of vehicles will be crossing above the structure(s) in question? What are the axle loads, tire / wheel type and spacings of the vehicle in question?
- What is the required minimum depth of cover above the chosen structure for such live loading?
Following this, identify what analysis and evaluation of the site has been conducted.
- Has a geotechnical study or evaluation of the site been made?
- What is the allowable bearing capacity of the foundation under the culvert / buried bridge?
- If this is a potential problem, can steps be taken to improve the foundation strength and long term stability?
- Lastly, identify any restrictions or environmental concerns.
Often there are strict regulations as to any disturbance of existing site conditions. Permitting issues and governing regulations controlling such site development must be considered. Right of way factors also come into play. Specific areas to focus on here include:
- Are closed shape structures – i.e., those with an invert / bottom – preferred or acceptable at the site?
- Or is an arch type structure with an open bottom and installed on footings more practical and appropriate?
These are simply a few considerations that should be identified during the selection of structure, type, shape, and size for a particular culvert or buried bridge structure. The next article will go into additional detail as to how those considerations relate to the selection of structure type, shape and size.