Highlights of 2013 CHBDC Supplement #3

The Canadian Highway Bridge Design Code (CHBDC) is Canada’s design standard document for designing bridge structures with spans exceeding three metres. Soil/steel structures fall into the category of buried structures, which is the subject of Section 7 of the Code. The CHBDC document is an invaluable tool, used extensively by bridge designers, public agencies, municipalities and departments of transportation.

The CSA publishes and maintains the CHBDC document. Its guidelines are adopted by relevant entities throughout most Canadian provinces and municipalities. Several provincial governments also publish amendments and/or addenda to the codes provided in the CHBDC to deal with unique regional considerations.

In April of 2013, the third supplement to the code was introduced to clear up some ambiguities in previous iterations and to update the 2006 supplement. Its additions and amendments are the product of a review committee comprising personnel representing industry, consultants, owners, academia and government. 

What’s New for 2013?

As it had been seven years since the previous update, the code needed to be reviewed and revised to ensure it properly acknowledged and addressed evidentiary field performance data collected over the past seven years; additionally, the document was amended to include code specifications for new steel/soil products (such as very deep corrugations) that didn’t exist when the last supplement was initiated. 

And, because deeper corrugations facilitate longer spans, their use makes installation easier and more economical. This can reduce the invasive impact of buried structures upon creeks, streams and rivers, resulting in more environmentally benign solutions. These benefits are important elements in the design approach to steel/soil structures.

This third supplement introduces several changes and enhancements to supplement #2 (2006) and is intended to further improve the practices and simplify the guidelines of those charged with designing and installing buried steel structures.  Owners will benefit from these changes in variety of ways, including reduced labour costs and simpler, faster construction.

Following is a brief overview/summary of some of the primary changes and their anticipated impact upon relevant stakeholders:

Minimum Depth of Cover Reduced

After reviewing empirical performance data of installations employing deep and deeper corrugated soil/steel structures, completed since the 2006 code, the minimum required depth of cover was reduced from 1.5 metres to 1.0 metres, which puts it more in-line with specifications for other bridge designs. In turn, this change promotes easier, faster construction and lower labour costs by facilitating:

  • An increase in the number of applications for which buried structures offer a viable solution. The reduction in the minimum cover required for buried structures also offers designers more freedom of choice in how they approach bridge designs.

  • The elimination of concrete relieving slabs in some low cover applications. Previously, for standard highway vehicle loads, concrete relieving slabs were required if minimum ground cover was less than 1.5 metres; this new iteration of the code eliminates the need for relieving slabs with a minimum cover of 1.0 metres.

  • A reduction of the amount of engineered backfill and approach embankment fill required for a solution – the backfill zone now extends 1.0 m above the conduit (the previous code required backfill of 1.5 m)

CHBDC Addresses Deeper Corrugated Plates

The new CHBDC addenda also support design methodologies that employ relatively new steel products, which feature large steel plates and much deeper corrugations than previous offerings. These deeper corrugations, in turn, provide owners and designers new options to specify larger and, hence, more economical spans while retaining all the installation and performance benefits of flexible buried structures.

New Approaches to Load Calculations for Footings

CHBDC supplement 3 includes clarifications relating to the methods used to calculate resultant loads on footings that support buried steel structures, thereby more accurately identifying performance demands on the footings and enabling designers to accomplish that role using significantly smaller footings. The intent of implementing these changes was to create a more uniform approach to footing designs, regardless of the material from which they’re manufactured; under the previous CHBDC supplement, buried concrete structures were, technically, able to utilize footings with lower loads than comparable steel structures.

Fine-Tuning The Code

In creating these new guidelines, the CHBDC committee has ensured the code now accommodates design methods (rigorous analysis) that are more accurate when applied to structures that feature multi-radius, deep, and deeper, corrugated buried structures.   

The old version of the code specified details of some design methods very descriptively. So, if you were looking to design a structure using deep corrugations, it provided specific, detailed instructions on how to do it. However, since that code was introduced it became clear that that those design instructions weren’t always appropriate, and sometimes resulted in final designs that were too conservative or, conversely, too aggressive a solution for the specific installation. In this update, the code recognizes that if you’re looking to design a deep corrugated structure with a low profile arch, you need to analyze the forces involved at a higher level, using rigorous analysis.

The new CHBDC guidelines define when the code equations are applicable and should be used – specifically, when designing with shallow corrugations (i.e., non deep/deeper corrugations), or when designing single radius arched structures with deep corrugations and H<Dh/2.  For all other structures, relevant forces must be calculated using rigorous analysis and modeling, which accounts for the effects of soil/structure interactions, as well.

For more information and details on the Canadian Highway Bridge Design Code (CHBDC), or the benefits of CSP, visit the Corrugated Steel Pipe Institute website at cspi.ca, or contact us at (519) 650-8080, or email info@cspi.ca