A variance design, often referred to as a design variance, is a formal approval process required in construction and engineering projects when proposed design values for new construction deviate from the standard or prescribed values outlined in official design guidelines or regulatory documents.
Design Variances are specifically required for utilization of design values for new construction that do not meet the design values prescribed in the RDG. This means if a project's technical specifications, dimensions, or other design parameters for new builds do not perfectly align with the established standards (such as those found in a Roadway Design Guide, or RDG), a variance must be sought and approved. These design values are distinct from the fundamental criteria that might necessitate a Design Exception.
Key Aspects of a Design Variance
Understanding design variances is crucial for ensuring compliance, safety, and project feasibility when standard designs cannot be strictly adhered to.
- Purpose: Design variances allow for necessary flexibility in design and construction, accommodating unique site conditions, innovative materials, or specific project constraints that make strict adherence to every prescribed value impractical or impossible. They ensure that deviations are formally reviewed and approved, maintaining overall project integrity and safety standards.
- Application: They apply to "new construction" where the design values—specific measurable parameters—do not match the established guidelines. This could involve dimensions, material specifications, performance metrics, or other technical details.
- Distinction from Design Exceptions: It's important to note that Design Variances are "separate from the basic controlling design criteria that require RDG Design Exceptions." While both involve deviations from standards, design exceptions typically relate to more fundamental or controlling design criteria, whereas design variances pertain to specific design values that don't meet prescribed values.
When is a Variance Design Needed?
A design variance becomes necessary when a project's proposed elements do not conform to the precise numerical or qualitative specifications set forth in official design manuals.
Here are common scenarios:
- Site-Specific Constraints: Geographical limitations, existing infrastructure, or environmental factors may necessitate a design that slightly deviates from standard values. For example, a road curve radius might need to be tighter than the prescribed minimum due to a protected wetland.
- Innovative Solutions: Sometimes, new technologies or construction methods offer superior performance or cost efficiency but do not perfectly match existing design values. A variance allows for the adoption of such innovations after proper review.
- Optimization and Efficiency: In certain cases, minor adjustments to design values can lead to significant cost savings or improved operational efficiency without compromising safety or functionality.
- Existing Conditions: When retrofitting or connecting new construction to existing infrastructure, it might be impossible to meet all new design values without major, unnecessary disruption.
Practical Implications and Examples
Securing a design variance involves a formal process, including justification, analysis, and approval by relevant authorities. This ensures that any deviation is well-reasoned and does not compromise the project's quality or safety.
| Feature | Design Variance | Design Exception |
|---|---|---|
| Required for | Utilization of specific design values for new construction that do not meet prescribed RDG values. | Deviations from basic controlling design criteria within the RDG. |
| Nature of Deviation | Focuses on specific, often measurable, design specifications or parameters. | Pertains to more fundamental or overarching design principles. |
| Example | A road's shoulder width being slightly less than the standard 10 feet due to right-of-way limitations. | A decision to omit a required design element like a minimum-grade separation for a new interchange. |
Example Scenario: Imagine designing a new segment of a highway. The official "Roadway Design Guide (RDG)" specifies a minimum clear zone width of 30 feet from the edge of the travel lane. However, due to an existing, non-removable historical monument or a steep rock face, the design team can only achieve a 25-foot clear zone in a specific section. In this instance, a design variance would be required to utilize this 25-foot design value, as it "does not meet the design values prescribed in the RDG" (30 feet). The design team would need to justify why this deviation is necessary and how safety will still be maintained (e.g., by implementing additional safety features in that specific section).
