The angle of the failure cone in concrete depends on the specific design method being utilized. The two most notable models that provide distinct angles for the concrete failure surface are the traditional 45° cone approach and the Concrete Capacity Design (CCD) method, which uses an angle of 35°.
Understanding Concrete Failure Cone Angles
In concrete design, particularly when dealing with anchors or breakouts, a conical or pyramidal failure surface is often assumed beneath the anchored element. The angle of this "failure cone" is a critical parameter that influences the calculated capacity of the connection in concrete. It represents the approximate shape of the concrete volume that would be pulled out under extreme loading conditions.
Key Models and Their Failure Cone Angles
Different design methodologies prescribe different angles for this failure surface, reflecting varied empirical data and theoretical assumptions. The provided reference highlights two prominent approaches:
| Method/Approach | Failure Cone Angle | Associated Standard/Context |
|---|---|---|
| Traditional Cone Approach | 45° | ACI Method 349-85 |
| Concrete Capacity Design (CCD) | 35° | Developed for anchor design [13] |
Traditional 45° Cone Approach
The traditional 45° cone approach, notably referenced in the ACI Method 349-85, assumes a failure surface that forms a 45° angle with the concrete surface.
- Principle: This model often simplifies the stress distribution within the concrete, assuming that failure occurs along a plane at 45 degrees to the applied load or anchor axis.
- Application: It has been a long-standing method for calculating the breakout strength of anchors in concrete, particularly in earlier design codes and simplified analyses.
- Practical Insight: While robust, this method can sometimes be more conservative or less accurate for complex loading scenarios compared to more advanced models.
Concrete Capacity Design (CCD) Method
The Concrete Capacity Design (CCD) method [11,12] offers a more refined approach to anchor design and, in this context, defines the angle of the failure surface (whether cone or pyramid) as 35° [13].
- Principle: The CCD method is based on more extensive experimental data and theoretical understanding of concrete behavior under concentrated loads. It aims to more accurately predict the actual concrete breakout strength.
- Application: Widely adopted in modern anchor design guidelines, including those by ACI (American Concrete Institute) and ETAG (European Technical Approval Guideline) documents for anchors.
- Practical Insight: The 35° angle used in the CCD method often results in more economical and efficient anchor designs, as it can yield higher concrete breakout capacities compared to the 45° approach for certain conditions, better reflecting observed failure patterns.
In summary, when discussing the angle of the failure cone in concrete, it is essential to specify the design method being referred to, as the angle can be 45° for traditional approaches like ACI Method 349-85 or 35° for the more contemporary Concrete Capacity Design (CCD) method.
