If you have been involved with a project that includes blast loads, you have probably heard of “Balanced Design.”   If the project had more than one blast consultant, you probably heard way more about Balanced Design than you ever wanted to hear.  This is because there is no single approach to incorporating it into a project.   So….the million dollar question is:  What the heck is Balanced Design?

Balanced design is a concept commonly used in blast and earthquake engineering that refers to designing supporting elements to the ultimate strength of the supported elements.

The key is to create structural fuses (just like an electrical fuse) designed to be a point of safe failure that will limit damage from a larger than design blast (or earthquake) from cascading to larger and larger areas.

In earthquake engineering, a typical application of this concept is the “strong column/weak beam” concept required in high seismic zones.  This requirement was instituted to avoid catastrophic failure by forcing the beam to fail before a column in case of a larger than design seismic load.  While there would still be a failure, the result of the failure would be less extensive than if the column failed.

From a blast perspective, balanced design can be applied to both structural and non-structural elements.  There are several ways to achieve balanced design:

• Vary the allowable response limits of elements based on their importance. For instance, while a secondary beam would be designed to no more than heavy damage, its supporting girder would need to be designed to allow only moderate damage. While this approach meets the intent of balanced design, when the overall design is not controlled by blast forces, the final structural system may be unbalanced.
• Design supporting elements to develop the static capacity of the supported elements. While this approach always results in a balanced system, it can result in a highly conservative design as this approach does not include the dynamic response of the system.
• Design the entire load path to the dynamic capacity of the first element loaded. While this approach results in a more precise solution, there are several issues related with the definition of failure of this first element and the associated time-history load.  When using this approach, project stakeholders should agree on definitions and approach prior to commencing analysis.

Balanced Design acknowledges that it is difficult to precisely predict the magnitude of an earthquake or the size of a blast, and it is therefore important to create a controlled failure mechanism to limit the extent of damage.