Seismic bracing of nonstructural components represents one of the biggest life safety hazards in our buildings. Nonstructural components include secondary building components that support the facility’s functions. Nonstructural damage historically accounts for 20% to 50% of the damage observed in recent earthquake events within the United States. This should not be too surprising considering the cost of nonstructural components within buildings is often greater than the cost of the primary structure itself.
Code requirements to seismically brace nonstructural components are well-documented and have been in the code for many years. Unfortunately, some requirements are often overlooked because the responsibility to meet the requirements is typically left to the sub-contractors installing the systems. This process has the potential to leave building owners at risk for systems that are not adequately braced.
Nonstructural Systems Required to be Seismically Braced
The governing code for the seismic bracing of nonstructural components is found in Chapter 13, ASCE/SEI 7-10. The code offers the following exemptions for seismic bracing:
Some common architectural components that require seismic restraint are:
Factors Influencing the Bracing Design
The equation governing the seismic bracing of nonstructural components is as follows:
Fp = (0.4 * ap * Sds * Wp) * (1 + 2 * (z/h))
(Rp / Ip)
As you can see, the primary factors that influence the design force are seismic acceleration, weight of the component, and the elevation of the component relative to the overall building height. Consequently, the force in the brace increases as seismic motion increase, component weight increases, and the closer the component gets to the structure’s roof.
Typical Bracing Methods
Post installed anchors are commonly used into concrete and masonry; however, the anchors must be prequalified for seismic applications. Power Actuated Fasteners are commonly used in both concrete and steel; however, power actuated fasteners cannot be used for sustained tension loads or for brace applications in SCD D, E, or F unless approved for seismic loading. It should be noted that power actuated fasteners can be used to support the following:
Unistrut, TOLCO and B-Line all have systems specifically design to resist seismic forces and are commonly used.
Who is Responsible to Design These Systems?
The building code requires drawings to show supports and attachments of nonstructural components with a quality assurance plan that needs to be prepared by a registered design professional for the use by the owner, authorities having jurisdiction, contractors, and inspectors. The big question is—who is responsible for this design and drawing preparation? The engineer of record, or the contractor installing the systems via a delegated design?”
As you might suspect, the correct answer might be a little of both. Below is a summary of the pros and cons of each path.
Engineer of Record
Non-structural components are the most valuable building assets. The building code provides clear direction on how to seismically brace these elements to protect the owner’s investment and life safety. The biggest challenges for the design team are to define who is responsible for this design and to ensure all code requirements are met during construction.