By: Yelena Straight, P.E Senior Researcher
For anyone who has felt an earthquake first-hand, they can attest that even the smallest of tremors can be jarring. Although we have no control over seismic activity, we do have control in the adoption and enforcement of up-to-date building codes and proper material installation.
In the past, seismic force levels were only considered in certain earthquake-prone locations like California and Alaska. But major revisions to the International Building Code (IBC) have changed the requirements, which now mandate that every construction project must include and meet a seismic design category, even those that have not required seismic installations in the past. It’s imperative for contractors to be up-to-speed on current seismic codes, especially when working in areas with high levels of seismic hazard.
Here we address the frequently asked questions our seismic experts receive from the field.
*Note: this post does not replace regulations dictated by the building codes maintained by the International Code Council (ICC). Please refer to current edition of IBC and ASCE/SEI 7.
Q: Are seismic specifications written based on how close they are to major fault lines or are they consistent throughout a state?
A: For each project, the engineer of record has to establish the Seismic Design Category (SDC), a classification assigned to a structure based on project location (proximity to faults), soil type and other factors. SDCs are identified on a scale of A-F. The installation requirements depend on the SDC identified. International Building Code (IBC) conveys these requirements through ASTM E580.
Q: Who writes seismic specification? Is it the county, state or federal government?
A: IBC has a procedure to determine Seismic Design Categories. ASTM E580 describes installation requirements for each category. Several agencies such as the Office of Statewide Health Planning and Development (OSHPD) have their own requirements that in some cases are more stringent then ASTM E580. Most states and local jurisdictions adopt the model building codes maintained by the ICC.
Q: What seismic zone is Southern California in?
A: Southern California’s Seismic Design Category must be determined by the project engineer, as categories depend on other factors in addition to location.
Q: If your job is under 1000SF of grid, are you required to use seismic bracing?
A: In Seismic Design Categories A, B and C, seismic bracing is not required. For Seismic Design Categories D, E and F, seismic bracing (splay wires and compression posts) is not required in areas under 1000SF provided the suspended ceiling is encased by four perimeter walls and perimeter closure angles or alternate methods.
Q: Should we use stabilizer bars or ACM7 clips with the MS274 Shadow Molding and USG Donn® Brand CentriciteeTM DXTTM?
A: Either component can be used. Stabilizer bars are not required if ACM7 clips are used. Refer to USG’s seismic technical guide for various combinations of seismic clip interface.
Q: Why don’t we use ACM clips with the M20SM2CT wall angle? Is it due to the width of the angle?
A: If 2” wall mold is used, the ACM7 clip is not required. The clip is designed to keep tees from falling off the narrow 7/8” angle. Since 2” can accommodate the tee movement during seismic activity, the clip is not needed. Please note that stabilizer bars and perimeter hanger wires are required, along with 2” angle.
Q: What types of grid can the ACM7 clip be used with?
A: ACM7 clip can be used with any USG Donn® Brand grid (DX®, DXTTM, DXFTM, DXFFTM, DXITM, etc.) Here is a great guide.
Q: What instances require a seismic compression post and how far apart should they be installed?
A: The seismic compression posts are required in Seismic Design Categories D through F when ceiling installation exceeds 1000SF. When installed, the compression posts and splay wires have to be a maximum of 6 ft. from each wall and 12 ft. on center in each orthogonal direction.
Q: Where can I find literature on why seismic compression posts are necessary in drop ceilings less than 12” from deck and spanning over 1000SF?
A: There is no such exemption in Building Code, or in ASTM E580. However, local jurisdiction in various states allows for omittance of the post and splay wires with plenums less then 6”.
Q: Where can I find in-depth literature on wire placement for seismic zones D, E, and F?
For more information related to seismic building codes and installations, check out the following resources: