Determining Earthquake Loads
Question:
We are currently developing a project which specifies “Seismic Design Category C.” We are an East Coast truss manufacturer and have not encountered seismic requirements before.
Answer:
We suggest that you contact the building designer (hopefully an architect or engineer) and have them specify the earthquake loads in more detail.
When building designers design a structure to resist earthquakes, they need information about the nature of the ground movement that is likely to occur below the building. This includes such things as the magnitude of the ground acceleration and the frequency of the expected motion. “Seismic Design Category C” is a designation used to specify the kind of earthquake ground motion that is expected in a particular area. This information, combined with the distribution of mass within the structure, is used to calculate the stresses that can be expected to develop in the structure when an earthquake hits. It is the responsibility of the building designer to determine the loads on a building. This is especially important when the determination of loads is quite complicated, as is the case with earthquake loads.
Earthquake loads are usually determined using the earthquake provisions specified in the building code that is used in a particular area. The building codes, in turn, usually reference a document called ASCE/SEI 7: Minimum Design Loads for Buildings and Other Structures. This document is produced by the American Society of Civil Engineers. Although SEI/ASCE 7 does provide guidance for determining earthquake loads, it is a fairly complicated procedure and is definitely outside the scope of work performed at most truss plants. In addition, by determining these loads yourself, you automatically assume some liability for the accuracy of those loads. You may not be comfortable with that.