by Kevin O’Neill

Seismic forces are used as a representation for the displacement and movement that a structure would undergo during a seismic event. The seismic forces are directly related to the height, seismic weight and number of levels of the structure. Seismic weight is determined from the dead load of the structure consisting of framing members, decking, railing and equipment securely attached to the structure.

Structures can be grouped into 6 different seismic design categories (SDC) with low seismic being classified as SDC’s A through C and high seismic being SDC’s D through F. The two most important factors in determining SDC are geographic location and site soil characteristics. High seismic classifications may require a site soils report but the authority having jurisdiction will have the final decision on whether this is required. Based on these factors, seismic response spectrum values can be determined. In combination with the framing system of the structure, the seismic forces can be determined for designing the structure. The Steele Solutions engineering team is able to help customers regardless of the SDC and are knowledgeable on how to handle different situations.

When discussing SDC the phrase lateral force resisting system (LFRS) is brought up; the LFRS selection is the most impacted by the SDC. Low seismic structures have the most freedom to utilize varying LFRS’s (i.e. braced frames, and cantilever column). The type of LFRS in turn dictates the response modification coefficient (R). The selection of the structural system is controlled by the height of the structure and the SDC. Low seismic structures with limited heights (up to 60 feet) are allowed to use an LFRS classified as “Steel System Not Specifically Detailed for Seismic Resistance” which has an R value of 3. This is the most common system for low seismic structures. Knowing the R value is important because it can help our engineering team create a safe yet simple and efficient structure.

High seismic structures are the most difficult and complicated to design. There are multiple systems that can be utilized that are classified as ordinary, intermediate and special. Ordinary systems have lower R values and special have the highest R values. Systems with R values greater than 3 require the connections of the structure to follow prescriptive requirements of AISC 341. These prescriptive requirements ensure there are ductile connections that would allow the steel members to undergo plastic deformations and dissipate energy in the structure. Designing high seismic structures is a balancing act of designing the steel structure with a higher R value and following the AISC 341 requirements.

There are a lot of different design paths to be taken when designing a structure for seismic loading. Two identical structures that have different SDC or even the same SDC can have drastically different framing sizes, connections and foundations. Seismic design is based on probabilities and statistics. Areas that are more prone to having earthquakes or seismic activity are more at risk than areas away from active faults. Steele Solutions works hard to ensure that all projects are up to code and are structurally sound when being designed. Our engineering team is ready to help with any seismic project you have!