ASCE 7-10 Section 11.2 defines Cantilevered Column Systems as follows: “A seismic force-resisting system in which lateral forces are resisted entirely by columns acting as cantilevers from the base.”

ASCE 7-10 Table 12.2-1, Item G, sets forth very low design coefficients and factors with strict height limits for cantilever column systems.

ASCE 7-10 Section 12.2.5.2 addresses them as follows:

12.2.5.2 Cantilever Column Systems. Cantilever column systems are permitted as indicated in Table 12.2-1 and as follows. The required axial strength of individual cantilever column elements, considering only the load combinations that include seismic load effects, shall not exceed 15 percent of the available axial strength, including slenderness effects.

Foundation and other elements used to provide overturning resistance at the base of cantilever column elements shall be designed to resist the seismic load effects including overstrength factor of Section 12.4.3.

The ASCE 7-10 expanded seismic commentary, which is based on the 2009 NEHRP Recommended Seismic Provisions for New Buildings and Other Structures (FEMA P-750), provides this helpful background information:

C12.2.5.2 Cantilever Column Systems. Cantilever column systems are singled out for special consideration because of their unique characteristics. These structures often have limited redundancy and overstrength and concentrate inelastic behavior at their bases. As a result, they have substantially less energy dissipation capacity than other systems. A number of apartment buildings incorporating this system experienced very severe damage and, in some cases, collapsed in the 1994 Northridge earthquake. Because the ductility of columns having large axial stress is limited, cantilever column systems may not be used where individual column axial demands from seismic load effects exceed 15 percent of their available axial strength, including slenderness effects.

Elements providing restraint at the base of cantilever columns must be designed for seismic load effects including overstrength so that the strength of the cantilever columns is developed.

Here is a question we often get regarding the application of the overstrength factor for cantilevered column systems:

Q: ASCE 7-05 Section 12.2.5.2 says that foundation and other elements used to provide overturning resistance at the base of cantilever column elements shall be designed to resist the seismic load effects including overstrength factor of Section 12.4.3. Where does the overstrength force stop for a steel column on a concrete footing?

A: We think the ASCE 7 language means that the column to base plate connection, the base plate, the base plate to footing connection and the footing must be designed using the load combinations with overstrength factor.

A particular aspect of cantilever column systems, as used in residential buildings, would be relevant to discuss here.

ASCE 7-10 Section 12.2.3.3 requires: “The value of the response modification coefficient, R, used for design in the direction under consideration shall not be greater than the least value of R for any of the systems utilized in that direction.”

Now consider a typical single-family dwelling with light-frame (wood) shear walls (R = 6½, when used as a bearing wall system) in the dwelling itself but with the front or part of the front supported on cantilever columns (R = 2½ for steel special cantilever column systems). By the above rule, the entire building will need to be designed for a punitive R-value of 2½.

However, a very important exception to ASCE 7-10 Section 12.2.3.3 states: “Resisting elements are permitted to be designed using the least value of R for the different structural systems found in each independent line of resistance if the following three conditions are met: (1) Risk Category I or II building, (2) two stories or less above grade plane, and (3) use of light-frame construction or flexible diaphragms. The value of R used for design of diaphragms in such structures shall not be greater than the least value of R for any of the systems utilized in that same direction.” This exception obviously alleviates the penalty mentioned above.