Free-standing walls, typically of concrete or masonry construction, have numerous functions such as separating adjoining properties, screening service areas, and protecting pools. No permit or inspection is required for such walls 6 feet or less in height per the 2006 International Building Code (IBC) Section 105.2.
However, how is a free-standing wall greater than 6 feet in height designed for seismic forces? Should the wall be considered a nonstructural component and designed in accordance with 2005 American Society of Civil Engineers/Structural Engineering Institute’s Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) Chapter 13, or should it be considered a nonbuilding structure and designed in accordance with ASCE 7-05 Chapter 15?
Answers to FAQs
It is surprising the code doesn’t directly address something as common as a free-standing wall, yet there is no clear answer to the question of how to design these elements for seismic forces.
In the 1997 Uniform Building Code (UBC), there is a very clear-cut entry for “masonry or concrete fences over 6 feet high” in Table 16-0, Item 2.H, where an ap = 1.0 and an Rp = 3.0 are specified. Unfortunately, such a clear-cut entry is not in ASCE 7-05 Table 13.5-1.
However, the entry “Wall Element” under “Exterior Nonstructural Wall Elements and Connections” in ASCE 7-05 Table 13.5-2 may be interpreted as an applicable entry for “masonry or concrete fences over 6 feet high.” And the corresponding ap = 1.0 and Rp = 2.5 are in line with values specified in the 1997 UBC. In this case, the seismic load is applied uniformly along the height of the wall with the resultant load applied at mid-height.
Others may interpret Table 15.4-2 as being applicable in this situation. The entry, which reads, “All other steel and reinforced concrete distributed mass cantilever structures not covered herein including stacks, chimneys, silos, and skirt-supported vertical vessels that are not similar to buildings,” most closely matches a free-standing wall, and an R = 3.0 is assigned. In this case, Chapter 15 dictates a triangular vertical distribution of the seismic load with the resultant load at a height of 2/3 the wall height above grade.
When compared with the Chapter 13 design, Chapter 15 results in a slightly higher moment at the wall base, which would govern the design of the footing and the wall reinforcing near the base of the wall.
This matter has been brought to the attention of those involved in making revisions to ASCE 7-05 seismic provisions, and hopefully in the next edition, there will be an entry in Table 15.4-2 for free-standing walls with an R = 3.0.
The issue of whether an item should be treated as a nonstructural component or a nonbuilding structure for purposes of seismic design comes up not only for free-standing walls, but for a host of other items. The following list of additional items can be considered either nonstructural components or nonbuilding structures: billboards and signs, bins, chimneys, conveyors, cooling towers, stacks, tanks, towers, and vessels. When in doubt, the designer always has the option of calculating the force demand in accordance with the nonstructural component equation in ASCE 7-05 Chapter 13 and the nonbuilding structure equation in ASCE7-05 Chapter 15 and using the more conservative design.
The design force equation for nonstructural components is indirectly dependent on the period of the item (is it flexible or rigid?), whereas the design force equation of nonbuilding structures is directly dependent on the period of the structure. For nonstructural components, the period of the item affects the determination of the amplification factor, ap. Because of differences in the form of the two sets of equations and the defined parameters, they can yield quite different results. A comprehensive article by Robert Bachman and Susan Dowty discussing ways to differentiate between the nonstructural components and nonbuilding structures is available at www.skghoshassociates.com/sk_publication/_Bachman,Dowty_reprint.pdf.
S.K. Ghosh Associates Inc., is a structural seismic, and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at firstname.lastname@example.org and email@example.com, respectively, or at www.skghoshassociates.com.