Alternative Design Approach for Earthquake-Resistant Masonry
In the premier issue of SMART|dynamics of masonry, structural engineers Steve Dill, now principal / CFO of KPFF Consulting Engineers in Seattle and Andres Lepage, PhD, currently at the University of Kansas, Department of Civil, Environmental & Architectural Engineering, introduced Limit Design for structural masonry, expanding the possibilities of masonry in earthquake-prone regions.
The 2013 MSJC, now known as TMS 402/602 Building Code Requirements and Specifications for Masonry Structures, added Appendix C: Limit Design of Masonry. Provisions described create code requirements that allow engineers to determine the usable strength and deformation capacity of perforated walls in a more rigorous way. Limit Design generally leads to masonry walls requiring less reinforcement than those obtained using conventional methods of Strength or Allowable Stress design.
Strong earthquakes generate ground motions that can induce large shaking forces in buildings. When subjected to reversed cyclic lateral loads, safe, well-proportioned and detailed structures have the capacity to deform without losing their integrity; they are tough. Toughness, in engineering jargon, is the ability to withstand displacement reversals without distress.
Historically, in building codes, toughness is considered through the assignment of numerical ratings (R-values, not to be confused with thermal resistance) to each common seismic system. Structural systems with high assigned R-values are designed for smaller lateral forces than those with low R-values. Toughness is achieved by providing redundancy, continuity and strength in the structural system and through proper detailing in structural components.
With masonry, the configuration of the structural system is often determined by aesthetic and functional envelope requirements – not structural requirements.
In other words, the need for adequate seismic performance must be balanced with the need for accommodating windows, doors, canopies, garage openings, etc. This combination of requirements frequently creates load paths that are more elaborate and easily depart from classic cantilever shear walls. For these more elaborate wall configurations, conventional linear-elastic analysis methods produce less useful design information – potentially leading to impractical designs with compromised seismic performance.
The Limit Design approach offers promising opportunity to avoid these shortcomings in a variety of building markets including multifamily residential, commercial office buildings, warehouses and schools, among others.