Evaluation of the Seismic Force Reduction Factor (R) for Adobe Walls Design

Earth-based construction (particularly adobe and rammed earth) continues to provide housing for nearly a quarter of the world's population, reflecting its economic accessibility and deep cultural roots. Yet, despite its widespread use, seismic design for earthen structures remains anchored in allowable-stress methods that mirror the material's brittle nature, rather than adopting performance-based frameworks centred on ductility, energy dissipation, and overstrength. Adobe's low tensile strength and limited deformation capacity constrain seismic energy dissipation, leading to brittle in-plane behaviour. Advancing force-reduction-based design thus requires experimental quantification of ductility and overstrength. This study analyses full-scale in-plane cyclic tests on adobe walls with intermediate slenderness. Capacity curves were used to estimate seismic reduction factors through three approaches: an energy-based method, a bilinear idealisation incorporating ductility and overstrength, and a demand-spectrum procedure aligned with SEAOC. Results indicate limited yet measurable ductility and moderate overstrength. Under controlled performance objective, a global reduction factor of R ≈ 1.5 is proposed for unreinforced adobe walls, assuming out-of-plane instability is prevented, thereby defining applicability limits. These findings provide an experimental basis for integrating earthen construction into contemporary seismic design frameworks, thereby supporting safer, more resilient applications in regions where adobe remains a primary building material for housing.