A Geometric Framework for Evaluating Compound Slopes Within the Wheelchair Footprint on a Curved Ramp

Abstract

Existing accessibility guidelines such as the Americans with Disabilities Act (ADA) and ICC117.1 (ICC) are based on the slope of the running surface of a ramp and the cross-slope. This is usually measured along the side of the ramp. This is suitable for straight ramps but when used on curved ramps three-dimensional geometry must be considered. This is something that the current accessibility evaluation approaches do not sufficiently address. The ADA does not provide much help regarding these issues, even though the ADA does advise against the use of curved ramps, because there is the possibility of compound slopes and unreliable wheel performance at the wheel contact points.

This thesis investigates the effect of the geometry of a curved ramp on the user's immediate slope of the ramp as experienced by the wheelchair user. Instead of considering the nominal running or cross slope, the proposed condition instead would be a diagonal compound slope between wheel contact points. This was done using a mixed method approach for the purpose of exploration. These methods included analysis of existing standards, geometrical analysis, construction of physical ramps models, in-situ observations of ramps, consulting disability experts and creating a basic spreadsheet tool to evaluate the ramps. The geometrical analysis served as the principal analysis method; physical and field models gave insights about the impact of different ramp designs on wheelchair movement. By analyzing the results of the research it is clear that the slope value, radius of the curve, ramp width and direction of the wheelchair are the significant factors that affect the amount of compound slope a ramp user might experience when using a curved ramp. Specifically, the study showed that enlarging the ramp would substantially reduce the degree of local diagonal slope of the wheelchair. This study also shows that the interaction between asymmetric wheels and a ramp can be observed even if ramp slopes would fulfil the established requirements. The work does not indicate that curvilinear ramps are inaccessible in themselves. Rather, the work suggests a geometric approach to assessing curved ramps as three-dimensional surfaces in a limited space. It outlines the start of a predictive approach that could be used for future design assessment, accessibility studies, and other related efforts in accessible design.