EXPERIMENTAL EVALUATION OF THE STRUCTURAL CHARACTERISTICS OF EXTRUDED ALUMINUM STEPLADDERS

摘要

Accidents associated with the use of portable step ladders are estimated to exceed 100,000 annually. Causes are often attributed to the actions of user, and not the mechanical or structural properties of the ladder. To assess the characteristics of modern ladder designs, a comprehensive experimental study was conducted in which the overall stability of the structure as well as the strains within specific structural members were measured during the normal use of extruded aluminum step ladders. Measurements of the force required to tip the ladder during use found that ladder stability was lower than expected. Stability was significantly reduced by user motion and an unlevel support surface. Commercially available Type III ladders were instrumented with strain gages on both the side front rails and steps; measured strains were used to compute dynamic stresses generated while users performed a variety of typical actions on the ladder. In general, stresses were much higher than those predicted from static analysis, and suggest a dynamic factor of at least 2.5 is necessary to adequately account for the loads applied to the structure. Overall, the actual factors of safety attained experimentally were significantly less than those required by ANSI Standard 14.2 for portable metal step ladders. This was attributed to the fact that the factor of safety defined in the standard is based on static loads alone, and does not adequately account for the dynamic stresses associated with the normal use of these structures.