Stepladder Spreader Bar Structural Integrity

This paper investigates the structural integrity of stepladder spreader bars and how their damage relates to ladder accidents. It finds that loads arising during a ladder tip-over can be sufficient to produce commonly observed post-accident spreader-bar damage.

Read the full article, “Stepladder Spreader Bar Structural Integrity and the Impact on Accidents,” published in Proceedings of the ASME International Mechanical Engineering Conference and Exposition, for the detailed findings.

Abstract

Ladder accidents involving stepladders usually reveal damage to the spreader bars. This paper addresses the sufficiency of the present stepladder safety standards, design and testing requirements related to spreader bars. 

Spreader bars are the hinge members affixed to the sides of the stepladder that facilitate folding. Post-accident observation of buckled spreader bars or detachment from the side rails is frequently suggested as the cause of a user’s fall and injury. 

In addition to complete detachment at an end of one or both spreaders, several different bending configurations to varying degrees have been observed during accident investigations. These include bars bent into an “S” shape, bars bowed out/in, and bars with compound bending. 

In order to study these various post accident spreader conditions, stepladders of different size, weight ratings (i.e. types III (200 lb.), II (225 lb.), I (250 lb.), etc.) and material (wood, aluminum and fiberglass) were instrumented with strain gages in relevant locations to monitor stresses during normal use and misuse, as well as during various load tests and during live user falls from ladders. 

This extensive measurement experience of multiple loading configurations empirically demonstrated that spreader bar forces were minimal both in normal use, and even some circumstances of misuse. The resulting stress does not result in disconnection or deformation. 

Conversely, the loading of the stepladder structure that occurs in a tip over accident was observed to be more than sufficient to cause the frequently-identified post accident spreader bar damage patterns. On ladders that meet the applicable safety standards, all post accident spreader bar damage was found to be the result of the accident and not the cause.