Licensed Professional Engineers
FORENSIC CLUES # 19 - "Ladder Racking Accidents" by John L. Ryan and L.D. Ryan
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 19: Vol. 18 August/September 2006
Safety Engineering Resources - (479) 549-4860
Ladder accidents that involve unintentional racking of the ladder followed by user loss of balance are very common. The ANSI A14 rationale states that in 1975 there were 211,000 injuries associated with ladders in the United States, as estimated by the Consumer Product Safety Commission. The Bureau of Labor Statistics shows that yearly occupational fatal falls from ladders increased from 78 in 1992 to 97 in 1995, a 24% increase. Out of 145 stepladder accident cases studied by the CPSC (Consumer Product Safety Commission), 87 were caused by instability of a stepladder. The American National Standards Institute (ANSI) committee in charge of the stepladder standard, ANSI A14, concluded that by decreasing ladder racking and torsional deflection the number of accidents due to instability could be reduced.
Figure1 - Racked Ladder
Anyone who has ever used a stepladder has probably experienced the racking phenomenon. The rear non-step legs of the typical stepladder can lift off the floor when the climber puts one foot on the first step and then uses one hand to pull his or her body up onto the stepladder. Figure 1 shows the normal configuration of stepladder racking. As the climber gets onto the first step with both feet, the stepladder’s rear legs comes down onto the floor. Often the stepladder’s rear legs are no longer lined up with the front step-legs. Ladder users often are not aware that the ladder is racked and unstable. In the racked position, the stepladder has only three legs on the ground, the fourth leg is off of the ground. This is shown in Figure 2.
Figure 2 - The Results of Racking
The result of racking is the stepladder is balanced on three legs. Racking puts lots of stress on the rear legs. Failure will occur easier with loading on a racked stepladder than on one that is not racked. Failure occurs because only three of the legs are now carrying the load that is designed to be carried by four legs. The deformation of the ladder may be temporary, or this deformation may become permanent, leaving the ladder permanently racked.
Figure 3 – Permanent failure due to racking
Another possible result of racking is the tipping of the ladder. Once the ladder is racked into a three-legged position, it is balanced on three legs. There is then an invisible line running diagonally across the ladder base between two of the legs on the ground. This line defines the tipping plane. While using the ladder, the user may shift his or her position on the ladder and move his or her center of gravity from one side of this line to the other. This may cause the ladder to tip so that the fourth leg touches the ground and another leg comes up. This movement can cause the user to lose his balance and fall off the ladder.
Figure 4 - If climber is directly over the yellow triangle the stepladder is unstable. The stepladder may jump when the climber is over the yellow triangle
It is also possible that as the forces change the ladder can “teeter”. The foot that was off of the ground would shift and come in contact with the ground while the foot diagonal to this foot would lift off of the ground. Both of these conditions (or combinations thereof) inevitably occur thus creating a quick, unstable tipping motion. Walking is this unstable tipping motion that can cause a person to lose their balance and fall.
Figure 4 - Bent lower leg caused by racking
The standard racking test required by the ANSI A14 standard involves placing 100 pounds on the lower step. The stepladder rear legs are lifted off the ground by 3 inches. A four pound side force is applied to the bottom of the rear legs and then released. The position where the ladder leg returns to will be the zero point for the racking tests. After this preload a 6-pound force is applied and the side movement of the rear legs is recorded.
The criteria for the ANSI racking test is displayed in Table 23 of the ANSI standard. Some of the criteria for common ladder sizes are shown below:
Figure 5: Allowed racking for different ladders
The allowed magnitude of deflection in the racking test is extremely large. A four-foot type III ladder can rack 13” and still pass the racking test! To put this in perspective, a four-foot type III stepladder fitted with Safety Engineering Resources’ anti-racking spreader plate racked 1.25 inches. Allowing such large racking performance ensures that ladders will be placed on the market that are ultimately unstable. A stepladder that racks 16 inches with a six-pound side load is highly unstable and extremely susceptible to tip-over accidents.
ANSI Racking Combined with ANSI Loading Tests:
ANSI’s stepladder standard has separate tests for racking and for the compressive strength of the ladder. The compressive strength test involves loading the top cap of the ladder with four times the rated load of the ladder. For a Type III ladder, 800 pounds is applied to the top cap. Safety Engineering Resources developed a composite racking/compression test. Since ANSI allows high magnitudes of rack, it seemed reasonable to expect the ladder to be able to support the necessary weight in the racked position.
Safety Engineering Resources began testing stepladder models involved in racking failures by testing the strength of the ladder by loading the top cap, while the ladder is racked. Safety Engineering found that many ladders that can pass ANSI’s racking test and compression test cannot pass the compression test while racked a small amount. As the stepladder racks, the ability of the ladder to carry the load is decreased.
Safety Engineering Resources developed an alternate design for the spreader bars of stepladders. Dr. L.D. Ryan developed a plate metal-based spreader panel in the 1990's for aluminum stepladders. Dr. Ryan modified his design originally developed for six-foot ladders for a four-foot stepladder model. The following photograph shows this spreader plate attached to a stepladder. The spreader plate acts to stiffen the stepladder and also prevents the ladder from collapsing, unlike the spreader bars found on many ladders. This four-foot stepladder racked 0.25 inches when subjected to the ANSI racking test.
Figure 7 – Solid plate spreader bar nearly eliminates racking
Closed Tubing Ladder Rails
One of the best step ladder designs in terms of racking that we have studied is manufactured by Leifheit, a German ladder manufacturer. This ladder is shown in the following photographs. The Leifheit exhibits a much smaller tendency to rack than ladders with open rails. This ladder is a more stable design and lacks the extreme flexibility and propensity to rack that is exhibited by open rail ladders. Leifheit Stepladder that allows the use of all steps and has a solid plate for a spreader bar
The Leifheit stepladder does not rack excessively, racking an average of about 1.0 inches in the ANSI racking test.
Figure 8 – Leifheit Ladder with closed rails control racking
Safety Engineering Resources has started a technical publishing company, Donegal Bay Publishing. We produce product litigation manuals among other things. These manuals provide in-depth information about specific products commonly involved in litigation cases. One of our manuals covers stepladders. This stepladder manual is a must-have for any attorney with a stepladder accident case or potential case.
For more information on warnings, purchase The Stepladder Manual - A Product Litigation Manual for attorneys at: www.donegalbaypublishing.com
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