Licensed Professional Engineers
FORENSIC CLUES -A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Volume 17 February/March 2007
By John L. Ryan
Hundreds of thousands of ladder accidents occur each year. Attic ladder accidents are not as common as some other ladder accidents, but the potential for serious injury still exists. People are injured on attic ladders when the ladder material fails catastrophically, ladder hinges fail, ladder rivets or other connections fail, the anchor points fail, or when they fall from the ladder, must be considered and dealt with.
Attic Ladder Hazards
Catastrophic Material Failure
Catastrophic failure of the ladder material is often seen in wooden ladders, probably due to the fact that attic ladders have traditionally been constructed of wood. Only relatively recently have other materials such as aluminum been used for attic ladder construction. If an attic ladder is not designed properly, members of the ladder may be overloaded at certain times. Ladder designers are notorious for failing to account for the large range of dynamic forces encountered when using a ladder. Attic ladders have even greater potential for high dynamic forces.
Attic ladders are normally designed to lie at an angle of about 60 degrees compared to about 75 degrees for step and extension ladders. This lower angle makes attic ladders possible to descend with the ladder user facing forward, facilitating the descent. The drawback of the low angle is that with ease of descent, come greater dynamic forces due to greater acceleration on the descent. Attic ladder users are also likely to be carrying heavy loads either up the ladder, down the ladder, or both. Ladder designers rarely consider carried weight in designing the structure of a ladder. When it is only hand or power tools being carried, the added force from the weight of the tools will be minimal. The heavier the loads carried into an attic, the greater the forces that act on the ladder will be. Boxes and other objects stored in attics can easily weigh 40 to 60 pounds. This additional weight will compound the forces acting on the ladder. If the ladder is not designed to withstand large dynamic forces, failure can result.
An attic ladder is only as strong as its weakest component. Many attic ladders have been designed and constructed with inferior hinges, hinges made out of lightweight, thin steel straps and rivets. These lightweight hinges are not designed to take the full range of forces encountered on an attic ladder.
Traditional attic ladders must be cut to length after installation. An attic ladder that is properly installed carries the bulk of force from a person climbing it by the wood sections butting against each other. This transmits the force from the ladder downward to the ground. A ladder that is not properly installed will not transmit the load through to the ladder rails, but through the hinges of the ladder. Attic ladder hinges are not designed to withstand all of the forces of climbing a ladder and can fail suddenly. Further problems develop when hinges are poorly designed mechanically. Some manufacturers in the past used metal strap for hinges that is bent out to accommodate the width of the other part of the hinge. The following figure illustrates such a design.
The material of the hinge has already yielded to be able to fit over the other steel straps. This hinge will not be able to withstand the forces that a hinge that was manufactured with a bend for clearance would be able to.
Fall from an Attic Ladder
Ladder users often fall from ladders when descending. Chances of falling from an attic ladder can be higher than other ladders. Facing downwards for the ladder descent, while convenient, makes the descent more treacherous. The lower angle of attic ladders makes users travel forward and down as they descend. While some manufacturers like to claim that their product is really a portable stairway, many of the products would not meet stairway requirements of tread rise and depth. Descending an attic ladder involves taking short steps. When people are carrying boxes down from an attic, as they often do, they may not be able to see their feet. Without a visual reference point, people will often descend the ladder as they would a steep stairway. This can lead to error in stride judgement, causing the person to miss a step, and fall down to the ground. Falls are made more serious with attic ladders, again, due to the loads that people will be carrying. The weight of the object a person is carrying will increase the person’s acceleration to the ground, which increases the forces the person will experience upon impact. Heavy objects dropped in a fall may cause further injuries to the user or people nearby.
Design Problems of Attic Ladders
Traditional attic ladders are problematic due to the varying heights of installation locations. Attic ladder manufacturers traditionally have provided a product that must be cut to length upon installation. If the ladder is not cut to the correct length, the attic ladder will not be safe. If the ladder is too short, gaps will form at the hinge locations, and the different sections of the ladder will be at different angles. This results in high concentration of stresses on the hinges and the connections that hold the hinges to the ladder material. A too-short ladder cannot be easily fixed either with a traditional attic ladder design. A ladder that is cut too long will also produce gaps in some of the hinges, increasing the loading on the various elements. Attic ladder length is absolutely crucial to effective functioning of the weight-supporting structure. Putting the final modification of the product in the hands of a consumer is not a valid method of accomplishing the task of making the ladder the correct length. Alternate designs, discussed later, have been developed that eliminate this problem.
Two ANSI standards appear to cover attic ladders. The minimum design load is given in section 18.104.22.168 of the ANSI A14.3 standard. This section states that “The minimum design live load shall be two loads of 250 pounds each concentrated between any two consecutive attachments”. Section 22.214.171.124 “All live loads shall be considered to be concentrated at such a point or points that will cause the maximum stress in the structural member in question.”
Section 126.96.36.199 states that “Wood ladders shall be designed and fabricated in accordance with applicable requirements of ANSI A14.1-1990…” The design loads shall be as specified in 4.2 of ANSI A14.3-1992…” Section 7.1.3 of ANSI A14.3 calls for the ladder to withstand a weight of at least 500 pounds dropped 18 inches.
Designs have been developed and are being manufactured that eliminate some of the problems associated with the traditional folding attic ladder. Hinge design has improved dramatically. Most attic ladders are now produced with plate hinges instead of the steel strap hinges. These plates distribute the load on the ends of the ladder rails evenly over a large area, making the connection points less likely to fail due to decreased stresses. Not only will this help prevent the hinges from failing, it will also help prevent wood failure at the connection points.
Attic ladder manufacturers are producing ladders that do not require cutting them to length after installation. Different designs have been developed to accomplish this, most of which have some sort of adjustable feet that can accommodate various ceiling heights.
These adjustable feet are similar in design to ladder levelers that have been produced for decades. Ladder levelers are after-market devices which attach to various types of ladders to make the ladder level when it is set up on uneven ground or on inclines. The following figure shows a patent for a ladder leveler dated 1906. This shows that the technology has existed since at least 1906 to make an attic ladder that could be adjusted for various ceiling heights.