Licensed Professional Engineers
FORENSIC CLUES # 44 - "Chair Accidents " by John L. Ryan
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 44: Vol. 1 July/August 2011
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Chair accidents can involve a variety of different causes. Even though the height of a chair is small, a person using a chair is in a compromised position, where the body is not positioned to safely absorb the shock forces involved in a fall. This issue of Clues examines the different causes of chair accidents, and the science behind these failures.
A common chair accident involves the tipping of a chair. A tipping chair can be the result of a design defect or a misuse of the chair.
Virtually any chair can be tipped if enough force is applied to the chair. Engineering experts can determine if a chair design is not stable enough to protect the chair occupant in all foreseeable positions of the chair using simple engineering principles and equations. ANSI/BIFMA standards have tests that establish a minimum stability requirement for chairs—exemplar chairs can be tested to this standard to see if it meets this minimum criteria.
Chair tipping may be a design flaw
Structural Support Failure
Many chair accidents involve a failure of a structural member of the chair that causes the chair to collapse, injuring the chair occupant. These types of structural failures are often a design flaw. Sometimes the chair legs are not sized properly, or not braced adequately. The material specified for the chair structure may not be the same material that the chair is constructed out of. Last minute design changes can be made that are not checked, or the production facility may use a material different than what is specified by the designer. The material may also not meet specifications due to a defect in the material.
Some failures involve the leg of a chair that buckles causing the chair to collapse. These types of failures are often due to design defects where the chair is simply not designed to adequately support the chair occupant. This can occur when an engineer designs a chair for the weight of the person, without taking into account dynamic forces.
Dynamic forces occur whenever motion is involved—for instance, often when people sit in a chair they drop down into it as opposed to easing into it very slowly. This natural tendency to drop into a chair because of the padding places high forces on a chair that it must be designed to withstand. Dynamic forces can easily increase the stress on a chair by 4 to 6 times.
Folding chairs including theater and stadium seating have moving components and bracing that can be more susceptible to failure than non-folding chairs. The strength of a standard folding chair depends on the bracing mechanism between the folding members. If this is not designed properly, deformation or failure of the brace can occur, resulting in chair failure.
Deformed folding chair brace
Seat Back Failure
Failure of a chair seatback can result in serious injuries. These failures are often due to the seatback being inadequately fastened to the rest of the chair structure.
Failure of structural chair connections can cause chair collapse. Weld defects in welds combining metal chair components can cause chair failure. This is a simple defect to identify after a chair collapse. Inadequate material penetration is a common cause of weld failure. Bolts and rivets can also fail due to inadequate sizing of these components, or due to a design defect that puts excessive stress on the connection.
Many chairs have various spring mechanisms incorporated into their design. This may involve an actual spring, or the spring effect may be obtained by yielding of the material—some chair backs are designed to bend when a person leans into the chair. This can be accomplished by using a material with enough elasticity to yield under load, but enough strength to prevent failure. If this is not designed correctly, fatigue failures can occur. Fatigue failures occur from repeated loading to an object., and often involve small cracks that grow until failure occurs.
Certain chairs that fold or recline have pinch points where hands or other body parts can get injured or amputated. Children are usually the victims of these types of accidents. Folding recliners may have pinch points that can harm a child. A particular CPSC recall involves a reclining chair with a gap between the seat cushion and the foot rest that was too big, allowing infants and small children to lean against the foot rest, fit their head in this space, and become strangled when the foot rest closed. High chairs can fail and collapse, not only injuring the child in the seat, but nearby children may get injured by the collapsing legs of the high chair.
Outdoor seating that is subjected to sunlight may be weakened by constant exposure to ultraviolet radiation. Plastics and fiberglass are particularly susceptible to sunlight. Prolonged exposure to sunlight with these materials will weaken the strength of these materials, which can result in failure.
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