Science Fair Project Encyclopedia
- For other uses of the term chair, please see chair (disambiguation).
A chair is a piece of furniture consisting of a seat, legs, back, and sometimes arm rests, for use by one person. Without back and arm rests it is called a stool. A chair for more than one person is a couch, sofa, settee, loveseat or bench. A footrest for a chair is known as an ottoman or hassock. A chair mounted in a vehicle or in a theatre is simply called a seat.
The back often does not extend all the way to the seat to allow for ventilation. Likewise, the back and sometimes the seat are made of porous materials or have holes drilled in them for decoration and ventilation.
The back may extend above the height of the head. There may be separate headrests. Headrests for seats in vehicles are important for preventing whiplash injuries to the neck when the vehicle is involved in a rear-end collision.
See history of the chair for an extended look at chairs from antiquity to the modern day.
Design and ergonomics
Chair design considers intended usage, ergonomics (how comfortable it is for the occupant), as well as non-ergonomic functional requirements such as size, stackability, foldability, weight, durability, stain resistance and artistic design. Intended usage determines the desired seating position. "Task chairs", or any chair intended for people to work at a desk or table, including dining chairs, can only recline very slightly; otherwise the occupant is too far away from the desk or table. Dental chairs are necessarily reclined. Easy chairs for watching television or movies are somewhere in between depending on the height of the screen.
Ergonomic designs distributes the weight of the occupant to various parts of the body. A seat that is higher results in dangling feet and increased pressure on the underside of the knees ("popliteal fold"). It may also result in no weight on the feet which means more weight elsewhere. A lower seat may shift too much weight to the "seat bones" ("ischial tuberosities").
A reclining seat and back will shift weight to the occupant's back. This may be more comfortable for some in reducing weight on the seat area, but may be problematic for others who have bad backs. In general, if the occupant is suppose to sit for a long time, weight needs to be taken off the seat area and thus "easy" chairs intended for long periods of sitting are generally at least slightly reclined. However, reclining may not be suitable for chairs intended for work or eating at table.
The back of the chair will support some of the weight of the occupant, reducing the weight on other parts of the body. In general, backrests come in three heights: Lower back backrests support only the lumbar region. Shoulder height backrests support the entire back and shoulders. Headrests support the head as well and are important in vehicles for preventing "whiplash" neck injuries in rear-end collisions where the head is jerked back suddenly. Reclining chairs typically have at least shoulder height backrests to shift weight to the shoulders instead of just the lower back.
Armrests will also support part of the body weight through the arms. They further have the function of making entry and exit from the chair easier. Armrests should support the forearm and not the sensitive elbow area. Hence in some chair designs, the armrest is not continuous to the chair back, but is missing in the elbow area.
A kneeling chair adds an additional body part, the knees, to support the weight of the body. A sit-stand chair distributes most of the weight of the occupant to the feet.
Padding will not shift the weight to different parts of the body (unless the chair is so soft that the shape is altered). However, padding does distribute the weight by increasing the area of contact between the chair and the body. A hard wood chair feels hard because the contact point between the occupant and the chair is small. The same body weight over a smaller area means greater pressure on that area. Spreading the area reduces the pressure at any given point. In lieu of padding, flexible materials, such as wicker, may be used instead with similar effects of distributing the weight. Since most of the body weight is supported in the back of the seat, padding there should be firmer than the front of the seat which only has the weight of the legs to support. Chairs that have padding that is the same density front and back will feel soft in the back area and hard to the underside of the knees.
There may be cases where padding is not desirable. For example, in hot climates, padding with fabric or plastic covers is often uncomfortable against the skin. Where padding is not desirable, contouring may be used instead. A contoured seat pan attempts to distribute weight without padding. By matching the shape of the occupant's buttocks, weight is distributed and pressure at any given point is reduced.
Actual chair dimensions are determined by measurements of the human body or anthropometric measurements. Individuals may be measured for a custom chair. Anthropometric statistics may be gathered for mass produced chairs. The two most relevant anthropometric measurement for chair design is the popliteal height and buttock popliteal length.
For someone seated, the popliteal height is the distance from the underside of the foot to the underside of the thigh at the knees. It is sometimes called the "stool height". (The term "sitting height" is reserved for the height to the top of the head when seated.) For American men, the median popliteal height is 16.3 inches and for American women it is 15.0 inches. The popliteal height, after adjusting for heels, clothing and other issues is used to determine the height of the chair seat. Mass produced chairs are typically 17 inches high.
For someone seated, the buttock popliteal length is the horizontal distance from the back most part of the buttocks to the back of the lower leg. This anthropometric measurement is used to determine the seat depth. Mass produced chairs are typically 38-43 cm deep.
Additional anthropometric measurements may be relevant to designing a chair. Hip breadth is used for chair width and armrest width. Elbow rest height is used to determine the height of the armrests. The buttock-knee length is used to determine "leg room" between rows of chairs. "Seat pitch" is the distance between rows of seats. In some airplanes and stadiums the seat pitch is so small that there is sometimes there is no leg room for the average person.
For adjustable chairs, the aforementioned principles are applied in adjusting the chair to the individual occupant.
Chair seats vary widely in construction and may or may not match construction of the chair's back. Some systems include: Solid center seats where a solid material forms the chair seat.
- Solid wood, may or may not be shaped to human contours.
- Wood slats, often seen on outdoor chairs
- Padded leather, generally a flat wood base covered in padding and contained in soft leather
- Stuffed fabric, similar to padded leather
- Metal seats of solid or open design
- Molded plastic
- Stone, often marble
Open center seats where a soft material is attached to the tops of chair legs or between stretchers to form the seat.
- Wicker, woven to provide a surface with give to it
- Leather, may be tooled with a design
- Fabric, simple covering without support
- Tape, wide fabric tape woven into seat, seen in lawn chairs and some old chairs
- Caning, woven from rush, reed, rawhide, heavy paper, strong grasses, cattails to form the seat, often in elaborate patterns
- Splint, ash, oak or hickory strips are woven
- Metal, Metal mesh or wire woven to form seat
Standards and specifications
Design considerations for chairs have been codified into standards. ISO 9241-5:1988, "Ergonomic requirements for office work with visual display terminals (VDTs) -- Part 5: Workstation layout and postural requirements " is the most common one for modern chair design.
There are multiple specific standards for different types of chairs. Dental chairs are specified by ISO 6875. Bean bag chairs are specified by ANSI standard ASTM F1912-98. ISO 7174 specifies stability of rocking and tilting chairs. ASTM F1858-98 specifies lawn chairs. ASTM E1822-02b defines the combustibility of chairs when they are stacked.
The Business and Institutional Furniture Manufacturer's Association (BIFMA) defines BIFMA X5.1 for testing of commercial-grade chairs. It specifies things like:
- chair backstrength of 150 pounds (68 kg)
- chair stability if weight is transferred completely to the front or back legs
- leg strength of 75 pounds (34 kg) applied one inch (25 mm) from the bottom of the leg
- seat strength of 225 pounds (102 kg) dropped from six inches (150 mm) above the seat
- seat cycle strength of 100,000 repititions of 125 pounds (57 kg) dropped from 2 inches (50 mm) above the seat
The specification further defines heavier "proof" loads that chairs must withstand. Under these higher loads, the chair may be damaged, but it must not fail catastrophically.
Large institutions that make bulk purchases will reference these standards within their own even more detailed criteria for purchase . Governments will often issue standards for purchases by government agencies (e.g. Canada's Canadian General Standards Board CAN/CGSB 44.15M  on "Straight Stacking Chair, Steel").
In place of a built-in footrest, some chairs come with a matching ottoman. An ottoman is a short stool to be used as a footrest but can sometimes be used as a stool. If matched to a glider, the ottoman may be mounted on swing arms so that the ottoman rocks back and forth with the main glider.
A chair cover is a temporary fabric cover for a side chair. They are typically rented for formal events such as wedding receptions to increase the attractiveness of the chairs and decor. The chair covers may come with decorative chair ties, a ribbon to be tied as a bow behind the chair. Covers for sofas and couches are also available for homes with small children and pets. In the second half of 20th century, some people used custom clear plastic covers for expensive sofas and chairs to protect them.
Chair pads are cushions for chairs. Some are decorative. In cars, they may be used to increase the height of the driver. Orthopedic backrests provide support for the back. Obus Forme is a major brand in this category and helped develop this market niche. Car seats sometimes have built-in and adjustable lumbar supports.
Chair mats are plastic mats meant to cover carpet. This allows chairs on wheels to roll easily over the carpet and it protects the carpet. They come in various shapes, some specifically sized to fit partially under a desk.
Remote control bags can be draped over the arm of easy chairs or sofas and used to hold remote controls. They are counter-weighted so as to not slide off the arms under the weight of the remote control.
English phrases relating to chairs
A movie or a story is said to keep you on the edge of your chair, if it is suspensful and engaging.
If you nearly fell off your chair, it was because you were very surprised.
Activites that are likely to be made insignificant or undone by some future event are said to be like moving the deckchairs on the Titanic.
- List of chairs for an extended list of various chair types and makes.
- Electric chair for information on chairs used in judicial executions.
- History of the chair
- Office Chair Reviews: User reviews of ergonomic office chairs
- See also definition at http://wiktionary.org/wiki/chair
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