18.104.22.168 Dimensional Requirements
Scientific progress in the field of analysing the seated position and adjusting furniture to the physiological characteristics of a human being began from Staffel’s claim (Izrael Abraham Staffel 1814-1884), that: chairs are almost without exception constructed more for the eye, for the beautiful form, than for the back. It was at that time when the principle was formed that the spine of a sitting person should be supported only in the lumbar region. The next step in formulating the principles of proper sitting was striving to ergonomically shape the upper part of the backrest, adjusted to the chest part of the spine (St^powski 1973). Currently, the common belief dominates among designers and users of furniture that regardless of the subjective perception of comfort of a seat, chair seat, armchair and sofa seat should be designed in accordance with the physiology of the human.
Some furniture for sitting (chairs, armchairs, stools), together with tables, benches or desks, can be used to work, study and dine, while soft armchairs and sofas generally constitute equipment for places to relax and rest. Each of these groups of furniture, due to the clear differentiation of purpose, should be characterised by different requirements: dimensional, mechanical and functional.
In the furniture for sitting, used at tables, not only the dimensions of the chair, armchair or table must be dealt with, but also the appropriate dimensional relations between these furniture pieces should be selected.
The correct dimensional proportions for the chair have been illustrated in Fig. 3.37 and listed in Table 3.2. Considering the need of free movement of the lower limbs, Fig. 3.38 shows the space in which the user can move them once bent in the knee joint, while in Fig. 3.39, the lower limbs are straight at the knee joint.
When designing tables, it is worth noting the correct height of the worktop, the height of the rails (restricting the movement of the lower limbs), the width of the board guaranteeing operational freedom on the worktop surface and the depth of the board conditioning convenient reach in a comfortable seated position (Fig. 3.40).
The optimal dimension of the worktop of a workstation or for eating meals should also depend on the hold reach of the upper limbs, taking into account the zones of precise, accurate, general and inaccurate work (Figs. 3.41, 3.42, 3.43, 3.44 and 3.45). Composing models of a chair and table also enables to determine the most appropriate dimensions for a set and demarcating the ergonomic space for the free use of the chosen arrangement (Fig. 3.46).
By building virtual models of chairs and tables, which can adjust their dimensions to individual types of human phantoms, already during the designing stage, the product being made can be validated and show the weakness of operational or constructional solutions adopted. By composing a virtual model of an actual dining room table with the model of a chair for the user, for example, of characteristics of the 50th centile (Fig. 3.46), the basic ergonomic and functional errors of this system can be eliminated. For a full analysis of the functionality of the object, it is possible
Fig. 3.37 Ergonomic dimensions of a chair
to slide out worktops and set up extra chairs (Fig. 3.47). A virtual synthesis of many probable functional systems of furniture enables to point out potential conflicts between the frame of the table and the comfort of the user.
Furniture intended for relaxation should be characterised by different mechanical and dimensional parameters than furniture for work and dining. Grandiean (1973, 1978) formulated the requirements in relation to the geometry of the seat and backrest of an armchair for healthy people and for people with spinal disorders (Fig. 3.47). The numerical values of these parameters have been provided in Table 3.15.
Preferred angles of seat and backrest inclination, depending on the purpose of the furniture, are also given in Table 3.16.
One of the basic dimensional parameters of furniture for sitting and resting is the height of the seat. It should be less than the distance between the popliteal bend and the base supporting a person sitting down, regardless of the adopted position during work, study or relaxation. In a seated position, the seat height should be 3-5 cm below the popliteal bend (St^powski 1973). A seat that is situated too high (Fig. 3.48a) can cause pressure on the artery, thus burdening the circulatory system
too much. And a seat that is too low (Fig. 3.48b) requires one to contract one’s legs, which causes pressure on internal organs and increased pressure on sciatic protuberances. Incorrect load of the spine in the lumbar region also has a negative impact on the well-being of the user of furniture for sitting. Usually, it is caused by a seat that is too deep (Fig. 3.48c). For this reason, it is recommended that the popliteal part of the thigh protrudes from the seat by 1/3 (St^powski 1973; Nowak 1993).