Operational Loads on Chairs and Stools

The relevant safety requirements for using chairs and stools have been provided in PN-EN 1335-2:2009. The tests described in this norm assume that a chair is used for 8 h a day by people weighing up to 110 kg. The locations of applying forces in durability and load stability tests are determined by using a special template

(Fig. 6.12). This template consists of two profiled segments connected articulately. Profiled surfaces are designed in such a way as to sink into the upholstery. To this end, a segment loading the seat should have a total weight of 20 kg, applied at the point of loading the seat.

The most commonly encountered schemes of load on chairs and stools are systems enabling the determination of:

• loss of balance by the front edge,

• loss of balance to the front,

• loss of balance to the sides of chairs without armrests,

• loss of balance to the sides of chairs with armrests,

• maximum distance of backrest displacement,

• loss of balance to the back of chairs with an untilting backrest,

• loss of balance to the back of chairs with a tilting backrest,

• rolling resistance,

• the durability of the seat and the backrest,

• the durability of backrests that rotate around a horizontal axis,

• the durability of the armrests.

An example of a static load on a seat and backrest is shown in Fig. 6.13. Forces should be located in points of load specified by the template (Fig. 6.12). By using an element or device for loading, the tests should be carried out as shown in Fig. 6.13a. The tested seat is loaded with a force of 750 N, while the static force acting on the backrest should not be less than 410 N.

Fig. 6.13 Determining the strength and durability of home and school furniture for sitting according to PN-EN 1728:2012, PN-EN 1729-2:2006: a load on the seat and the backrest, b, c load on the armrests and side headrests with b horizontal forces, c vertical forces (mm)

By testing the durability of armrests, a force directed outwards in the point in which it is easiest to cause damage should be applied, no less than 100 mm from both ends of the armrest structure (Fig. 6.13b). In the case of vertical loads, the force is applied to the armrests in the location where it is most likely to cause damage (Fig. 6.13c), but no closer than 100 mm from one of the ends of the armrests.

Impact tests usually aim to determine the durability of backrests, armrests and seats. By testing the durability of a backrest, the product should be set with the front legs at the safety blocks preventing from moving forwards and then struck from the outside in the middle of the construction of the top part of the backrest with an impact hammer weighing 6.5 kg (Fig. 6.14a). In carrying out the impact test of the backrest (Fig. 6.14b), the blow should be carried out from the outside on the surface of one armrest, in the place where causing damage is most likely. The durability of the seat is determined by a free fall of the device to hit the seat (Fig. 6.14c) from a specific height on the point of loading the seat marked using a template.

By testing the loss of stability of office chairs to the front, a vertical force of 600 N must be applied, from a distance of 60 mm from the edge of the front load-carrying construction, in places where the loss of stability is most likely. Then, a horizontal force of 20 N should be applied, acting on the outside from the point in which the base of the element shifting load is in contact with the top surface of the seat (Fig. 6.15a).

In attempting to assess stability, the sides of chairs without armrests should be applied with a vertical force of 600 N on the seat, at a distance of 60 mm from the edge of the load-carrying construction of the side closer to the blocked points of supports, in those places where the loss of stability is most likely, and then apply a

Fig. 6.14 Impact tests of home and school furniture for sitting according to PN-EN 1728:2012, PN-EN 1729-2:2006: a backrest, b armrests and side headrests, c seat (mm)

Fig. 6.15 Determining stability of office chairs PN-EN 1335-3:2009: a loss of stability to the front, b loss of stability to the side, c loss of stability to the side of a chair with armrests (mm)

horizontal force of 20 N directed on the outside, from the point in which the base of the element shifting load is in contact with the top surface of the seat (Fig. 6.15b).

In testing a chair with armrests, a vertical force of 250 N should be applied at a distance of 100 mm from the middle plane, on the side of the blocked points of support (Fig. 6.15c), and at a distance from 175 to 250 mm to the front of the rear edge of the seat, as close as possible to the side edge. Then, a vertical force of 350 N should be applied acting on the armrest in points that are up to 40 mm from

Fig. 6.16 Determining the maximum distance of displacement of the backrest of office chairs according to PN-EN 1335-3:2009 (mm)

the outer edge, directed inwards, but not going beyond the middle of the armrest in the most unfavourable place of its length. Subsequently, a load with a value of 20 N should be applied, directed outwards from the point where the base of the loading element comes into contact with the upper surface of the armrest.

When testing the maximum distance of displacement of the backrest of office chairs, the base of the chair should be secured against lifting, by placing a weight of 75 kg on the seat. Then, to the backrest, at a point lying at a height of 220 mm, a force with a value of 315 N should be applied (Fig. 6.16). The distance of dis­placement of the backrest is the distance measured horizontally between the point of back support on a loaded backrest and the axis of rotation of the chair.

By specifying the conditions of stability loss to the front of home and school furniture for sitting, a vertical force should be applied with a value from 200 to 600 N, at a distance of 60 mm from the edge of the front load-carrying construction, in places, where the loss of stability is most likely to occur. Then, a horizontal force of 20 N should be applied, acting outwards from the point in which the base of the element shifting load is in contact with the top surface of the seat (Fig. 6.17a). In attempting to assess stability to the sides of chairs without armrests, a vertical force of 200-600 N should also be applied on the seat, at a distance of 60 mm from the edge of the load-carrying construction of the side closer to the blocked points of supports, in those places where the loss of stability is most likely to occur. Then, a vertical force of 20 N should be applied directed outwards from the point in which the base of the element shifting load is in contact with the top surface of the seat (Fig. 6.17b). In testing the stability of a chair backwards, a vertical force of 200­600 N should be applied, at a distance of 180-300 mm from the front edge of the seat, and a horizontal force of 50-180 N, at a height of 120-180 mm, counting from the top surface of the seat (Fig. 6.17c).

The strength and durability of home and school furniture for sitting is determined by applying a vertical force of 1300-2000 N, a horizontal force of 410-700 N (Fig. 6.18a) and by loading the front bar with a force equal to 1000 N (Fig. 6.18b).

Fig. 6.17 Determining the stability of home and school furniture for sitting according to PN-EN 1728:2012, PN-EN 1729-2:2006: a loss of stability to the front, b loss of stability to the side, c static load on the seat and the backrest (mm)

Fig. 6.18 Determining the strength and durability of home and school furniture for sitting according to PN-EN 1728:2012, PN-EN 1729-2:2006, load on a the seat and the backrest, b bars

An important operational load of furniture for sitting is also loading the legs, enabling us to determine the durability of the rack construction. To this end, a vertical force of 1300-1600 N should be applied to the seat and a horizontal force at the back of the seat in its axis, directed forwards (Fig. 6.19a, b) through an element shifting local loads. In furniture for sitting without legs (e. g. chairs with castors or

sliders, which are attached directly to the construction), two forces of 300-600 N going in opposite directions should be applied simultaneously to the pair of corners of the product which lie opposite to one another (Fig. 6.19c).