Constructions of case furniture are mainly made from board elements or frame-board elements. Due to the high shearing stiffness of boards in comparison with their torsional stiffness, case furniture subjected to torsion forces has a regular, characteristic way of deforming, which can be caused by various force systems (Fig. 7.9).
Fig. 7.9 Equivalent systems of forces causing torsional deformation of the body: a shifting, b unevenness of the base, c moving
With regard to operational requirements, the quality of the furniture is determined by a stiffness and strength test both of the whole structure and on its individual elements and joints, as well as impact on mass and operational loads (Fig. 7.10).
The stiffness of case furniture can be defined as the quotient of the values of external load Pz applied to the body’s side wall, at a height of the top (Fig. 7.11) to the value of the displacement APz measured in the direction of the effect of this load:
k stiffness of the furniture body,
Pz external load and
APz displacement in the direction of the load Pz.
The experiments of the testing and validation station of furniture show that the value of this coefficient should not be smaller than 10,000 N/m. In engineering practice, however, it is worth taking into account two values:
• k > 10,000 N/m for house furniture and
• k > 20,000 N/m for library bookcases, kitchen furniture, office furniture and other heavily loaded structures.
The support and load scheme of the body contained in Fig. 7.11 differ, however, from laboratory tests, in that the external load Pz is not applied 50 mm below the top or at the height of 1600 mm, but exactly at the height of the top, in the plane of the front case. Therefore, as an external operational load Pz it should be assumed:
for furniture of height C < 1.65 m,
for furniture of height C > 1.65 m, where
C furniture height and Pv operational load.
The operational load Pu constitutes a reduced sum of operational loads and weight of the furniture piece, expressed in the form of the equation:
Pu = (Qm + Qu), (7-4)
a width of the furniture body,
H height of applying force, where H = 1.6 m for C > 1.65 m, H = C – 0.05 m for C < 1.65 m.
And the operational load Qu represents the sum of fixed evenly distributed loads, acting on the board of the bottom, partition, shelf, drawer and bars of the furniture piece, calculated according to Table 7.1, depending on the surface of the board element, the length of the bar or the volume of the container, from the equation:
Qu = ^Z (qviVi + qAiAi + qiLt), (7.5)
qvi volume load, qAi surface load, qi linear load,
vi container or drawer capacity,
Ai surface of bottom, shelves, partitions and Lt length of bars.
Mass load Qm is mostly determined by weighing the furniture piece. In the absence of relevant scales, the value of this load can be determined as follows:
Qm = 1.05 £ PiVig, (7.6)
g gravity acceleration,
Pi density of material and vi volume of element,
by summing up the weight of all structural elements and adding the weight of fittings constituting around 5 % of Qm.