Strength of Case Furniture

7.2.3.1 State of Internal Forces in Corner Joints

The primary determinant of the strength of a furniture body is the strength of its joints. Therefore, the forces acting on the nodes of a loaded construction must be established and the strength of joints specified. The previously assumed reserva­tions that the boards of the furniture body are connected articulately and only in the corners allow to conclude that the impacts between the boards are also focused in the corners. Such assumptions correspond well to the constructional solutions using separable joints like eccentric joints, minifix or confirmat, distributed in ones in the corners (Fig. 7.60a). In the case of joints distributed in more than two, on the length of the edge of the board, e. g. dowels or more confirmants, we recognise that calculation angular corner forces are displaced by connectors distributed on half the length of the edge (Fig. 7.60b). From precise solutions, it is known that in joints

Fig. 7.60 Corner forces shifted by: a one connector in the corner, b a few connectors in the middle of the length of the edge

between boards, forces P occur perpendicular to each of the joined boards (Fig. 7.61). The force Pi, causing twisting of the ith board of the dimensions (/il2)i, amounts to

Pz

n Gidj a2

i=1 3(l1l2)i ai

Figure 7.61 also shows that connectors of external elements (bottom, top and side walls) are loaded by edge forces Pk, originating from an external load Pz, with a value of

Pk = aPz. (7.192)

b

For corners not loaded by external forces Pz, the force Pi is a force shifted by the connector of the construction’s node. And in the case of a corner loaded by external forces, the values of internal torsional force of the board are the difference between external load and force Pi:

Pb = Pz – Pi. (7.193)

Connectors of vertical partitions of course only shift Pi loads perpendicular to its plane, while a group of connectors of corner horizontal partitions, in addition to torsional loads Pi, also shifts the fourth part of surface loads attributed to this partition (Fig. 7.62) according to the following equation:

Pa = Pi + Ri = Pi + 4 qAi (l 112) i ■ (7.194)