5.2Main Joint Categories

5.2.1 Overview

The definition for a particular joint configuration depends on the geometry of the clamped parts and fasteners, and the effective loading applied at the fastener (or fastener group). As stated in Section 1 it is assumed that the user has knowledge of the system of loads acting on the joint in the vicinity of the fastener(s); these being derived by analysis of the overall structure prior to detailed consideration of joint design. Adequate definition of the applied loading system is essential as this can determine the particular category to which a joint belongs and hence the method of analysis adopted. This is particularly critical when distinguishing between concentric and eccentric axially loaded joints. The selection of the appropriate analysis for the combined loading case is also dependent on adequate specification of the loading system. More complex situations arise where combined loading occurs, typically axial, shear and bending. However, where a single load is dominant in a combined loading case and other load(s) are small it can be possible to assume the joint falls into one of the first three categories outlined in 5.2.2, 5.2.3 or 5.2.4, thereby simplifying the analysis without significant reduction in accuracy. Criteria are included to determine when such an assumption is applicable.

5.2.2 Concentric Axially Loaded Joints

The distinguishing feature of this joint category is that the line of action of the applied loading on the joint is parallel to and coincident with the longitudinal axis of the fastener. Therefore, any combination of joint geometry and system of applied loads which conform to this definition can be analysed by the methods and procedures specified for this category.

Examples of joints for this category are illustrated in Figure 5-1 to Figure 5-3. For clarity a single fastener is shown, although in many cases the same analysis can be applied to a group of fastener if they are in a symmetric pattern. However, it is important to note that the effect of flange flexibility can lead to eccentric fastener loads in joints that with symmetric fastener pattern (see Figure 5-3, Joint 17).

5.2.3 Eccentric Axially Loaded Joints

For this joint category the line of action of the applied loading on the joint, whilst being parallel to the fastener longitudinal axis, is not coincident, but offset. The result of this is that a “prying” (or “prising”) action occurs between the clamped parts of the joint such that bending loads are introduced under the bolt head and in the shank. Three fundamental loading case variants can be identified depending on the relative position of the bolt axis, line of action of the applied load, and the joint centroid.

This category of joint and its variants represent a large proportion of the joints encountered in practice. The main examples of these are illustrated in Figure 5-1 to Figure 5-3. For clarity a single fastener is shown, although in practice this can be a group.

5.2.4 Shear Loaded Joints

The principle feature of this joint category is that the line of action of the applied loading on the joints is in the plane of the clamped parts immediately adjacent to the fastener, and therefore normal to the fastener longitudinal axis. The forces need not be coplanar if the joint is of the non-symmetrical or single lap shear type. Joints of this type can be further subdivided into friction grip or bearing categories, depending on whether load is transferred through the joint by friction at the faying surfaces or by transverse shear in the fastener(s). Examples of joints in this category are illustrated in Figure 5-1 to Figure 5-3. For clarity a single fastener is shown, although in practice this can be a group if the joint has multiple fasteners.

5.2.5 Combined Loaded Joints

The essential feature of this joint category is that more than one system of loads act on the joint relative to the fastener axis. In the most general case features of all the preceding categories are combined. Examples of joints for this category are illustrated in Figure 5-1 to Figure 5-3. For simplicity a single fastener can often be considered, although in practice the same can be a group if the joint has multiple fasteners.

During the design process, joints initially being placed in the (most general) category of combined loaded joints can be reclassified into a sub-category, for the purpose of analysis. This guideline does not provide specific criteria for determining when this simplification can be assumed. Rather, the appropriate analysis method is determined using engineering judgment considering the relative magnitudes of the shear and axial loads, the configuration of the joint and any other relevant attributes of the design.

5.2.6 Low Duty Joints

Joints in this category have loadings and configurations that fit into one of the preceding categories, however they form a unique category since they have small external loading with respect to the fastener strength. In many cases this is readily apparent due to the particular application, e.g. hold-downs, access panel attachments, etc. Some example diagrams of joints that frequently fall into this category are shown in Figure 5-1 to Figure 5-3.

Figure 5-1 - Joint Categories (1 to 6)

Figure 5-2 - Joint Categories (7 to 12)

Figure 5-3 - Joint Categories (13 to 17)

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