Structural elements are connected to form structural systems; the connections thus constitute an intermediate condition between elements and systems, and are not, strictly speaking, part of the elements themselves. Such connections, however, do have a direct bearing on the types of assumptions made when the individual elements (or systems) are analyzed. Specifically, when the various elements of structure — columns, beams, and so on — are considered individually, we show them either as constrained by hinges and rollers, free to translate and rotate, or fixed in such a way that all relative movement is prevented.
These abstract constraints are models of the actual conditions encountered by such elements when they are connected within actual structural systems. For example, beams are attached to girders, walls, or columns; columns are attached to foundations, transfer girders, or other columns; and tension elements are hung from beams, or inserted within truss systems. It may seem surprising that the conventional means of attaching structural elements to each other with nails, screws, bolts, welds, and reinforcing bars corresponds to the abstract hinges, rollers, or fixed constraints that will be encountered in the discussion of individual elements of wood, steel, or reinforced concrete (or as discussed earlier in this chapter): we rarely see connections in typical building structures that look anything like the diagrammatic representation of the constraints shown in Figure 1.14.
In fact, the relationship between the reality of a connection and the abstract modeling of it as hinge, roller, etc. is quite interesting. On the one hand, it is possible to design a real connection so that it both appears and behaves just like the abstract model. More commonly, however, one starts with a convenient means for connecting real materials, and then chooses a constraint model that approximates the behavior of this connection. Of course, such typical and "convenient" connections have evolved over time so that their behavior is in line with the assumptions we make about the types of movement, and the magnitude of forces and moments transmitted, between the elements being connected.
It is the latter group of typical connection strategies that will be discussed in the chapters about wood, steel, and reinforced concrete. These connections must resist the same sort of forces already encountered in the design of the structural elements themselves: direct compression and tension, as well as shear. Bending does not often show up directly in the design of fasteners, as it can usually be resolved into the other forces already mentioned.
© 2020 Jonathan Ochshorn; all rights reserved. This section first posted November 15, 2020; last updated November 15, 2020.