Braced versus rigid frames

The fundamental structural requirement governing the design of connections in building frames is related to the strength and stiffness

Types Braced Frame Steel Construction

3.6 Various forms of steel connections: (a) examples of effectively 'rigid' connections; and (b) examples of effectively 'pinned' connections

Offset Braced Frame Connection

of the connections between the members, or of members to the foundations.

The connections may be one of three configurations defining these degrees of strength (or more correctly 'resistance') and stiffness:

1. Rigid (also called fixed or moment-resisting) connections (Figure 3.6(a)).

2. Pinned (also called simple) connections (Figure 3.6(b)).

3. Semi-rigid (also termed partial strength) connections.

Rigid frames require rigid connections in order to provide for stability at least in one direction. Braced frames are stabilised by vertically oriented bracing, and require only pinned connections. Rigid frames are often termed 'sway frames', because they are more flexible under horizontal loads than braced frames.

The characteristics of these connections are presented in more detail in Chapter 5 and may be summarised as follows.

In a 'rigid' connection there is complete structural continuity between any two adjacent members. Moment (or rigid) connections are used in frames where there is a desire to omit vertical bracing in one or both directions. The main advantage of rigid frames is that an open space between columns can be created, which offers flexibility in choice of cladding, etc. (e.g. in glazed fa├žades). However, the achievement of full continuity between members at the connection requires an extensive amount of fabrication and, as a consequence, this system is relatively expensive.

To achieve a nominally 'pinned' joint, the connections are made so as to permit the transfer of axial and shear forces, but not bending moments. Nominally simple connections may provide some small degree of rigidity, but this is ignored in structural design and these connections are treated as pinned. Examples of pinned connections are cleated, thin or partial depth end-plates, and fin-plate connections as illustrated in Figure 3.6(b).

Pinned connections are usually simple to fabricate and erect, and are the least expensive type of connection to produce. As a consequence, lateral stiffness must be introduced into the frame by other means.

Semi-rigid (and also partial strength) connections achieve some continuity through the connections, but are not classified as full strength, as they do not achieve the bending resistance of the connected members. These forms of connections are illustrated later on in Figure 5.5. They are used for low-rise frames in which horizontal forces are not so high, or in beams where some end fixity is beneficial to the control of deflections.

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