portal frame
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6. THE PITCHED-ROOF PORTAL FRAME
The pitched-roof portal frame, often plastically analysed, is a very economical and popular building type. Since it is probably the largest market for the type of connections with which this lecture is concerned, it deserves special mention.
Haunched end plate connections are almost universal in portal frames; these adapt readily to angles of intersection other than 90
°. It is customary to make the eaves haunch depth (almost) equal to that of the beam, and to extend it some way along the span. The haunch geometry is determined by overall frame design rather than purely a matter of connection detailing. Apex haunches are usually of more modest size. Figure 6 illustrates typical portal frame connections.
With extended eaves haunches, it becomes a moot point whether the connection is full strength (relative to the plain beam section) or partial strength (relative to the section as increased by the haunch). This is usually resolved by ensuring that the latter is sufficiently 'oversized' to force the plastic hinge to occur at the haunch end, and designing the connection for the maximum moment that this (determinate) situation can induce.
For the usual range of roof pitches, the eaves connection may be designed in the same way as an equivalent 90
° beam-to-column connection, with the compression taken as the horizontal component of haunch flange force. Axial compression in the beam will generally be non-negligible; this can be added to bottom flange force with the design moment adjusted to account for its offset.
Except at the interior columns of multi-bay frames, web panel shear is likely to exceed the capacity of the column section. (The column is very likely to be an I rather than an H section.) Stiffeners are usually called for, and a common choice is the 'Morris' stiffener shown in Figure 6a. This acts similarly to a conventional diagonal stiffener, with the advantage that access for the bolts is not impeded.
Additional 'rib' stiffeners may be used to reinforce the column flange between lower bolt rows. The end plate thickness can of course be chosen to avoid the need for such stiffening, but they are sometimes used on the beam side to enhance web tension resistance.