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Steel Designers' Manual - 6th Edition (2003)
Therefore, by rotating the reactant line through point O, the positions of the bending
moment of equal magnitude in the positive and negative regions can be found and
the member sized accordingly based on this value of bending moment.
Having found the positions of the reactant line which gives the number of hinges
required for a mechanism to form, in this case two, the reactant line for the stanchions
can be drawn through point O, a distance h1 from the end of the free-bending
moment diagram. The unknown reaction, H, is then calculated as H = MD/h1.
In a majority of instances, portal frames are constructed with a haunch at the
eaves, as shown in Fig. 1.23(a).
Depending on the length/depth of the haunch, the plastic hinges required for a
mechanism to form are shown in Figs. 1.23(b) and (c). The dimensional details for
the haunch can be readily investigated by the graphical method by superimposing
the dimensions of the haunch on the free-bending moment diagram. The reactant
line can then be rotated accordingly until the required mechanism is achieved and
members sized accordingly.
Haunches are generally fabricated from parallel beam sections (Fig. 1.24). In all
cases, the haunch must remain in the elastic region. A detailed check is required
along its length, from a stability point of view, in accordance with the requirements
laid down in the relevant code of practice. In some instances, bracing of the bottom
flange must be provided from a purlin position within its length, as shown in
Fig. 1.25.
haunch
V /H-Cut
Fig. 1.24 Haunch fabrication
Design of common structural forms 27
(a) (b) (c)
Fig. 1.23 Alternative hinge locations for haunched portal frames
tines