RIGID FRAMES - HEAVY TIMBER CONSTRUCTION

The cruck (Figure 4.2), cut from a  bent tree, was a form of  rigid frame or  portal frame. Today's rigid frames are glue-laminated to shape and find wide use in longer-span buildings.  Standard configurations are readily available (Figures 4.22-4.24), or the  designer may order a custom shape.  Rigid frames exert a horizontal  thrust, so they must be tied together  at the base with steel tension rods, also  called  tie rods. In laminated wood construction, rigid frames are often called arches, acknowledging that the  two structural forms act in very nearly  the same manner.
Figure 4.2 European timber house forms
generally followed a progression of
development from crude pit dwellings,
made of earth and tree trunks, to
cruck frames, to braced frames. The
crucks (curved timbers), hewn by hand
from appropriately shaped trees, were
precursors to the laminated wood arches
and rigid frames that are widely used
today.
Figure 4.22 Three-hinged arches of glue-laminated
wood carry laminated wood roof purlins.
The short crosspieces of wood between
the purlins are temporary ladders for
workers.
Figure 4.23 Typical details for three-hinged arches
of laminated wood. The tie rod is later
covered by the fl oor slab.
Figure 4.24 The shear plates in the crown connection
of the arch, shown here in a larger-scale
detail, are recessed into grooves in the
wood and serve to spread any force
from the steel rod across a much wider
surface area of wood to avoid crushing
and splitting.

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