Page 492 - Aircraft Stuctures for Engineering Student

11.5 Constraint of open section beams 473

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Fig. 11.30 Section of axially constrained open section beam under torsion.

The beam is loaded by a pure torque so that the axis of twist passes through the
shear centre S(R) of each section. We shall take the origin for s at the point 1 and
initially plot 2AR-o against s to determine rR (see Fig. 11.31). The position of the
centre of gravity, (2A’,), of the wire 1’2’3‘4’ is found by taking moments about the
s axis. Thus

(3 (7) (hqd)
t(2d+h)2A’, = td - + th - + rd -

from which
hd(h + d)
3A‘, =
2(h + 2d)