When a body in the form of a rod or a tube, albeit a short rod, is placed
under torsion, ie., it is subjected to a TWISTING MOMENT, then SHEAR
stresses are set up in it.


The shear stresses are in the plane of the cross-section - adjacent
cross-sections tending to slide over each other.


When the body experiences a sideways thrust, as from wind, then a BENDING
MOMENT is set up in it. One side of the body is under vertical TENSION and
the other side is under vertical COMPRESSION.


Brickwork and concrete are much the weakest when under tension. Not so weak
when under shear. Much the strongest when under compression.


The weight of brickwork exerts a uniform compressive stress over the
cross-section. Initially there is no tensile stress. But the bending moment
due to wind on ONE side of the structure eventually overcomes the
compression. On THAT side the stress becomes tensile and the brickwork parts
company with itself, ie., the structure initially fails under tension.


It is only AFTER failure has occurred due to tension that shearing takes
place and bricks begin to slide sideways, one over the other.


The art in the design of brick and concrete structures, eg., as for
chimneys, bridges and gravity damns, is NOT to allow any TENSILE stresses to
be set up. Hence the frequent use of steel reinforcement.


Memo: In view of the variability in such materials apply large factors of
safety.
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Reg, G4FGQ