Forces and reactions |
. |
Like a
bridge, a building must resist the internal stresses induced in its
structural members |
by the
dead and live loads acting on it. Dead loads include the weight of
the structure itself |
plus all
of the fixed equipment. Internal live loads include the people, moveable
furnishings |
and
equipment, and vehicles occupying the building. External live loads
include wind, rain, |
snow,
earthquakes, etc. Besides keeping the building from being
permanently damaged by |
these
loads, the structure must be strong enough that no part of the building is
significantly |
deformed
by them. Spongy floors or sagging ceilings are undesirable, as is
significant |
swaying
of the upper levels of high rise buildings. |
. |
Wind
loads exert a positive lateral pressure on the windward side of the
building (the side |
facing
the wind), and a negative, sucking, pressure on the leeward side (the side
opposite |
the
wind's direction). The height of a building and its profile are
important in determining |
the
effective load of the wind. Wind velocity generally increases with
height. And the |
higher
up the wind strikes the greater the bending moment it exerts. Thus
tall structures |
are
affected by winds more so than shorter ones. Also whether the sides
are vertical or |
sloped
affects whether the generally horizontal force of the wind strikes it
head-on or just a |
glancing
blow. As shown in the exercise on vector forces, a force perpendicular to
a surface |
exerts a
greater load on it than one that is oblique to it. |
. |
The same
can be said for the snow load on a roof. Recall that a load applied
to an A-frame |
with a
steeper slant, or pitch, and therefore a larger H/S ratio, induces less
compressive |
stresses
in its legs than one with a shallower pitch. |
. |
Like a
bridge, the individual structural members of a building must be selected
so as to |
have the
proper materials and dimensions for the stresses they will be subjected
to. And |
they
should be arranged together so as to maximize the efficiency of the
structure in |
supporting the anticipated load. The structural members that
experience tensile stresses |
mainly
include the lower chords of trusses, rafter ties, and drag struts. Mainly
compressive |
stresses are experienced by posts, wall studs, columns,
diagonal struts, and the upper chord |
of trusses. Members that experience a combination of
stresses include beams, joists, rafters, |
and girders. |
. |
You can
use the load equations for columns and beams presented earlier to estimate
the |
tensile and compressive stresses experienced by a building's members.
Keep in mind |
though that the joints are usually rigid. Therefore you should use
the load equations for |
fixed beams and columns. Recall that fixing the ends of a beam to
its supports reduces |
the
amount it will deflect under load by a factor of four to five compared to a
simply loaded |
beam.
And the maximum bending moment experienced by a fixed beam is two to three |
times less than that for
a simply supported beam. The maximum bending stress allowed for |
a beam depends on
whether it carries the load by itself or whether it is part of a support |
framework such as
flooring joists or roof rafters. The maximum allowable stress is
greater |
for beams that are
part of a repeated structure. This is because the load is
distributed |
between a number of members via tributary areas and thus the structure has
some |
redundancy. Recall also that fixing both ends of a column lowers its
tendency to buckle by |
a factor of four
compared to a freestanding column. |
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to Knowhere |
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- Building stability - Building forces and reactions |
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