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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Page 123 - Building stability - Building forces and reactions

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