The most common lateral load is a wind load. The Eiffel Tower is one example
of a building which has a structure that was designed to resist a high wind
load. Wind against a building builds up a positive pressure on the windward
side and a negative pressure (or suction) on the leeward side. Depending
upon the shape of the structure it may also cause a negative pressure on
the side walls or even the roof. The pressure on the walls and roof is not
uniform, but varies across the surface. Winds can apply loads to structures
from unexpected directions. Thus, a designer must be well aware of the dangers
implied by this lateral load. The magnitude of the pressure that acts upon
the surfaces is proportional to the square of the wind speed.
Wind loads vary around the world. Meteorological data collected by national
weather services are one of the most reliable sources of wind data. Factors
that effect the wind load include the geographic location, elevation, degree
of exposure, relationship to nearby structures, building height and size,
direction of prevailing winds, velocity of prevailing winds and positive
or negative pressures due to architectural design features (atriums, entrances,
or other openings). All of these factors are taken into account when the
lateral loads on the facades are calculated. It is often necessary to examine
more than one wind load case.
For this course, it will be assumed that wind loads, as well as the pressure
they develop upon wall and roof elements, are static and uniform. They actually
not only pound a structure with a constantly oscillating force, but also
increase as a building increases in height. The loading of a tower can be
very roughly approximated by an evenly distributed load. It is a vertical
cantilever. The applet below allows you to investigate the variables which
influence the structural behavior of a tall, thin tower. It does not represent
actual methods of calculating the total wind force on a tall building. It
is intended to demonstrate the interaction between the variables of the
equations which govern the structural behavior.
Earthquake loads are another lateral live load. They are very complex, uncertain,
and potentially more damaging than wind loads. It is quite fortunate that
they do not occur frequently. The earthquake creates ground movements that
can be categorized as a "shake," "rattle," and a "roll."
Every structure in an earthquake zone must be able to withstand all three
of these loadings of different intensities. Although the ground under a
structure may shift in any direction, only the horizontal components of
this movement are usually considered critical in a structural analysis.
It is assumed that a load-bearing structure which supports properly calculated
design loads for vertical dead and live loads are adequate for the vertical
component of the earthquake. The "static equivalent load" method
is used to design most small and moderate-sized buildings.
The lateral load resisting systems for earthquake loads are similar to
those for wind loads. Both are designed as if they are horizontally applied
to the structural system. The wind load is considered to be more of a constant
force while the earthquake load is almost instantaneous. The wind load is
an external force, the magnitude of which depends upon the height of the
building, the velocity of the wind and the amount of surface area that the
wind "attacks." The magnitude earthquake load depends up the mass
of the structure, the stiffness of the structural system and the acceleration
of the surface of the earch. It can be seen that the application of these
two types of loads is very different.
This movie is a representation of the movement of a free standing water
tower in an earthquake. It can be seen that the as the ground moves, the
initial tendency is for the water tower to remain in place. The shifting
of the ground is so rapid that the tower cannot "keep up."
After a moment, the tower moves to catch up with the movement of the
ground. The movement is actually an acceleratoin. From Newtonian Physics,
it is know that an applied force=mass x acceleration. Thus, the force which
is applied to the water tower depends upon the mass of the tower and the
acceleration of the earth's surface.
The force in this last diagram may be thought of as the "equivalent
static load" for which the structure would be designed. This idealized
situation demonstrates a concept; it requires modification for actual buildings.
These modifications account for building location, importance, soil type,
and type of construction. This movement can also be seen in the following
movie of lateral earth movement. Note how the mass slowly reacts to the
movement of the earth. Eventually, the bending strength of the stem of the
tower would be exceeded and it will fail.
Simulation of a water tower in an earthquake
It remains very difficult to imagine the destruction which can be wrought
by an earthquake. The lessons learned from the Los Angeles Earthquake of
1994 helped structural designers change design strategies.
Fluid and Earth Pressure Loads
Liquids produce horizontal loads in many structures. The horizontal pressure
of a liquid increases linearly with depth and is proportional to the density
of the liquid. This is similar for earth pressures. These last are a bit
more complex in that the load due to earth pressure varies with its depth,
any surcharge, the type of soil and its moisture content. The design live
load for this soil pressure must not be less than that which would be caused
by a fluid weighing 30 pcf.