All structures are
subjected to forces that are imposed by gravity, wind and seismic events (see
Figure 1). The combination and anticipated severity of these forces will
determine the maximum design force the member can sustain. The structural
engineer will then select a member that meets all of the strength as well as
serviceability issues such as deflection and/or vibration criteria for any
specific project. The following is a brief discussion on each of the types of
loads and how these loads are transferred to the other structural components.
Gravity Loads
Gravity loads include
all forces that are acting in the vertical plane (see Figure 2). These types of
forces are commonly broken down into dead loads and live loads in a uniform
pounds per square foot loading nomenclature. Dead loads account for the
anticipated weight of objects that are expected to remain in place permanently.
Dead loads include roofing materials, mechanical equipment, ceilings, floor
finishes, metal decking, floor slabs, structural materials, cladding, facades
and parapets. Live loads are those loads that are anticipated to be mobile or transient
in nature. Live loads include occupancy loading,
office equipment and
furnishings.
The support of
gravity loads starts with beams and purlins. Purlins generally refer to the
roof while beams generally refer to floor members. Beams and purlins support no
other structural members directly. That is to say, these elements carry
vertical loads that are uniform over an area and transfer the uniform loads
into end reactions carried by girders.
Girders generally
support other members, typically beams and/or purlins, and span column to
column or are supported by other primary structural members. Girders may support
a series of beams or purlins or they may support other girders. Forces imposed
on girders from beams, purlins, or other girders are most often transferred to
the structural columns. The structural column carries the vertical loads from
all floors and roof areas above to the foundation elements.
Horizontal Loads
Forces created by
wind or seismic activity are considered to act in the horizontal plane. While
seismic activity is capable of including vertical forces, this discussion will
be based only on horizontal forces. The majority of this section will address
wind forces and how they are transferred to the primary structural systems of
the building (see Figure 3).
Wind pressures act on
the building's vertical surfaces and create varying forces across the surface
of the façade. The exterior façade elements, as well as the primary lateral
load resisting system, are subjected to the calculated wind pressures
stipulated by code requirements. This variation accounts for façade elements
being exposed to isolated concentrations of wind pressures that may be
redistributed throughout the structural system. Design wind pressures can be
calculated using a documented and statistical history of wind speeds and
pressure in conjunction with the building type and shape. Calculated wind
pressures act as a pushing force on the windward side of a building. On the
leeward (trailing) side of the building, the wind pressures act as a pulling or
suction force. As a result, the exterior façade of the entire building must be
capable of resisting both inward and outward pressures.
Roof structures made
up of very light material may be subjected to net upward or suction pressures
from wind as well. Roofs typically constructed of metal decking, thin
insulation and a membrane roof material without ballast have the potential to
encounter net upward forces. Roof shape may also determine the net uplift
pressures caused by wind. Curved roofs will actually exhibit a combination of
downward pressures on the top portion of the curve and upward pressure on the
lower portion of the curve. This distribution of downward and upward pressures caused
by the curve is similar to the principles of air pressure and lift acting on an
airplane wing.
As the wind pressures
are applied to the exterior of the building, the façade (actually a structural
element to some degree), transfers the horizontal pressures to the adjacent
floor or roof. As these pressures are transferred, the floor and roof systems
must have a means to distribute the forces to the lateral load resisting
systems. Floors and roofs that are generally solid or without large openings or
discontinuities may behave as a diaphragm. A diaphragm is a structural element
that acts as a single plane with the connecting beams and columns. When experiencing
a force, this single plane causes the beams and columns to displace
horizontally the same amount as the diaphragm. This can be exemplified by a
sheet of paper or cardboard that is supported by a series of columns. Should
the paper, a flexible diaphragm, be pushed horizontally, all points in contact
with the paper will move laterally by the same amount. The metal roof decking
on most projects will behave as a flexible diaphragm.
Substituting a piece
of cardboard for paper in our example, the paper will behave more like a rigid
diaphragm. A typical floor decking and composite structural slab are examples
of a rigid diaphragm.
Horizontal diaphragms
are an efficient means to transfer the horizontal loads at each level of a
building to the lateral load resisting systems (see Figure 4). Should large
openings, such as atriums, skylights, raised floors or other discontinuities
exist to interrupt the diaphragm, the lateral or horizontal loads may not flow
easily to the lateral load resisting systems. As a result, the structural
engineer will investigate the need for a horizontal truss system utilizing the
floor beams and/or girders as chord members. Secondary web members will be
added to complete the truss concept. This is particularly common in roof areas
where there may be very long continuous skylights on a relatively narrow or
long roof area.
Seismic
Seismic activity
induces horizontal forces, and at times, vertical loads. The discussions in
this publication will focus on horizontal forces imposed during seismic
activity. Forces created during a seismic event are directly related to weight
or mass of the various levels on a specific building. During seismic activity horizontal
diaphragms behave like wind load transfers with respect to the primary lateral
load resisting systems. However, the induced forces are much more sensitive to
the shape of the building and the positioning of the lateral load resisting
systems. It is advantageous to consider a very regular building plan in areas
of the country with significant seismic activity.
(Design with Structural Steel - A Guide for Architects)
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