Download Load Paths in a Braced Frame Steel Building

Load Paths in a Braced Frame Steel Building
Kristen M. Lechner
November 17, 2009
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What is a Load Path?
A load path is a passageway that transfers loads
on a building structure into the foundation
system.
The function of a load path is to ensure that any loading on the structural system is
transmitted into the foundation and distributed to the ground. This ensures that the
building structure is not in danger of failing or falling down due to fatigue. Structural
engineers always design the structural system of a building with the concern of how the
loads will get to the ground. This issue must be resolved at the onset of the design.
Load Path for a Gravity System
Load Path for a Lateral System
How Does a Load Path Work?
The two types of loading that a building will encounter are gravity loads and lateral loads.
Gravity loads include the self weight of the structure and building finishes, such as
carpeting or tile. They are supported by a part of the structure called the gravity system.
The gravity system in a typical building consists of beams, girders, and columns.
Lateral loads are created by the wind or an earthquake. These loads are transferred to the
ground by following a load path through a lateral system. The material used for the
structure of the building, steel or concrete, will determine what is used for the lateral
system. For simplicity, only a steel building utilizing braced frames will be discussed in
detail.
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What steps do
the gravity
loads follow?
The gravity loading within a building will accumulate
within the floor and distribute itself into the infill beams
framing into the girders. Please reference the image
below.
Becomes
Girder
Beam
Girder
Beam
Column
Column
Next, the load distributes itself throughout the length of the beams. As seen in the image
below, half of the load will go one direction and half will go the other direction. This is
because the structure likes to be balanced. The loading is naturally evenly distributed to all
elements of the structure.
Beam
Girder
Column
The girders are the next logical step within this process because they are directly
connected to the beams. The load will travel throughout the length of the girder, and
distribute itself into the columns. This is because the girders are directly connected to the
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columns. The same idea of even distribution is seen again in the load path process. Each
column is responsible for half of the total load taken by the girders.
Beam
Girder
Column
The loading will travel down the length of the column, distribute into the foundation
system, and get transferred into the ground. This completes the load path process for
gravity loads.
Beam
Girder
Column
What steps do
the lateral
loads follow?
As previously stated, a steel braced frame system will be
selected to discuss in detail, since there are many different
options engineers use for lateral systems. Since lateral
loads are generated from wind or an earthquake, they hit
the side of the building horizontally rather than vertically
like gravity loads.
Diaphragm
Girder
Column
Beam
Brace
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Because these loads hit the building horizontally, they accumulate within the diaphragm, or
floor, of a building. As they accumulate, they follow their load path according to stiffness
within the structure. Stiffness within the structure is created by the braces within this
example because they are the lateral elements. Half the load within the floor will distribute
itself into the first diagonal brace, while the other half will distribute itself into the second
diagonal brace.
Diaphragm
Girder
Column
Beam
Brace
Next, the loading travels throughout the length of the brace.
Diaphragm
Girder
Column
Beam
Brace
Finally, the lateral loads transfer into the foundation system and get distributed into the
ground. This completes the load path process for lateral loads.
Diaphragm
Girder
Column
Beam
Brace
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Importance of the Load Path Process
It is definitely clear that loads within a building must find their way to the ground by some
method. If this does not happen, being inside of a building would never be a safe situation.
Without a clear gravity system, the loads due to the self weight of the structural system and
building materials would distribute themselves randomly. This situation would be like if
all cars on the road drove off in any direction, not following the road at all. Many people
would be injured or even killed as a result of unpredicted behavior. Without a lateral
system, a building would fall over like a stack of cards if a big gust of wind came, or if an
earthquake hit. This would also lead to many injuries and deaths. Therefore, the load path
process is essential to structural engineering and safety of the public.
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