1

Load Paths in a Braced Frame Steel Building

Kristen M. Lechner

November 17, 2009

2

What is a Load Path?

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.

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.

A load path is a passageway that transfers loads

on a building structure into the foundation

system.

Load Path for a Gravity System Load Path for a Lateral System

3

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.

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.

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

What steps do

the gravity

loads follow?

Becomes

Beam Girder

Column

Beam Girder

Column

Beam Girder

Column

4

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.

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.

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.

What steps do

the lateral

loads follow?

Beam Girder

Column

Beam Girder

Column

Column

Brace

Beam

Girder

Diaphragm

5

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.

Next, the loading travels throughout the length of the 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.

Column

Brace

Beam

Girder

Diaphragm

Column

Brace

Beam

Girder

Diaphragm

Column

Brace

Beam

Girder

Diaphragm

6

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.