Structural Engineering 101

Foundations are the first thing to be built during construction. Therefore, they must be the first thing to be designed, right? WRONG! This is a common misconception. Surprising as it may be, buildings are built from the ground-up, but are designed from the top-down.


Follow the Load Path

Engineered structures are composed of multiple types of members that connect in order to move the loads from the top to the bottom of the structure. Loads in any building have to travel from the roof and upper floors down to the ground. This effect is simply termed “Load Path.” Load paths need to be continuous. Each consecutive member needs to support itself and the previous members and loads that connect to it.


Gravity Loads

Gravity loads are the vertical forces that act on a structure. The weight of the structure, human occupancy and snow are all types of loads that needs to have a complete load path to the ground.

Gravity-Loads

The simple structure in the above picture can be used to demonstrate how gravity loads move from the top of a structure to the ground.

  1. A floor slab is designed to support the imposed gravity load.
  2. This load travels from the floor slab to the beams that support it.
  3. Upon reaching the beam, the load travels to the end of a beam, which is connected to a girder.
  4. This girder is supporting the accumulated loads from the floor slab and beams and transmits the load to a connecting column.
  5. The load then travels down the column to the foundation and is distributed to the ground.

Lateral Loads

Gravity loads are not the only type of load that is considered when designing a structure. Lateral loads (wind and earthquake loads) must also have a complete load path to transfer them to the ground. Unlike gravity loads, which act in a downward direction, lateral loads can act in a horizontal direction or even cause an uplift effect.

lateral-loads

A shear wall is a compilation of smaller structural elements into one larger element that is used to resist lateral loads. The shear wall in the above picture shows how a lateral load moves from the top of the structure to the ground.

  1. The lateral load is distributed throughout the top of the wall.
  2. This load travels through the shear wall and is output at the base of the wall.
  3. The connection between the base of the wall and the foundation forces the load into the foundation and is eventually transferred to the ground.

A complete and continuous load path is necessary to safely move the loads from the top of a structure to the ground. If a correct path isn’t designed to move the load to the ground, then the loads will find other means of making it to the ground; usually in an unfavorable way. It’s not quite as simple as an apple falling on Newton’s head, but it’s essentially the basic law of gravity.

Untitlhhed

And that concludes today’s lesson!

Thanks for reading,

Jillian


Allegheny Design Services

Structural Engineering is a field of engineering dealing with the analysis and design of structures that support or resist loads. Structural engineers create complex structural systems through the use of simple components.

Allegheny Design Services (ADS) works closely with architects and contractors to provide the engineering support that turn their vision into reality.   To see some of our latest work, click here!

Written by Jillian Maston, EIT, Junior Structural Engineer

jillianJillian is a resident of Morgantown, WV. She graduated magna cum laude from West Virginia University College of Engineering and Mineral Resources with her bachelor’s degree in civil engineering in 2011. She was inducted into the civil engineering honor society, Chi Epsilon, during her senior year of college. Presently, she is completing her master of civil engineering at North Carolina State University with a focus in structural engineering. Jillian gained experience in the industry through internships at Burgess and Niple and the North Carolina Department of Transportation. In January 2013 she began her career at Allegheny Design Services where she has broadened her experience in the analysis and design of structural materials.

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