Unit 2: Lateral Design of Structures


An introduction to lateral design in modern structural engineering.

While dead, live, and snow loads act vertically downward on a structure, another class of loads designates those that act horizontally. These are known as lateral loads, and in this unit we will give a brief overview and discuss their historical significance.


Learning Objectives

After completing this unit students will have an introductory grasp of:

  • Wind loads
  • Seismic loads
  • Modern seismic design
  • Investigation of frame stability using Robot Structural Analysis
  • Hand sketching deflection and forces on a steel frame


Structural engineering by nature must adhere to strict, standardized codes. Simply because of the human lives dependent on the stability of the structure, every aspect of the design must be detailed and mastered. Consequently, distinct specializations have developed within the field of structural engineering. In Unit 1 the concept of loads was introduced, specifically the field of vertical loads. While dead, live, and snow loads act vertically downward on a structure, another class of loads designates those that act horizontally. These are known as lateral loads, and in this unit we will give a brief overview and discuss their historical significance.

Lateral Loads

Within the study of lateral design several types of lateral loads exist but the most notable are seismic and wind loads. Both terms need little description. Wind loads are the lateral forces applied by wind, and seismic loads are the lateral loads that result from earthquakes. Though these terms seem easy to explain, their study and design applications remain challenging. Even though it is a narrow sub-field of structural engineering, lateral design constitutes a large field of study.

Tacoma-narrows-bridge-collapse". Licensed under Public domain via Wikimedia Commons.

Wind Load: Wind load has been recognized long before the academic study of structural engineering. No sooner had man fabricated the first shelter then it was blown over by next storm. No sooner had he mastered dead load than he discovered wind load. The basic principle behind a shelter is to protect the inhabitants against the natural elements. It makes sense therefore to design the structure.

Seismic Load:

If wind load design seems a study of ancient origin, seismic design must therefore be a modern advancement. Though seismic failure has affected man as long as any other load type, it is only through recent innovation that engineers are now able to design against it. There are several examples of these tragedies, one will be discussed later, which resulted in total collapse of a building and many deaths.

Seismic Load.

Example of earthquake destruction

See page for author [Public domain], via Wikimedia Commons

Naturally, engineers wish to protect the lives of occupants and the financial investment in a building.









This chapter seeks to demonstrate the vast scope and level of detail included in such a specific field of engineering. From the primitive need to withstand wind load, to modern endeavors in preventing seismic failure, building design continues to evolve and adhere to our needs. The importance of lateral design is paramount in structural engineering, and the recent developments in seismic design, especially, prove to make the buildings of our future safer still.





Seismology is the study of earthquakes. This violent movement deep in the earth propagates elastic waves that, if large enough, cause destruction in surrounding structures.

Northridge Earthquake Example

Northridge Earthquake Example

On January 17, 1994, an earthquake with a moment magnitude of 6.7 hit the Reseda neighborhood in Los Angeles, California.  The amazing amount of destruction came as a shock to everyone.  But an even greater shock came after a thorough analysis of the structural failures.

Lateral Design

Video Exercises: Lateral Design & Structural Stability

 Learn how to calculate the stability of a steel frame based on the loads it is subjected to.