Lateral Force Resisting System 101 – Basics

Introducing the Basics

This article is the first in a series discussing the lateral force resisting system (LFRS) for buildings. There is much to cover on this topic and thus the basics of a lateral system must be discussed first. To understand clearly how it works, we will discuss the following items:

  • Define what an LFRS is and the different types
  • Answer a few basic but important questions
  • Learn what materials can be used and their corresponding building codes
  • Illustrate the application of an LFRS

A lateral force resisting system (LFRS), or lateral system for short, can be defined as a specifically designed structure as part of a building that resists against lateral loads such as wind or seismic. There are three main types used in a building – braced frames, moment frames, and shear walls. Below are examples of each. The three types will be discussed in further detail in later articles.

A computer-generated image of a braced frame, a basic structure of a lateral force resisting system. Ballasts hold two vertical steel beams, with a third closing the top. Two smaller steel beams form an X, going from the top joint on one side to the joint with the ballast on the other.
Example of a braced frame.
A computer-generated image of a moment frame, a basic structure of a lateral force resisting system. Ballasts hold two vertical steel beams, with a third closing the top.
Example of a moment frame.
A computer generated image of a shear wall, a basic structure of a lateral force resisting system, with the words "reinforced concrete shear walls" in the bottom right corner. The image shows a wall of solid concrete, with the top left corner exposing a metal lattice forming a grid inside the concrete.
Example of a shear wall.

These illustrations depict the basic structure of each type of lateral system. For multi-story buildings, each frame or wall is built and connected at the top and bottom to resist the lateral loads from top to bottom of the building.

Why is a Lateral Force Resisting System (LFRS) Important?

The two main lateral loads an LFRS is designed to resist are wind and seismic. An LFRS is essential not only to the stability and longevity of the structure to resist these loads but also to the lives of its inhabitants. Without an LFRS designed specifically for each building it could suffer extensive damage, and worse, could cost lives. In short, a lateral system is critical to any structure/building.

How Does an LFRS Work?

The three main types of lateral systems, illustrated above, all work in basically the same way, in that, they all “brace” the building and transfer the lateral load to the foundation and then is dissipated into the earth.

Where is an LFRS Located in the Building?

There is much consideration as to where the lateral system is placed within the building, but it is usually preferred to be located on the perimeter of the building and can either be exposed or concealed within the walls depending on the architect’s preference. Lots of communication between the architect and structural engineer is had in the early stages of design to figure out the best location for the lateral system.

Steel, concrete, wood, and concrete masonry units (CMU) are the four main building materials, of which two or more are often used in the construction of a building. The pre-requisite to designing a building with any of these materials is to know their governing codes they must comply with, in which, each building material has its own code, below:

  • Steel – Specification for Steel Buildings, by American Institute of Steel Construction (AISC 360)
  • Concrete – American Concrete Institute (ACI 318)
  • Wood – National Design Specification (NDS) for Wood Construction, by American Wood Council
  • Concrete Masonry Units (CMU) – Building Code Requirements and Specification for Masonry Structures, by The Masonry Society (TMS 402/602)

These codes are the minimum standard to which the structure must conform. Below are a few basic illustrations of how wind and seismic loads affect buildings and the lateral systems resisting the loads.

A computer-generated image of a six-story office building is show with blue arrows on the right, indicating wind load, and red lines running in a grid over the facade of the building, with a red arrow pointing both directions at the bottom indicating seismic load.
An overlaid frame with wind and seismic loads on a multi-story office building.
A computer-generated image of a five-story building depicted in simple lines. There is a shadow-image of the building completely vertical in the background. The more clear image of the building is depicted in wavy lines, tilted to the side. An arrow pointing left and right at the bottom is labeled "seismic load".
Seismic load on a multi-story building.
A computer-generated image depicting a simple house-shaped building. Two arrows illustrate wind loads, one pointing at the side of the building, and one going over the peaked roof and showing uplift on the far side of the peak.
Wind load on a single-story building.

As illustrated above, the lateral system is integrated into the building and braces the building against wind and seismic loads. The example of an overlaid frame is a real building with a multi-story lateral frame overlaid to show a basic illustration of the lateral support structure that is built behind the brick façade.

This concludes some of the basic items of a lateral force resisting system. Further in-depth discussion will be continued in subsequent articles.


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