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Lateral Force Bracing

July 17, 2011

There are two types of loads that are particularly damaging to a building. Wind and earthquake loads, more commonly known as lateral forces.

Wind loads are most extreme in areas of the country that are close to the water. For example hurricanes tend to effect the south-east coast the most so some of the strongest codes governing wind loads are found there. Typically located in the building code are maps that show the wind speeds that need to be designed to. For example southern florida has wind speeds of 150 MPH and the midwest 90 MPH. (2006 IBC) There are also special wind regions located throughout the country where microclimate need to be given special consideration.

Miami-Dade county, in florida, has basically written the standard on wind loads for window and curtain wall systems. Their code actually requires a missile test to comply with their codes.  A wood 2×4 is basically shot at a wall system and if it doesn’t fall apart it passes.

 

 

Earthquakes happen all over the US, but in California because it has fault lines running through it along major population centers has the strictest codes. Similar to wind loads, the building code contains maps of the US showing the earthquake information for different regions of the country.

There are three ways to counteract lateral force:

  1. Shear walls
  2. Diagonal bracing
  3. Moment connections

Shear Walls

Shear walls are provided by installing a continuous wall in between two supporting members, for example, installing a CMU wall between two steel columns. The solid wall will prevent the columns from moving if a lateral force is present. Installing drywall or plywood on metal or wood studs also acts to prevent lateral forces in the same way. Window and door openings are best avoided in sheer walls, but are possible with carefull placement and heavy reinforcing around the opening.

Diagonal Braces

Diagonal braces are installed in steel frame structures. Typically a piece of steel is installed diagonally in between two columns, at the base of one column and at the top of the other. Bracing in the form of an “X” is also common. When the bracing does not meet with both the base and top of a column it is called excentric bracing. In order for the lateral force to prevail it would have to bend the steel bracing between the columns.

Moment Connections

To understand moment connections I need to explain what a moment forces is. A moment force is a force that causes something to rotate about a point. The easiest descriptions of this would be if you held a 10 pound weight at arm’s length. Your arm want’s to naturally rotate towards your waist. To counteract a moment force in steel construction requires heavy bolting or continuous welding of the beams to columns. A moment connection is the most expensive lateral bracing option available, but it allows for clear opening without the need for a wall or bracing.

 

On one final note, lateral bracing needs to be provided in both directions of the structural system to be effective. It should be provided in both the north/south direction as well as the east.west direction.

 

Good luck on your next exam!

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