Moment Columns, Strong Walls, And Moment Frames
For many years garage door openings could be braced with shear walls that were only 18″ wide. The Northridge Earthquake revealed that these shear walls acted like posts and tipped over. The building code therefore changed in the 1997 Uniform Building Code such that walls on either side of a garage had to be a minimum of 4′ wide rather than the 18″ previously allowed.
Sometimes builders did not want this 4′ wide wall on either side of the garage because it meant he had to build an even bigger garage with its added expensive.
Let’s say a builder wants the walls on either side of the garage to be only be 12″ wide. He can’t building a standard shear wall because the building code tells him it must be 4′ wide. In order to fill this need engineers started designing moment columns and manufacturers started strong walls that were very narrow and still resisted very strong earthquake forces. This webpage will be looking at both moment columns and strong walls.
The first column in the table “Allowable Load” tells us how much earthquake force a certain size moment column can resist. From a contractors point of view the weight is probably the most important factor because a contractor must figure out how to erect a very heavy steel column perfectly vertical that weighs 1600 lobs. Try and pick up a 50lb bag of concrete, multiply that by 32 and you get 1600lbs, and you will see what I mean. Moment columns also require an extensive amount of steel reinforcing, engineering, and concrete. Strong Walls on the other hand are pre-engineered and if you apply rudimentary retrofit design principles it is easy to know which one to use. We figure out what size Strong Wall we need, and then go up one size “just to be sure.”
This is another option called a Strong Wall. This table is from the manufacturer’s catalog.
The first column in this table under “Model No.” tells us the width and height of the strong wall. For example, SSW24x7 means the strong wall is 24″ wide and 7 feet tall. The numbers boxed in with red tells us how much earthquake force they can resist. For example, the SSW24x7 can resist 5,405 lbs of earthquake force. A SSW12x7 can resist 955lbs.
One of the best things about the strong walls is that you can put two of them next to each other back to back. This is called a “Double Strong Wall”. If we install two SSW24x7s back to back we will be able to resist 10,990lbs of earthquake force and two men can lift them and put them in place. This greatly reduces the labor expense. Notice how the Double Shear Wall resistance of 10,990lbs can resist almost as much as the strongest practically-impossible-to-install moment column. It should be obvious why these are usually the best choice.