Lateral Bracing & Stability
Some people may think building design is easy. They may think that anyone can build a house or a deck and that they don’t need any engineering background or knowledge. Put a beam here. A post there. Some concrete here. Some nails there. But what if the big bad wolf came and huffed and puffed and tried to blow that structure down?
Like I mentioned in our article about loads, there are two main types of lateral loads that can be applied to a structure: wind loads and seismic loads. Many know what structural elements resist vertical loads. That is beams and columns. But what if a huge gust of wind is applied to the building, or if Godzilla came stomping around your neighborhood and caused the ground to shake/vibrate? What elements do you need to resist those lateral loads so the structure doesn’t collapse?
A structure needs a lateral force resisting system (LFRS) to provide lateral stability in the event of lateral loads. These lateral loads are based on worst case loading conditions, like a 1 in 50 year gust of wind. If we are lucky, the building may never have to deal with resisting these loads, but it is necessary to design for this in case something like that ever happens. No matter what type of material the structure is built out of, there is likely a way to stabilize that structure to resist lateral loads.
If you were standing on one foot and someone gave you a big push in any direction, chances are you would topple over. If you stood on two feet shoulder width apart and someone gave you a big push, you may find yourself either standing tall or flat on your stomach depending on what direction the push came from. Getting pushed from the front or behind, your two legs won’t provide much stability unless your shoes were anchored to the floor. If you were to get pushed from your side, your legs along with your upper body act as a braced frame that provides lateral stability.
One of my summer jobs during college was working for a local company washing windows. For years, when I was out for a walk or sitting down at a restaurant and looked at a window I would think to myself: “How would I wash that window? What ladder would I use? Those are some dirty windows!” Now that I have been working as a structural engineer, when I go out for walks or go to a restaurant with my wife I think to myself “How was this structure designed? What system did they use to support the vertical loads? How is the structure supported when lateral loads are applied?” I built a picnic table a few years back with a friend and I wanted to make sure it wasn’t going anywhere when someone leaned on it or decided to jump on top of it and start doing the Macarena… This is the same thought that goes through a structural engineer’s mind when designing a building.
When it comes to the design of a structure, there are different lateral force resisting systems (LFRS) that are used. Sway and non sway frames are the most common ways to resist lateral load when dealing with a steel framed building. A great video that shows how these types of frames work can be seen here https://www.youtube.com/watch?v=P3l6iwXaWUc. You may be out for a drive one day and notice a giant X on the side of a building in one or many locations. You may think this is an architectural feature, but there is a good chance that the X, or cross brace, is being used to resist lateral loads applied to the building. This is an example of a non sway frame.
When cross bracing or diagonal bracing is not used, the structure’s connections need to be more rigid or “fixed” so that the vertical elements don’t deflect too much when lateral load is applied. This often results in bulkier and more expensive connections but allows for more flexibility for an architect when designing the layout and elevations of the building. For steel framing, these connections often consist of multiple bolts, large plates, and big welds.
A rigid connection in a concrete structure would consist of lots of additional steel reinforcing extending from a column and hooked into a beam. Although the steel reinforcing is not visible, the concrete elements likely need to be wider/deeper in order to accommodate the additional steel needed to resist the lateral loads.
Have you ever tried to build a bridge or tower out of spaghetti or popsicle sticks? I wish you good luck trying to build one of those without diagonal members. Apply a bit of lateral force and watch it tumble to the ground. Unless the connections are rigid you will have a hard time building a tall tower.
Using marshmallows to connect each segment is an example of a pinned connection where the elements framing into it can rotate freely. This video does a great job of showing what fixing the connection does to laterally support a structure creating a moment or sway frame by using super glue! It also gives some pros and cons of each type of frame.
In wood, concrete and masonry structures, the most common element used to resist lateral loads are shear walls or cores walls. A shear wall is a segment of wall that is stiff enough to attract the lateral loads applied to a structure and transfer them down to the buildings foundation. Often these walls are also used to resist vertical loads.
For a wood framed shear wall, the connection of the sheathing to the studs plays a big role in its ability to attract and resist lateral loads. The stronger the fasteners and the tighter the spacing of those fasteners, the stiffer that wall is going to be. The thickness and type of sheathing also plays a role in the capacity of the shear wall and whether the wall is sheathed on one side or both.
When we design a concrete or masonry shear wall, their ability to attract and resist lateral loads are based on the length and width of the wall, the compressive strength of the concrete or masonry, and the horizontal reinforcing provided in the wall.
Core walls are segments of walls that connect together to form a box or a “core”. When wall segments connect in this manner, the segments as a whole become much stiffer than if we were to look at each wall segment individually. For tall buildings, the walls surrounding the elevator shafts and stairwells are often used as one of the main lateral load resisting systems for the structure.
Hope this gives you a bit of a background on what elements of a structure resist lateral loads so you can take this into account when working on your next project or so you can better understand the function of these elements next time you are looking at a structure. If you have any questions or comments on the content, feel free to leave a message in the comment section below!
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