Have you ever wondered: how do buildings stand up, and resist the forces of time and weather? Well, we have the answer for you, and it’s bracing!
There are many hidden parts of a structure that give it strength and integrity, and without them, they would collapse. A bracing system is a secondary but essential part of any structure, from buildings to bridges. Bracing systems include wood or steel components that help evenly distribute loads and increase the safety of the structure. For bridges, a bracing system serves to stabilize the main girders during construction, to contribute to the distribution of load effects and to provide restraint to compression flanges or chords where they would otherwise be free to buckle laterally. While traditional framing in homes can support the weight of the roof and floors above, it is not able to resist lateral stresses caused by wind, earthquakes or other forces. Bracing requirements are set by the International Residential Code, and have been adopted into the majority of building code laws. It is a necessary part of building any structure, and there are many different types.
There are two main types of structures that require bracing: bridges and buildings. Between the two, types vary. Within each of these two categories, engineered or prescriptive bracing can be used, depending on the building codes. Engineered bracing is a system designed by a structural engineer based on the needs of a specific project. Prescriptive bracing is a system chosen based on the experience of the builder or developer, and is less complex and more flexible than engineered systems. Generally, an engineered system is required in areas with high levels of seismic activity or for structures that will be exposed to wind speeds greater than 110 MPH. In addition, temporary or permanent bracing can be used: temporary bracing uses metal poles, cables, wooden frames or pre-engineered bracing components to keep a structure stable during construction. These items are removed once the permanent bracing is installed. The primary purpose of a temporary bracing system is to keep workers and the public safe during construction. Permanent bracing may be installed at any point during construction, and is designed to keep occupants and the public safe over the life of the building.
With most residential buildings, wood is used for the structure, so wood bracings are also used. Some larger and more industrial or corporate buildings use steel, in which case the bracings would also be in steel. Even for your home, it is vital to consult engineers in order to make sure you are choosing the right kind of bracing, and meeting the building codes/structural requirements. While designing bracing systems, building height can influence the building’s stability. Buildings of eight stories or less can be considered stable with well-designed steel structures. Taller buildings require concrete or braced steel cores for stability. Increasingly, architectural issues and visual impact have to be addressed, and leading architects are involved with the design of aesthetically appealing bracing systems.
1. Horizontal Bracing
Horizontal, meaning running from side to side, goes in tandem with vertical bracing. The bracing at each floor (in horizontal planes) provides load paths for the transference of horizontal forces to the planes of vertical bracing. Horizontal bracing is needed at each floor level, however, the floor system itself may provide sufficient resistance. Roofs may require bracing.
2. Vertical Bracing
Vertical, meaning running from top to bottom, is the other half of the bracing system with horizontal braces. Bracing between column lines (in vertical planes) provides load paths for the transference of horizontal forces to ground level. Framed buildings require at least three planes of vertical bracing to brace both directions in plan and to resist torsion on a vertical axis.
3. Rigid Foam Bracing
Now, for many residential buildings, all that is needed is rigid foam bracing; one of the most popular methods used to brace exterior walls. Using this system, framing studs are covered by sheets of foam insulation that are at least 1-inch thick and 4 feet wide. In addition to bracing the structure, rigid foam bracing actually insulates the building at the same time! Especially for buildings in the northern part of the world, this is a real advantage. It can help to block sound transmission through walls as well! Foam bracing can only be used in one or two-story buildings, and is fairly labor-intensive.
4. Structural Sheathing
Structural sheathing, instead of using beams, uses sheets for support. They rely on OSB or plywood sheets to provide lateral support to the framing system. These sheets of wood are nailed to regular framing members, and are covered by moisture barriers and exterior siding or finishes. The boards are relatively strong, and flexible for bracing, which is perfect. They can be installed in any direction, and sizes are not typically regulated. Structural fiberboard panels may be used for additional strength and insulation.
5. Drywall Sheathing
Drywall boards used to be called gypsum board, which is why this method is also/often referred to as gypsum sheathing. It is a popular method of bracing homes from the inside. Rather than use traditional 3/8-inch drywall, a thicker 1/2-inch sheet is used to provide structural support and lateral strength. The major distinction is that instead of installing the bracing on the outside of the building, the drywall is installed on the inside of the framing. These sheets are then finished and painted to provide a normal wall appearance. This system eliminates the need for exterior sheathing, and can help lower project costs.
Most commercial building of a larger size use metal and concrete, and depending on the height of the building will use one of the following methods of bracing. You may have heard of the phenomenon of some skyscrapers swaying in the wind. In addition to the vertical force of gravity, skyscrapers also have to deal with the horizontal force of wind. Most skyscrapers can easily move several feet in either direction, like a swaying tree, without damaging their structural integrity. To keep these buildings from swaying heavily, engineers have to construct especially strong cores through the center of the building. In the Empire State Building, the Chrysler Building and other skyscrapers from that era, the area around the central elevator shafts is fortified by a sturdy steel truss, braced with diagonal beams. Most recent buildings have one or more concrete cores built into the center of the building. Some buildings already use advanced wind-compensating dampers. The Citicorp Center in New York, for example, uses a tuned mass damper. In this complex system, oil hydraulic systems push a 400-ton concrete weight back and forth on one of the top floors, shifting the weight of the entire building from side to side. So, at this point with the heights we have reached in construction, there are more advanced technologies than just bracing systems to keep them safe, and comfortable for the people inside. However, bracing remains a core tennet of construction. Here are the main types of bracing used with steel beams:
1. Single Diagonals
Trussing, or triangulation, is formed by inserting diagonal structural members into rectangular areas of a structural frame, helping to stabilize the frame. If a single brace is used, it must be sufficiently resistant to tension and compression. This is one of the simpler methods of bracing but can be just as strong if done right.
2. Cross Bracing
When two diagonal members cross each other, this is known as cross-bracing, or X-bracing. These braces need to be tension-resistant, where each brace resists horizontal forces. Steel cables can be used for this kind of bracing. Care must be taken so that external cross-bracing does not clash with the placement and purpose of windows. Cross-bracing also has the potential to bend floor beams in some cases. On the outside face of a building, however, it can also interfere with the positioning and functioning of window opening. Some skyscrapers by engineer Fazlur Khan, such as the 1969 John Hancock Center, have a distinctive X-bracing exterior, allowing for both higher performance from tall structures and the ability to open up the inside floorplan (and usable floor space) if the architect desires.
Typically, K-bracing connects to columns at mid-height and is thus more flexible to working around windows and generally results in reduced bending in floor beams. However, K-bracing may potentially cause column failure in case the compression brace buckles and thus may not be encouraged in earthquake-prone regions.
In the V-bracing style, two diagonal elements are framed in a V-shape stretching down from the top two corners of a horizontal element and join at the centre of a lower horizontal element. When the V is inverted, also known as chevron bracing, the two elements in the V meet at the centre of an upper horizontal element.
Both systems can significantly reduce the buckling capacity of the compression brace so that it is less than the tension yield capacity of the tension brace. This can mean that when the braces reach their resistance capacity, the load must instead be resisted in the bending of the horizontal member.
The way that buildings are constructed and held up is often something we don’t think about in our day to day life. It is, however, truly a backbone of our society and the way we live our lives. Having sturdy buildings that can withhold the test of the elements and time is vital. There are so many people behind the scenes involved in making buildings stand up, from engineers, to architects, to builders. For simple cabins throughout history, large wooden beams stacked on each other or stones would be enough to create a structure to live in. Our standards have gone up over time, and we have also invented systems that make our buildings much more fuel efficient, with insulation.
At the moment, the Burj Khalifa building in Dubai is the tallest in the world, with a total height of 829 metres. Burj Khalifa’s construction used 330,000 m3 (431,600 cu yd) of concrete and 55,000 tonnes (61,000 short tons; 54,000 long tons) of steel rebar, and construction took 22 million man-hours. The structural system can be described as a “buttressed core”, and consists of high-performance concrete wall construction. Each of the wings buttresses the others via a six-sided central core or hexagonal hub. This central core provides the torsional resistance of the structure, similar to a closed pipe or axle. Corridor walls extend from the central core to near the end of each wing, terminating in thickened hammer headwalls. These corridor walls and hammer head walls behave similarly to the webs and flanges of a beam to resist the wind shears and moments. Perimeter columns and flat plate floor construction complete the system.
While most buildings don’t require this kind of elaborate planning to be executed, bracing is always an invaluable part of the building structure, and even though it’s invisible, one we couldn’t live without.