How Bracing Plans Ensure the Structural Fitness of a Building

They look like geometric patterns, a patchwork quilt design or even the connecting-dot children’s game of yesteryear, but bracing plans or layouts are no child’s game. They are serious stuff. Bracing plans are a segment of larger structural plans created to determine where to place the components of a bracing system to withstand wind loads and seismic pressure. Making sure that these plans are calculated and prepared with precision is vital to the structural fitness of a building. Qualified, experienced and accurate partners are needed to provide architectural design drafting, with high quality and more detailed architectural CAD services, to ensure that bracing plans are beyond reproach.

Although building stability can be provided either entirely or partially by reinforced concrete cores, bracing elements, or members, can help with this. Bracing elements bolster the strength of a building against winds, seismic activity, such as earthquakes and tremors, and the load of the building’s components.

Steel-braced frames ensure the stability of the building when the steel bracing is bolted, and steel-braced cores can be economical when construction is on a tight deadline.
In addition, steel-braced cores can be installed along with the remainder of the steelwork.

Grid lines are drawn, or bracing plans generated, across the length and width of buildings to position bracing elements. To facilitate a robust seismic performance, certain precautions in developing bracing plans must be considered:

· They must run in two orthogonal directions. Typically, they match the wall layout. In timber-framed buildings, they must be parallel to the external walls.

· The lines must be spaced evenly, when possible.

· Lateral loads can be transferred between bracing lines in timber-framed floor and ceiling construction with standard or typical detailing.

· Bracing lines should accommodate bracing elements anywhere on the lines.

Braced frames, a structural system in buildings subject to lateral loads, generally consist of structural steel, which works well for the requirements of tension and compression on these frames. The primary function of bracing is to provide stability and resist lateral loads, either from diagonal steel members or from a concrete ‘core’. For bracing frames, beams and columns are designed only to support vertical load, since the bracing system should carry all lateral loads. Sometimes, the placement of bracing frames or elements may clash with the façade and the location of doors and windows. Those structures that display post-modernistic styles sometimes feature bracing elements as part of their internal or external design.

There are two bracing systems that resist horizontal forces acting against the structure of the building: vertical and horizontal bracing. To transfer horizontal forces safely to ground levels, bracing provides load paths between column lines, in vertical planes. Buildings with bracing frames need to have vertical bracing in three planes to deal with directions in plan and to resist torsion around a vertical axis. Now, to transfer horizontal forces to vertical planes of bracing, bracing is required in horizontal planes on each floor as load paths. In some cases, the floor itself can provide the resistance required. Also, roofs may need bracing.

It is important that vertical bracing resists the forces of wind loads. To do so, bracing members should ideally be sloped at approximately 45°, which effectively braces the systems against member, vertical and lateral forces, and the connections between bracing and beam/column junctions should ideally be compact. If the bracing is too narrow and the inclination of bracing members are too steep, the sway sensitivity of the building will increase. Bracing systems that are wide will result in greater building stability.

When it comes to horizontal bracing systems, two types are used in multi-storey braced frames. They are:

· Diaphragms

· Discrete triangulated bracing

Usually made of plywood or oriented strand board in timber construction, metal deck or composite metal deck in steel construction or a concrete slab in concrete construction, a diaphragm acts to distribute horizontal forces in a bracing frame. It is required to connect two levels. Typically, the floor may be a sufficient diaphragm, without using a bracing system. If forces acting on a diaphragm are transferred to columns directly (a slab on columns), it is crucial that the capacity of the columns to hold the slab load must be precisely calculated.

The transfer of wind loads to the bracing system from the walls and roof are dependent on ceiling and floor diaphragms, specifically the depth of the diaphragms. A deep diaphragm transfers wind loads more effectively than narrow or long diaphragms.

Bracing on the roof level is known as wind girders, and they may be needed to carry horizontal forces on top of columns in the absence of diaphragms.

Primary to its efficiency, a diaphragm must be continuous over an area. When the ceiling of one room is higher than the rest of the building’s ceiling, modifications must be calculated due to the loss of continuity. Even steps on a floor can break this continuity. Other features that break continuity include bends in ceilings, ridges, hips, valleys and coved ceilings.

In circumstances where the floor cannot be used as a diaphragm, discrete triangulated bracing is employed. In its basic form, this consists of using triangulated steel bracing in a horizontal system, acting along each orthogonal direction. These systems span the vertical bracing.

For both vertical and horizontal bracing, there are different types of bracing that can be used. Horizontal elements of bracing must not be bent. This may occur when braces reach their resistance capacity, which is why the correct type of bracing is used. They can be broadly categorised as follows:

· Single diagonals

When diagonal structural elements are inserted into rectangular areas of a structural frame, this is called trussing, or triangulation. This helps stabilise the frame. A single brace needs to be resistant to tension and compression.

· Cross-bracing (or X-bracing)

When two diagonal members cross each other, this is known as cross-bracing, or X-bracing. Such braces need to be tension-resistant, where each brace resists sideways 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-tracing also has the potential to bend floor beams in some cases.

· K-bracing

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.

· V-bracing

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.

· Centric bracing

Used in seismic regions, centric bracing is similar to V-bracing, with the difference that bracing elements do not join at a centre point. Instead, there is a gap between them at the lateral connection. The gap, or ‘link’, between the bracing elements absorbs energy from seismic activity. Single diagonals may also be used in this kind of bracing and are called ‘eccentric’ single diagonals.

While developing bracing plans, or bracing layouts, these bracings are shown in 2D along the building’s elevation to help decide how best to reduce drift. The X-bracing is preferred to reduce drift, and the V-bracing reduces drift while being less heavy and with reduced joints. The inverted V-bracing is the most cost-effective type for this function. Another consideration for load applied is the geographical region, the tributary width of the braced sections and the tributary height of the floors.

No system is perfect. Considerations must be made for imperfections. In structural design, these come in the form of geometric imperfections, such as a lack of verticality, straightness, flatness, fit and joint irregularities. Also, imperfections, such as global faults in frames and bracing systems and local imperfections of individual bracing frame members, must be taken into account. These should be applied for each building storey and each frame and function combination, including vertical and horizontal loads.

With so many considerations, the process to design bracing systems may seem complex, but they need not be so if certain fundamentals are kept in mind for a medium-size building with bracing frames.

Bracing System Design in a Nutshell

1. Select the right beam section sizes
2. Select the right column section sizes
3. Calculate equivalent horizontal forces (EHF) for each floor
4. Calculate equivalent horizontal forces (EHF) of wind loads
5. Determine the total shear of the bracing base (adding the total wind load to the total EHF)
6. Share shear appropriately between the bracing systems
7. Size bracing members
8. Determine frame stability (combination of EHF and wind loads in conjunction with vertical loads)
9. Ensure effectiveness of floor diaphragms in distributing forces to bracing system
10. Determine total force the bracing should resist locally
11. Ensure that each floor’s bracing can handle the forces on the individual floor and forces of external loads

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. Building designers have a great responsibility to effectively design bracing, and contractors must then ensure the correct installation of the bracing system. If the bracing plan is not accurate, it may lead to the absence of adequate bracing members, thereby putting the building at considerable risk. For example, even a truss with a long span can be potentially risky, as the size and weight of the truss can lead to instability, buckling and collapse of the truss, if it is not braced properly.

Structural engineers and architects ensure effective bracing is in place for a building.

Software, such as GIB EzyBrace and Bracing software, with ArchiCAD, is used to determine how many bracing panels are required and where they should be placed, by calculating the drift associated with the wind velocity and the corresponding bracing layout, with the ultimate goal of developing a system that provides minimal lateral drift in an efficient manner. A frame stability tool is currently available to assist with the calculation of EHF. For cost-effective, accurate and timely delivery of bracing plans, many Western firms use the architectural CAD outsourcing services of a dedicated offshore CAD team, ultimately ensuring the structural fitness of the building.