A structure does not only transfer gravitational forces to the ground, it must do so for several other lateral forces applied . The components that hold the structure standing against these lateral forces is called the ‘stability system’ or, often, the ‘lateral load-resistant system’.
Achieving lateral stability in structures is usually dependent on the geometry and the materials from which the structure is constructed. For instance, steel beams support a concrete floor slab, or a timber-roof frame rests on a masonry wall. Every one of these elements must work together to safely move horizontal loads to the ground.
This post is set-out to introduce developing and identifying defined load paths within structures in order to ensure lateral stability.
Components of a Lateral Stability System
For every structure resisting lateral loads, there are four types of elements that can be found:
- Steel bracing
- Shear cores and walls
For certain cases, they are used in tandem with each other to produce a more stable structure.
These elements can be categorized into the horizontal bracing elements and vertical bracing elements.
Horizontal Bracing Elements
Horizontal elements are those that are in a horizontal, or near-horizontal plane (typically associated with floor and roof planes). They are required to transfer lateral forces to the vertical stability elements. There are two types of horizontal system: diaphragms and triangulated bracing (the latter being sometimes referred to as a wind girder).
A diaphragm is the part of a structure that provides bracing in its plane. Usually, these are floor slabs and roof cladding, but they can also be in vertical cladding elements. Diaphragms are tied back to vertical elements of the structure that provide lateral stability. They prevent structures from ‘racking’ or rotating about an axis. There might be some instances where diaphragms may not have sufficient strength to resist lateral loads. In such instances, additional horizontal bracings must be provided or the floor diaphragms strengthened.
In the absence of a sufficiently stiff diaphragm, Horizontal bracing may be used, in the form of steel bracing. The bracing is arranged in either the modified warren truss or the modified fink truss format as shown in Figure 1. The modified fink truss is more suited for tension bracing, in both cases the floor or beams act as part of the horizontal stability system and therefore must be designed for tension or compression in addition to flexure and shear.
Vertical Bracing Elements
The vertical elements are those that are in a vertical or near-vertical plane. They are the parts of the lateral stability system responsible for transferring the lateral loads down to the foundations. These vertical elements consist of one or more vertical elements with individual elements fixed or pinned at the foundation.
This is one of the most well-known methods of providing lateral stability. Bracing (Figure 2) consists of diagonal elements, working from the ground up in a similar fashion to a cantilevering vertical truss. In order for bracings to be effective, it must be present at each and every level of the structure down to the foundations. If bracings are discontinuous at one level significant lateral forces may be passed into the elements, which could be quite catastrophic if they weren’t designed to withstand such forces.
Shear Wall & Cores
Shear wall and cores are concrete vertical elements within a structure providing lateral stability (Figure 3). The rest of the structure is built around them and they normally work in combination with floor plates and roofs acting as diaphragms. These can also be combined with steel bracing. Cores generally serve as vertical access to the entire structure through lifts and stairs and are usually placed in line with the centre of the structure in an effort to reduce torsional effects. However, in reality, there will always be a difference between the centre of the stiffness of the structure and the centroid of the applied wind loads. As a result, some torsional stresses may be induced in the shear walls and cores as a result of eccentric lateral loads
Portalization is another method of lateral stability, it is based on the concept of portal frames whose connections are designed to resist lateral forces
Positioning of Lateral Restraints
As mentioned in the preceding section, the location of a vertical element within a structure has an effect on their response to lateral forces. Simply dispersing them within the structure does not necessarily guarantee lateral stability, as excessive twisting could be generated if significant eccentricities exist between the centre of area of the vertical elements and the point of application of the lateral loads.
Positioning Steel Bracings
Vertical steel bracing is usually used in combination with other components that provide lateral support, be it a shear wall, a core or a sway frame. These typically have an influence on the architecture of the structure, and their location is generally dictated by both the architectural needs and the spatial constraints of the building.
Positioning Shear Walls & Cores
The relative location of the shear cores and walls against the resulting force produced by the applied horizontal loads should ideally be as near as possible to the centre of the applied forces. It is required to minimize the effect of combined bending and torsion.
When positioning vertical bracing elements, it is important to remember their stiffness compared to the other vertical elements in the structure that also provide lateral stability. This is because the lateral loads are being shared according to their relative stiffnesses.
Strategies for ensuring Lateral Stability
A certain understanding of alternative options (and their consequences) is needed before attempting to implement a particular strategy for lateral stability. A system must always:
- Provide linear resistance in two orthogonal horizontal axes.
- Provide torsional resistance about a vertical axis.
- Provide the said resistance to all parts of the structure.
To buttress the points made above, consider figure 4 showing various lateral stability solutions for a building.
Structure ‘A’ boasts a solution which orthogonally braces the structure in both directions. However, there is the possibility that if one of the vertical bracing elements is damaged unintentionally, the entire structure may become unstable.
Structure ‘B’ is the most effective, but not architecturally sound solution. Nonetheless, it does have flexibility so that if one of the vertical bracing systems fails, it would not leave the building in an unsafe condition.
Structure ‘C’ is a poor solution as off-centre wind forces within the structure can produce major cant torsion. If the structure was also made predominantly from concrete, then shrinking inside it would also produce major stresses that can not be alleviated
Structure ‘D’ is marginally better than ‘C,’ but is still susceptible to off-center wind forces and would also cause substantial torsion in the structure.
References & Further Reading
The Institution of Structural Engineers (2014) -(Part 1 & 2) Stability of Buildings-General Philosophy and Framed Bracing: The Institution of Structural Engineers
BS EN 1990: UK National Annex to Eurocode: Basis of Structural Design.
The Institution of Structural Engineers (2012) -Principles of Lateral Stability-Technical Guidance Notes(level 2): The Institution of Structural Engineers