11 - Principles of Lateral Stability

› Note 10 Level 1

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TheStructuralEngineer July 2012

Technical Technical Guidance Note

Principles of lateral stability Introduction

This Technical Guidance Note concerns the concepts of lateral stability within structures. A key component to the design of structures is a sound understanding of stability. As building and bridge construction has become increasingly ambitious, so the principles of stability are continually tested. This guide explains the various methods that can be adopted to ensure that lateral stability to structures is achieved. This note also highlights the need for robustness in structures as it is regarded as an aspect of structural design that can have an impact on strategies adopted for lateral stability. All of the guides in this series have an icon based navigation system, designed to aid the reader.

ICON LEGEND

W Design principles

W Applied practice

W Worked example

W Further reading

W Web resources

Design principles The chosen method of achieving lateral stability of a structure is normally driven by both geometry and the materials the structure is constructed from. For example, a concrete floor slab is supported by steel beams, or a timber roof frame sits on a masonry wall. All of these elements are required to work together in order to transfer horizontal loads to the ground in a safe manner. This note aims to guide the reader in developing and identifying defined load paths within structures that maintain their lateral stability. It is considered to be good practice to have a single designated engineer within a design team who is responsible for overseeing a structure's lateral ability during its design. By having such an individual, all design development of the structure is referred to one designated engineer, and thus the lateral stability aspect of the design is maintained. As a general rule, any vertical element that is a key contributor to a structure's lateral stability should be well spaced out from other similar elements. This is to ensure that no significant proportion of the structure

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is tied to one cluster of vertical restraint elements. Vertical bracing elements that have significantly different magnitudes of stiffness from each other can cause torsional effects within the structure when it is subjected to lateral loads. A good example of this is a diagonally braced bay that is paired with a portal frame. This must be recognised and addressed during the design process. Finally, it is imperative that all of the forces from any vertical element that provides the lateral restraint to the structure are fully resolved and taken into the foundations of the structure.

Components contributing to lateral stability There are four forms of components that can be found in a structure that contribute to lateral stability: Bracing; Shear cores and walls; Portalisation and Diaphragms. In most cases they are used in combination with one another in order to achieve a stable structure.

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Vertical elements

Horizontal elements

Bracing This is one of the most well-known methods of providing lateral stability. Bracing (Figure 1) consists of diagonal elements and acts in a similar way to a cantilevering vertical truss, from the ground up. It is for this reason that bracing should be present at every level of the structure down to the founding level in order for it to be effective. If bracing is discontinuous, significant lateral forces are generated and need to be transferred from one bracing system to another, which can exert high localised lateral loads onto elements of structure. Additionally, the transfer system that is adopted for this purpose needs to have adequate stiffness. It is not uncommon to see bracing working in conjunction with other vertical elements to achieve overall lateral stability of a structure.

Diaphragm A diaphragm is an area of the structure that provides bracing in its plane (Figure 4). Typically these are floor slabs and roof cladding, but can also be in vertical cladding elements. If cladding is used as a diaphragm then careful consideration must be given to the temporary condition of the structure during erection. This is also true for the maintenance of the structure if the cladding has a shorter design life than the structure. 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 are instances where diaphragms do not have enough strength to resist the lateral loads that can build up within them. In such cases, either the diaphragm is strengthened or horizontal bracing is installed to either supplement or replace the diaphragm completely. Figure 2 Shear cores

Portalisation/Sway Frame Portalisation, also known as sway frame is based on the concept of portal frames whose connections are designed to withstand forces generated from lateral and vertical loads (e.g. Figure 3). This negates the need for any vertical bracing elements and therefore large clear span spaces are created. This does however add significant complexity to the construction of the frame as well as its weight. This is due to members tending to be larger than their simple construction counter-parts and connections becoming more onerous in their design and installation. This leads to a limit to the number of storeys sway frames can be constructed due to these practical considerations. Figure 1 Bracing

Shear Cores/Walls Shear cores and walls are vertical elements within a structure that provide lateral stability (Figure 2). The rest of the structure is framed around them and they typically work in conjunction with floor plates and roofs that act as diaphragms. They can also be paired with braced based systems. Shear cores typically act as vertical access throughout the structure via lifts and stairs and are usually located in line with the centroid of the structure in an attempt to minimise torsional effects. There will however always be a difference between the centre of stiffness of the structure and the centroid of the applied wind load. As a result, some torsion does develop within the shear cores due to eccentric loading.

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Additionally, the adoption of portal frames as a bracing solution may require careful consideration of second order effects within the structure. This adds to the complexity of its analysis and design significantly.

Figure 4 Floor and roof diaphragms. All other lateral stability elements omitted for clarity

Location of lateral restraints As has already been explained in the section on shear cores, the location of lateral restraints within the structure impacts the way it behaves when subjected to a lateral load. Simply placing them within the structure does not necessarily lead to a stable structure, as excessive torsion due to twisting can occur if significant eccentricities are introduced.

Bracing location

Figure 3 Portal frame with braced bay

Vertical bracing typically works in conjunction with other elements that provide lateral stability, be it a shear core, a wall or a sway frame. They usually have an impact on the architecture of a structure, so their placement is normally driven by both the structural needs as well as the geometric restrictions of a building. When placing bracing elements, it’s important to take note of their stiffness relative to the other vertical elements in the structure that are also providing lateral stability.

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› Note 10 Level 1

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Technical Technical Guidance Note

TheStructuralEngineer July 2012

Shear core and wall location The relative location of shear cores and walls against the resultant force generated from applied horizontal loads should preferably be as close as possible to the centroid of the applied forces. This is done in order to limit the effect of combined bending and torsion. There is also the effect of expansion and contraction of the structure due to thermal effects to consider when placing shear cores and walls. In the case of larger structures it is important to take these into account when placing vertical bracing elements, such as shear cores. Their placement must encourage the structure to move in sympathy to thermal effects, otherwise there is a risk of large forces being locked into the structure.

Lateral Stability Strategies Before attempting to adopt a particular strategy for lateral stability, some appreciation of alternative solutions (and their consequences) is required. Consider Figure 5:

Structure ‘D’ is slightly better than ‘C’, but is still susceptible to off-centre wind forces and would also generate significant torsion within the structure.

Robustness The robustness of a structure is linked to its stiffness, as generally the stiffer the structure, the more robust it is. Typically in terms of load path, the shorter its length, the less stress the members within the structure are subjected via any applied horizontal forces. This is why it is important that in order to make an efficient structure, the applied loads must be transmitted to the foundations via the shortest load path. This is dependent on the form of the structure and as such, efficiency often gives way to function and, in some cases, aesthetics. In such instances a good engineer will do their utmost to maintain the philosophy of robustness, regardless of the form of the building they are designing the structure for.

Eurocode 0.

Applied practice

The applicable codes of practice for lateral stability are as follows: BS EN 1990: Eurocode Basis of Structural Design BS EN 1990: UK National Annex to Eurocode: Basis of Structural Design

Figure 5 Plan of various lateral stability solutions

Structure ‘A’ does boast a solution that braces the structure orthogonally in both directions. There is the risk however that if one of the vertical bracing elements is subjected to accidental damage, then the entire structure will become unsafe. Structure ‘B’ is the most efficient solution, but is not normally architecturally sound. It does however have redundancy in that if one of the vertical bracing elements were to fail, it would not leave the building in an unsafe state. Structure ‘C’ is a poor solution as off-centre wind forces would generate significant torsion within the structure. If the structure were also primarily made from concrete, then shrinkage within it would also create significant stresses that could not be relieved.

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Glossary and further reading Sway frame – Similar to ‘Portalisation’ Further Reading The Institution of Structural Engineers (2010) Practical Guide to Structural Robustness and Disproportionate Collapse in Buildings London: The Institution of Structural Engineers The Institution of Structural Engineers (2010) Manual for the design of steelwork building structures to Eurocode 3 London: The Institution of Structural Engineers The Institution of Structural Engineers (1988) Stability of Buildings London: The Institution of Structural Engineers Owens, G.W. and Davison, B. (Eds) (2012) Steel Designers Manual. 7th ed. Chichester: Wiley-Blackwell

Eurocode 0.

Web resources

For more information on this subject, visit: www.istructe.org/resources-centre/library

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Worked example Figures 6 and 7 are two structural forms. Suggest appropriate methods of lateral stability for each structure.

Fig. 7 (below) replicates Fig. 6 but with the addition of a single storey structure:

Figure 6 Multi-storey building with stair core attached to an extreme face

In this instance the stair core can be used as a component to achieve lateral stability. It cannot do this alone however as it is eccentric to the centre of the structure in one direction and thus any lateral load would likely generate torsion within the shear core. One viable solution would be to install bracing in the opposing face of the building from the stair core, thus: Figure 7 Multi-storey building with single storey section attached

One viable lateral solution would be to install a shear wall on the opposing side to the stair core and to portalise the single story structure. The latter will have an isolation joint between the taller portion of the building and be braced in the direction that is perpendicular to the portal frames. The roof of the single storey structure can act as a diaphragm:

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