TheStructuralEngineer28

Technical Guidance Note

Technical

July 2012

Note 10 Level 1

›

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.

W Design principles

W Applied practice

W Worked example

W Further reading

W Web resources

ICON LEGEND

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 fl oor 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 defi ned 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 signifi cant proportion of the structure

is tied to one cluster of vertical restraint elements.

Vertical bracing elements that have signifi cantly diff erent magnitudes of stiff ness from each other can cause torsional eff ects 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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Horizontal elements

Diaphragm

A diaphragm is an area of the structure that provides bracing in its plane (Figure 4). Typically these are fl oor 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 4 Floor and roof diaphragms. All other

lateral stability elements omitted for clarity

Location of lateral restraints

As has already been explained in the sec-tion 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 struc-ture does not necessarily lead to a stable structure, as excessive torsion due to twist-ing can occur if signifi cant eccentricities are introduced.

Bracing locationVertical 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 stiff ness relative to the other vertical elements in the structure that are also providing lateral stability.

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 signifi cant 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.

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

Figure 3 Portal frame with braced bay

Vertical 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 eff ective. If bracing is discontinuous, signifi cant 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 stiff ness. It is not uncommon to see bracing working in conjunction with other vertical elements to achieve overall lateral stability of a structure.

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 fl oor 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 eff ects. There will however always be a diff erence between the centre of stiff ness 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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TheStructuralEngineer30

Technical Guidance Note

Technical

July 2012

Note 10 Level 1

›

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

Shear core and wall locationThe 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 eff ect of combined bending and torsion.

There is also the eff ect of expansion and contraction of the structure due to thermal eff ects 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 eff ects, 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:

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 effi cient 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 signifi cant torsion within the structure. If the structure were also primarily made from concrete, then shrinkage within it would also create signifi cant stresses that could not be relieved.

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

Robustness

The robustness of a structure is linked to its stiff ness, as generally the stiff er 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 effi cient 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, effi ciency 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

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.

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:

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

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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