TheStructuralEngineer30

Technical Guidance Note

Technical

September 2012

Note 14 Level 1

›

Structural analysis methodologyIntroduction

When analysing structures it is important to adopt a methodical approach wherever possible. By breaking down the structure into manageable portions, the complexity of the analysis is reduced and thus becomes easier to control and review. By adopting such an approach, a seemingly insurmountable task becomes a much more approachable one.

This Technical Guidance Note is a good practice guide for analysing and designing structures. It explains how structures are given form, modelled, analysed and designed. Mention is made of the need to rationalise the analysis process, but not at the expense of an economic design.

W analysis

W Further reading

W Web resources

ICON LEGEND

Structural analysis principles

The analysis and design of structures starts with the form they are to take. This is based on the parameters that a building has to adhere to and is typically defi ned by an architect and/or client. The shape of the building is defi ned, from which a set of design criteria is generated. The structural engineer must do their utmost to work within these restrictions and where they cannot, they must advise the architect and client of this and develop an alternative solution. This is a crucial element of the design process and more often than not continues throughout the lifespan of the project.

Examples of the design criteria that structures must incorporate, and the impact that these have on the fi nal analysis and design is given in Table 1. Once these criteria are established and understood by the structural engineer, a concept of the structure is developed. This includes an initial layout of vertical elements and a method of lateral stability that is to be adopted.

Geometry, loading and ground conditionsThere are three fundamental components to the analysis and design of building structures that must be accurately modelled.

however, especially for transfer structures, as an error in the location of a vertical support element along the length of a transfer beam can have a signifi cant impact on the stresses within the structure.

Loading is very sensitive to error and can cause a cascade eff ect if it appears early in the analysis process. Take for example an area load; an assumption is made that the ceiling structure will be no heavier than 25 kg/m2 (0.25 kN/m2). During the design process this load is incorrectly grouped with

They are; geometry, loading and ground conditions. Geometry and loading are elements that have a very small margin of error. So much so that signifi cant deviations can result in the outcome from analysis becoming suspect.

Geometry is less sensitive to errors than loading. For example, a beam that is modelled 200mm longer than it will span when constructed, will have little impact on its design, other than it will likely be slightly over capacity. This is not always the case

E Figure 1Plan of beams to be analysed showing rationalisation

Structural

principles

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Table 1: Project criteria and impact on structural analysis and design

Criteria Impact

Planning restrictionsOverall height of building and external appearance

leading to changes in structural solutions

Building useMagnitude of imposed loads and building

services requiring plant rooms and voids within the structure for containment

Floor movement and vibration Depth of beams and fl oor structure

Vertical access Stair and escalator locations creating large voids

Occupancy management/egress Location of doorways and columns

Aesthetics Form of structural elements and their connections

Column and wall location Span of beams and slabs as well as size of columns

Partition form and location Loading onto fl oor structure and defl ection limits

Form of fl oor structure Depth of fl oor and primary beams

Floor to ceiling height Height and eff ective length of columns

LocationWind, snow load and extreme loading conditions

from seismic events and blasts

Ground conditions Foundation type and size

Service zoneEff ective length of columns and size of openings in

fl oors and walls

Service lifeReturn period for wind and snow load as well as

probability of seismic events

Size

The larger the footprint of the structure, the more likely it is to need the inclusion of movement joints due to thermal eff ects that cause expansion

and contraction

the building services containment load, which amounts to 50 kg/m2 (0.50 kN/m2). This error negates the additional load allowance for the ceiling. Some would argue that 0.25 kN/m2 is not a signifi cant increase from 0.50 kN/m2, but when you consider its impact on the overall load to a beam structure that is a reasonable assumption, as can be demonstrated thus:

A 10m long beam supports an 8m span fl oor structure. The total load on the beam from services (including the ceiling structure described previously) is 4 kN/m. The correct load, with the ceiling structure kept separate from the building services, equates to 6 kN/m. This error fi lters down the whole structure to the point where the columns and foundations are designed for a smaller load than they should be.

Although less sensitive to error, ground conditions are no less infl uential in the design of structures, as they dictate the form and extent of foundations that are to be installed. The greater the accuracy of data concerning ground conditions, the more appropriate the foundation solution becomes. This data should include soil composition, level of water table, condition of bearing strata, presence of underground services and signifi cant excavations such as mines.

Rationalising structuresWhen developing a strategy for carrying out the analysis and design of a structure, the primary drivers behind it should be to develop a safe, robust and economic design. The elements in the structure must be sized economically, yet be safe and easy to construct. This leads to the analysis becoming rationalised to avoid overly complex structures with a myriad of diff erently sized elements. While such structures may be optimised, there is a risk that the structure will become overly complex and as a result can add to the overall cost of the structure due to the diffi culty in building it.

To overcome this, elements within structures are rationalised into types. Within these types there are bands of elements. This reduces the workload of the structural engineer as they now only have to design a percentage of the elements and not each one individually. Figure 1 is a plan of a steel frame with beam types labelled for analysis.

Compartmentalising structuresWhen considering sections of the structure that are designed to resist lateral forces, such as shear core walls and bracing, it is

advisable to create a compartmentalised version of them. This creates an isolated model that will have all the lateral loads that the rest of the structure places upon it, as well as all the vertical loads from any beams and fl oor slabs that frame into it.

Sway frames, which are structures that have moment resisting connections, also need to be compartmentalised in this way in order for them to be accurately analysed. The bending moments at the connections

depend on the stiff ness of the elements that frame into them and thus the sway frame needs to be analysed to include these elements. A good example of this is the sub-frame, which is commonly used in concrete frame analysis and design and for rigid and semi-rigid steel frame analysis. Sub-frames are a form of models that represent the full structure. They consist of two lifts of columns and a series of beams. A maximum of fi ve approximately equal beam

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TheStructuralEngineer32

Technical Guidance Note

Technical

September 2012

Note 14 Level 1

›

Glossary and further reading

Compartmentalisation – Discreet sections of the structure that are modelled in sections while being mindful of the eff ects the overall structure will have on the section being considered.

Design criteria – The parameters within which a structure must be designed to adhere to.

Rationalisation – The grouping of elements within a structure during the analysis process into pre-defi ned types.

Sub-frame – A section of a structure that has fully fi xed rigid connections within it and consists of two heights of columns and a maximum of fi ve approximately similar spans of beams.

Further Reading Brohn, D.M.: (2005) Understanding Structural

Analysis 3rd Ed. New Paradigm Solutions

The Institution of Structural Engineers (2006) Manual for the Design of Concrete

Building Structures to Eurocode 2 London: The Institution of Structural Engineers

The Institution of Structural Engineers (2002) Guidelines for the Use of Computers

for Engineering Calculations London: The Institution of Structural Engineers

Eurocode 0.Web

resources

The Institution of Structural Engineers’ Structural Analysis and Design Education Study Group:www.istructe.org/about-us/

organisation-structure/study-groups/

structural-analysis-and-design-education

The Institution of Structural Engineers library: www.istructe.org/resources-centre/library

or other structures not included in the analysis.

Releases – Where connections are pinned i.e. not capable of transmitting a bending moment, releases are installed within the model. These releases create a connection that has no stiff ness in relation to bending moments in some or even all directions. This includes torsional moments.

Material properties – The properties of the material the structure is made from are defi ned and applied to each appropriate element, e.g. concrete, steel, composite, masonry, timber, and glass.

Section properties – The size of the elements are established along with their orientation and elevation in relation to the structure’s gridlines, which were defi ned at the geometry phase.

Loading – Accurately inputting the loading into the model is crucial. All of the loads are installed individually, located onto the model and then grouped. These groups typically fall into dead, imposed and wind headings and are factored for the design cases. Load combinations are then created and applied to generate the worst cases.

Design parameters – Where computer models are being used for design, the design parameters defi ne the boundaries within which the design can be carried out. These boundaries include the restraint of members to resist lateral torsional buckling, stress value limits, and defl ection limits. The capacity limits that the elements are to be assessed against, are also defi ned.

spans would normally be included within them in order to reduce the complexity of the analysis; a frame any larger than this would require a proportionally higher amount of analysis to be carried out on it. With concrete frames, columns are modelled to have fully fi xed supports, unless it can be demonstrated that the supports to the columns can be treated as pins or partially fi xed. Figure 2 is an example of a sub-frame for a concrete structure.

When analysing multi-storey frames, it is necessary for some allowance to be made for the transfer of bending moments and shear forces between vertical and horizontal elements of the structure. This is especially true for concrete framed structures as the joints between vertical and horizontal elements are continuous and therefore moment resisting. To address this, the structure can be split into manageable sub-frames that can be quickly analysed and designed.

Modelling of structuresWhen creating models of structures in a computer analysis and design application, it is important to note the logical fl ow of information that is required to feed into it. Typically they abide by the following logic:

Geometry – The geometry of the structure must be defi ned fi rst. This sets out the location of elements in relation to each other in the structure. Their form can be a notional centreline, a plate or a solid element.

Supports – The support conditions are established with pinned, fully fi xed, partially fi xed and springs, placed where the structure is either supported off foundations

"Accurately inputting the loading into the model is crucial"

N Figure 2Sub-frame of a concrete structure

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