Timber-Frame-Building-Eurocode 5 (EC5): Design of Timber Structures

Eurocode 5 (EC5): Design of Timber Structures

An Overview & Comparison with the BS 5268-2 Method
Effective from late 2006 Compulsory from 2009 / 2010
1 Introduction
The development of a structural building code is an im- mensely important matter because human safety is in- volved. This was so, even as early as the 18th century BC. Article 229 in the Code of Hammurabi, reads: “The builder has built a house for a man and his work is not strong: if the house he has built falls in and kills a house- holder, that builder shall be slain.” Interestingly, the death penalty did not apply if the cause of the collapse
Fig 1 Saville Building, Windsor Great Park - designed using Eurocode 5: External decking
was a hurricane or an earthquake, because such ex- traordinary events were not considered to be under the control of the designer or builder.
The Eurocode process started in the 1970s. The then European Economic Community issued a Construction Products Directive (CPD), which declared that the exis- tence of different national codes and standards consti- tuted “a barrier to trade”. It was decided to produce, by consensus, a new set of unified design codes and sup- porting material standards to cover all the major building materials – a hugely ambitious project, but now largely completed.
A design code like Eurocode 5 is a rule book for struc- tural engineers. It sets out agreed calculation methods for checking the strength, stiffness and stability of build- ings and other structures. EC5 is a design code for timber structures and currently co-exists with BS 5268- 2, which it will fully replace in 2009 /10. It has already been used to design a number of beautiful buildings in the UK, including the Sheffield Winter Garden (See Fig 3). This indoor park was opened in 2003 and has won four major awards. The glulam arches for the roof were designed to EC5. The roof of the Saville Building in Windsor Great Park (See Figs 1, 5 & 6) was also de- signed using EC5.
2 Scope of briefing
This briefing is aimed at the semi-technical reader who wants to better understand the changes that lie ahead for timber as we make the transition from the current system (largely to BS 5268-2 for timber) to the new sys- tem of Eurocodes. The briefing concentrates, therefore, on Eurocode 5 (BS EN 1995) and in particular Part 1-1: General – Common rules and rules for building. There are two further significant parts, which are not within the scope of this briefing:
• Part 1-2: General – Structures and fire design • Part 2: Bridges
EC5 Summary and Comparison with BS 5268-2
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At a technical level the subject is vast. TRADA recog- nises that thousands of design professionals and suppli- ers will need in-depth technical guidance to work to the new rules and make the transition. To this end, TRADA is publishing in conjunction with the Institution of Struc- tural Engineers (IStructE) a comprehensive technical guide to EC5 (Design of timber structures) in Autumn 2007.
This briefing covers the following topics: • Purpose of the new Eurocode system • Timescales for implementation • An overview of the Eurocode documents / standards • Limit state design philosophy
• Comparison of approach: EC5 vs BS 5268-2 • Advantages and disadvantages of the EC5 approach • Further help
3 Purpose of the Eurocodes
The Eurocodes are to become the new rule books for structural engineers. But what is wrong with the old / current codes for structural design? Are they technically inadequate for our current needs?
Although the new rules are technically very different in approach, the purpose for introducing them is not pri- marily because of technical concerns over our current codes. The reason is linked to the raison d’être for the European Union. In particular the goals for the Euro- codes are to:
• Remove obstacles to the free movement of goods and services within the European Union by providing com- mon rules for calculation and design;
• Serve as a basis for reference in contracts both within and outside the European Union; and
• Improve the competitiveness of the European con- struction industry by offering advanced concepts of calculation and design.
4 Timescales for implementation 4.1 General
It has formally been possible to design using EC5 since the end of 2006 and informally some considerable time before that. It has equally been possible to design to the Eurocodes in other materials. The formal requirement, set by the Department for Communities and Local Gov- ernment (DCLG), formerly ODPM, is for both the specific
Eurocode standard (eg EC5) and the National Annex to be published. These two criteria were reached for tim- ber at the end of 2006. The National Annex is an im- portant document, which provides the necessary data allowing each member state to calculate certain pa- rameters where particular geographical conditions (eg weather), serviceability requirements or levels of work- manship may be taken into account.
Fig 2 shows the sequence of events and timescales set by DCLG for introducing each Eurocode (ie relating to each material sector).
Fig 2 Illustration of general timetable from Date of Avail- ability of Eurocode to withdrawal of National Codes. Taken from ‘Implementation of structural Eurocodes in the UK’, by ODPM, Feb 2003.
4.2 The Building Regulations
The regulations in England & Wales (Part A) and Scot- land (Section 1) have all been amended to allow struc- tural design requirements to be calculated to the Euro- codes. In other words we are formally in the period of co-existence shown in Fig 2.
Section 1 of the Technical Handbook to the Scottish Regulations, which was amended in May 2007, clearly states the period of co-existence will end in Scotland by 2010 at the latest.
Given that EC5 and the National Annex had both been published by the end of 2006, and that there is a maxi- mum three-year transition period, we should see the withdrawal of the British Standards for timber design by the end of 2009.
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4.3 Making the transition
Although it is fairly obvious, it is probably worth making the point that it is very easy to make the transition from using only the old system to being able to use both. Those who wish to grapple with the new rule book on certain projects can, while those who do not wish to do so are under no compulsion to change. However, mov- ing from voluntary adoption of the new to compulsory use is a different matter altogether!
5 An overview of the Eurocode
documents
Four tables / charts are provided as follows:
Table 5.1 List of Eurocodes (those relevant to timber are shown in bold)
Notes: 1 EN 1990 sets out the design principles for all the structural building materials. It is usually referred to as “Eurocode 0”, even though its correct title is simply “Eurocode”. 2 EN 1991 is the loading code, equivalent to BS 6399. It includes snow and wind loads and a vast array of self- weights. These include dry chicken manure, raspber- ries and mercury at 133kN/m3 – so it should be possible to design structures for virtually every purpose!
6 Limit state design philosophy
A structure must be designed to sustain safely the loads and deformations which may occur during construction and in use, and should have adequate durability during the life of the structure. The design method aims at guaranteeing adequate safety against the structure be- ing rendered unfit for use. A structure, or part of a structure, is rendered unfit for use when it reaches a limit state, defined as a particular state in which it ceases to fulfil the function or to satisfy the condition for which it was designed. There are two categories of limit states:
Standard
Title
EN 19901
Eurocode - Basis of structural design (“Eurocode 0”)
EN 19912
Eurocode 1 - Actions (loads) on struc- tures
EN 1992
Eurocode 2 - Design of concrete structures
EN 1993
Eurocode 3 - Design of steel structures
EN 1994
Eurocode 4 - Design of composite steel & concrete structures
EN 1995
Eurocode 5 - Design of timber structures
EN 1996
Eurocode 6 - Design of masonry structures
EN 1997
Eurocode 7 - Geotechnic design
EN 1998
Eurocode 8 - Design of structures for earth- quake resistance
EN 1999
Eurocode 9 - Design of aluminium struc- tures
National Annex for the UK
Table 5.1
Table 5.2 Table 5.3
Fig 4
This is a list of all the top level documents. Those required in order to design in timber are shown in bold.
Contents of the UK National Annex to EC5.
This provides a list of the sections within EC5 with some explanatory commentary.
This shows the hierarchy of documents relevant to timber design.
Fig 2 Sheffield Winter Gardens, courtesy of Buro Hap- pold, Structural Engineers: Internal view
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• An ultimate limit state is reached when the structure (or part of it) collapses. Collapse may arise from the rupture of one or more critical sections, from the trans- formation of the structure into a mechanism, from elas- tic or inelastic instability, or from loss of equilibrium as a rigid body, and so on.
• The serviceability limit states are those of excessive deflection, cracking, vibration and so on.
Normally, three limit states only are considered in de- sign: the ultimate limit state and the serviceability limit states of excessive deflection and cracking under ser- vice loads. The structure is usually designed for the ultimate limit state and checked for the serviceability limit states. Structural collapses often have serious conse- quences; therefore in design the probability of reaching the ultimate limit state is made very low, say, 10-6. Since the loss resulting from unserviceability is generally much less than that from collapse, a probability much higher than 10-6, of reaching a serviceability limit state may still be acceptable. In limit state design, the engineer’s aim is that the probability of each limit state being reached is about the same for all the members in a structure and is appropriate to that limit state.
Table 5.2 Contents of the UK National Annex to EC5
In contrast to timber, other material types (eg concrete and steel) have been designing using the ‘limit state design’ approach for many years. For concrete and steel the Eurocodes represent a significant change in detail but not a change in philosophy.
Contents of National Annex
• Assignment of loads to load duration classes • Assignment of timber constructions to service
classes • Partial factors for material properties • Limiting values for deflections • Limiting values for vibrations • Design method for domestic floor vibrations • Advice on nailed timber-to-timber connections • Choice of method for design of wall diaphragms
• Modification factors for bracing of beam and truss systems
Eurocode 0 Basis of design
Eurocode 1 Actions on structures
Part 1-1 Densities, self-weight & imposed loads
Part 1-3 Snow loads
Part 1-4 Wind loads
Etc
Eurocode 5 Design of timber structures
Part 1-1 Common rules & rules for buildings
Part 1-2 Structural fire design
Part 2 Bridges
Fig 4 Hierarchy of Eurocode documents relevant to timber design.
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Table 5.3 List of the sections within EC5 (EN 1995) Part 1-1
EN 1995-1-1 is the largest part of EC5 and is itself divided into the 10 sections described below. EC5 assumes a knowledge of EC0 which sets out the general design principles which apply to all structural building materials.
Fig 5 Saville Building, Windsor Great Park: External roofline
Section of Part 1-1
Commentary on the contents
1 General
Consists mainly of definitions.
2 Basis of design
Supplements Eurocode 0 with timber-specific matters – service classes, load duration classes, creep and slip, plus partial safety factors for different timber based materials.
3 Material properties
This section does not include properties as such – these have to be sought in various materi- als standards which are published separately – but gives tables of values to allow for the ef- fects of service class, load duration and creep.
4 Durability
This is a one-page chapter on durability which includes a very useful table of specifications for the corrosion protection of metal fasteners.
5 Basis of structural analysis
This section is also short, covering mainly the analysis of assemblies including frames and arches.
6 Ultimate limit states
An important section covering the design of beams and columns, and tapered, pitched and curved beams.
7 Serviceabilitylimit states
An explanation on how to calculate the initial and final slip in mechanically fastened joints, and how to check the vibration of domestic floors – the satisfactory vibration performance of floors is a requirement for floor design in Eurocode 5.
8 Connections with metal fasteners
The longest chapter in the code - a reminder that the design of connections is one of the most important, and most difficult, aspects of timber design. Much of this chapter has already been incorporated into BS 5268, but in Eurocode 5 only the formulae are provided: there are no pre-calculated tables of basic loads which a designer can look up for a quick first design.
9 Components and assemblies
Very useful guidance for the design of items such as I-joists, stressed skin panels and bracing. It also contains the biggest problem area for Eurocode 5 - two methods for designing wall pan- els to resist racking loads, neither of which is very helpful for conventional UK timber framing. Fortunately an international group, including UK experts, has almost completed a third Euro- code 5-compatible approach which will meet our national needs and which we hope will be included or referenced in the EC5 National Annex as an acceptable alternative.
10 Structural detail- ing and control
Like its counterpart in BS 5268, this is fairly short, and includes quite a bit on connections.
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7 Comparison of approach: EC5 vs BS 5268-2
There are many differences in approach between the two codes. In fact the only area where there is not a great deal of difference is the resulting designs! The main attributes of the codes are compared in the table below.
Attribute (including definitions)
EC5
BS 5268
1 General philosophy
‘Limit state design’.
Largely formulaic with very few tables of results.
Broadly speaking the design process is the same irrespective of the material type used.
‘Permissible stress design’.
Mainly calculated using tables with few formulae.
The approach differs depending on the material type used for the struc- ture.
2 Presentation of the strength properties of materials The inherent strength perform- ance of individual species / mate- rials when tested under load.
The properties are presented as ‘characteristic test values’. These are sim- ply test values, giving the low 5th percentile value, with no in-built safety factors – these must be added by the designer.
The properties are presented as ‘grade stresses’. These have built-in long-term duration factors required for service classes 1 and 2 (see item 7 below).
3 Load durations Factors applied to take account of how well a material performs un- der load over longer periods of time.
The characteristic values referred to above are based on 5-minute duration tests. Because timber’s resistance to load is af- fected by duration, a reduction factor must be applied by the EC5 designer. As an example, in the case of long-term or per- manent loads, a reduction factor of 0.6 for solid timber is required for service classes 1 and 2 (see item 7 below).
The long-term effect of load is already taken into account in the tabulated values.
4 Stiffness properties (elastic moduli) The measured extent to which a material will bend under a load.
Defines a mean (Emean) and a characteris- tic value (Ex).
Uses the Ex for column stability (strength) and Emean value for member deflection cal- culations.
Defines a mean (Emean) and minimum value (Emin). Emin is equivalent to Ex in EC5.
Uses Emin for column stability and sin- gle member deflection calculations and Emean for deflection calculations in load sharing systems.
5 Inclusion of safety factors Factors applied to allow for mar- gins of error elsewhere.
Specified factors are applied both to loads and material properties, increasing the loads and decreasing the characteristic materials properties.
Timber-based materials which are manu- factured to produce less variability eg LVL, are given a lower (or better) safety factor.
All the safety factors are built into the tabulated grade stresses. One conse- quence of this is that it is difficult to establish what the factors are.
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Attribute (including definitions)
EC5
BS5268
6 Load duration classes A factor which takes account of the period of time that loads are likely to be imposed. Structures taking a load for long periods need to be designed to higher levels of performance.
EC5 distinguishes between ‘permanent loads’, nominally having a duration of 50 years (eg self-weight) and long-term loads, corresponding to a duration up to 10 years (eg storage).
EC5 combines the result of the load dura- tion class and service class (see below) in a single combined strength factor.
Load duration is calculated as a sepa- rate factor.
7 Service classes This refers to the environmental conditions, which affect the mois- ture content, which in turn affects the strength and stiffness of tim- ber. Class 1 = heated indoors, Class 2 = unheated indoors / covered outdoors, Class 3 = ex- posed outdoors.
EC5 combines the result of the load dura- tion class (see above) and service class in a single combined strength factor.
BS5268 adopted the EC5 service classes in a previous revision.
Service class is calculated as a sepa- rate factor.
8 Creep Additional deflection (bending) to that experienced initially and which occurs after a period of time (eg a bookshelf with heavy books after one year).
Creep must be calculated separately and added to the figure for initial deflection to provide a value for total deflection.
Generally it is considered unnecessary to calculate for creep.
Annex K does provide a method of calculating creep deflection in beams, for information only.
9 Limit states Limits placed on what is consid- ered acceptable in terms of safety (eg risk of collapse) and comfort / appearance (eg risk of plasterboard cracking).
Ultimate Limit States (ULS): Addresses strength and stability issues critical to the safety of the building. Be- yond these limits the building may fail.
Serviceability Limit States (SLS): Addresses such issues as deflection, joint slip and vibration, critical to the comfort / appearance / functioning of the building.
BS 5268 addresses ULS with regard to strength in a similar manner to EC5 (with the exception that safety factors have already been added).
With regard to seviceability, BS 5268 only addresses initial deflections (without creep). In a simplistic man- ner, it also sets an upper limit of 14mm for static deflection to avoid too much vibration in floors.
10 Deflection limits The design limit set on the ac- ceptable deflection distances for any structural member.
These are advisory in EC5 on the basis that deflection is not normally a safety is- sue.
EC5 provides suggested ranges for deflec- tion limits.
The UK National Annex provides some recommendations. TRADA and other bod- ies may provide others. The client and designer must agree on what is appropri- ate.
Deflection limits are mandatory for floors.
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Attribute (including definitions)
EC5
BS 5268
10 Combinational values of variable loads When two or more variable loads (eg snow and wind) act on a member, it is unlikely that both will be at their full characteristic value simultaneously.
EC5 allows for a non-conservative ap- proach to calculate the optimal value of the design load by: • taking each factor and assuming it is at
its full characteristic value; and then • taking all other factor(s) and applying a
given reduced factor for each.
As the number of simultaneous variable loads increases, so the number of permu- tations multiplies geometrically. In the case of trussed rafter design there can be well over 100 combinations and load cases to consider – making it necessary to use spread sheets to keep time to a minimum.
Typically all the loading factors are added up to give a conservative result.
11 Irreversible vs reversible limit states
EC5 makes the following distinctions: An irreversible limit state involves irre- versible damage when it is exceeded – eg excessive deflection on a plasterboard ceiling causing it to crack. A reversible limit state involves no per- manent change – when the load is re- moved components return to their original state – eg snow on a rafter.
Since the consequences of breaching a reversible state limit are less serious, it is ‘safe’ to reduce the accompanying variable loads. Psi factor values for this are given in the UK National Annex to EC0.
No such distinction is made.
BS 5268 provides little guidance on this.
A static deflection limit of 0.003 times the span is given as guidance to pre- vent irreversible limit state (which is the worst of the two cases).
13 Vibration response of floors to impact loading In other words, does the floor vibrate when you walk over it?
EC5 provides formulae but they are com- plicated to use.
BS 5268 does not cover vibration cal- culations. Instead it sets a maximum deflection limit of 14mm to address the problem.
14 Beam and column formulae
The EC5 method for dealing with beams allows for an effective reduction in the bending strength to allow for lateral- torsional instability.
The method for columns allows for bend- ing moments in both directions as well as the axial forces.
BS 5268 does not allow for lateral- torsional instability.
BS 5268 only gives formulae for com- bining axial load with bending in one direction.
15 Tables for fastener loads
No tables are provided in EC5 – the ap- proach is entirely based on formulae. For timber to timber constructions the for- mulae are the same as those given in An- nex G of BS 5268. Special formulae are also given for steel to timber connections.
Tables of values are provided for a range of fasteners. Some formulae are provided for timber to timber con- nections in Annex G. Only a “rough and ready” method is provided for steel to timber connections.
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8 Advantages and disadvantages of the EC5 approach
The advantages
The same design basis is used for all materials in- cluding timber. Once designers become familiar with Eurocodes it should become much easier to switch between design- ing in timber and other materials. At present, a relatively small percentage of engineers are experienced in de- signing in timber and there is an in-built inertia to de- velop one’s knowledge in this area.
Note: In contrast to timber, other materials have been using a form of limit state design for many years.
The safety factors are transparent. Because the safety factors are all kept separate, it is easy to modify them when there is reason to do so. Sometimes this will yield more efficient material solu- tions.
The levels of reliability are more logical and consis- tent.
Designs can sometimes be more economical. This is because all the required factors are built up sepa- rately, rather than sometimes including them unneces- sarily in a conservative approach.
Some aspects are handled in a better way. For example the approaches to beams and columns, creep and steel to timber connections.
The formulaic approach is ideal for spreadsheets and software. Once you have developed the necessary formulae within a spreadsheet, it is much easier to iterate towards opti- mum solutions.
EC5 Part 2 is the first design code for timber bridges in the UK. This could provide the impetus for choosing timber for smaller access bridges as well as for footbridges in parks and leisure centres.
Fig 6 Saville Building, Windsor Great Park: Underside of roof in gift shop
The disadvantages
It is more complicated to use than BS 5268. This is because all issues are addressed separately. No issues / factors are built into tabulated values. From a commercial point of view it is impossible to design without the efficient use of a computer.
More additional documents are required. For example, documents covering strength properties of timber, plywood and glulam etc.
No tables are given for permissible fastener loads. The tables in BS 5268 can make calculations very quick in comparison.
Some areas lack practical guidance. Areas that need more practical guidance include:
• lateral restraint for beams • trussed rafter bracing • masonry wind shielding • methods of calculating spans for domestic members
Some subjects are missing. For example there is nothing on the design of glued joints or glued in rods.
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9 Further help
TRADA Wood Information Sheet: Eurocode 5 – An introduction Further introductory guidance available at: www.trada.co.uk
IStructE / TRADA EC5 Design Manual Comprehensive practical guidance on designing to EC5 with plenty of tables. Available from TRADA bookshop from Autumn 2007.
BS PD 6693 – A ‘rump standard’ UK timber professionals have agreed to produce a publi- cation to include all the practical information in the vari- ous parts of BS 5268 which would otherwise be lost.
Software to assist in designing to EC5 Various companies will be producing software design tools. A working trial version of TRADA’s ‘Actions pre- processor’ is available at: http://research.ttlchiltern.co.uk/pii304 (click on ‘outputs’)
Timber Design Knowledge Free downloadable general guidance on designing in timber including case studies, is available at: http://research.ttlchiltern.co.uk/pif294
Fig 7 Sheffield Winter Gardens, courtesy of Buro Hap- pold, Structural Engineers: Internal view
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Further help
TRADA members may contact the Members’ Helpline for free on t: 01494 569601.
Other useful websites
Eurocode website: www.eurocodes.co.uk
Institute of Structural Engineers (IStructE): www.istructe.org
TRADA Construction Briefings
This document is part of a series of briefings for TRADA members on the key elements of building regulations and codes and how they relate to timber construction. Copies of all briefings are available at www.trada.co.uk.
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