KNOWLEDGE UPGRADE ON EUROCODE AND THE EFFECTS OF USING OLD BRITISH STANDARD AS DESIGN GUIDANCE IN NIGERIA

Being a technical paper on the need for knowledge enhancement for Nigerian engineers

 

 

Publication:            National Development Initiative

Release date:          25th June 2012

 

By

Olugbenro Falola

MSc.(Structural Engineering, UK), B.Eng.(Ogbomoso)

Correspondence e-mail: olugbenro@structuralsolution.co.uk

Editing/Proof: Olatunji Ariyomo & Olugbenro Falola

Published by NDi

www.nd-i.org

publications@nd-i.org

25TH JUNE 2012


 

1.0      INTRODUCTION

It is over two years now that the use of British Standard has been replaced with the 58 parts of Eurocodes (March 31, 2010 – BSI, 2009). The British standard committee has made it clear since that time that all public works should be designed in accordance with the new Eurocode (BSI, 2009). They also made it known to the public that there would be no amendment or revision of the British standards (BSI, 2009) which suggests that the design code would be obsolete in the nearest future. The advantages of the Eurocode cannot be over emphasized. Numerous literatures and experience have shown that the Eurocode provides economical design than British Standard (Mostley et al., 2007, Narayanan 2010, Concrete centre, 2010). Aside this, the code is less restrictive and allows choice of safety parameters to be used by each country knowing that design and construction practices, standard and quality of materials, climatic conditions, human behaviour to structures among other factors differ from country to country.

Recent short survey has shown more than 85% of Nigerian engineers, lecturers, students and graduates with no exception of companies are not aware of this replacement and dangers associated with it (Falola, 2011). The aim of this article is to alert users of the British Standard to the effects of using the code, identify areas that need public attention in the old British Standard, enumerate the advantages of adopting the new Eurocode and suggest how we can develop the Nigeria’s indigenous code.

2.0      BENEFITS OF ADOPTING THE NEW EUROCODE

2.1 General Benefits

  • Provide a common basis for research and development.
  • Allow the preparation of common design aids and software.
  • Eurocode 2 should result in more economic structures for clients.
  • Eurocode 2 is less restrictive than British Standards.
  • Eurocode 2 is extensive and comprehensive.
  • The new Eurocodes are reputed as the most technically advanced codes in the world (Concrete Centre, 2010). 

2.2 Benefits to Companies:

  • Eligibility to design and work freely in all European countries
    Lagos: Partial Collapse Of 21-Storey Skyscraper

    Lagos: Partial Collapse Of 21-Storey Skyscraper

  • Enhance career opportunity for the company and staff.
  • Improve design competency.
  • Abreast with latest design standard.
  • Enable companies to bid for international projects especially in all EU.
  • Improve companies’ standard and quality.
  • Economical design – reduced construction cost. (Concrete Centre, 2010)

2.3       Benefits to Students & Graduates:

  • Eligibility to design and work freely in all European countries.
  • Enhance international career opportunity.
  • Improve design competency.
  • Allow academic research in higher institutions to meet current global challenges.

3.0      WHY IS IT NECESSARY TO ADOPT THE NEW EUROCODE?

It is obvious that many Nigerians will feel reluctant to adopt the new eurocode. Answers to the following questions will help to form the right judgment:

  1. Do we know the consequences of using the old British standards on the standard and quality of our academic research, design and quality of our projects?
    Do we know the flaws in BS8110?
    Do we think that our academic research in the nation’s universities will still be acceptable in the global knowledge using the old code as references?
  2. What would be the fate of our students and graduates even if engineers are reluctant to adopt new knowledge knowing that the British Standard is now obsolete?
  3. Are we working towards the development of our own code, If yes, How?
  4. Are we developing our own code from the previous code without resolving the flaws in the code?
  5. Are we developing our own codes based on non-empirical factors of the ideas of some practising engineers knowing that a code should be developed along with experimental research?
  6. Are we involving academic scholars or researchers to channel their research towards the development of our own code?
  7. Do we even have funds available for this?
    Are we developing just an aspect of the code such as design of concrete buildings?
  8. What of other areas such as highway, geotechnical, steel design, etc?
    Do we even welcome contributions from scholars towards the development of our own code knowing that nobody is an embodiment of knowledge?
  9. However the adoption of eurocode will not only answer the aforementioned questions but also provide the benefits as stated earlier.

I am not of the opinion that we should suddenly drop the old British standard but it is high time we got acquainted with new ideas. There are several systematic ways we can adopt the new eurocode rather than continue using the obsolete British Standards.

We should also have in mind that there are sections that need further clarification in the British Standards especially the BS8110. Further articles and research papers by the same author will reveal some of these in due course. In the mean time, it is of great necessity to point the art of column classification in the BS8110 to the public. Several arguments have been raised to know the specific parameters that influence column classification. The enclosed short term paper at the back of this article will further shed light upon this.

3.1      NATIONAL ANNEX AS A WAY TO DEVELOP OUR OWN CODE

Having known that the eurocode is flexible and allow each country to choose their safety parameters through what is called Country National Annex to Eurocode, the National Annex is concise and easy to develop than a whole code. It gives us the opportunity to select our own safety parameters such  as load factors, material factors, etc which are mainly dependent on our construction practices, materials standard and quality, human behaviour to structures, etc in the country – although this still requires some practical works and experimental findings. It is obvious that this huge advantage is not available in the old British Standards wherein the safety factors were mainly developed based on the design and construction practices in the United Kingdom.

Do we even know that this system was also adopted by the owner of old British Standard? The United Kingdom now uses “UK National Annex to Eurocode”. Similar procedure is in place in South Africa where they already have their own design code (SABS). The country still intends to adopt the Eurocode alongside the SABS knowing the benefits and the importance of global knowledge. This means that developing an effective “Stand Alone” design code in Nigeria might take years.

The “Nigeria National Annex to Eurocode” will not only provide us the integration to global knowledge and design practices but can also be used as a foundation for the development of our own code after extensive design practices and qualitative researches.

4.0      SAMPLE PROBLEM AND COMPUTATION

4.1 COLUMN CLASSIFICATION IN ACCORDANCE WITH BS8110 & EUROCODE 2 COMPARED

Columns are structural elements which are primarily used to provide either temporary or permanent support to compressive loads, and sometimes used to resist bending. It may be referred to as “spinal cord” of structures due to the fact that their failure may trigger the collapse of the whole structure if not properly designed. However, this poor design may be due to inappropriate guidance procedures given in the adopted design code rather incompetency of the designer alone.

The design of columns perhaps in any design code in the world is primarily based on column classification (either as short or slender). Inappropriate classification may lead to failure of the column especially when local buckling and second moment is expected to be considered in the design which will affect the anticipated design load capacity of the column. This error may be due to the adopted design code as aforementioned. It is obvious that the main essence of column classification is to know the failure types of the column. Therefore it is necessary to understand those parameters that govern such failure which in turn affects column classification.

 4.2 Research Objectives

  • To investigate column classification in accordance with BS8110 in comparison with Eurocode.

4.3 Research Significance

The results of this study will validate further applicability of the present column classification approach described in BS8110. It will also give general notice to the insisted present users of BS8110 to make adequate revision of the code where necessary or get acquainted with the latest Eurocode.

4.4 Research methods

In order to achieve the objective of this research, a critical analysis of column classification in accordance with BS8110 and Eurocode 2 were thoroughly examined. A questionnaire was also developed to examine the art of column classification in accordance with BS8110.

4.5 Terms definitions:

Short column– one in which the ultimate load capacity is dictated by the material strength and cross-section. This type of column fails by crushing of the material.

Slender column– one in which the ultimate load is influenced by material strength, cross-section as well as slenderness. This type of column can fail either by material crushing or instability (buckling) of the column.

(a)               BS 8110 approach (BSI, 1997):

According to the above design code, a column is regarded as short when both the ratios lex/h and ley/b are less than 15 and 10 for braced and unbraced respectively, otherwise is slender. Where lex and ley are the effective length in major and minor axes respectively; ‘h’ and ‘ b’ are the larger and smaller dimension of the column section respectively (BS8110, 1997). The effective length is given as:

le = β l                                                                                 (4.1)

Where lo is the clear length between end restraints and not exceeding 60 times the minimum thickness of a column according to section 3.8.1.7 of BS8110, 1997. For a cantilever column, this should not exceed:

                                                                                 (4.2)

 

The value of β depends on end conditions and lateral restraint against sideway (braced or unbraced) as shown below.

Table 2.1 Values of β for (a) Braced column and (b) Unbraced column.

  End condition at top End condition at bottom
1 2 3
1 0.75 0.80 0.90
2 0.80 0.85 0.95
3 0.90 0.95 1.00

(a)

 

 

  End condition at top End condition at bottom
1 2 3
1 1.2 1.3 1.6
2 1.3 1.5 1.8
3 1.6 1.8
4 2.2

 

 

(b)

 

 

Source: Table 3.19 and 3.20 of BS8110, Part 1(1997).  For conditions 1 to 4 see section 3.8.1.6

(b)               Eurocode approach (EN, 1992; Mosley, et al., 2007)

 

In contrast to BS8110 slenderness approach for column classification, EC2 takes into account the contribution of design ultimate axial load on the column, concrete strength, longitudinal reinforcement as well as creep effect along with aforementioned parameters when classifying a column.  In this approach, columns are classified as either short or slender by placing a limit on slenderness ratio. Columns having slenderness ratio λ not exceeding slenderness ratio limit lim) are regarded as short, otherwise as slender and second order effects must be taken into account in design (EN, 1992; Mosley, et al., 2007). The slenderness ratio of a column bent about an axis is given as:

λ =                                                               (4.3)

Where lo is the effective length, ‘i’ is the radius of gyration (uncracked section) and ‘I’ is the second moment of area of section about the axis in consideration, and ‘A’ is the cross-sectional area of the column.

For braced columns;

lo =                                                (4.4)

For unbraced columns, the larger of:

lo =                          or                                                              (4.5a)

lo =                                                                                    (4.5b)

where k1 and k2 are the relative flexibilities of the rotational restraints at ends ‘1’ and ‘2’ of the column respectively. See EC2 (EN 1992) and Mosley, et al. (2007) for further reading.

Relative flexibilities k [EC 2: 5.8.3.2 (3) – (5)]

( =   If top & bottom columns does not contribute to column rotational restraint. But if top & bottom columns contribute to column restraint, k is given as:

+ (. Refer to EC 2: 5.8.3.2 (3) for their definitions.

As previously mentioned, EC 2 places a limit on slenderness ratio and this is given as:

λlim =                                                                              (4.6)

A =  and A can be taken as 0.7 if  is not known.                (4.6.1)

B =   and B can be taken as 1.1 if ω is not known.         (4.6.2)

C = 1.7 – rm and C can be taken as 0.7 if rm is unknown.                              (4.6.3)

n =                                                                                                   (4.6.4)

ω =                                                                              (4.6.5)

Where:

   = effective creep ratio

= design yield strength of the concrete

= design compressive strength of the concrete

= total longitudinal reinforcement area

NEd = design ultimate axial load in the column and rm is the ratio of first order moments at the end of the column. Refer to EN (1992) and Mosley, et al. (2007) for proper definitions.

Therefore, for a short column, λ < λlim otherwise as slender; whereas the code does not states any specific guidance if λ lim.

4.6 Comparison between the two design codes

As aforementioned that short column fails by material crushing and this failure depends on the column geometry, properties (concrete strength), creep, reinforcement area and strength, design ultimate axial load on the column and so on. It is not understood how these factors were accounted for when classifying column as short or slender in BS8110 apart from the effective height of the column, column size, and degree of fixity at ends as well as limiting values given for slenderness ratio.

It is clearly shown from equation 4.6 on how these factors were considered in EC 2 by placing an upper limit on the slenderness ratio.

4.7 Summary of on-going results from questionnaire:

About 44.44% were civil/structural engineers, 11.11% were professors/lecturers, 11.11 % were graduates and 33.33% were students.  Approximately 80% of them have used BS8110 for column design. About 90% of the respondents agreed that short column fails by material crushing and slender column fails by either material crushing or instability (buckling). The same number agreed that the term failure will be dictated by column geometry, concrete strength, reinforcement grade and design applied load on the column. However about 77% of them did not know how these aforementioned parameters were considered in the classification of column with BS8110.

Based on the findings from several literatures, thorough comparison of the two design codes and the results from current questionnaire, this principal conclusion was drawn.

  • Column classification should also be based on design axial load, concrete strength, reinforcement grade and area, aside from slenderness ratio (which is calculated from column geometry, fixity, end conditions and side restraints). These parameters were clearly considered in Eurocode 2 & ACI 318 (American code). However it is not yet understood how BS8110 incorporated these parameters.

Conclusion

Nigerian engineers have to start from somewhere. In this age of convergence, they must adopt a technology philosophy that promotes catching up. This can be accomplished if they understand the details of research and development carried out by others before them and adapt this to suite local needs especially in the area of standard.

References & Bibliography:

a)      British Standards Institution, 1997. BS8110: 1997 Code of practice for design and construction. London: BSI.

b)      British Standards Institution, 1990. BS EN 1990: Basis of structural design. London: BSI.

c)      British Standards Institution, 1991. BS EN 1991: Actions on structures-Densities, selfweight, imposed loads for buildings. London: BSI.

d)      British Standards Institution, 1992. BS EN 1992: Design of concrete structures-General rules and rules for building. London: BSI.

e)      British Standards Institution Shop, 2009. Available at : http://shop.bsigroup.com/en/Browse-By-Subject/Eurocodes/

f)       Concrete Centre, 2010.  Available at: http://www.concretecentre.com/codes__standards/eurocodes/eurocode_2/background_to_ec2/benefits_of_eurocode_2.aspx

g)      Falola, O. O., 2011. Investigation of Column Classification Parameters Survey. kwiksurveys.com

h)      Falola, O. O., 2010. Comparison between Reinforced Solid Column and the Equivalent Hollow Column. London: University of East London.

i)        Mosley, B.  Bungey, J. & Hulse, R., 2007. Reinforced concrete design. 6th ed. China: BookPower.

j)        Narayanan, R. S, et al. eds., 2010. How to Design Concrete Structures using Eurocode 2. London: Concrete Centre.

 

The author Olugbenro Falola is a PhD student at the Eastern Mediterranean University, Cyprus
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