Eurocode 6: Design of Masonry Structures

 

Answers to FAQs

1. The current version of BS EN 1996-1-1:2005 + A1:2012 was published by BSI in 2013 and incorporates corrigenda issued in February 2006 and July 2009.

2. The current version of the National Annex to BS EN 1996-1-1:2005+ A1:2012 was published by BSI in 2013.

3. The agreement between CEN (The European Standards Organisation on which BSI represents the UK) and the member states required the conflicting national standards to be withdrawn in 2010.

Withdrawn standards will not be maintained by BSI but may still be used provided that clients are happy to accept them.

4. The selection of appropriate values for Nationally Determined Parameters in Eurocode 6 was meant to ensure that the overall economy in the use of masonry should remain the same as design to BS 5628. Experience since its introduction suggests that adjustments need to be made for units of higher aspect ratios such as the concrete blocks typically used in the inner leaves of cavity walls. A revision of the UK National Annex to BS EN 1996-1-1 is currently being considered by the Code committee.

5. Thin layer mortars are fully covered by Eurocode 6 and the supporting BS EN standards. The equation used in Eurocode 6 to evaluate the Characteristic Compressive Strength of Masonry is modified when thin layer mortar is used and the supporting BS EN 771 group of standards provides requirements for the dimensional accuracy required of the units.

6. Residual standards are a particular form of NCCI (Non Contradictory Complimentary Information) intended to provide information not otherwise contained in the Eurocodes. In the case of masonry this is particularly necessary because custom and practice varies so much across Europe. Much of the extensive information contained in BS 5628 Part 3, for example, would have been lost if it had not been retained by the publication of PD 6697 in 2010.

7. Special Category of manufacturing control was replaced by the publication of the BS EN masonry standards and the revisions to BS 5628 which were published towards the end of 2005. Masonry units are now described either as Category I or Category II.

Category I masonry units are units with a declared compressive strength with a probability of failure to reach it not exceeding 5%. This may be determined via the mean or characteristic value.

Category II are masonry units not intended to comply with the level of confidence of Category 1 units.

8. Fire resistance is covered in BS EN 1996-1-2 and the corresponding National Annex. Information is provided on the passive fire resistance of masonry walls. Tabulated data covers loadbearing and non-loadbearing walls including cavity and separating walls. For loadbearing walls a value needs to be determined for α, the proportion of load on the wall (i.e. the ratio between the load on the wall and the load capacity of the wall). A change point occurs in the tabulations of fire resistance at values of α less than or equal to 0,6, which for certain constructions, results in a reduction in the wall thickness for a given period of fire resistance when compared to a more heavily loaded wall.

9. The lateral load clauses in Eurocode 6 are based on those contained in BS 5628 and will therefore be familiar to UK engineers. The main difference is that the limiting dimensions for serviceability are now presented in graphical form. For some panel configurations there are some significant changes in these limiting dimensions.

10. The Eurocodes remain subservient to Building Regulations and in England and Wales Approved Document A will still be available for the design of masonry buildings. BS EN 1996-3 contains simplified calculation methods for unreinforced masonry based on the principles contained in BS EN 1996-1-1 and should not be confused with simple rules developed on the basis of experience. In general the rules in Part 3 are conservative and unlikely to result in the economy of solution to be found in Approved Document A.

11. There are no specific recommendations in Eurocode 6 for assessing existing buildings or structures. The situation remains that sensible engineering judgment needs to be applied when making such assessments. Remember that both the methods of test for masonry units and the test for determining the characteristic compressive strength of masonry changed with the introduction of the BS EN standards.

12. The structural design of low-rise buildings is covered by BS 8103 which has been updated to incorporate the BS EN material and test standards. This standard will continue to be used and is not affected by the introduction of Eurocode 6.

13. In Eurocode 6 the primary method of specifying mortar is by the strength grade. An M12 mortar should have a compressive strength at 28 days of 12 N/mm2. This strength is used in the relationship to determine the characteristic compressive strength of the masonry. The mortar manufacturer will provide information on the strength that a particular mortar will achieve and its expected durability when subjected to given exposure conditions.

Historically in the UK mortar is often specified as proportions by volume (i.e. prescribed proportions rather than designed to achieve a specified strength). The UK National Annexe indicates the following approximate equivalence:

Compressive strength class and equivalent Mortar designation

 M12 .................(i)

M6 ...................(ii)

M4................... (iii)

M2................... (iv)

 

14. Shell bedded masonry is constructed so that the masonry units are bedded on two or more strips of mortar. Normally there are two strips which are at the outside edge of the bed face of the units. When shell bedding is used the procedure for determining the characteristic compressive strength of the masonry is modified.

15. The normalised compressive strength of masonry units is the compressive strength of the masonry units converted to the air dried compressive strength of an equivalent 100mm wide by 100mm high masonry unit. It is the normalised strength that is used to determine the characteristic compressive strength of the masonry. More detailed information may be found in BS EN 771-1,2,3,4,5&6 and BS EN 772-1.

16. Masonry units are designated Group 1,2,3 or 4 according to the the percentage, size and orientation of holes in the units when laid. This enables the properties of the unit to be characterised for structural design purposes.(For details see Table 3.1 of BS EN 1996-1-1)

17. There are five exposure classes recognised in Eurocode 6 which relate to the micro conditions of exposure of the completed masonry. The five conditions may be summarised as follows:

MX1 In a dry environment

MX2 Exposed to moisture or wetting

MX3 Exposed to wetting plus freeze/thaw cycling

MX4 Exposed to saturated salt air, seawater or de-icing salts

MX5 In an aggressive chemical environment.

More detailed information and examples are provided in Annex A of BS EN 1996-2. Annex B provides acceptable specifications of masonry units and mortar for durability but this is not applicable to the UK. Full details of the durability requirements for masonry and mortar in the UK are contained in Table 15 of PD 6697.

18. The partial factors for all materials, including ancillary components such as ties and straps, may be found in Table NA.1 of the National Annex to BS EN 1996-1-1.

19. To look for help on other Eurocodes visit Eurocode Expert http://www.eurocodes.co.uk/

20. A Normative Annex forms part of the Eurocode and therefore has to be followed. An example is Annex B of BS EN 1996-1-2. It is a matter of National Choice as to which of the Informative Annexes may be used in a particular country and the country's National Annex states which Informative Annexes may be used.

21. The designations P, M & S are used to specify the durability of mortar. Masonry mortar is specified for durability using the terms defined in BS EN 998-2. The designations provide a useful abbreviation as follows:

P   - mortar for use in masonry subjected to passive exposure

M  - mortar for use in masonry subjected to moderate exposure

S  - mortar for use in masonry subjected to severe exposure

Table B.2 in Annex B of BS EN 1996-2 which relates these designations to the class of exposure is not applicable to the UK where PD 6697 should be used.

22. Although recommendations for the corrosion protection required for different ancillary components, when subjected to a particular class of exposure, are provided in Annex C of BS EN 1996-2 this is not to be used in the UK where PD 6697 should be used .

23. The design of collar jointed walls is covered in Design Note 1. Link

24. The design of deep beams is covered in Design Note 2. Link

25. Information on the design of laterally loaded panels is given in Design Note 3. Link

26. Information on wall tie capacity is given in Design Note 4. Link

27. Shape factors for normalised strength are given in Design Note 5. Link

28. Tables showing normalised strengths for UK masonry units are provided in Design Note 6. Link

 

Questions relating to the pre-corrigenda version of BS EN 1996-1-1

A.1. Figure G1 lacks some of the annotation. The correct version is shown below.

fig2

 

A2.

fig1

A3. This is an error. Equation 8.1 of BS EN 1996-1-1 provides the straight anchorage length required for a reinforcing bar. The design strength and design anchorage strength of the reinforcing steel are used and the partial factor for material is not required.

A4.Equation C.1 in Annex C of 1996-1-1 is incorrect. In the formula h3 and h4 should be changed to l3 and l4respectively.

A5.The title of Annex E of 1996-1-1 is incorrect. It should refer to α 2 and not α 1. The values in the tables are values for α 2.