Guidance on Eurocode 6
The purpose of this series of guides is to introduce designers to the basic approach adopted in Eurocode 6. This is the first guide in the series of three and provides:
■ A brief outline of the scope of Eurocode 6.
■ An introduction to design, including fire resistance and movement. ■ Assessment of actions and combination of actions using Eurocode*. ■ How to specify mortar and masonry units.
■ Glossary of Eurocode 6 terms.
The second guide in the series, Vertical resistance1, explains how to design for vertical actions, whilst the third guide, Lateral resistance2, covers the design of laterally loaded masonry panels. Further information on Eurocode 6 can be found at www.eurocode6.org.
Eurocode 6
Eurocode 6 comprises the following parts:
■ Part 1–1, General rules for reinforced and unreinforced masonry structures3. ■ Part 1–2, Structural fire design4.
■ Part 2, Design considerations, selection of materials and execution of masonry5. ■ Part 3, Simplified calculation methods for unreinforced masonry structures6. Each part also has a National Annex (NA) which provides the Nationally Determined Parameters (NDPs) to be used in the application of Eurocode 6 in the UK. The UK NDPs have been used throughout this guide. In addition PD 6697 contains useful guidance complementary to Eurocode 67. This series concentrates on Eurocode 6, Part 1–1 but includes material from Part 2 to explain the exposure and durability requirements. The scope and content of Part 1–2 and Part 3 are also briefly explained.
Eurocode 6 is intended to be used with Eurocode*: Basis of structural design8, Eurocode 1: Actions on structures9 and, where appropriate, the other Eurocodes and relevant European Standards. The guide Introduction to Eurocodes10 provides more information on the Eurocode family.
Eurocode 6 has been developed to enable the designer to use the following types of masonry unit: clay, calcium silicate, aggregate concrete, autoclaved aerated concrete (aircrete), manufactured stone and natural stone. European standards for these materials have been published by BSI and form part of an array of standards relating to masonry produced under the auspices of the European Committee for Standardisation (CEN), committee TC/125 (Masonry). *BS EN 1990 is entitled ‘Eurocode’, but is often referred to as Eurocode 0.
How to design masonry structures using Eurocode 6
1. Introduction to Eurocode 6
Eur Ing, Prof.
J J Roberts
BSc(Eng), PhD, CEng, FIStructE, FICE, FIMS, FCMI, MICTO Brooker
BEng, CEng, MICE, MIStructEIntroduction
This publication is part of a series of three guides entitled How to design masonry structures using Eurocode 6. The aim is to make the use of Eurocode 6, Design of masonry structures as easy as possible by drawing together in one place key information and commentary required for the design of typical masonry elements.
The Concrete Centre (and, originally, The Modern Masonry Alliance) recognised that effective guidance was required to ensure that the UK design profession was able to use Eurocode 6 quickly, effectively, efficiently and with confidence. Therefore a steering group, with members from across the masonry industry (see back cover for a list of members), was established to oversee the development and publication of the original guides. This second revision addresses the publication of PD6697 in 2010 and revised National Annex to BS EN 1996-1-1 in 2013. It was overseen by a reconstituted steering group from industry (see back cover).
How to design masonry structures using Eurocode 6
2
Basis of design
Masonry structures are required to be designed in accordance with the general rules given in Eurocode, which requires that:
Ed ≤ Rd where
Ed = design value of the effect of actions Rd = design value of the resistance
The basic requirements of Section 2 of Eurocode are deemed to be satisfied for masonry structures when the following are applied: ■ Limit state design in conjunction with the partial factor method
described in Eurocode.
■ Actions as given in Eurocode 1. (See ‘Assessment of actions’ below.) ■ Combination rules as given in Eurocode.
■ The principles and application rules given in Eurocode 6.
Thus using the partial factor method, the design value for a material property is obtained by dividing its characteristic value by the relevant partial factor for materials as follows:
Rk Rd = gM where
Rd = design value of resistance
Rk = characteristic value of the resistance gM = partial factor for a material property
Partial factors for materials
The partial factors for use with masonry are given in Table NA.1 of the National Annex to Eurocode 6, Part 1–1 and shown here as Table 1. Two levels of attestation of conformity are recognized, Category I and Category II and this will be declared by the manufacturer of the masonry units. There are also two classes of execution control that are recognized: 1 and 2.
Assessment of actions
The Eurocodes use the term action to refer to a set of forces, deformations or accelerations acting on the structure; this includes horizontal and vertical loads. The guide How to design concrete structures using Eurocode 2: Introduction to Eurocodes9 gives guidance on determining the design value of actions and should ideally be consulted. However, a brief explanation on how to determine the partial factors for masonry design to Eurocode 6 is given below. There are a number of combinations of actions that are described in Eurocode, but for masonry design (excluding retaining structures) the ultimate limit state, STR (STR represents an internal failure or excessive deformation of the structure or structural member) will normally be used. For plain masonry, Eurocode 6 indicates that,
Scope of Part 1–1 of Eurocode 6
Part 1–1 describes the principles and requirements for safety, serviceability and durability of masonry structures. It is based on the limit state concept used in conjunction with a partial factor method. For the design of new structures, BS EN 1996–1–1 is intended to be used together with the other relevant Eurocodes.
Scope of Part 1–2 of Eurocode 6
This part deals with the design of masonry structures for the accidental situation of fire exposure and identifies differences from, or supplements to, normal temperature design. Only passive methods of fire protection are considered and active methods are not covered. It addresses the need to avoid premature collapse of the structure and to limit the spread of fire.
Scope of Part 2 of Eurocode 6
This part gives the basic rules for the selection and execution of masonry to enable it to comply with the design assumptions of the other parts of Eurocode 6. It includes guidance on factors affecting performance and durability, storage and use of materials, site erection and protection, and the assessment of the appearance of masonry.
Scope of Part 3 of Eurocode 6
This part provides simplified calculation methods to facilitate the design of a range of common wall types under certain conditions of use. The methods are consistent with Part 1–1 but result in more conservative designs, and other methods are available in the UK; see 'Simplified calculation methods' on page 7. The simplified methods are not applicable to design for accidental situations, which should be designed for in accordance with CI 5.2 of Part 1–1.
Scope of PD 6697
This Published Document contains non-contradictory complementary information and additional guidance for use in the UK with Part 1-1 and Part 2 of Eurocode 6.
Supporting standards
There are European Standards that support Eurocode 6, and whilst they were developed within a common framework, it has not proved possible to standardise all the test methods used for the different materials. Words like brick and block have disappeared from the European vocabulary and they are all referred to as masonry units. Products should be specified by their performance requirements. The Standards that support the use of masonry in Eurocode 6 are published by BSI. Two key factors that changed from previous UK practice are:
■ The six masonry unit standards introduced methods for determining the compressive strength of masonry units11. ■ The method of determining characteristic compressive and shear
strengths of masonry changed.
3
Table 1
Values of gM for ultimate limit state
Material Class of execution
control gM
1a 2a
Masonry
When in a state of direct or flexural compression Unreinforced masonry made with:
Units of category I 2.3 b 2.7 b Units of category II 2.6 b 3.0 b Reinforced masonry made with mortar M6 or M12:
Units of category I 2.0 b – c Units of category II 2.3 b – c When in a state of flexural tension
Units of category I and II but in laterally loaded wall panels when removal of the panel would not affect the overall stability of the building
2.3 b 2.0 b
2.7 b 2.4 b When in a state of shear
Unreinforced masonry made with:
Units of category I and II 2.5 b 2.5 b Reinforced masonry made with mortar M6 or M12:
Units of category I and II 2.0 b – c Steel and other components
Anchorage of reinforcing steel 1.5 d – c Reinforcing steel and prestressing steel 1.15 d – c Ancillary components – wall ties 3.0 b 3.0 b Ancillary components – straps 1.5 e 1.5 e Lintels in accordance with EN 845-212 See NA to
BS EN 845–2f See NA to BS EN 845–2f
Key
a Class 1 of execution control should be assumed whenever the work is carried out following the recommendations for workmanship in BS EN 1996–2, including appropriate supervision and inspection, and in addition:
i the specification, supervision and control ensure that the construction is compatible with the use of the appropriate partial safety factors given in BS EN 1996–1–1;
ii the mortar conforms to BS EN 998-2, if it is factory made mortar. If the mortar is site mixed, preliminary compressive strength tests, in accordance with BS EN 1015-2 and 1015-11, are carried on the mixture of sand, lime (if any) and cement that is intended to be used (the proportions given in Table NA.2 may be used initially for the tests) in order to confirm that the strength requirements of the specification can be met; the proportions may need to be changed to achieve the required strengths and the new proportions are then to be used for the work on site. Regular compressive strength testing is carried out on samples from the site mortar to check that the required strengths are being achieved.
Class 2 of execution control should be assumed whenever the work is carried out following the recommendations for workmanship in BS EN 1996–2, including appropriate supervision.
b When considering the effects of misuse or accident these values may be halved. c Class 2 of execution control is not considered appropriate for reinforced masonry
and should not be used. However, masonry wall panels reinforced with bed joint reinforcement used:
i to enhance the lateral strength of the masonry panel, ii to limit or control shrinkage or expansion of the masonry,
can be considered to be unreinforced masonry for the purpose of class of execution control and the unreinforced masonry direct or flexural compression gM values are appropriate for use.
d When considering the effects of misuse or accident these values should be taken as 1.0. e For horizontal restraint straps, unless otherwise specified, the declared ultimate
load capacity depends on there being a design compressive stress in the masonry of at least 0.4 N/mm2. When a lower stress due to design loads may be acting, for example when autoclaved aerated concrete or lightweight aggregate concrete masonry is used, the manufacturer’s advice should be sought and a partial safety factor of 3 should be used.
f Yet to be published.
provided the ultimate limit state is satisfied, no checks for the serviceability limit states are required. This assumes compliance with the limiting dimensions and ratios specified in Eurocode 6.
There are three combinations that can be used for the STR limit state Expression (6.10), which is always conservative, or the most onerous of Expressions (6.10a) or (6.10b) (see Table 2). For laterally loaded masonry walls, where self-weight is usually beneficial, it will be sufficient to use Expression (6.10) only. For vertically loaded walls there will be some benefit in using Expression (6.10b), which, for members supporting one variable action (except storage loads) is the most economical of the three expressions, provided that the permanent actions are not greater than 4.5 times the variable actions. In Table 2 c0 is a factor that reduces the design value of a variable action when it acts in combination with another variable action (i.e. when it is an accompanying action). The value of c0 can be obtained from Table 3. The UK NA to Eurocode values can be applied to Expression (6.10), and this is shown in Table 4, which also shows the factors to be used when wind loads act in combination with imposed loads. Note that wind loads and imposed loads are both considered to be variable actions.
Table 2
Design values of actions, ULS (Table A1.2 (B) of Eurocode) Combination
Expression reference
Permanent actions Leading variable action
Accompanying variable action Unfavourable Favourable
Exp. (6.10) gG,j,sup Gk,j,sup gG,j,inf Gk,j,inf gQ,1 Qk,1 gQ,i c0,i Qk,i Exp. (6.10a) gG,j,sup Gk,j,sup gG,j,inf Gk,j,inf gQ,1 c0,1 Qk,1 gQ,i c0,i Qk,i Exp. (6.10b) jgG,j,sup Gk,j,sup gG,j,inf Gk,j,inf gQ,1 Qk,1 gQ,i c0,i Qk,i Notes
Gk =characteristic value of a permanent load
Qk =characteristic value of the variable action
gG,sup=partial factor for permanent action upper design value gG,inf =partial factor for permanent action lower design value gQ =partial factor for variable actions
c0 =factor for combination value of a variable action j =reduction factor/distribution coefficient
Where a variable action is favourable Qk should be taken as 0.
Table 3
Recommended combination values of variable actions (c0) buildings (from UK National Annex to Eurocode)
Action c0
Imposed loads in buildings (see BS EN 1991–1–1)
Category A: domestic, residential areas 0.7
Category B: office areas 0.7
Category C: congregation areas 0.7
Category D: shopping areas 0.7
Category E: storage areas 1.0
Category F: traffic area, vehicle weight < 30 kN 0.7 Category G: traffic area, 30 kN < vehicle weight < 160 kN 0.7
Category H: roofs a 0.7
Snow loads on buildings (see BS EN 1991–3)
For sites located at altitude H > 1000 m above sea level 0.7 For sites located at altitude H < 1000 m above sea level 0.5 Wind loads on buildings (see BS EN 1991–1–5) 0.6 Key
a See also 1991–1–1: Cl 3.3.2
How to design masonry structures using Eurocode 6
4
Imperfections
Eurocode 6 also recognizes that imperfections should be taken into account in design and requires that, at the ultimate limit state, the horizontal forces to be resisted at any level should be the sum of 1 and 2 below.
1. The horizontal load due to the vertical load being applied to a structure with the following notional inclination angle v to the vertical:
1
v = radians 100 Rhtot
where
htot = the total height of the structure in metres.
Each vertical action therefore produces a horizontal action to which the same load factor and combination factor as the vertical load apply.
2. The wind load derived from Eurocode 1, Parts 1–4 multiplied by its partial factor and distributed across the elements resisting the load in proportion to their stiffness.
Mortar
The way in which mortar is specified is shown in Table 5. The primary method of designation for mortar is the strength grade. Thus an M12 mortar should have a minimum compressive strength of 12 N/mm2 at 28 days. Eurocode 6 recognises three types of masonry mortar: general purpose, thin layer and lightweight mortar, and they may all be either designed or prescribed (see Glossary).
The use of mortars should be in accordance with the recommendations given in Eurocode 6, Part 2. For site made mortars, the mixing of the mortar should be in accordance with Part 2 (PD 66977 provides more detailed information). For factory made, semi-finished factory made and pre-batched masonry mortars, BS EN 998–213 applies.
Table 4
Design values of actions derived for UK design, ultimate limit state Combination
expression reference
Permanent actions Leading variable action (unfavourable) Accompanying variable actions (unfavourable) Unfavourable Favourable
Combination of permanent actions and one variable action Exp. (6.10) 1.35 Gka 1.0 G ka 1.5 Qkb – Exp. (6.10a) 1.35 Gka 1.0 G ka 1.5 c0-1b Qk Exp. (6.10b) 0.925 x 1.35 Gka 1.0 G ka 1.5 Qk
Combination of permanent actions, wind load (Qk,W) and imposed load (Qk,I) Exp. (6.10) Case 1 1.35 Gk a 1.0 G ka 1.5 Qk,W 1.05 c Qk,I Exp. (6.10) Case 2 1.35 Gk a 1.0 G ka 1.5 Qk,I 0.75 d Qk,W Key
a Where the variation in permanent action is not considered significant Gk,j,sup and Gk,j,inf may be taken as Gk
b Where a variable action is favourable Qk should be taken as 0
c The value of c0 has been taken as 0.7, for storage loads c0 = 1.0 and a factor of 1.5 must be used
d The value of c0 has been taken as 0.5
For designed mortars, the compressive strength of the mortar provides the control of the hardened mortar quality, whereas prescribed mortars use set proportions. When samples are taken from a designed mortar in accordance with BS EN 1015–214, and tested in accordance with BS EN 1015–1115, the compressive strength of the mortar should not be less than the declared compressive strength.
Durability of materials
Eurocode 6, Part 2 gives the basic rules for selection of mortar and masonry units for durability. Exposure conditions are defined in CI 2.1.2(3) of Part 2, with further guidance given in Annexes A, B and C. The UK NA to Part 2 advises that Annexes B and C should not be used because the information is not as extensive as that given in PD 6697; this advice is included in Table 6.
Structural fire design
Eurocode 6, Part 1–2 provides information on the passive fire resistance of masonry walls so that the designer can ensure that the loadbearing performance is maintained for the necessary period of time and that the fire is appropriately contained.
Designers will find that the tabulated data covers most situations but there is also the provision for testing and calculations. (Calculation methods are excluded by the UK NA to Part 1–2.) The tables cover loadbearing and non-loadbearing walls, single leaf, cavity and separating walls.
Table 5
Acceptable assumed equivalent mixes for prescribed masonry mortars for use with Class 2 of Execution control
Com-pressive strength
classa
Prescribed mortars (traditional proportion of materials by volume) (see Note)
Mortar design-ation Suitable for use in environ-mental condition Cementb: lime: sand with or without air entrain-ment Cementb: sand with or without air entrain-ment Masonry cementc: sand Masonry cementd: sand M12 1 : 0 to ¼: 3 1 : 3 Not
suitable Not suitable (i) Severe(S)
M6 1 : ½: 4 to 4½ 1 : 3 to 4 1 : 2½ to 3½ 1 : 3 (ii) Severe(S) M4 1 : 1: 5 to 6 1 : 5 to 6 1 : 4 to 5 1 : 3½ to 4 (iii) Moderate(M) M2 1 : 2: 8 to 9 1 : 7 to 8 1 : 5½ to 6½ 1 : 4½ (iv) Passive(P) Key
a The number following the M is the compressive strength for the class at 28 days in N/mm2 that may be assumed for the proportions given in columns 2 to 4; site compressive strength testing is not required for these traditional mixes. Checking of prescribed mortars should only be done by testing the proportions of the constituents. b Cement or combinations of cement (which include CEM I and many CEM IIs) in accordance
with NA.2.3.2, except masonry cements
c Masonry cement in accordance with NA.2.3.2, (inorganic filler other than lime) d Masonry cement in accordance with NA.2.3.2 (lime)
Notes
When the sand portion is given as, for example, 5 to 6, the lower figure should be used with sands containing a higher proportion of fines whilst the higher figure should be used with sands containing a lower proportion of fines.
For Class 2 of execution control site compressive strength testing is not required for these traditional mixes and checking of prescribed mortars should only be done by testing the proportions of the constituents.
5
Table 6
Durability of masonry in finished condition Masonry condition
or situationa Quality of masonry units and appropriate mortar designations
Clay unitsb Calcium silicate units Aggregate concrete bricks Aggregate concrete
and autoclaved aerated concrete blocks A – Work below or near external ground level
A1 – Low risk of saturation Without freezing: LD – F0 and S0 or HD: F0, F1 or F2 and S0, S1 or S2 in M12, M6 or M4 With freezing: HD – F1 or F2 and S0, S1 or S2 in M12, M6 or M4 unless a manufacturer advises against the use of HD-F1
Without or with freezing: Compressive strength class 20 or above in M4 or M2
Without or with freezing: Compressive strength 16.5 N/mm2 or above in M4
Without or with freezing: As Notes 4A, 4B, 4C or 4Dc. All in M4 or M2d
A2 – High risk of saturation
without freezinge HD – F1 or F2, and S1 and S2 in M12 or M6 Compressive strength class 20 or above in M6 or M4 Compressive strength 16.5 N/mm 2
or above in M6 or M4 As for A1 in M6 or M4 A3 – High risk of saturation
with freezinge HD – F2 and S1 or S2 in M12 or M6 Compressive strength class 20 or above in M6 or M4 Compressive strength 22 N/mm 2
or above in M6 or M4 As for A1 in M6 B – Masonry d.p.c.s
B1 – In buildings D.P.C units, max. water absorption 4.5% in M12fg Not suitable Not suitable Not suitable B2 – In external works D.P.C. units, max. water absorption 7 % in
M12fg Not suitable Not suitable Not suitable
C – Unrendered external walls (other than chimneys, cappings, copings, parapets and sills)h
C1 – Low risk of saturation HD – F1 or F2 and S1 or S2 in M12, M6 or
M4 Compressive strength class 20 or above in M4 or M2 Compressive strength 7.3 N/mm2 or above in M4 Any in M4 or M2 C2 – High risk of saturation HD – F2 and S1 or S2 in M12 or M6 Compressive strength class 20
or above in M4 Compressive strength 18 N/mm2 or above in M4 Any in M4
D – Rendered external walls (other than chimneys, cappings, copings, parapets, sills)ij
Rendered external walls HD –F1 or F2 and S1 or S2 in M12, M6 or M4 Compressive strength class 20
or above in M4 or M2 Compressive strength 7.3 N/mm2 or above in M4 Any in M4 or M2 E – Internal walls and inner leaves of cavity walls above d.p.c level
Internal walls and inner leaves of cavity walls
LD – F0 and S0 or HD: F0, F1 or F2 and S0, S1 or S2 in M12, M6, M4 or M2
Compressive strength class 20 or above in M4 or M2
Compressive strength 7.3 N/mm2 or above in M4 or iv
Any in M4 or M2
F – Unrendered parapets (other than cappings and copings)klm
F1 – Low risk of saturation e.g. low parapets on some single storey buildings
HD – F1 or F2 and S1 or S2 in M12, M6 or M4
Compressive strength class 20 or above in M4
Compressive strength
22 N/mm2 or above in M4 As Notes 4A, 4B, 4C or 4D c in M4.
R2 – High risk of saturation e.g. where a capping only is provided for the masonry
HD – F2 and S1 or S2 in M12 or M6 Compressive strength class 20 or above in M4
Compressive strength
22 N/mm2 or above in M4 As for F1 in M6
G – Rendered parapets (other than cappings and copings)jo
Rendered parapets HD – F1 or F2 and S2 in M12, M6 or M4 or HD – F1 or F2 and S1 in M12 or M6
Compressive strength class 20 or above in M4
Compressive strength
7.3 N/mm2 or above in M4 Any in M4
H – Chimneysn
H1 – Unrendered with low risk of saturation
HD – F1 or F2 and S1 or S2 in M12, M6 or M4
Compressive strength class 20 or above in M4
Compressive strength
12 N/mm2 or above in M4 Any in M4 H2 – Unrendered with high risk
of saturation HD –F2 and S1 or S2 in M12 or M6 Compressive strength class 20 or above in M4 Compressive strength 16.5 N/mm2 or above in M4 As Notes 5A, 5B, 5C or 5D c in M6.
H3 – Rendered HD – F1 or F2 and S2 in M12, M6 or M4 or
HD – F1 or F2 and S1 in M12 or M6 Compressive strength class 20 or above in M4 Compressive strength 7.3 N/mm2 or above in M4 Any in M4
I – Cappings, copings and sillskl
Cappings, copings and sills HD – F2 and S1 or S2 in M12 Compressive strength class 30
or above in M6 Compressive strength 33 N/mm2 or above in M6 As Notes 4A, 4B or 4C in M6. J – Freestanding boundary and screen walls (other than cappings and copings)
J1 – With coping HD – F1 or F2 and S1 in M12 or M6 or HD –
F1 or F2 and S2 in M12, M6 or M4 Compressive strength class 20 or above in M4 Compressive strength 16.5 N/mm2 or above in M4 Any in M4 J2 – With capping HD – F2 and S1 or S2 in M12 or M6 Compressive strength class 20
or above in M4
Compressive strength
22 N/mm2 or above in M4 As Notes 5A, 5B, 5C or 5D in M6.
K – Earth-retaining walls (other than cappings and copings)p
K1 – Waterproofing retaining
face and coping HD – F1 or F2 and S1 or S2 in M12 or M6 Compressive strength class 20 or above in M6 or M4 Compressive strength 16.5 N/mm2 or above in M6 As Notes 5A, 5B, 5C or 5D c in M4.
K2 – With coping or capping but no waterproofing on retaining face
HD – F2 and S1 or S2 in M12 Compressive strength class 30 or
above in M6 Compressive strength 33 N/mm2 or above in M12 or M6
As for K1 but in M12 or M6q
L – Drainage and sewage, e.g. inspection chambers and manholes L1 – Surface water Engineering bricks or F1 or F2 and S1 or S2
in M12 Compressive strength class 20 or above in M6 or M4 Compressive strength 22 N/mm2 or above in M4 As Notes 5A, 5B or 5C in M4. L2 – Foul drainage (continuous
contact with masonry) Engineering bricks or F1 or F2 and S1 or S2 in M12 Compressive strength class 50 or above in M6r Compressive strength 48 N/mm 2 or above with cement content ≥ 350 kg/m3 in M12 or M6
Not suitable
L3 – Foul drainage (occasional
contact with masonry) Engineering bricks or F1 or F2 and S1 or S2 in M12 Compressive strength class 20 or above in M6 or M4r Compressive strength 48 N/mm 2 or above with cement content ≥ 350 kg/m3 in M12 or M6
Not suitable
1. Introduction to Eurocode 6
How to design masonry structures using Eurocode 6
6
Reinforced and
prestressed masonry
Eurocode 6, Part 1–1 contains information relating to the design of reinforced masonry, but provides no application rules on the design of prestressed masonry. Annex J of Part 1-1 provides additional guidance for reinforced masonry when subjected to shear. PD 6697 extends the information provided in Part 1-1 for both reinforced and prestressed masonry as well giving guidance on the use of bed joint reinforcement to enhance resistance to lateral loads.
Masonry movement
The potential for movement in completed masonry needs to be allowed for in design, and Eurocode 6, Part 2 makes recommendations for controlling differential movements; for example, the use of movement tolerant ties between the leaves of a cavity wall. Movement joints need to be provided to deal with the effects of moisture, temperature and movement caused by other agents. The position of movement joints needs to be considered with care to ensure that the structural integrity of the wall is maintained. Factors affecting the location of joints include:
■ The type and group of masonry unit. ■ The geometry of the structure. ■ The degree of restraint.
■ The effect of loading, thermal and climatic conditions. ■ Requirements for fire, sound and thermal performance. ■ The presence of reinforcement.
Movement joints that pass through the full thickness of the wall should be provided. The maximum horizontal distance, lm, between vertical movement joints for use for all walls in the UK (in the absence of other guidance from the manufacturer) is shown in Table 7.
Execution of masonry
Eurocode 6, Part 2 gives guidance on the execution of masonry including: ■ Permissible deviations.
■ Jointing and pointing.
■ Storage, preparation and use of materials on site. ■ Masonry protection during execution.
Further detail on the first two points is given in the following paragraphs.
Permissible deviations
The permissible deviations of the constructed masonry from the position in which it is intended to be built should form part of the design specification. The permissible deviations should not normally be greater than the values shown in Table 8 for structural imperfections.
Mortar pointing
Most masonry is constructed in such a way that the bedding mortar also forms the tooled surface of the finished mortar joint. If it is necessary to point the mortar joint the unhardened mortar should be raked out to a depth not less than 15 mm, but no more than 15% of the wall thickness.
How to design masonry structures using Eurocode 6
Table 6 – Continued from page 5
Notes
1 Table 6 is a reduced version of the information provided in PD 6697 which should be consulted when detailed guidance is required. 2 For designations (i), (ii), (iii) and (iv) see Table 5.
3 LD - clay masonry unit with a low gross dry density for use in protected masonry. HD - clay masonry unit for unprotected masonry as well as clay masonry unit with a high gross dry density for use in protected masonry. 4 Categories used for clay masonry for freeze/thaw
are as follows: F0 Passive exposure F1 Moderate exposure F2 Severe exposure.
Categories used for clay masonry for active salt content are as follows:
S0 No requirement for active salt content
S1 Limited active salt content (see BS EN 771–1, Cl. 5.3.9) S2 Limited active salt content (see BS EN 771–1, Cl. 5.3.9). 5 Concrete block options:
A Of net density ≥ 1500 kg/m3
B Made with dense aggregate conforming to BS EN 12620 C Having a compressive strength of 7.3 N/mm2
D Most types of autoclaved aerated block (consult manufacturer).
Key
a Refer to PD 6697 for guidance on sulfate bearing ground.
b Consult masonry unit manufacturer for use below or near ground level.
c Consult masonry unit manufacturer. d M2 mortar does not meet the minimum requirements of Approved Document A and if used refer to PD 6697 for limitations and guidance.
e Masonry is very vulnerable between 150mm above and 150mm below finished ground level. Refer to PD 6697 and consult manufacturer as required.
f Masonry dpcs do not resist water percolating downward.
g Dpcs of clay units are not normally suitable for use with other types of unit. h Protect with good roof overhang and detailing.
i Rendered walls are suitable for most wind driven rain conditions. j For rendered walls follow guidance in PD 6697.
k AAC blocks are not suitable for cappings, copings and cills.
l Bed dpcs for cappings, copings and cills in same mortar as the masonry units. m Parapets are likely to be severely exposed - coping and dpc should be provided. n For chimneys follow guidance in PD 6697. o Single leaf walls should be rendered only on one face.
p Masonry in retaining walls particularly prone to frost and sulfate attack. Check with unit manufacturer. q Some types of aggregate concrete units are not suitable for capping and copings. r Some types of calcium silicate units are not suitable for foul drainage.
7
1. Introduction to Eurocode 6
References
1 ROBERTS, J J & BROOKER, O How to design masonry structures using Eurocode 6: Vertical resistance (TCC/03/36). The Concrete Centre, 2013. 2 ROBERTS, J J & BROOKER, O How to design masonry structures using Eurocode 6: Lateral resistance (TCC/03/37). The Concrete Centre, 2013.
3 BRITISH STANDARDS INSTITUTION. BS EN 1996–1–1: Eurocode 6 – Design of masonry structures. General rules for reinforced and unreinforced masonry structures. BSI, 2005. Including NA to BS EN 1996-1-1:2005+A1:2012.
4 BRITISH STANDARDS INSTITUTION. BS EN 1996–1–2: Eurocode 6 – Design of masonry structures. General rules. Structural fire design. BSI, 2005. Including NA to BS EN 1996-1-2:2005:2007.
5 BRITISH STANDARDS INSTITUTION. BS EN 1996–2: Eurocode 6 – Design of masonry structures. Design considerations, selection of materials and execution of masonry. BSI, 2006. Including NA to BS EN 1996-2:2006:2007.
6 BRITISH STANDARDS INSTITUTION. BS EN 1996–3: Eurocode 6 – Design of masonry structures. Simplified calculation methods for unreinforced masonry structures. BSI, 2006. Including NA to BS EN 1996-3:2006:2007.
7 BRITISH STANDARDS INSTITUTION. PD 6697. Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2. BSI, 2010. 8 BRITISH STANDARDS INSTITUTION. BS EN 1990: Eurocode – Basis of structural design. BSI, 2002. Including NA to BS EN 1990:2002+A1:2005:2004. 9 BRITISH STANDARDS INSTITUTION. BS EN 1991: Eurocode 1 – Actions on structures. BSI (10 parts). Including their NAs.
10 NARAYAN, R S & BROOKER, O. How to design concrete structures using Eurocode 2: Introduction to Eurocodes (TCC/03/16). The Concrete Centre, 2005. 11 BRITISH STANDARDS INSTITUTION. BS EN 771: Specification for masonry units. BSI, 2011 (6 Parts).
12 BRITISH STANDARDS INSTITUTION. BS EN 845–2: Specification for ancillary components for masonry – Lintels. BSI, 2013. 13 BRITISH STANDARDS INSTITUTION. BS EN 998–2: Specification for mortar for masonry: Masonry mortar. BSI, 2013.
14 BRITISH STANDARDS INSTITUTION. BS EN 1015–2: Methods of test for mortar for masonry: Bulk sampling of mortars and preparation of test mortars. BSI, 1999. 15 BRITISH STANDARDS INSTITUTION. BS EN 1015–11: Methods of test for mortar for masonry: Determination of flexural and compressive strength of
hardened mortar. BSI, 1999.
16 DEPARTMENT FOR COMMUNITIES AND LOCAL GOVERNMENT, Building regulations (England and Wales) Approved Document A (2004). DCLG, revised 2006. 17 SCOTTISH BUILDING STANDARDS AGENCY. Scottish building standards technical handbooks: domestic – for compliance with 'Building Scotland
regulations 2004'. SBSA, 2007.
18 THE STATIONERY OFFICE. The Building Regulations (Northern Ireland) 1994 Technical Booklet D: Structure. TSO, 2000.
19 BRITISH STANDARDS INSTITUTION. BS 8103–2: Structural design of low-rise buildings– Code of practice for masonry walls for housing. BSI, 2005. 20 BRITISH STANDARDS INSTITUTION. BS 772: Methods of test for masonry units. BSI (20 parts).
Simplified calculation
methods
Eurocode 6, Part 3 contains simplified calculation methods for unreinforced masonry structures. These methods are based on the principles contained in Part 1 and should not be confused with simple rules developed on the basis of experience. In general, these methods are more conservative than design based on Part 1 and have not,
historically, been used in the UK, unlike some European countries. In the UK guidance on the Building Regulations16–18 and BS 8103–219 provide a very effective and economic set of simple rules for low rise masonry and it is anticipated that these will continue to be the primary method of demonstrating compliance for many small buildings.
Table 8
Permissible deviations for structural design purposes
Position Maximum deviation
Verticality
In any one storey ± 20 mm
In total height of building of three
storeys or more ± 50 mm
Vertical alignment ± 20 mm
Straightness a
In any one metre ± 10 mm
In 10 metres ± 50 mm
Thickness
Of wall leafb ± 5 mm or ± 5 % of the leaf thickness, whichever is the greater Of overall cavity wall ± 10 mm
Key
a Deviation from straightness is measured from a straight reference line between any two points.
b Excluding leaves of single masonry unit width or length, where the dimensional tolerances of the masonry units govern the leaf thickness.
Table 7
Maximum horizontal distance between vertical movement joints in walls (in the absence of other guidance from the manufacturer)
Type of masonry lm (m)
Clay masonry – unreinforced 15a
Calcium silicate masonry 9 b
Aggregate concrete and manufactured stone masonry 9 b Autoclaved aerated concrete masonry 9 b
Natural stone masonry 20 c
Key
a The value for clay masonry walls containing bed joint reinforcement may be greater than 15 m subject to expert advice.
b This value applies when the ratio, length to height of panel, is 3 to 1 or less. It should be reduced for long horizontal panels of masonry which lie outside this ratio. c When using this figure, movement joints should be located at not more than 8 m
All advice or information from MPA The Concrete Centre (TCC) et al is intended only for use in the UK by those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by TCC or their subcontractors, suppliers or advisors. Readers should note that the publications from TCC et al are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.
Ref: TCC/03/35. ISBN 978-1-904818-56-4 First published November 2007
(in partnership with the Modern Masonry Alliance) revised January 2009 and June 2013
Price Group M
© MPA The Concrete Centre™
Glossary of Eurocode 6 terminology
Term Definition
Adhesion The effect of mortar developing a tensile or shear resistance at the contact surface of masonry units. Category I masonry unit 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 masonry unit Units not intended to comply with the level of confidence of Category 1 units.
Confined masonry Masonry provided with reinforced concrete or reinforced masonry confining elements in the vertical and horizontal direction. (Not usually used in the UK.)
Designed masonry mortar A mortar whose composition and manufacturing method is chosen in order to achieve specified properties (performance concept).
General purpose masonry mortar Masonry mortar without special characteristics.
Griphole A formed void in a masonry unit to enable it to be more readily grasped and lifted with one or both hands or by machine.
Groups 1, 1s, 2, 3 and 4 Group designations for masonry units, according to the percentage, size and orientation of holes in the units masonry units when laid. Note: The group designation will normally be declared by the manufacturer. Historically only Groups
1, 1s and 2 units have been used in the UK. Group 1s is referred to in BS EN 1996–1–2.
Hole A formed void which may or may not pass completely through a masonry unit.
Lightweight masonry mortar Designed masonry mortar with a dry hardened density below a prescribed figure.
Normalized compressive The compressive strength of masonry units converted to the air dried compressive strength of an equivalent strength of masonry units 100 mm wide x 100 mm high masonry unit (see BS EN 772-120).
Orthogonal ratio, m The ratio of the flexural strength of masonry when failure is parallel to the bed joints to that when failure is perpendicular to the bed joints.
Prescribed masonry mortar Mortar made in predetermined proportions, the properties of which are assumed from the stated proportions of the constituents (recipe concept).
Shell The peripheral material between a hole and the face of a masonry unit.
Shell bedded wall A wall in which the masonry units are bedded on two or more strips of mortar, two of which are at the outside edge of the bed face of the units.
Thin layer masonry mortar Designed masonry mortar with a maximum aggregate size less than or equal to a prescribed figure.
Web The solid material between the holes in a masonry unit.
1. Introduction to Eurocode 6
For more information on Eurocode 6 and other questions relating to the design, use and performance of concrete units, visit www.eurocode6.org
Published by The Concrete Centre
Gillingham House, 38-44 Gillingham Street, London, SW1V 1HU Tel: +44 (0)207 963 8000 | www.concretecentre.com
Members of the steering group
Ali Arasteh, Brick Development Association; Owen Brooker, The Concrete Centre; Ken Fisher, International Masonry Society; Cliff Fudge, Aircrete Products Association; Charles Goodchild, The Concrete Centre; Gerry Pettit, Concrete Block Association; John Roberts, Consultant.
Members of the steering group for 2nd revision
Cliff Fudge, Aircrete Products Association; Charles Goodchild, The Concrete Centre; Simon Hay, Brick Development Association; Andy Littler, Concrete Block Association; John Roberts, Consultant; Guy Thompson, The Concrete Centre.Acknowledgements
This publication was jointly sponsored by the following organisations: ■ Aircrete Products Association - www.aircrete.co.uk
■ Brick Development Association - www.brick.org.uk ■ Concrete Block Association - www.cba-blocks.org.uk ■ MPA - Mortar Industry Association - www.mortar.org.uk ■ MPA - The Concrete Centre - www.concretecentre.com
Guidance for vertical resistance
This guide is the second in a series of three giving guidance on the design of masonry structures to Eurocode 61. The first guide, Introduction to Eurocode 62 gives an introduction to design and assessment of actions using Eurocode 6 and also covers the specification and execution (workmanship) of masonry. This guide explains how to design for vertical actions and determine vertical resistance. The third guide in the series3 covers the design of laterally loaded masonry panels. Throughout this guide the Nationally Determined Parameters (NDPs) from the UK National Annexes (NAs) have been used. These enable Eurocode 6 to be applied in the UK.
Design procedure
This guide explains how to determine the design resistance for a vertically loaded wall. The first guide in the series, Introduction to Eurocode 6, should be referred to so that the design load can be determined. In essence, when using the Eurocodes the designer should check that the resistance is greater than or equal to the effect of the actions. A flow chart for the design of masonry walls to resist vertical actions is shown as Figure 1.
Compressive strength
Eurocode 6 introduces some new concepts when dealing with the design of masonry for vertical loads. The first of these relates to the way the compressive strength of the masonry units is expressed. For design purposes the normalized compressive strength, fb, of the masonry units is used. This is the compressive strength of the units converted to the air-dried compressive strength of an equivalent 100 mm wide by 100 mm high masonry unit. The detail is contained in Part 1 of BS EN 772, Methods of test for masonry units4. The advantage to the designer is that the normalized strength is independent of the size and shape of the units used in the final construction.
Grouping of masonry units
The second change relates to the way in which masonry units are classified. This is dealt with by grouping masonry into one of four groups as shown in Table 3.1 of Eurocode 6. The group designation will normally be declared by the manufacturer. The designation depends upon the volume and direction of holes in the unit and the thickness of webs and shells. Historically only Group 1 and Group 2 units have been used in the UK, so only values for these groups are given in the UK NAs.
How to design masonry structures using Eurocode 6
2. Vertical resistance
Eur Ing, Prof.
J J Roberts
BSc(Eng), PhD, CEng, FIStructE, FICE, FIMS, FCMI, MICTO Brooker
BEng, CEng, MICE, MIStructEIntroduction
This publication is part of a series of three guides entitled How to design masonry structures using Eurocode 6. The aim is to make the use of Eurocode 6, Design of masonry structures as easy as possible by drawing together in one place key information and commentary required for the design of typical masonry elements.
The Concrete Centre (and, originally, The Modern Masonry Alliance) recognised that effective guidance was required to ensure that the UK design profession was able to use Eurocode 6 quickly, effectively, efficiently and with confidence. Therefore a steering group, with members from across the masonry industry (see back cover for a list of members), was established to oversee the development and publication of the original guides. This second revision addresses the publication of PD6697 in 2010 and revised National Annex to BS EN 1996-1-1 in 2013. It was overseen by a reconstituted steering group from industry (see back cover).
How to design masonry structures using Eurocode 6
2
For blocks laid flat, Table 8 of the National Annex to Eurocode 6, Part 1–1 contains a specific value for K to be used in Equation (3.1) of Eurocode 6, Part 1–1.
The following limitations are placed on Equation (3.1):
¢ The masonry is detailed and constructed in accordance with the requirements of BS EN 1996–1–1, section 8.
¢ fb is taken to be not greater than 110 N/mm2 when units are laid in general purpose mortar and 50 N/mm2 when laid in thin layer mortar (fb is determined in the normal direction of loading). ¢ fm is taken to be not greater than fb nor greater than 12 N/mm2
when units are laid in general purpose mortar or 10 N/mm2 when units are laid in lightweight mortar.
¢ The coefficient of variation of the strength of the masonry unit is not more than 25%.
For masonry made with general purpose mortar, adjustments are made to the value of K as shown in Figure 2.
In addition the following points should be noted:
¢ For masonry made of general purpose mortar where Group 2 and Group 3 aggregate concrete units are used with the vertical cavities filled completely with concrete, the value of fb should be obtained by considering the units to be Group 1 having a compressive strength corresponding to the compressive strength of the units or of the concrete infill, whichever is the lesser.
The characteristic
compressive strength of
masonry
The characteristic compressive strength of masonry (other than shell bedded masonry) is determined from the results of tests in accordance with BS EN 1052–15. The tests are carried out on small wallette specimens rather than the storey-height panels used in the past. The designer has the option of either testing the units intended to be used in a project or using the values determined from a database. Values from a large database are provided in the UK NA to Eurocode 6, Part 1–1 in the form of the constants to be used in the following equation: fk = K fba f
mb [Equation (3.1) of Eurocode 6, Part 1–1]
where
fk = characteristic compressive strength of the masonry, in N/mm2
K = constant – see Table 1 and Figure 2 a, b = constants – see Table 2
fb = normalized mean compressive strength of the units, in the direction of the applied action effect, in N/mm2
fm = compressive strength of the mortar, in N/mm2
Figure 1
Flow chart for the design of masonry walls to resist vertical actions
.
Obtain gM from table 1 of
Introduction to Eurocode 6 Determine requirements
for mortar strength and durability. See tables 5 & 6 of Introduction to
Eurocode 6
Determine effective height, hef, of the wall (see page 4)
Check slenderness ratio hef /tef ≤ 27 Check area ≥ 0.04 m2
Determine eccentricity (see page 5)
Determine capacity reduction factors, Fm and Fi from (see page 6)
Determine normalized compressive strength, fb.
Characteristic vertical actions
Check Ed ≤ NRd
Check complete Calculate design resistance
(per unit length) from least favourable of: NRd = Fm t fk / gM
and NRd = Fi t fk / gM
Where cross-sectional area, A < 0.1 m2, factor f
k by (0.7 + 3A)
Determine design value of vertical actions (per unit length), Ed, using Expression (6.10), (6.10a) or (6.10b) of Eurocode (see Introduction to
Eurocode 6) Masonry unit properties
• Type and group • Dimensions • Strength
Determine characteristic compressive strength of masonry, fk, from Equation (3.1) of Eurocode 6 and Tables 1 & 2
Determine effective thickness, tef, of the wall (see page 4)
2. Vertical resistance
3 ¢ For collar jointed aggregate concrete masonry made with general
purpose mortar, with or without the collar filled with mortar, the unit shape factor correction to obtain the normalized strength should use the width of the wall as the unit width and the height of the masonry units. ¢ Where action effects are parallel to the direction of the bed joints, the
characteristic compressive strength may be determined from Equation (3.1) with fb derived from BS EN 772–1, where the direction of application of the load to the test specimens is in the same direction as the direction of the action effect in the masonry, but with the shape factor, d, as given in BS EN 772–1 taken to be no greater than 1.0. For Group 2 and 3 units, K should then be multiplied by 0.5.
Table 1
Values of K to be used with equation (3.1) Masonry
unit General purpose mortar Thin layer mortar (bed joint ≥ 0.5 mm and ≤ 3 mm) Lightweight mortar of density (kg/m3) 600 ≤ rd ≤ 800 800 < r≤ 1300d Clay Group 1 0.50 0.75 0.30 0.40 Group 2 0.40 0.70 0.25 0.30 Group 3 and 4 – a – a – a – a Calcium silicate Group 1 0.50 0.80 – b – b Group 2 0.40 0.70 – b – b Aggregate concrete Group 1 0.75 0.90 0.45 0.45 Group 1c
(units laid flat) 0.50
d 0.70d 0.40d 0.40d
Group 2 0.70 0.76 0.45 0.45
Group 3 and 4 – a – a – a – a
Autoclaved aerated concrete
Group 1 0.75 0.90 0.45 0.45
Manufactured stone
Group 1 0.75 0.90 – b – b
Dimensioned natural stone
Group 1 0.45 – b – b – b
Key
a Group 3 and 4 units have not traditionally been used in the UK, so no values are available. b These masonry unit and mortar combinations have not traditionally been used in
the UK, so no values are available.
c If Group 1 aggregate concrete units contain formed vertical voids in the normal direction, multiply K by (100 – n) /100, where n is the percentage of voids, maximum 25%. d When aggregate concrete masonry units are to be used laid flat the normalised strength
of the unit should be calculated using the width and height of the unit in the upright position along with the compressive strength of the unit tested in the upright position. Note
Where a mortar joint is parallel to the face of the wall K should be modified (see Figure 2)
Table 2
Values to be used in Equation (3.1)
Type of mortar Values to be used
General purpose mortar a = 0.7 and b = 0.3
Lightweight mortar a = 0.7 and b = 0.3
Thin layer mortar in bed joints of thickness 0.5 to 3 mm (using clay units of Group 1, calcium silicate units, aggregate concrete units and autoclaved aerated concrete units)
a = 0.85 and b = 0
Thin layer mortar in bed joints of thickness 0.5 to 3 mm (using clay units of Group 2)
a = 0.7and b = 0
¢ When the perpendicular joints are unfilled, Equation (3.1) may be used, with consideration of any horizontal actions that might be applied to, or be transmitted by, the masonry. (See also CI. 3.6.2(4) of BS EN 1996–1–1.)
The characteristic
compressive strength of
shell bedded masonry
Shell bedding provides two strips of mortar rather than a full mortar bed. It serves to improve rain penetration resistance but reduces the strength of the masonry. A typical shell bedded unit is shown in Figure 3. For Group 1 and Group 4 units the procedure above may be used to obtain the characteristic compressive strength of the masonry.
Figure 2
Modifications to K for units laid with general purpose mortar Plan sections of bonded masonry
a) K from Table 1
b) K from Table 1
c) K from Table 1 multiplied by 0.8
d) K from Table 1 multiplied by 0.8
Masonry thickness Masonry thickness Masonry thickness Masonry thickness Figure 3 Shell bedding
How to design masonry structures using Eurocode 6
4
provided that:
¢ The width of each strip of mortar is at least 30 mm.
¢ The thickness of the masonry wall is equal to the width or length of the masonry units so that there is no longitudinal mortar joint through all or part of the length of the wall.
¢ The ratio g/t is not less than 0.4 where
g = total width of the mortar strips t = the thickness of the wall.
¢ K is taken as above when g/t = 1.0 or half this value when g/t = 0.4. Linear interpolation may be used for intermediate values.
Groups 2 and 3 may be designed as non-shell bedded masonry provided that the normalized mean compressive strength of the units used in Equation (3.1) is obtained from tests carried out in accordance with BS EN 772–14 for shell bedded units.
Effective height
The effective height of a masonry wall is obtained by applying a factor to the clear height of the wall such that:
hef = rn h where
hef = effective height of the wall h = clear storey height of the wall
rn = reduction factor, where n = 2, 3 or 4, depending upon the edge restraint or stiffening of the wall
The reduction factor to be applied depends upon the restraint offered by adjoining elements. Masonry walls may be stiffened by a number of rigid structural elements such as floors, roofs and other walls. Stiffening walls should have a length of at least 1/5 of the clear height
Figure 4
Minimum length of stiffening wall with openings
h2 (window)
h2(door) h
Stiffened wall Stiffening wall
>h/5 h1
t
1 (h1+ h2) 5 2 > t
and have a thickness of at least 0.3 times the effective thickness of the wall to be stiffened. When the stiffening wall contains openings, the minimum length of wall should be as shown in Figure 4 and the stiffening wall should extend a distance of at least 1/5 of the storey height beyond each opening.
Where a wall is restrained at the top and bottom by reinforced concrete floors or roofs spanning from both sides at the same level or by a reinforced concrete floor spanning from one side only and having a bearing of at least 2/3 of the thickness of the wall then:
r2 = 0.75
unless the eccentricity of the load at the top of the wall is greater than 0.25 times the thickness of the wall, in which case r2 = 1.0. Where the wall is restrained by timber floors or roofs spanning from both sides at the same level or by a timber floor spanning from one side having a bearing of at least 2/3 the thickness of the wall but not less than 85 mm, then:
r2 = 1.0.
For walls restrained at the top and bottom and stiffened on one vertical edge, use rn = the value r3 from Figure 5 and where both vertical edges are stiffened, use rn = the value r4 from Figure 6. Note that Equations (5.6), (5.7) and (5.8) in Eurocode 6, Part 1–1 may be used as an alternative to the use of the graphs.
Effective thickness
For a single-leaf wall, a double-leaf wall (with ties at a density of 2.5 per m2 or greater), a faced wall, a shell bedded wall and a grouted cavity wall, the effective thickness, tef, is taken as the actual thickness
Figure 5
Graph showing values of r3 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0 1 2 3 4 5 Ratio hef/tef Re duction factor , r3 r2 = 1.0 r2 = 0.75
How to design masonry structures using Eurocode 6
2. Vertical resistance
5
of the wall (t), provided this is greater than the minimum thickness, tmin. The value of tmin for a loadbearing wall should be taken as 90 mm for a single-leaf wall and 75 mm for the leaves of a cavity wall. For a cavity wall the effective thickness is determined using the following equation:
tef = 3
R
t13 + t23 ≥t2 where
t1 = actual thickness of the outer or unloaded leaf t2 = actual thickness of the inner or loaded leaf
Note that the effective thickness of the unloaded leaf should not be taken to be greater than the thickness of the loaded leaf and that ties should be provided at a density of 2.5 per m2 or greater.
When a wall is stiffened by piers the effective thickness is enhanced by using the following equation:
tef = rtt where
tef = effective thickness
rt = coefficient obtained from Table 3 t = thickness of the wall
Slenderness ratio
The slenderness ratio of the wall is obtained by dividing the effective height by the effective thickness and should not be greater than 27 for walls subjected to mainly vertical loading. Note also that the effects of creep may be ignored in walls with a slenderness ratio up to 27.
Figure 6
Graph showing values of the reduction factor, r4
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0 1 2 3 4 5 Ratio h/l Re duction factor , r4 r2 = 0.75 r2 = 1.0
Assessment of eccentricity
When a wall is subjected to actions that result in an eccentricity at right angles to the wall, Eurocode 6 requires the resistance of the wall to be checked at the top, mid-height and bottom. The eccentricity at top or bottom of the wall is:
Mid
ei = + ehe + einit ≥ 0.05t Nid
where
Mid = design value of the bending moment at the top or the bottom of the wall resulting from eccentricity of the floor load at the support
Nid = design value of the vertical load at the top or the bottom of the wall
ehe = the eccentricity at the top or bottom of the wall resulting from the horizontal loads
einit = initial eccentricity for construction imperfections, which may be taken as hef/450, with a sign that increases the absolute value of ei and em as appropriate
t = thickness of the wall The mid-height eccentricity, emk, is: emk = em + ek ≥ 0.05t
where
Mmd
em = + ehm + einit Nmd
ek = 0, when the slenderness ratio ≤ 27 (ie. ignoring creep) Mmd = design value of the greatest moment at the mid-height
of the wall resulting from the moments at the top and bottom of the wall, including any load applied eccentrically to the face of the wall (see Figure 7) Nmd = design value of the vertical load at the mid-height of the
wall, including any load applied eccentrically to the face of the wall
ehm = the eccentricity at mid-height resulting from horizontal loads A sub-frame analysis may be used as a simplified method for obtaining the moments at the top and bottom of vertically loaded walls, as given in Annex C in Part 1–1 of Eurocode 6.
Table 3
Stiffness coefficient, rt, for walls stiffened by piers Ratio of pier spacing
(centre to centre) to pier width
Ratio of pier thickness to actual thickness of wall to which it is bonded
1 2 3
6 1.0 1.4 2.0
10 1.0 1.2 1.4
20 1.0 1.0 1.0
Note
How to design masonry structures using Eurocode 6
6
Capacity reduction factors
At the top or bottom of the wall, the reduction factor for slenderness and eccentricity is given by:
ei Fi = 1 – 2 t where
Fi = reduction factor at the top or bottom of the wall ei = eccentricity at the top or bottom of the wall t = thickness of the wall
A method for calculating a capacity reduction factor at the mid-height of the wall, Fm, is given in Annex G of Eurocode 6, Part 1–1, which simplifies the principles given in Cl. 6.1.1. This is shown graphically in Figure 8, which shows the corresponding capacity reduction factors for different values of slenderness and eccentricity for an elastic modulus 1000 fk, which is the value recommended in the UK NA.
The least favourable value of Fi and Fm should be used to calculate NRd.
Vertical load resistance of
solid walls and columns
The design resistance of a single-leaf wall per unit length, NRd, is given by the following:
NRd = F tfd where
F = capacity reduction factor allowing for the effects of slenderness and eccentricity of loading
Figure 7
Moments from calculation of eccentricities
N1d M1d (at underside of floor) Mmd (at mid-height of wall) M2d
(at top of floor) Nmd
N2d h
h2
h2
a) Section b) Bending moment diagram
t = thickness of the wall
fd = design compressive strength of the masonry ( fk/gM) For sections of small plan area, less than 0.1 m2, f
d should be multiplied by (0.7 + 3A)
where
A = loadbearing horizontal cross-sectional area of the wall in m2 In the case of a faced wall, the wall may be designed as a single-leaf wall constructed entirely of the weaker material with a longitudinal joint between leaves.
A double-leaf (collar-jointed) wall may also be designed as for a single-leaf wall provided that the leaves are tied together adequately and both leaves carry similar loads and the cavity does not exceed 25 mm, or it may be designed as a cavity wall with one leaf loaded. In the case of cavity walls, check each leaf separately using a slenderness ratio based on the effective thickness of the wall.
Concentrated loads
For a Group 1 unit (not shell bedded) the vertical load resistance is: NRdc = bAbfd
where a1 Ab b = 1 + 0.3 1.5 – 1.1 hc Aef
= enhancement factor for load that should not be less than 1.0 nor taken to be greater than:
1.25 + a1 or 1.5, whichever is the lesser 2hc
Figure 8
Capacity reduction factor, Fm at the mid-height of the wall
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 5 10 15 20 25 30 Ratio, hef/ltef
Capacity reduction factor
, F m Eccentricity = 0.05t 0.10t 0.15t 0.20t 0.25t 0.30t 0.35t 0.40t
Values of Fm at the mid-height of the wall against slenderness ratio for different eccentricities, based on E =1000 fk
