Design principles

Before any design of a masonry element can be initiated, some understanding of its geometry, support conditions and material properties must be established. All of these facets have a signifi cant eff ect on the design of masonry and therefore must be established from the outset.

Once the basic geometry is confi gured an iterative process begins with an initial thickness of the wall considered and then checked against the applied stresses.

Material propertiesThe material properties are entirely dependent on the type of masonry and mortar selected. In the UK, the National Annex for BS EN 1996-1-1 provides values for the material partial factors and the shear and fl exural strength, based on the types of masonry and mortar strength selected. Specifi cally Table NA.1 provides values of partial factors γM, Table NA.2 provides the defi nition of mortar types and Table NA.5 describes the shear strength of masonry. Table NA.6 defi nes the values of fl exural strength of masonry depending on its mode of failure.

All of these tables will be referenced

Designing a laterally loaded masonry wallIntroduction

Until relatively recently, masonry was the major load bearing component in a building structure. With the advent of steel and concrete frame technologies, masonry has become a part of a building’s cladding envelope and as such is more prone to being exposed to lateral loads than vertical ones.

This Technical Guidance Note concerns the design of masonry walls that are subject to lateral loads i.e. they are being used as a cladding element. It will discuss the way in which the material is assessed against how it is being restrained and its geometry. All of these factors have an impact on the design of masonry walls as well as the mortar within them and the exposure conditions. This is discussed in Technical Guidance Note 27 (Level 1) and should be read in conjunction with this guide.

W Design principles

W Worked example

W Applied practice

W Further reading

W Web resources

ICON LEGEND

throughout this guide, but not reproduced. It is advisable that you have access to both BS EN 1996-1-1 and its UK Annex when referring to this guide.

Geometry and stabilityWith masonry generally being constructed in slender forms, the geometric properties are of paramount importance. In recognition of this, BS EN 1996-1-1 applies coeffi cients to distribute the load between perpendicular directions through the masonry, based on its dimensions and support conditions. One of the fi rst checks carried out should be to determine whether the wall is too slender. This is based on the ratio of height vs. thickness (h/t) of the wall. If it is more than 30 and the wall is spanning vertically, then it is too thin and its thickness needs to be increased. The value of this ratio is referred to again when determining the bending moment coeffi cients.

The next item for review is the Serviceability

Slenderness check. This is described in a series of fi gures in Annex F of BS EN 1996-1-1. Depending on the support conditions of the wall, the ratio of height to length (h/l) is compared against the ratio of length to thickness (l/t) where h is the height of the wall, l is the length of the wall and t is the thickness of the wall. Provided the combination of h/t and l/t ratios are below and left of the line on the appropriate chart, the wall is satisfactory from a serviceability point of view.

Support conditionsModelling the support conditions accurately is very important when designing masonry as their rigidity impacts on the capacity of the wall to resist lateral loads. Figure 1a-c is a series of typical restraint conditions for masonry walls. These should be referred to when determining the bending moment capacity of the wall when using the fi gures contained in Annex F of BS EN 1996-1-1.

Bending moment capacityIn the case of laterally loaded walls, the bending moment capacity is dependent upon the geometry of the wall, its support conditions and the material it is constructed from. For failure that is parallel to bed joints fxk1 is the fl exural strength based on the mortar and masonry type. Similarly the value of fxk2 is the fl exural strength of masonry based on failure that is perpendicular to bed joints.

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27TheStructuralEngineerTechnical

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Technical Guidance Note June 2013

Note 6 Level 2

"Geometric properties are of paramount importance"

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Technical Guidance Note

Technical

June 2013

Note 6 Level 2

›

A 2.5m high wall, spanning between piers at 2.025m centres has to withstand a design wind action of 1 kN/m2. It is founded on a mass concrete strip footing and is bonded to the piers, forming a continuous connection. The top of the wall has no lateral support and is free to move. The wall is to be made from clay bricks with moisture absorption of 9% and a Class M2 mortar that is 10mm thick. Determine if a single skin of brick can withstand the wind action.

Worked example

Applied bending momentHaving found the values of fxk1 and fxk2 in Table NA.6, the Orthogonal Ratio μ can be calculated thus:

Once this ratio is determined, you are directed to Annex E of BS EN 1996-1-1 where a series of tables provide the value of bending moment coeffi cient α2

. These tables plot the values of h/l against μ .

The bending moment capacity of a wall that is subject to lateral forces MRd is defi ned thus:

The applied bending moment MEd is calculated using the coeffi cients defi ned in Annex E. The applied bending moment that is parallel to the bed joints is defi ned as:

Where:α1 is the orthogonal ratio μ multiplied by

the bending moment coeffi cient α2

WEd is the ultimate design load applied to the wall

l is the length of the wall

The bending moment that is perpendicular to the bed joint is defi ned as:

Bending parallel to the bed joint:

Where:σd is the applied compressive stress in N/mm2

Z is the elastic modulus of the wall

Bending perpendicular to the bed joint:

Shear capacityFor walls, the shear capacity VRd is defi ned thus:

Where:fvd is the shear capacity of the wall and

those that have fully fi lled mortar joints are defi ned as fvk0+0.4σd. The value of fvk0 is drawn from table NA.5 in the UK National Annex of BS EN 1996-1-1

t is the thickness of the walllc is the length of wall under compression

This is then compared against the applied shear load VEd and provided it is equal to or greater than the applied shear, the wall is adequate.

ff

xk

xk

2

1n =

n

n

M W lEd Ed1 12a=

n

M W lEd Ed2 22a=

V f tlRd vd c=

Mf

ZRdM

xkd1

1

c v= +c m

Mf

ZRdM

xk2

2

c= c m

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Further reading

The Institution of Structural Engineers (2008) Manual for the design of plain

masonry in building structures to

Eurocode 6 London: The Institution of Structural Engineers

Morton J. (2011) Designers’ Guide to

Eurocode 6: Design of Masonry

Structures: EN 1996-1-1 London: Thomas Telford Ltd

The International Masonry Society (1996) Eurocode for Masonry EN 1996-1-1 and

EN 1996-2 Guidance and worked examples Surrey, UK: The International Masonry Society

Eurocode 0.Web resources

Brick Development Association: www.brick.org.uk

Concrete Block Association: www.cba-blocks.org.uk

Eurocode 0.Applied practice

BS EN 1996-1-1 Eurocode 6: Design of Masonry Structures – Part 1-1: General Rules for Buildings

BS EN 1996-1-1 UK National Annex to

Eurocode 6: Design of Masonry Structures – Part 1-1: General Rules for Buildings

BS EN 1996-2: Design of Masonry Structures - Design considerations, selection of materials and execution of masonry

PD 6697: Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2

S Figure 1cExample support conditions to top of masonry walls

E Figure 1bSimply

supported masonry walls

W Figure 1aContinuous

support to masonry walls In situ concrete slab spanning

parallel to wall. Fixity limited to masonry’s fl exural properties

Presence of DPC impacts shear and moment resistance of wall. Enhanced by vertical load and properties of DPC material

Precast concrete spanning parallel to wall. Simple support to top of wall

Technical Guidance Note 4, Level 2 – Designing a concrete beam:

• In continuous beams it is necessary to calculate the width of the tension

flange in a ‘T’ and ‘L’ beam, as defined in Figure 5.2 of BS EN 1992-1-1 and

Figure 1 of the Technical Guidance Note

• In the worked example, the tension reinforcement at the support should

be distributed within the tension flange, as described in Clause 9.2.1.2(2)

of BS EN 1992-1-1, thus:

• In the ‘Detailing requirements’ section, the expression used to

determine maximum spacing only applies to beams that are designed

with partial factors 1.35 γG and 1.5 γQ applied to permanent and variable

actions, rather than the more common 1.25 γG and1.5 γQ

• In the ‘Detailing requirements’ section, Clause 8.8 of BS EN 1992-1-1

highlights special rules when using 32mm bars and it is not permitted to

use 6mm bars as a reinforcement element within concrete as they do not

comply with Annex C of BS EN BS EN 1992-1-1

• Maximum bar spacing check for worked example is as follows:

Errata

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