Masonry - ICE Manual of Structural Design: Buildings - page 395

Movement joints need to be located to ensure that they do not
create instability of the structure. A holistic design approach
is required when detailing the location of movement joints to
ensure that assumptions made in the design are refl ected in
the construction details. For example, if movement joints are
located in the centre of a long wall, the wall cannot be assumed
as continuous. The design and specifi cation for movement
joints should also consider the following aspects:

Design
shear
resistance:

Clause 3.6.2
Equation 3.5

f vk = f vko + 0.4 σ d < 0.065f b orf vit

Table NA.5
Equation 3.5

f vk = f vko + 0.4 σ d = 0.15 +
(0.4 × 0.08) = 0.18 N/mm 2

0.18
2.0

f 0.09 N/mm

2

= = =

vd
f vd

Fire resistance

■

Therefore shear OK.

Thermal performance

■

20.5.3 Other aspects of fi nal design
20.5.3.1 Movement
Masonry construction can be intolerant of movement, leading to
cracking. Movement can occur for a number of reasons such as:

Acoustic performance.

■

20.5.3.2 Tolerances
Tolerances in masonry construction are dealt with BS
EN1996 - part 2, clause 3.4 and Table 3.1. These are summa-
rised in Table 20.11 .

■ Thermal loads : Masonry will be subject to changes in thermal
loads. This is of particular concern in external facades where dir-
ect radiant solar gains may mean temperature changes of 50 o C.
Movement joints to accommodate thermal loads can be deter-
mined from fi rst principles based on a knowledge of the coeffi -
cient of thermal expansion of masonry. Typical values are given
in Table 20.9 .

20.5.3.3 Health and safety considerations
The design and construction of masonry, as with any material,
should consider the health and safety of the operatives on site.
The following list gives some guidance, albeit not exhaustive,
of considerations which should be made when designing and
specifying masonry construction:

■ Creep and shrinkage : Creep and shrinkage affect all construction
materials which are subject to loading. This is particularly preva-
lent in masonry where the units and mortars contain moisture.
From the National Annex to BS EN1996, table NA.7, the fi nal
creep coeffi cients for all masonry types should be taken as 1.50.

■ Over restraint : Cracking can occur when masonry is over restrained.
This is typically when masonry is closely packed or tied to other
construction elements with differing movement criteria. Examples
of this are masonry infi lls to reinforced concrete frames and ma-
sonry with concrete slabs cast directly to the upper face.

Masonry unit
type

Distance
between joints

Comments

Clay masonry
units

15 m (12 m) This may be increased where

bed-joint reinforcement is used.
Tables published with the BRC
website suggest this may be
increased to 18 or even 20 m.

■ Dimensional or support irregularities : Cracking in masonry can
occur when it is founded or supported by fl exible structures or
foundations which suffer movement. Poorly designed supports,
such as steel beams with incorrect defl ection criteria, would allow
movement and subsequently cracking. Settlement in foundations
may occur for a number of reasons, which can be identifi ed by the
type of cracking in the masonry facades. Changes in height are
also often changes in stiffness which can lead to cracking.
The difference in coeffi cient of thermal expansion of clay,
calcium silicate and concrete units means that the location of
movement joints will vary in different constructions. Guidance
is given in the National Annex. Some examples are given in
Table 20.10 . This varies from the previous guidance given in
BS 5628 (given in brackets).

Aggregate
concrete
blockwork

9 m (6 m) Applicable when L/H is equal to

or less than 3.
Similar to clay units, the distance
may be increased may where
bed-joint reinforcement is used.
Tables published with the BRC
website suggest this may be
increased to 12 or up to 15 m.

Table 20.10 Distance between movement joints

Maximum deviation
(mm)

Unit type Coeffi cient of thermal expansion

Verticality of one storey +/- 20

Clay brickwork 6 x 10 − 6 /K

Verticality of up to three storeys +/- 50

Aggregate concrete blockwork 10 x 10 − 6 /K

Straightness in one metre +/- 10

Straightness in ten metres +/- 50

For further guidance refer to NA to BS EN1996-1, Table NA.7 (© BSI, London, UK)
Adapted from and courtesy of the British Standards Institution (BSI)
Table 20.9 Coeffi cients of thermal expansion. Permission to
reproduce extracts from British Standards is granted by BSI

Overall thickness of cavity wall +/- 10

Table 20.11 Tolerances