Civil Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 20.2
Masonry jointing (a) Curved/recessed; (b) Square recessed; (c) Struck or weathered; (d) Over struck
coordination, tolerance between the masonry units and for the
size of the sand particles used (Thomas, 1996).
BS EN1996 defi nes limits for general purpose mortar (and
lightweight) of 6 to 15 mm thick. Guidance for thin layer joints
of 0.5-3 mm is also given, with modifi cation factors to increase
the strength of the masonry construction accordingly.
The leading edge of masonry joints may be fi nished depending
on the aesthetic desired. This can be either as work continues
(jointing) or by raking out semi-rigid mortar and replacing
with a facing or pointing mortar (pointing) (Thomas, 1996).
Pointing can allow pigmented mortar to be used, yet reduces
the area of load carrying mortar.
Joint fi nishing can be fl ush or as shown in
Figure 20.2
(Chudley and Greeno, 2008).
Jointing will often depend on architectural and durability
requirements. Generally tooled joints (struck or recessed) can
be prone to frost damage and are less water resistant (IStructE,
2008).
Repointing can be used to improve durability of old masonry,
where joints have recessed due to weathering. Careful consid-
eration is required for repointing to ensure compatibility of the
repointing mortar with the existing masonry units and mor-
tar. Repointing mortar must be suited to the environment and
should not be stronger than the existing masonry units or mor-
tar (BRE 1994).
strengths. Ties were classifi ed in the UK within BS 5628-1
as types 1 to 4. This code of practice gave the type of tie in
relation to geographical location, suitable type of construction,
density of ties and compressive and tensile resistance. UK
manufactures often relate their products to these code of prac-
tice references. Some examples are given in
Table 20.3
.
BS EN1996, clause 6.5, gives guidance on the number of
wall ties per square metre as equal to the factored wind load
per unit area divided by the declared strength of the wall ties.
The declared strength is the characteristic (published) strength
divided by the factor of safety,
m
, for ancillary products.
Standard practice from the UK is defi ned in the previous
British Standards and generally equates to 2.5 to 3.4 ties per sq m
with an increased number of ties local to door reveals and free
edges, as indicated in
Figure 20.3
.
Frame cramps is a generic term for ties used to tie masonry
to other elements, for example, steel, timber or concrete
frames. Manufacturers give declared strengths which are used
to determine the number of ties to provide 'simple' support to
a panel. It is important to ensure an adequate load path if such
Type of structure
Compressive
resistance
(M2 mortar)
Tensile
resistance
(M2 mortar)
Type 1
Heavy Duty Ties -
Suitable for most types
of construction. Not very
fl exible, therefore not
suitable for applications
where large differential
movements may occur,
i.e. masonry cladding to
timber frames
2500
2500
20.2.8 Ancillary products
20.2.8.1 Wall ties (and frame cramps, etc.)
Wall ties are used to join two leaves of masonry together, join
masonry to other materials (frame cramps) or to start masonry
walls from existing masonry construction (starter ties).
For cavity wall construction, ties transfer the wind loads
and stability forces between leaves such that the lateral resist-
ance and the slenderness can be based on the two leaves acting
together.
There are many manufacturers of wall ties and each pro-
vides data for the compressive, tensile and shear resistance of
particular types of tie. Typically the compressive strength of
wall ties is critical as they can be slender. BS EN845-3 gives
guidance for the compressive strength and suggests that the
manufacturer of the wall ties provides declared compressive
Type 2
General Purpose Ties -
Suitable for small domestic
or commercial buildings
up to 15 m high
1300
1800
Further guidance is given in Table C1 and C3 from BS 5628-1 (© BSI, London, UK)
Adapted from and courtesy of the British Standards Institution (BSI)
*Values for 1 mm displacement
Table 20.3 Typical wall tie properties. Permission to reproduce
extracts from British Standards is granted by BSI
