Civil Engineering Reference
In-Depth Information
The height/thickness ratio can be increased where piers are
introduced or reinforced cavities are used. Refer to BRE GBG 14
for specific limitations/assumptions.
The clause also gives guidance on the design methods for
masonry buildings including:
Minimum wall lengths and thicknesses (table 9.2).
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Brickwork walls height 0.5-2.2 m height/thickness = 4.5-6.5
Blockwork walls height 0.4-1.8 m height/thickness = 4-6
The values above are based on moderately exposed locations.
Refer to the BRE GBG for specific limitations/assumptions.
Permitted material factors of safety under seismic (typically 2/3 of
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the normal values - clause 9.6).
Analysis methods (typically assuming moment resisting frames
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with the bending and shear stiffness of elements taken as 50% of
the actual values).
Construction requirements for masonry to floor diaphragm
20.4 Seismic design
Buildings within seismic or earthquake zones need to be
designed with suitable resistance to the horizontal forces gen-
erated and/or a suitable level of detailing to accommodate the
potentially large movements. The design of structures within
seismic zones is discussed in Eurocode 8 (BS EN1998-1:2004)
and the National Annex to this code (NA to BS EN1998).
Seismic forces are primarily horizontal (lateral) and must be
resisted by a building's lateral stability system. In most cases
seismic loads will exceed other lateral loads, and are normally
a critical load case. In simplistic terms, seismic forces applied
to a structure are proportional to the stiffness and mass of the
structure, the ground/site conditions, the building geometry
and the type/use of the building. The seismic forces deter-
mined by the site and ground conditions are modified by a fac-
tors related to the building stiffness and an importance factor
which defines a buildings use/risk should it fail.
Masonry structures are used in three forms of construction:
unreinforced, reinforced and confined masonry structures,
with unreinforced and reinforced masonry structures covered
in clause 9 of Eurocode 8.
Unreinforced masonry can be problematic under seismic
loads given the lack of ductility of this form of construction.
The ability of unreinforced masonry to sustain lateral seismic
loads depends on the strength of the units, mortar and the qual-
ity of construction. Typically unreinforced masonry buildings
can only be used in low seismicity zones with building heights
limited to less than four storeys (Chalson, 2008).
Reinforced masonry construction has greater ductility than
unreinforced masonry buildings and therefore can be used for
taller buildings, up to six storeys (Chalson, 2008).
Confined masonry is the description given to masonry panels
located within beam and column frames, such as steel or concrete
frames. In this form of construction, masonry panels act as stiffen-
ing elements to frames. The masonry acts as diagonal struts within
each frame to sustain lateral loads in compression/crushing.
In a load-bearing masonry frame, shear walls often form the
lateral force resisting system. For this form of construction clause 9
gives the following minimum values to be used in the design:
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junctions.
Clause 9.7 gives simple rules for the design and construction
of buildings which have an importance category I or II (clause
9.7 and table 4.3). The Simple Building clauses would apply to
ordinary buildings (II), for which the risk of collapse would not
have dramatic consequences, and for low risk buildings (I).
These simple rules can be used to determine the percentage
of area required to be shear wall, based on the floor area, storey
height and site acceleration. This information is presented in
table 9.3, which indicates that unreinforced masonry can really
only be used at low site accelerations and for less than three
storeys.
The clauses also state that shear walls should:
Carry 75% of the vertical load.
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Be a minimum of 30% of the length of the building in the par-
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ticular orthogonal direction.
Be spaced at least 75% of the building width apart.
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Buildings should be regular in shape (i.e. rectangular with limited
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recesses or projections).
For non-load-bearing masonry in framed construction, spe-
cial consideration is required for the detailing of the junctions
between the masonry panel and the framed construction. As
seismic loading and deflections are dynamic and are often
large, masonry infills to structures often need large movement
joints to the sides and head. This will often mean complete
separation of the frame and the panel of in excess of 50 mm
per storey.
20.5 Final design
20.5.1 The Eurocode system
BS EN1996 deals with the structural design of masonry. This
Eurocode is split into four parts, which are as follows:
Part 1-1
General - Rules for reinforced and unreinforced
masonry structures
Part 1-2
General - Structural fire design
Masonry strength (fb) = 5 N/mm
2
(or 2 N/mm
2
parallel to bed
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Part 2-0
Design considerations, selection of materials and
execution of masonry
face)
Mortar strength (fm) = 5 N/mm
2
(or 10 N/mm
2
for reinforced)
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Part 3-0
Simplified calculation methods for unreinforced
masonry structures.
