Civil Engineering Reference
In-Depth Information
Slab type
Description
Advantages
Design considerations
Double 'T' units
A thin slab supported in narrow ribs,
or beams. Units are usually 2.4 m wide
with two ribs per unit. Structurally very
efficient for long spans
Speed of construction
Although structurally very efficient,
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Economy
the depth is significant factor for
shorter spans
Beams required to support the
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Structurally very efficient,
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Long spans
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Holes can be formed
units
Usually supplied with pre-camber,
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Low self-weight
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Aesthetically pleasing soffit■■
which increases with span
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Off-site construction
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Two-way spanning slabs
Solid slabs spanning in two-directions
and supported on beams
Good for heavy loads
Often only used for long-span or
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Long spans
heavily loaded situations because
the formwork is complex
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Holes can be formed
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Low self-weight
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Inherent robustness
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Low deflection■■
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Table 17.1 Concrete floor systems (cont.)
17.3 Preliminary sizing
To enable some initial sizes to be put to concrete members
some rules-of-thumb are provided in
Tables 17.2
and
17.3
;
the notes to the tables should be read and understood. These
rules are only intended to be a quick reference guide and more
detailed guidance can be found in
Economic Concrete Framed
Elements
(Goodchild
et al
., 2009).
their reaction in the ground. This will enable potential gaps in
the structural resistance to be identified.
A moment frame relies on the stiffness of the connections between
the vertical and horizontal members, so a frame with deep beams
will have less deflection than one using flat slabs. It is generally
considered that three storeys is a sensible maximum height for a
moment frame, although clearly this will vary depending on the
structural system the floor-to-floor height and the depth of the floor
plan (for more discussion on stability, see Chapter 15:
Stability
).
17.4 Stability
The stability of the building is of paramount importance and
should be considered at the early stages of a project. Assuming
the building does not have a basement or part basement the
main considerations for stability are lateral forces which can
be caused by:
17.5 Detailed design
Before commencing the final design, the structural engineer
should ensure that all the design data are place, including:
fixed geometry for the elements;
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wind loads
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finalised actions on the structure, especially the finishes;■■
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geometric imperfections
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position of any openings;
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accidental loads
fire resistance periods;
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any particular requirements for serviceability, for example, vibra-
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earthquakes.
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tion limits;
material classes.
A concrete framed building should be designed to resist these
actions, and this can be achieved either with shear walls or
by using a moment resisting frame. Detailed consideration of
earthquake loading is not considered here (for a discussion of
earthquake loading, see Chapter 10:
Loading
).
Where there are shear walls for lateral stability, they should
ideally be arranged so that their shear centre coincides with
resultant of the applied forces. In practice this is often not
achievable and therefore torsion/twisting moments should be
considered. There is also an inherent assumption that the floor
acts as a horizontal diaphragm; however, there are occasions
when this is not the case: for instance when precast floor units
are used without an
in situ
concrete topping, or where there are
large openings on a particular floor. The designer should sketch
out the load path for all lateral loads from their source down to
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The guidance below follows the Eurocode 2 approach which
can then be applied as appropriate to various elements.
Figure 17.1
gives the overall design process for an element.
17.5.1 Cover
The cover to the reinforcement is an important aspect of con-
crete durability and fire resistance, especially when exposed to
the elements or for long fire periods. Accordingly Eurocode 2
gives guidance on determining cover for:
bond
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durability
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fire resistance.
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