Civil Engineering Reference
In-Depth Information
Start design process for reinforced concrete.
Assess actions on element, which can be determined from EN 1991
Determine the combinations of actions which apply from EN 1990
Arrange the actions in the most unfavourable way - see CI. 5.1.3 of EN 1992-1-1.
Carry out analysis to determine the appropriate forces on the member:
Axial force,
N
ED
N
ED
Moment,
M
ED
M
ED
M
Shear force,
V
ED
V
ED
Determine cover requirements for
Fire resistance (EN 1992-1-2)
Durability (CI. 4.3 of EN 1992-1-2)
Bond (CI. 4.4.1.2(3) of EN 1992-1-2)
Carry out design at ultimate limit state for flexure, shear and axial forces.
Check deflection and cracking at serviceability limit state
Carry out detailing of the reinforcement
Figure 17.1 Design process for concrete elements
17.5.1.4 Fire resistance
Guidance for fi re resistance is provided in a separate standard -
BS EN1992-1-2 (BSI, 2004). Although this is an extensive
document, covering a variety of approaches, the simplest
approach is to use the tabular method in section 5. A variety
of tables are provided for a range of concrete element and sup-
port conditions. These tables give minimum dimensions and
a minimum axis distance. The axis distance is measured from
the face of the concrete to the centre of the principal reinforce-
ment, i.e. it is not a cover distance. The relevant axis distance
can then be compared with the required nominal cover. For
more discussion of fi re resistance, please refer to Chapter 11:
Structural fi re engineering design
.
Bar/tendon type Minimum bond requirement,
c
min,b
Single reinforcing bar
Bar diameter,
ϕ
Bundled reinforcing bars Equivalent bar diameter
(See EN1992-1-1, Cl. 8.9.1)
Circular post-tensioning ducts Duct diameter*
Rectangular post-tensioning ducts Greater of smaller dimension and
half the greater dimension*
Pre-tensioned strand or wire tendon 1.5 times diameter*
Pre-tensioned indented wire tendon 2.5 times diameter*
*These values may be amended by a country's National Annex
Table 17.4 Minimum bond requirements
17.5.2 Flexure
The fl exural design of reinforced concrete members is well
established and is based on assuming that concrete has no
tensile capacity. The tension in the section is resisted by the
reinforcing steel and the compression by the concrete. Both the
concrete and the reinforcement are assumed to act plastically
at the ultimate limit state (ULS). A variety of approaches are
taken for the shape of the concrete compression zone, com-
monly referred to as the concrete 'stress block'. The stress
block shape can be parabolic, bi-linear or rectangular. Since
there is little advantage from use of the more complex stress
blocks, generally a rectangular stress block is used. The shape
of the stress block may vary between codes of practice. In
Eurocode 2 only the stress block limits are presented; design-
ers are expected to work from fi rst principles or turn to text-
books for design equations.
A design process, including design equations, is presented in
Figure 17.2
. In this approach, a limit is placed on the normal-
ised bending resistance,
k
, to ensure that the reinforcement in the
section yields before the concrete crushes. This ensures more
ductility close to the ultimate limit state giving more warning of
an impending failure. The values given for
α
cc
,
k
1
,
k
2
,
k
3
and
k
4
