Chapter 5 - Detailing Durability
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Transcript of Chapter 5 - Detailing Durability
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Detailing and durability requirements are to ensure
that a structure has satisfactory durability and
serviceability performance under normal
circumstances throughout its lifetime.
These requirements will involve aspects of design,
such as concrete mix selection and determination of
cover to reinforcing bars, as well as selection of
suitable materials for the exposure conditions which
are expected .
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EC2 recommends simple rules concerning the
concrete mix and cover to reinforcement,
minimum member dimension, and limits to
reinforcement quantities and spacing which
must be taken into account at the member
sizing and reinforcement detailing stage.
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The nominal cover can be assessed as follows:
Cnom = Cmin + Cdev
where Cmin shall be provided in order to ensure:
•The safe transmission of bond forces
•The protection of steel against corrosion (durability)
•An adequate fire resistance
And Cdev is and allowance which should be made in the
design for deviation from the minimum cover. It should be
taken as 10 mm. It is permitted to reduce to 5 mm if the
fabrication subjected to a quality assurance system.
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Minimum cover for bond, Cmin, b (EN 1992-1-1)
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Arrangement of
bars
Minimum cover Cmin,b*
Separated Diameter of bar
Bundle Equivalent diameter
n = nb 55 mm
where nb is the number of bars in the bundle, which
is limited to
nb 4 for vertical bars in compression
nb 3 for all other cases
* If the nominal maximum aggregate size is 32 mm, cmin,b should be
increased by 5 mm
Minimum cover for bond, Cmin, b (EN 1992-1-1)
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Minimum cover for durability, Cmin, dur (EN 1992-1-1)
Recommended structural class
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Minimum cover for fire resistance, Cmin, fire
Rather than giving a minimum cover, the tubular method based on
nominal axis distance is used. This is the distance from the centre of the
main reinforcement bar to the top or bottom surface of the member.
a > Cnom + link + bar/2
asd = a + 10 mm
From Table 5.5 and 5.6 BS EN 1992-1-2: 2004
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The minimum area of reinforcement that must be
provided within tensile zone is
As, min = kck fct, eff Act / fyk
The minimum area of reinforcement for beam also
specified in Cl. 9.2.1 as follows:
As, min = 0.26(fctm/fyk)btd but not less than 0.0013btd
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As, max
To ensure the structural member is not congested
with reinforcements and for ease during concrete
compaction work
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hagg + 5 mm
hagg + 5 mm
The minimum distance between
bars is to permit concrete flows
around reinforcement during
construction and to ensure that
concrete can be compacted
satisfactorily for the development of adequate bond.
The clear distance between bars should not be less
than the maximum of (i) the maximum bar size, (ii) the
maximum aggregate size + 5 mm, or (iii) 20 mm.
(Specified in section 8.2 EC2).
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It is a common practice to cut off bars where they are no
longer required to resist moment.
Each curtailed bar should extend a full anchorage length
beyond the point at which it is no longer needed.
The basic required anchorage length given in EC2: Cl.
8.4.3 is as follows;
lb,rqd = ( / 4) (sd / fbd) = ( / 4) (fyk / 1.15) / fbd)
= (fyk / 4.6 fbd)
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The curtailment of the tension reinforcement is based upon the enveloped of
tensile forces, Fs derive from the bending moment envelop as shown in Fig. 9.2:
EC2 such that at any location along the span, Fs = MEd/z + ∆Ftd
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When considering the curtailment the following rules must be applied:
At least one-quarter of the bottom reinforcement should
extend to the support.
The bottom reinforcement at the end support should be
anchored into the support as shown in Fig. 9.3: EC 2.
At an end support where there is little or not fixity the
bottom steel should be designed to resist a tensile force of
0.5VEd to allow for the tension induced by the shear
with a minimum requirement of 25% of the reinforcement
provided in the span.
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At and end support where there is fixity but it has been
analysed as a simple support, top steel should be
design and anchored to resist at least 25% of the
maximum mid-span moment.
At internal supports the bottom steel should extend at
least 10 beyond the face of support. To achieve
continuity and resistance to such factors as accidental
damage or seismic forces, splice bars should be
provided across the support with a full anchorage lap on
each side as shown in Fig. 9.4: EC 2.
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The design anchorage length lbd mentioned above is
given by,
lbd = 1 2 3 4 5 lbd, rqd lb, min
where
1, 2 ,3, 4 and5 = coefficient given in Table 5.5
lbd, rqd = Eq. (5.4)
lb, min = the minimum anchorage length
for tension bars: max {0.3 lbd, rqd; 10 ; 100 mm}
for compression bars: max {0.6 lbd, rqd; 10 ; 100 mm}
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Value of allows for the effect of Type of anchorageReinforcement in
Tension Compression1 The shape of the bars Straight 1.0 1.0
Other than straight 0.7 if cd > 3.0or 1.0 if not
1.0
2 Concrete cover to reinforcement Straight 1.0 – 0.15(cd - )/but 0.7 and ≤ 1.0
1.0
Other than straight 1.0 – 0.15(cd - 3)/but 0.7 and ≤ 1.0
1.0
3 Confinement of transverse reinforcement not welded to the main reinforcement
All types of reinforcement
1 – Kbut 0.7 and ≤ 1.0
1.0
4 Confinement of transverse reinforcement welded to the main reinforcement
All types, position and sizes of reinforcement
0.7 0.7
5 Confinement by transverse pressure
All types of reinforcement
1 – 0.04ρbut 0.7 and ≤ 1.0
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Table 5.5: Values of 1, 2, 3, 4 and 5 coefficients
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Laps are required when bars placed short of their
required length need to be extended. Laps are
also required when the bar diameter has to be
changed along the length. The purpose of lapping
is to transfer effectively the axial force from the
terminating bar to the connecting bar with the
same line of action at the junction.
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Requirements for laps are discussed in EC2: Cl.
8.7. The code recommends that;
Laps between bars should be staggered
and should not occur in regions of high
stress.
The arrangement of lapped bars should
comply with Fig. 8.7: EC2 below.
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Transverse reinforcement must be provided around
laps unless lapped bars are less than 20 mm
diameter or there is less than 25 % lapped bars. In
these cases minimum transverse reinforcement
provided for other purposes such as shear links will
be adequate. Otherwise transverse reinforcement
must be provided, as shown in Fig. 8.9: EC2, having
a total area of not less than the area of one lapped
bar.
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Figure 8.9: Transverse reinforcement for lapped splices
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The length of laps should be based on the minimum
anchorage length modified to take into account
factors such as cover, etc. The design lap length
required is given by;
lo = 1 2 3 5 6 lbd, rqd lo, min
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where
1, 2 ,3, 4 and5 = coefficient given in Table 5.5
For the calculation of 3 , Ast, min should be taken as 1.0As(sd/fyd), with As =
area of one lapped bar
6 = (ρ1/25)0.5 but not exceeding 1.5 nor less than 1.0 and ρ1 is the
percentage of reinforcement lapped within 0.65lo from the centre of the lap
length being considered.
lbd, rqd = Eq. (5.3)
lo, min = the absolute minimum lap length max {0.36 lbd, rqd ; 15; 200 mm}
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