Civil Engineering Reference
In-Depth Information
3.4.2.2 Full wrapping in
fibre-reinforced material
The strength of full wrapping in a fibre-reinforced material is calculated using Equation
3.78:
f
Lwd
;
GF
k
R
?
α
time
?
f
Ld
(3.78)
Where such shear wrapping is made from a CF sheet, it is the tensile strength of the
fibres that governs. However, several effects mean that this strength must be reduced.
On the one hand, the change in direction leads to transverse pressure on the CF sheet. On
the other, the unevenness of the concrete surface and holes left behind in the concrete by
dislodged aggregate lead to a loss of strength (see [11]). This is taken into account by the
reduction factor
k
R
according to Equation 3.79:
8
<
r
c
<
60 mm
r
c
60 mm
r
c
60 mm
0
:
5
?
2
k
R
(3.79)
:
0
:
5
r
c
60 mm
The reduction factor
k
R
, which takes into account the reduction in the static short-term
strength caused by transverse pressure, is based on an evaluation of numerous change-
of-direction tests carried out on concrete and metal cross-sections [56]. The value of 0.5
for this factor represents the characteristic value of an evaluation of these tests (see [11]).
As numerous different products were included in this evaluation, it could be that this
value is more favourable for sheets that are not affected by transverse compression.
However, a reduction to half the strength of the fabric in the case of shear strengthening
is also proposed in many other international publications (see evaluations in [7]
and [11]).
Moreover, the DAfStb guideline [1, 2] includes a creep rupture factor
α
time
to take into
account the effects on adhesive joints over time. This is because when using shear
wrapping made from a fibre-reinforced material there are always adhesive joints or laps
between various layers. According to [56] it is assumed that this factor allows for a lap
length of min. 80mm, which is guaranteed by the requirement for a 250mm long lap in
the detailing provisions of the DAfStb guideline [1, 2]. The creep rupture factor
α
time
assumes the use of customary epoxy resins, but other values might emerge when using
totally different resins.
3.4.2.3 U-wrapping
When using U-wrapping, the strength of the wrapping is the minimum of the strength of
full wrapping and the bond of the surface-mounted shear wrapping:
f
Lwd
min f
bLwd
;
f
Lwd
;
G
(3.80)
The tests carried out in [11] revealed that the structural response of U-wrapping also
depends very much on the position of the surface-mounted shear wrapping in relation to
the internal links. If we assume that a shear crack forms at an angle of 45
°
and the top end
of this crack is very close to one of the internal links, then an external strap attached in
the vicinity of another link will have no effect, as is shown in Figure 3.11. This is
because the effective length of the adhesive joint (shaded area of wrapping) becomes
Search WWH ::
Custom Search