Civil Engineering Reference
In-Depth Information
Fig. 7.6 Section taken in
consideration in web shear
tension failure calculations
[I-50-3.1, II-0-2.3 and II-70-2.3b]. Nevertheless in HCS I-50-3.0a, which was also
performed with a critical length below the required transfer length, there was no
anchorage failure; the reason was that the crack which caused the failure was at a
long distance from the cantilever. If there is not enough anchorage, prestressing
contribution should not be considered but if, in this case, it is not considered in the
calculations very high SM will be obtained. It is, therefore, evident that when the
cantilever span is short, there is a partial transfer of prestressing. In Table
7.5
, the
parameters used in the basic value of the transmission length (l
pt
) calculation,
l
pt
¼ a
1
a
2
/ r
pm0
=
f
bpt
ð
7
:
1
Þ
where:
ʱ
1
= 1.25 for sudden release;
ʱ
2
(Series I) = 0.19 for 3 and 7-wire strands;
ʱ
2
(Series II) = 0.25 for tendons with circular cross section;
˕
is the nominal diameter
of the tendon;
˃
pm0
is the tendon stress just after release and f
bpt
is the constant bond
stress at which the prestress is transferred to the concrete at release of tendons. f
bpt
is
determinate with the Eq. (
8.15
) of EC2 (here (Eq.
7.2
)):
f
bpt
¼ g
p1
g
1
f
ctd
ðÞ
ð
7
:
2
Þ
ʷ
p1
is a coef
where:
cient that takes into account the type of tendon and the bond
situation at release (
ʷ
p1
= 2.7 for indented wires and
ʷ
p1
= 3.2 for 3 and 7-wire
strands);
ʷ
1
= 1.0 in this case (good bond conditions) and f
ctd
(t) is the design tensile
value of strength at time of release. All these values are presented in Table
7.5
.
Recently, Elliott et al. [
4
] has proposed an equation for prestressed hollow core
units without steel
bers manufactured using long-line techniques. The effect of this
on a typical slab design is to increase V
Rd,c
by about 10 % and reduce the zone of
transmission around holes by about 350 mm, which in some circumstances may
bene
t the failure criteria at holes.
Search WWH ::
Custom Search