Nonlinear Material Behavior of the Bridge Components - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 79

accordance with EN 10138 [ 2.30 ] . For steels complying with this Eurocode,
tensile strength, 0.1% proof stress, and elongation at maximum load are spec-
ified in terms of characteristic values; these values are designated, respec-
tively, f pk , f p0.lk , and e uk . It should be noted that EN 10138 [ 2.30 ] refers
to the characteristic, minimum and maximum values based on the long-term
quality level of production. In contrast, f p0.lk and f pk are the characteristic
proof stress and tensile strength based on only the prestressing steel required
for the structure. There is no direct relationship between the two sets of
values. However, the characteristic values for 0.1% proof force, F p0.lk ,
divided by the cross-sectional area, S n , given in EN 10138 together with
the methods for evaluation and verification provide a sufficient check for
obtaining the value of f p0.lk . No welds in wires and bars are allowed. Indi-
vidual wires of strands may contain staggered welds made only before cold
drawing.
2.5.2 Properties
According to EC2 [ 2.27 , 2.28 ], the properties of prestressing steel are given
in EN 10138 [ 2.30 ]. The prestressing tendons (wires, strands, and bars) shall
be classified according to strength, class, size, and surface characteristics. The
strength denotes the value of the 0.1% proof stress ( f p0.lk ) and the value of
the ratio of tensile strength to proof strength ( f pk / f p0.lk ) and elongation at
maximum load e uk . On the other hand, the class indicates the relaxation
behavior. The actual mass of the prestressing tendons shall not differ from
the nominal mass by more than the limits specified in EN 10138 [ 2.30 ] .
EC2 specifies three classes of relaxation, which are class 1 (wire or strand
with ordinary relaxation), class 2 (wire or strand with low relaxation),
and class 3 (hot-rolled and processed bars). The design calculations for the
losses due to relaxation of the prestressing steel should be based on the value
of r 1000 , the relaxation loss (in %) at 1000 h after tensioning and at a mean
temperature of 20 C. It should be noted that the value of r 1000 is expressed
as a percentage ratio of the initial stress and is obtained for an initial stress
equal to 0.7 f p , where f p is the actual tensile strength of the prestressing steel
samples. The values for r 1000 can be either assumed equal to 8% for class 1,
2.5% for class 2, and 4% for class 3 or taken from the certificate. The relax-
ation loss may be obtained from the manufacturers' test certificates or
defined as the percentage ratio of the variation of the prestressing stress over
the initial prestressing stress, which should be determined by applying one of
the expressions in the succeeding text. Expressions (2.33) and (2.34) apply to