Info

Table 8.1. Basic value of factor fab for transmission length according to ENV, table 4.7

Table 8.1. Basic value of factor fab for transmission length according to ENV, table 4.7

Two design values of the transmission length are defined, 0,8 and 1,2 times the basic value, of which the more unfavourable one should be used depending on the design situation.

For anchorage at the ultimate limit state, the bond is assumed to be the same as at release of prestress.

The ENV rules did not take into account the following parameters:

a) the initial prestress in the tendons1

b) the way of release of prestress (gradual or sudden)2

c) the difference in bond conditions with regard to the position of tendons etc.

d) the difference in bond conditions with regard to "push-in" and "pull-out"3

e) the difference between strands and indented wires.

The national comments on clause 4.2.3.5.6 were mainly editorial, but the rules needed a thorough update based on MC90, which represents a more up-to-date knowledge, and takes into account all the important parameters.

The present rules in clause 8.10.2 of the EN are basically a different formulation of clause 6.9.11 in MC90. There is one difference, however. MC90 gives two design values of the transmission length, with a ratio 2,0 between the upper and lower value. The rules in 8.10.2 have been calibrated to give the same mean value as MC90, but with a ratio 1,2/0,8 = 1,5 between the upper and lower values, like in the ENV. Thus, the upper value will be o lower and the lower value somewhat higher according to the EN, compared to MC90.

Among the ENV rules the alternative parabolic development of prestress, given in ENV 1992-1-3 should also be mentioned. This has not been explicitly included in the EN, since a linear development is considered to be more realistic. However, an opening for "alternative build-up of prestress" is given, without details.

8.10.2.2. Rules in MC90 and EN 1992-1-1 8.10.2.2.1 Summary of rules

The rules in MC90 and EN 1992-1-1 are summarized in table 8.2. They are described and commented more in detail in the following clauses.

1 It is stated in the ENV that table 4.7 (table 1 above) is valid for the maximum allowable prestress, but nothing is said about reducing the transmission length for lower values of prestress

2 A "neutralized zone" for the case of sudden release is mentioned in 4.2.3.5.6 (5), but no value is given

3 "Push-in" refers to the situation at release of prestress, whereas "pull-out" refers to anchorage in ULS

Expression

0 0

Post a comment