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C | Q I I I I I I I I I'l'l'l'l_I_ 1 I I I I ll|lll|lll

' 2 4 6 810 1520 40 6080100 span length L [ml

C | Q I I I I I I I I I'l'l'l'l_I_ 1 I I I I ll|lll|lll

' 2 4 6 810 1520 40 6080100 span length L [ml

Figure 6.9: Limits of natural frequencies n0 in [Hz] as a function of span length L in [m]

(2) In a bridge the natural frequencies of an element are related to the deflected form under permanent action. For a simply supported structure subjected to bending, the natural frequency may be obtained from the formula:

n 17,75

V6 o where:

60 is the deflection at mid span due to permanent actions in [mm].

Note: S0 is calculated, using a short term modulus for concrete bridges, in accordance with a loading period appropriate to the passage of the train.

6.4.3.2 Dynamic factor 4>

(1)P This dynamic factor <|> which enhances the static stresses and deflections under Load Model 71 is taken as ^ or 4b whichever is appropriate as follows:

(a) For carefully maintained track:

(b) For track with standard maintenance:

L^ (length associated with 4>) is the "determinant" length in [m] as defined in Table 6.2.

Note: The dynamic factors were established for simply supported girders. The length L4 allows these factors to be used for other structural members with different support conditions.

(2)P The dynamic factor to be used should be specified. If none is specified <t>3 shall be adopted.

Note: The dynamic factor should be specified by the relevant authority.

(3)P This dynamic factor is to be used also for Load Models SW.

6.4.3.3 Determinant lengths

(1)P The determinant lengths L^ to be used are given in the Table 6.2 below:

(2) If the resultant stress in a structural member depends on several terms, each of which relates to a separate structural component, then each term should be calculated using the appropriate determinant length.

Table 6.2: Determinant lengths L+

Case

Structural element

Determinant length Lé

Deck Plate (Steel) closed deck with ballast bed (orthotopic deck plate) (for local stresses)

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