Conclusiones

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9.3 Discussion

For design load combinations involving wind in the current British Standard, load combination 2 considers two load situations: wind only and wind plus traffic.

9.3.1 Wind Only

The ULS partial load factors in the British Standardwhere wind acts alone are γfL = 1,40 and γf3= 1,10 giving an equivalent ULS factor = 1,54. The Eurocode partial factor value for wind alone isγfL = 1,50⁶ . Therefore the comparison factor (assuming the actions are equal) for the applied ULS factors is 1,03.

The SLS partial factors for this case are all 1,00 ( i.e. the characteristic values). For the typical structures considered, subject to wind only, the Eurocode is more onerous with comparison factors ranging between 0,77 and 0,96 at SLS (characteristic) and 0,62 and 0,77 at ULS. The differences are primarily due to a greater wind force coefficient in the Eurocode.

9.3.2 Wind (Leading) and Railway Traffic

(i) ULS

Where traffic is considered acting with the wind, for the wind component, the ULS partial factors in the British Standardare γfL = 1,10 and γf3= 1,10, which is equivalent to a ULS factor of 1,21. For the railway traffic component the factors are γfL = 1,20 and γf3= 1,10 which is equivalent to a ULS factor of 1,32.

The current British Standard only considers the case where wind is the leading action. The equivalent Eurocode partial factor at ULS considered is γQ = 1,50⁶ for the wind action, not the value of 1,70 recommended in the draft National Annex for BS EN 1990 (A1):2005, Annex A2. Applying the load classification factorα = 1,1 to the railway traffic component, along with a partial factor γQ= 1,45 and a combination factor ψ0 = 0.80, results in an equivalent factor of 1,28 at ULS. Assuming the actions are equal, the comparison factors for the applied ULS actions are 0,81 for the wind and 1,03 for the railway actions.

For the typical structures considered, the wind applied in accordance with the Eurocodes is generally greater than the current British Standard with comparison factors between 0,57 and 0,81 at ULS. The differences are due to, a greater wind force coefficient, partial factor and, wind area, in the Eurocode.

(ii) SLS

The SLS partial factors are 1,00 and the combination factors are the same as for the ULS. For the typical structures considered, the wind applied in accordance with the Eurocodes is generally greater than the current British Standard with comparison factors between 0,70 and 1,00 at SLS (characteristic). The differences are due to, a greater wind force coefficient, partial factor and, wind area, in the Eurocode.

Where the railway loading is the leading action, the comparison factor for the SLS vertical load is 1,00 and where the wind is the leading action, the comparison factor is 1,14. The difference is attributed to the load combination factor applied in the Eurocode.

6  Following the completion of this study, the partial factor for wind load has been confirmed as 1,70 in NA EN 1990(A1):2005..

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9.3.3 Railway Traffic (Leading) and Wind

The Eurocode allows wind to be combined when the railway traffic is the leading action. In this case the Eurocode ULS factors are γQ = 1,45 for the railway traffic component and application of the load classification factor α = 1,1, gives an overall equivalent factor at ULS of 1,60. The coexistent wind action partial factors are γQ= 1,50⁷ and ψ0= 0,75 which is equivalent to a ULS factor of 1,13.

The SLS partial factors are equal to 1,00 and the combination factors are the same as for the ULS.

As the load combination involving railway traffic as the leading action and wind as the accompanying action does not exist in the current British Standard, it is not possible to make an equivalent comparison. This additional case could lead to an increase in the size of structural elements which are primarily designed to resist railway traffic actions but which are susceptible to wind actions. The design of wind susceptible structural elements to the British Standard would normally involve designing the element to resist the railway traffic actions. The element would then be checked to establish that the stresses due to wind, combined with the reduced stresses due to railway traffic actions within combination 2, are within the permissible limits.

For design to the Eurocodes, structural elements such as bearings, transverse bracing, main girders, stiffeners (end and intermediate U frames) and their connections, may have to be enlarged to carry full railway traffic as the leading action coexistent with wind as the accompanying action. The change in the section sizes for the structural elements of continuous bridges or integral (e.g. portal frame) structures, could be subject to a further increase in stress if thermal effects are also considered. This is explained in section 10.

It is recommended that the partial factor adopted in the National Annex,γQ, is taken as 1,50 rather than the recommended value of 1,70 in the draft National Annex, to avoid further conservatism. (Refer to footnote).

7  Following the completion of this study, the partial factor for wind load has been confirmed as 1,70 in NA EN 1990(A1):2005.

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10 Temperature Effects

British Standards Eurocodes (incl. National Annex)

BS 5400-2:2006 BS EN 1991-1-5:2003

BS EN 1990:2002(A1) Annex A2 BS EN 1991-1-1:2002

BS EN 1991-2:2003 The Standards referred to in Section 10 are listed above.

There are two temperature effects to consider:

Global effects (expansion and contraction) Effects of temperature difference

10.1 Ultimate Limit State Actions

In accordance with BS EN 1990:2002, the design effect, E_d, is calculated from equation (6.10). The recommended values of the partial factors, load classification factor, combination factors and dynamic factors specified in BS EN 1990:2002(A1) Annex A2, BS EN 1-1:2002 and BS EN 1991-2:2003, are summarised in Table 33:

Action γ (Gor_Q) α Φ Ballast

depth factor

Leading Action

ψ0 ψ1

Permanent Self weight (steel)

1,20 Superimposed

Ballast 1,35 30%

Other 1,35

Settlement 1,20

Transient LM71 1,45 1,10 Ф3 1,0 0,80 0,80*

SW/0 1,45 1,10 Ф3 1,0 0,80 0,80*

Temperature

global 1,50⁸ 

1,0 0,60 0,60

Temperature difference

1,50⁸  1,0 0,60 0,60

*decks considered are single track or decks where a single track effect governs.