Clasificación y comparación de propuestas

British Standards Eurocodes (incl. National Annex)

BS 5400-2:2006 BS EN 1991-1-4:2005

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

BS EN 1991-2:2003 The Standards referred to in Section 9 are listed above.

The calculation and application of wind actions on typical railway bridges (see Part 2) was studied to complete the work undertaken by Scott Wilson for Network Rail. Only the wind action on railway structures and wind coexistent with railway traffic actions has been considered. A full review of BS EN 1991-1-4:2005 and the draft National Annex to BS EN 1991-1-4:2005 dated 23^rd June 2005, has not been undertaken.

The wind actions were calculated in accordance with the current British Standard and the Eurocodes for the typical railway structures and compared. It is noted that the draft National Annex modifies key clauses of the Eurocode and the study has considered the proposed modifications in the National Annex, in the calculations for this study. Explanation of the differences between the published Eurocode and the amendments made in the National Annex, should be available from the BSI committee responsible for BS EN 1991-1-4:2005 (B525/1). For the purposes of this study, it was assumed that the structures are located in a rural location near Sheffield, 50km from the sea at an altitude of 30m with the bridge 10m above the ground and topography factors were not considered.

The approach to the calculation of the wind actions is similar for both the current British Standard and the Eurocode in that the basic wind velocity is factored to account for environmental conditions and the probability of occurrence. However, the factors accounting for the environmental conditions are not directly comparable. The Eurocode combines a number of the individual factors contained in the current British Standard. For example, the Eurocode roughness factor is a function of the altitude, terrain and wind direction, all of which are separate factors in the current British Standard.

The Eurocode also includes factors not considered in the current British Standard, including the application of a seasonal factor and, in calculating the peak velocity pressure, the Eurocode considers wind turbulence. The draft National Annex simplifies the calculation of the peak velocity pressure and provides figures and correction factors. The resultant environmental factors can be compared to the British Standard, BS 5400-2:2006, environmental factors, which is the product of several factors squared (Sg.Sp.Sa.Sd)^2. The resulting value can be considered to be equivalent to the Eurocode exposure factor Ce. Therefore the dynamic pressure head, q, based on the calculation method in the British Standard can be expressed as 0,5.ρ.vb

2.c_e. The comparison factor for the environmental factors or wind pressures, considering the assumed location and environment for the typical structures, was 1,01.

Furthermore, different terminology is used in the Eurocode, for example, what is referred to as topography in the current British Standard is referred to as orography in the Eurocode.

Design of Railway Structures to the Structural Eurocodes

70

The principal difference between the Eurocode and the current British Standard is in the calculation of  wind actions on railway bridges with railway vehicles on them. The key factors contributing to this difference are:

The maps showing the basic wind speed are not the same, with the Eurocode values for the fundamental basic wind velocity generally less than the basic wind speed to BS 5400-2:2006.

The Eurocode has a maximum wind speed in this situation whereas the current British Standard does not.

The height of the railway vehicles is also greater in the Eurocode than the current British Standard.

The calculation of the wind force (drag) coefficients is different.

The ULS combination factors are different and a combination including transient railway traffic loading as the primary action acting together with wind as a secondary action is possible.

Some important aspects affecting the limiting values of wind speed on railway bridges coincident with railway traffic are as follows:

The Eurocode recommends a cut off limiting the fundamental value of the characteristic basic wind velocity to a value of 25m/s. Depending on the location of the structure and assuming orography is not significant, this equates to a peak velocity pressure of approximately 980kPa which is the equivalent pressure due to a maximum characteristic gust wind speed of 40m/s in the current British Standard.

The limiting fundamental value of the basic wind velocity in the Eurocode is appropriate, as the maximum gust speed for overturning of trains, clause B10.1 b), of GM/RT2149 'Requirements for Defining and Maintaining the Size of Railway Vehicles', sets a limit of 35 m/s in order to limit pantograph sway when trains are operating at maximum speed and maximum cant deficiency.

Furthermore GM/RT2142 'Resistance of Railway Vehicles to Roll-Over in Gales', sets limits on wind speed of 40.8 m/s for typical passenger trains and 31 m/s for typical freight trains.

However, this standard is under review and the values are being revised to 36.5 m/s and 30.5 m/s respectively.

Network Rail Company Standard RT/LS/S/021, Issue 2, October 2004, 'Weather - Managing the operational risks', sets a limit of wind gust speed of 90 mph (40 m/s), at which train services should be suspended.

Although, for the design of bridges, there is a case for adopting the lower limits set for train operation in GM/RT2142, additional conservatism is achieved by adopting a higher value. Therefore, a higher limit for the maximum characteristic gust wind speed of 40 m/s is recommended for adoption in the National Annex to BS EN 1991-1-4:2005. Note that for all locations, with the exception of central and northern Scotland, the fundamental basic wind velocity (specified on the wind action contour map in the National Annex to BS EN 1991-1-4:2005) is less than the 25m/s limiting value specified in BS EN 1991-1-4:2005.

Where the fundamental value of the basic wind velocity exceeds the limiting value in the Eurocode, the limiting value should be used when wind and railway traffic acting together is considered. If the railway traffic action is the leading action, the combination factor for the maximum wind force with traffic action is ψ0= 0,75. The maximum wind force ψ0FWk that can act simultaneously with railway traffic is limited toψ0FW**. In the latter case, a combination factor with a value ψ0= 1,00 applies.

71

The height of the railway vehicles in the Eurocode is greater than for the current British Standard.

When calculating the wind area, the depth to be considered, in both the Eurocode and the current British Standard, is the height of the train plus the depth of the bridge below the rails. The comparison factor for the wind area is a minimum of 0,93.

The effective depth of the bridge considered, d, also affects the b/d ratio used in calculating the force (drag) coefficients. The current British Standard and the Eurocode have different relationships and are not directly comparable. The Eurocode calculates the force coefficient on the total depth of the structure plus the vehicle height whereas the current British Standard calculates the drag coefficient based on the vehicle height only. As the two charts used to determine the coefficients are different, the effect of the difference is difficult to determine without further analysis. However, the force factor in the Eurocode is generally greater than the drag coefficient calculated using the current British Standard. The drag factor comparison factors range between 0,80 and 1,00 where there is no live load and between 0,73 and 1,05 with live load.

Design of Railway Structures to the Structural Eurocodes

72

9.1 Wind - Ultimate Limit State

In accordance with BS EN 1990:2002 the design effect, Ed, 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 1990:2005(A1) Annex A2, BS EN 1-1:2002 and BS EN 1991-2:2003, are summarised in Table 27. The wind action partial factors are as recommended in the Eurocode and not as set out in the draft National Annex.

Action γ (GorQ) α Φ Ballast

depth factor

Leading Action

ψ0 ψ1

Permanent Self weight (steel) 1,20 Superimposed

Ballast 1,35 30%

Other 1,35

Transient LM71 1,45 1,10 Ф2 1,00 0,80 0,80*

Wind + live load^#5 

1,50 1,00 0,75 0,50