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ANALYSIS AND COMPARISON OF EUROCODE AND SNIP TRAFFIC

LOAD MODELS FOR RAILWAY BRIDGES

Algirdas Jonas Notkus1, Zenonas Kamaitis

2

Dept of Bridges and Special Structures, Vilnius Gediminas Technical University,

Sauletekio al. 11, LT-10223 Vilnius, Lithuania.

E-mails:[email protected];

[email protected]

Abstract. The traffic loads and consequently the load effects in the bridge structures determined according to the

Eurocode (EN) have been found to be substantially different from the values obtained by the SNiP code model used

in Lithuania. For highway bridges the SNiP loads are lighter (sometimes 1,5-2,0 times), for railway loads they are

considerably heaver. It was decided to investigate whether railway bridges designed to EN and SNiP load models

have sufficient capacity and how to achieve good agreement between the load effects determined according to both

codes. In this article the results of comparison for simply-supported bridge decks with span lengths ranging

from 2 to 80 m. are presented. Rail traffic load models from EN and SNiP are analysed. The objective of

the study was to modify the Eurocode rail traffic load factors to fit them to the SNiP loading provisions

and to determine the consequences for bridge supestructures, if the bridges were designed according to

the code SNiP and assesed according to the Eurocode. It is shown that the actions imposed on bridges of

10-40 m length of spans (that is common in Lithuanian practice) due to rail traffic according to the EN

code are 20-40%

lower than those determined according to the SNiP. The EN code gives even five traffic load

models in comparison with only one load model in the SNiP. However the effects of load model SW/2 are sometimeslower than those calculated by LM71. The recommendations are presented how to adjust the loading models for both

codes in order to insert the specifications of loads on bridges in new national Annex actually in prepara-

tion for Lithuanian standard LST EN 1990/A1:2006 lt.

Keywords: railway bridges, traffic load models, Eurocode, SNiP, flexural moments, shear forces, recommendations.

1. Introduction

The railway transport forms an important part of a

whole transportation system of each country. Lithuania is

being crossed by Trans-European Rail Corridors connect-

ing Baltic countries via Warsaw to the rest of EU as wellas Lithuania with Byelorussia, Ukraine and Russia be-

coming a part of major Trans-Asian corridors (Vasili-

auskas and Barysiene 2008). The main problem facing

the railway bridges is that national and EU territory rail-

way networks operate to different track and load stan-

dards.

Actual traffic loads on bridges are quite complex be-

cause of the randomness of traffic process. Normally,

they are represented in the codes by simple load models,

reflecting the worst loading that can be caused on the

bridge by actual and future traffic. Bridge loadings de-

scribed in codes of practice vary from country to country

(e.g., OConnor and Shaw 2000). It is evident that therehas been a trend towards the increase in design loads and

loading configurations over the years in most countries.

Problems occur when different codes or standards are

combined together.

In Lithuania many existing bridges were designed

using codes SNiP (

84). Design of road bridges

according to Eurocodes started from 2000. Lithuanianstandards LST EN 1991-2:2005 lt and

LST EN 1990/A1:2006 lt were issued which give rules

and methods for establishing combinations of actions for

road and railway bridges. Preliminary analysis and com-

parison of traffic load effects determined according to

SNiP and first pre-standards ENpr at that time showed

that the Eurocode loads for existing highway bridges are

heaver sometimes up to 1,5-2 times (Notkus 1998, 2005)

and for railway bridges are considerably lighter (Notkus

1998, 2003; Cypinas 2003, Cypinas et al. 2005). This

means, for example, that old highway bridges need repa-

ration and strengthening that is not always reasonable.

The same situation is observed and in other countries.


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Comparison of national rail traffic loads which

have been found to be significantly different in the dif-

ferent codes (Gu et al. 2006; Heiden et al. 2003;

OConnor and Shaw 2000) to the EN provisions is of

vital importance. Some publications on this subject can

be found (e.g., James 2003; Sieffert et al. 2006). Mod-

ernization of European rail network with the aim toincrease axle loads up to 33 tones for freight traffic and

the maximum speeds up to 350 km/hour for passenger

traffic (Cho et al. 2010) is leading to the need to re-

evaluate loading conditions for high-speed railway

bridges. Note that there are significant cost benefits in

the use of heavier and rapid trains.

In this paper some comparisons of the SNiP code

and the Eurocode (EN) railway bridge loadings are pre-

sented in order to insert the specifications of loads on

our bridges in new national Annex actually in prepara-

tion for Lithuanian standard LST EN 1990/A1:2006 lt.

2. Presentation of EN and SNIP traffic loads

In EN 1991-2 (EN 2003) five models of railway

static vertical loading are given:

Load Model 71 (and Load Model SW/0 forcontinuous bridges) to represent normal rail

traffic on mainline railways;

Load Model SW/2 to represent heavy rail traf-fic;

Load Model HSLM to represent the loadingfrom passenger trains at speeds exceeding 200

km/h;

Load Modelunloaded train to represent theeffect of an unloaded train.

N o l i m i t a t i o n N o l i m i t a t i o n

1 , 6 1 , 6 1 , 6

0 , 8

0 , 8

Q

v k

q

v k

= 8 0 k N / m

q

v k

= 8 0 k N / m

2 5 0 k N

2 5 0 k N

2 5 0 k N

2 5 0 k N

d i s t a n c e , m

Fig 1. Load Model 71 and characteristic values for

vertical loads

Load Model 71 represents the static effect of vertical

loading due to normal rail traffic. The characteristic val-

ues given in Fig 1 shall be multiplied by a factor , on

lines carrying rail traffic which is heavier or lighter than

normal rail traffic. The factor

may be specified in the

National Annex and shall be one of the following:

= {0,75; 0,83; 0,91; 1,00; 1,10; 1,21; 1,33; 1,46}.

Load Model SW/2 represents the static effect of ver-

tical loading due to heavy rail traffic (Fig 2 and Table 1).

Fig 2. Load Models SW/0 and SW/2

Table 1. Characteristic values for load Models SW/0 and SW/2

Load model qvk, kN/m a, m c, m

SW/0 133 15,0 5,3

SW/2 150 25 7,0

The effect of lateral displacement of vertical loads

should be taken into consideration. The eccentricity of

vertical loads for LM71 and SW/0 e r/18 (ris distance

between wheel loads), the ratio of wheel loads on all axis

on one track 1,25:1,0 or 0,5556:0,4444 (section 6 of EN

1991-2 or LST EN 1991-2:2005 lt). Partial load factor f

is 1,45 for LM71, and 1,2 for SW/2 (Table A2.4(b) of EN1990:2002/A1:2005).

The dynamic amplification factor 3 which en-

hances the static load effects under load models LM71,

SW/0 and SW/2 for track with standard maintenance is

determined as follows:

73,02,0

16,23 +

=

L

,

,0,20,1 3 (1)

where

L is the length specified for particular elements.

For example, for simple supported main girders,

L is

the span length.A static analysis of the bridge structures is carried

out with the load models described above using the dy-

namic factor 3 , partial load factor f, and 1 (if

required).

The rail traffic load of the SNiP is based on the load

model CK14. The load is given as a uniformly distributed

line load, with different values depending on the loading

length and shape of influence line (different for bending

moments and shears forces). The CK14 line loads are

presented in SNiP ( , 1984) (see Table 2). Partial

load factor is taken as 1,3 for loading length 0, 1,15 for

loading length 50 m. and 1,1 for loading length 150 m.

Intermediate values are determined by interpolation.

Wheel loads are uniformly distributed on the track rails

(0,5:0,5).

Table 2. Characteristic values for load model CK14 (Extract

from Table 1 Annex 5 of

2.05.03-84)

Length,

m

kN/m

for V

kN/m

for M

Length,

m

kN/m

for V

kN/m

for M

2 428 374 20 207 181

4 304 266 40 168 147

6 273 239 60 151 137

10 245 214 80 144 137

15 223 195 100 140 137


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The dynamic factors of SNiP for steel deck is

+

+

+=+

.15,1)1(

;30

181)1(

(2)

for concrete deck is

+

+

+=+

.15,1)1(

;20

101)1(

(3)

where

is the length of loading.

3. Analysis and comparison of traffics loads

The load models of EN ir SNiP have been compared

according to maximum characteristic and design bending

moments and shear forces on simply-supported bridge

decks with span lengths ranging from 2 to 80 m. carryingsingle lane of rail traffic. For spans up to 50 m the girder

decks are considered, for longer spans the truss decks are

analysed. Bridge decks are composed of two girders or

two trusses carrying only one track. Decks of steel

bridges are located between the main trusses near the

bottom chords. Comparison was performed including

partial load factors, impact factors, and eccentricities

when designing new bridges as is presented in section 2.

The comparison is shown in Figs. 3, 4, 5, and 6. In

these figures bending moments and shear forces are

presented as the ratios between the bending moments

(shear forces) due to EN and SNiP code live load models

and to the load model CK14 for concrete beams (MCK,conor VCK,con). In figures they are noteded as the ratios ofcharacteristic Mk,L/Mk,CK,con (Vk,L/Vk,CK,con) and design

Md,L/ Md,CK,con (Vd,L/ Md,CK,con) values.

As can be seen from presented figures, discrepancy

of moments and shear forces determined according to the

EN and the SNIP load models is significant.

Fig 3. Comparison of deck characteristic flexural

moments for different code traffic load models and

deck span lengths

Fig 4. Comparison of deck design flexural moments

for different code traffic load models and deck span

lengths

Figure 3 and 4 compare bending moments. For rein-

forced concrete decks spanning 10-40 m characteristic

Mk,LM71 and Mk,SW/2 are approximately 20 % and designMd,LM71 is 10% and Md,SW/2 is 20% lower then Mk,CK.con

and Md,CK,con, respectively. The major difference between

moments is seen to occur on spans less then approxi-

mately 5 m. For shorter spans (1-5 m) LM71 model

shows increased relative values of deck bending moments

due to influence of much higher dynamic factor (ap-

proximately 0,23 = ) in relation with (1+) = 1,45 for

reinforced concrete decks and (1+) = 1,56 for steel

decks according to code SNIP.

The bending moments for load model SW/2 for deck

spans up to 10 m are up to 2 times lower nevertheless that

about 1,4 time higher dynamic factor was used.Much more discrepancy of calculated values is ob-

served when comparing shearing forces. Figures 5 and 6

illustrate this phenomenon. For concrete bridge spans

ranging from 10 to 40 m. characteristic shear forces in-

duced by LM71 are from 40 to 20 % respectively lower.

Load model SW/2 also gives approximately 40 % lower

values. A similar lower value (40-20 % ) is observed also

for design forces.

Fig 5. Comparison of deck characteristic shear forces

for different code traffic load models and deck span

lengths


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Fig 6. Comparison of deck design shear forces for dif-

ferent code traffic load models and deck span lengths

Eurocode load model SW/2 seeks to model heavy

rail traffic. As follows from figures, for deck spans 10-40 m. load model SW/2 only slightly influences load

effects with regard to load model LM71: characteristic

moment values are slightly higher, design moment values

are even lower (approximately 15%

), characteristic and

design values of shear forces are approximately equal.

For the bridge decks up to 10 m. the load effects are from

1,5 to 3 times lower then those calculated by SNiP load-

ing. Hence, in most cases, the SW/2 heavy load model

may not provide the required load carrying capacity of

bridges for current and future rail traffic in Lithuania. As

a result, load and speed restrictions may be imposed to

prevent overloading of railway bridges. Inevitable some

modifications on application of load model SW/2 fornational bridges are needed.

4. Recommendations

The National Standards implementing Eurocodes

may be followed by the National Annexes. They may

include, for example, alternative load models, values of

and dynamic factors (see section 2). In an attempt to har-

monise EN and SNIP traffic loads three alternatives were

analysed.

Case 1. Adjusted values of the LM71 load model

can be obtained using Eurocode factor

, for example 1,1

or 1,3. However, constant value of

may lead to a largeerror because calculated action effects according to both

codes were substantially different for different lengths of

spans. Constant value of factor

may be used only for a

given span length.

Case 2. Different values of based on the length of

span or the length of influence line could be used. How-

ever, the span lengths should be divided into intervals.

The observed range of factor for flexural moments was

0,9 to 1,1 for span lengths 1-20 m., 1,0 for spans 20-50

m, 1,1 to 1,33 for spans 50-70 m., and 1,33 for longer

spans (see Fig 7). Unfortunately, these values do not fit

for shearing forces: for span lengths 10-50 m. the dis-

crepancy is of the order of 25-20 % (Fig 8). On the otherhand, in this case the SW/2 load model becomes not use-

ful.

Fig 7. Comparison of adjusted according to case 2

deck design flexural moments for different code traffic

load models and deck span lengths

Fig 8. Comparison of adjusted according to case 2deck design shear forces for different code traffic load

models and deck span lengths

Case 3. The heavy rail traffic SW/2 model loads as

well as those of LM71 were multiplied by the same par-

tial load factor f = 1,45 with the same lateral eccentricity

of vertical loads 18/re=

(the ratio of wheel loads

0,5556:0,4444).


deck design flexural moments for different code traffic

load models and deck span lengths


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deck design shear forces for different code traffic loadmodels and deck span lengths

The comparison of load effects is shown in Fig 9

and Fig 10. As can be seen that there is relatively a good

agreement between the bending moments and shear

forces imposed by the SW/2 design load and the designload used in Lithuania. The difference is in the range of 5

% for design bending moments and 5 to 15 % (for spans

7-25 m.) for design shear forces. Characteristic values of

internal forces induced by SW/2 and presented in Fig 3

and Fig 5 would be increased of the order of 10% (influ-

ence of eccentricity). Hence, the proposed load adjust-

ment is simple, based on consideration of national traffic

regulations.

Conclusions

1. For concrete rail bridge spans ranging from 10 to 40

m., that is typical in Lithuania, bending momentsand shear forces induced by the Eurocode LM71 and

SW/2 load models are up to 40 % lower then thosedetermined according to SNiP.

2. The SNiP traffic load produces the highest values of

bending moments and shear forces indicating that

Lithuanian bridges designed for live load C14 have

sufficient capacity to carry the Eurocode rail traffic

loads.

3. Heavy rail traffic load model SW/2 for deck spans of

1-50 m only slightly influences load effects with re-

gard to load model LM71.

4. For design of new railway bridges on Lithuanian

railways with different from European track stan-dards it is recommended to adjust the heavy rail traf-

fic SW/2 load model for national application. New

national Annex for Lithuanian standard

LST EN 1990/A1:2006 lt actually is in preparation.

The rail traffic SW/2 model loads as well as those of

LM71 should be multiplied by the same partial load

factorf = 1,45 with the same lateral eccentricity of

vertical loads 18/re = (the ratio of wheel loads

0,5556:0,4444). Two design situations with the load

models LM71 and SW/2 should be analysed and the

most unfavorable case identified that will be used

for the bridge design.

References

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darto projekto prLST EN 1991 2 nacionalinis priedas.[Lithuanian prestandard project prLST EN 1991 2nd Na-

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Cypinas, I. 2003. Euronorm eismo apkrov palyginimas suklasifikuotomis eismo apkrovomis [Comparison of Euro-

code traffic loads with classified traffic loads], i Statyba

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