On the Need to Increase the Design Load and Adoption of a Uniform Service Load for Bridges in Europe

Abstract

Since 2010, the same standard has been used in EU countries, which includes design load models for road bridges. However, some countries have adopted load factor values other than those recommended in the standard, which has resulted in large differences in the design load of bridges, including bridges located on trans-European network routes. In addition, the standard design load is close to that of a column of actual heavy vehicles when congestion occurs on a bridge. This chapter proposes the adoption of a uniform increased design load for bridges, guaranteeing the safe transfer of European traffic. Currently, vehicle traffic on the trans-European road network traverses thousands of bridges designed to different and outdated load standards. The future of these bridges depends on what load is adopted when checking their service load capacity. This chapter proposes adopting a service load based on the characteristics of the vehicles allowed on European roads.

1. Introduction

The smooth operation and development of the European market and Europe’s economic, social and territorial cohesion depend on the functioning of the trans-European transport networks. These networks concern road, rail, maritime and inland waterway traffic. Within the transport networks, the road network occupies an important place. There are thousands of bridges and viaducts (hereafter referred to as “road bridges”, “bridges” or “structures”) on the main public roads in Europe that make up the trans-European road network. The usability of these bridges is determined by their load-bearing capacity. Some bridges with a load capacity sufficient to carry the current traffic will be able to continue to be used without reconstruction; some will require reconstruction (strengthening or widening), and some will be demolished, and new bridges will be built in their place. How a particular road bridge in service should be dealt with: leave it without reconstruction, reinforce it or demolish it, depends on both its design load capacity and its service load capacity. These are two load-bearing capacities that depend on each other, but are completely different.

1.1 Design load capacity and service load capacity of a bridge

The design load capacity of road bridges is the load capacity that the structure has when designed to a given load standard and for the load class specified in the standard. Until 2010, each European country had its own national load standards. So, today, vehicles on trans-European road network traffic over bridges are designed to meet different standards. As a rule, the later the standard was issued, the higher the design load of the bridge. Without reconstruction, a bridge has (and will always have) only one design load capacity – the one it was designed for, regardless of which load standard it was designed to, for which load class and over what period.

Since 1 April 2010, in Europe, only the European standards for the design of structures for civil engineering works, the so-called Eurocodes [1, 2] with their national annexes [3, 4, 5, 6], have to be used for the design of bridges. These standards are currently used in 33 countries that are members of the European Committee for Standardization.

Road bridges have a service load capacity after commissioning. It can be assumed that the load-carrying capacity of a bridge is the load of vehicles with such a total vehicle weight that the load-carrying capacity and serviceability conditions of the structure are met. If the weight of the vehicles allowed to traffic on the structure is less than the weight of the vehicle allowed to traffic on the public road without restrictions, the bridge should be marked. The sign shall bear a number indicating the actual total weight, in tons, of the vehicle authorized to use the structure. Thus, the carrying capacity of a bridge is determined by the weight of the vehicle allowed to drive on that structure without restrictions. In Poland, the rules for determining the load capacity of road bridges were developed by the author of this chapter and recommended for use in 2004 by the General Director of National Roads and Motorways [7].

1.2 Difference between design load and service load of a bridge

Fundamental to the determination of both the design load and the service load of bridges are the load models. The variable design load for road bridges is specified in EN 1991–2 Eurocode 1 [2]. This standard gives the values of the forces and how the design load models are located on the roadway of the structure. These models represent a simulation of road traffic.

The design load should be used to determine the service load capacity of bridges in service. Eurocode 1 [2] provides some information related to service load but does not provide models for such load. In this situation, it is sometimes assumed that bridges in service should meet the design requirements and carry the design load. Since the internal forces in a bridge structure under design load are generally greater than under service load, the use of design requirements to determine the service load capacity of in-service structures is unjustified. An in-service structure designed to an outdated standard will never meet the current design requirements because it was designed for a lower load.

According to the Regulation of the European Parliament and of the Council [8], each structure during its foreseeable lifetime should have an adequate load-bearing capacity and should ensure safety in use. For structures in service, the safety of their use should be ensured by specifying the conditions of that use, that is, the indication of the service load, including the maximum weight of the vehicle allowed to traffic on the structure without restriction, and the positioning on the roadway of the vehicles allowed.

2. Design load models for bridges according to the European standard

Variable design load models for bridges are contained in the European Standard Eurocode 1 [2] (in this chapter, the term “European Standard” or “Standard”, without reference to a bibliographic entry, means Eurocode 1 [2]). It should be noted that, in discussing design loads, the provisions of this standard have been used, giving them in inverted commas without any authorial commentary.

In the standard, the design load models have been adopted in such a way that their impact is that of “the actual in the year 2000 traffic in European countries” (Section 4.2.1(1)). A dynamic surplus (dynamic factor), determined “for a medium pavement quality and pneumatic vehicle suspension” (Section 4.2.1(1)), is included in the load values.

The load values are given as characteristics. According to the European Standard for the fundamentals of structural design, the characteristic value of a variable action corresponds to a nominal value, which can be taken when the statistical distribution is unknown. Thus, the values of loads as characteristic values will be accepted for analysis without additional safety factors ([1], Section 4.1.2(7)).

Four vertical force design load models are given in the standard:

• Model 1, which is the basic model and concerns the loading of the bridge with vehicles;

• Model 2 concerns the loading of the bridge with one vehicle axle;

• Model 3 is for the loading of the bridge with special standard vehicles;

• Model 4 concerns the loading of the pavement with a crowd of pedestrians.

Since, of the above four models, Model 1 is the basic loading model and is related to vehicle loading, and this model was adopted for further analysis.

2.1 Characteristics of the basic load model

Model 1 consists of a characteristic load (indicated by the subscript “k” when describing the loading) of the roadway with concentrated forces Q_ik and a uniformly distributed load q_ik, where “i” is the lane number. In this model, the concentrated forces are two axles (tandem TS) located in each lane. The load values are given in Table 1.

Lane No.	Load Model 1
	Qk – concentrated forces; - tandem TS	qk – uniformly distributed load UDL
	[kN]	[kN/m^2]
1	2 × 300	9.0
2	2 × 200	2.5
3	2 × 100	2.5
>3	—	2.5

Table 1.

Characteristic load values in model 1.

According to the standard, the lane width is 3.00 m, the tandem wheelbase is 1.20 m and the wheelbase per axle is 2.00 m. Only full tandem sets are to be considered for loading, and uniformly distributed loads are to be placed at the most unfavorable point of the impact surface. The order in which the lanes are positioned on the roadway should be such that the effects caused by the loads are most unfavorable (Section 4.2.4(2)). For span lengths greater than 10 m, the tandem arrangement is replaced by a single-axle concentrated load equal to the sum of the two axle loads (Section 4.3.2(6b)). A diagram of the load model, in side and top views, is shown in Figure 1. The load, according to Model 1, is the product of the characteristic load and the adjustment factors.

Figure 1.

Diagram of load model 1.

2.2 Values for the adjustment factors recommended in the European standard

In the European Standard, the adjustment factors are given for the characteristic load in Model 1:

• α_Qi – for concentrated forces – tandem system TS;

• α_qi – for uniformly distributed load UDL.

The standard recommends the following minimum adjustment factors: α_Qi ≥ 0.8 and α_qi ≥ 1.0, with i ≥ 1 (Section 4.3.2(3)). “For road bridges, Load Models 1 and 2 (…) and taken into account with adjustment factors α and β equal to 1, are deemed to represent the most severe traffic met or expected in practice, other than that of special vehicles requiring permits to travel, on the main roust of European countries” (foreword). With this value of factors, “a heavy industrial international traffic is expected, representing a large part of the total traffic of heavy vehicles. For more common traffic compositions (highways or motorways), a moderate reduction of α factors applied to tandem systems and uniformly distributed loads on Lane 1 may be applied (10 to 20%)” (Section 4.3.2(3)).

In Table 2.1 of the European Standard, the characteristic load values in Model 1 are assumed such that, with adjustment factors α equal to 1.0, the probability of them being exceeded on Europe’s major roads in 50 years is 5%, indicating a “1000-year return period.”

2.3 Values for adjustment factors adopted in some European countries

Nationally Determined Parameters are allowed in the European Standard. These parameters are given in the national annex to the standard (national annexes are denoted by the acronym NA). Such parameters include the adjustment factors α_Qi and α_qi, which increase or decrease the characteristic load in Model 1.

In European countries, including EU member states, the national annexes to the standard have adopted the adjustment factors α_Qi and α_qi with different values [9]. In some countries, factor values of 1.00 have been adopted, and in some countries, such as Denmark, France, Germany, Poland and the United Kingdom, among others, values other than 1.00 have been adopted.

Table 2 summarizes the adjustment factors used in some European countries for the design of bridges located on the trans-European road network.

European country	Adjustment factors
	αQi		αqi
	i = 1	i > 1	i = 1	i = 2	i > 2
Most countries	1.00	1.00	1.00	1.00	1.00
Denmark [5]	1.00	1.00	0.67	1.00	1.00
France [6]	1.00	1.00	1.00	1.20	1.20
Germany [4]	1.00	1.00	1.33	2.40	1.20
Poland [10]	1.00	1.00	1.33	2.40	1.20
United Kingdom [3]	1.00	1.00	0.61	2.20	2.20

Table 2.

Summary of adjustment factors for the design of bridges over the trans-European road network.

Designations adopted in the table:

i – lane number;

α_Qi – adjustment factor for concentrated forces;

α_qi – adjustment factor for uniformly distributed load.

Analyzing the values of the factors given in Table 2, it can be seen that:

• in Denmark, the value of the factor for the load uniformly distributed on the first lane was reduced by 33%;

• France increased the factor for the load uniformly distributed on lanes other than the first lane by 20%;

• in Germany and Poland, the factors for the load uniformly distributed on all lanes have been increased by 33% for the first lane, 140% for the second and 20% for the other lanes;

• in the United Kingdom, the factor for the load uniformly distributed on the first lane was reduced by 39% and the factor for the other lanes was increased by 120%.

The adoption of different adjustment factors by different European countries (in the appendixes to the standard) results in different design load values being adopted for bridge design in these countries. This, in turn, results in different design loads for bridges, that is, European road traffic is not provided with the same level of safety. For example, if the values of the adjustment factors given in Table 2 are adopted, with design loads according to the same European standard, the load-carrying capacity of bridges designed in Denmark will be lower than those designed in France, and in France will be lower than those designed in Germany and Poland [9].

3. Proposal for adoption of increased equal design load by European countries

Equal loading of bridges designed on the trans-European road network can be achieved by adopting equal values for the adjustment factors in Model 1. In this respect, there should be no freedom if traffic in Europe were to be carried by structures with the same level of road safety.

The study [11] gives the history of the work on the European load standard. Work on the standard started in 1987, but traffic surveys were carried out in several European countries as early as 1977. It was these surveys that formed the basis for the adoption of design load models, including Model 1. It should be noted that almost 50 years have passed between the time the surveys were carried out and the present. During this period, traffic has changed fundamentally: vehicles have more weight and their share of traffic is greater than it was a few decades ago [12].

In the Eurocode design system, according to Table 2.1 in EN [1], the design of new bridges should take into account a minimum service life of 100 years. In this case, suitably high design loads should take into account the progression of service loads over this period and the progressive loss of service properties over time, while maintaining the assumed operation and ongoing maintenance of the structure.

Given that:

• Model 1 design load for bridges was adopted from traffic studies of 50 years ago,

• bridges designed on the basis of this load model should have a service life of at least 100 years,

European countries in their annexes to the standard should adopt adjustment factors that will result in the highest possible values of internal forces induced by the design load. Such factor values were adopted in Germany and Poland – these factor values were adopted for the analysis.

Table 3 summarizes the concentrated force (TS) and uniformly distributed load (UDL) values, with the standard factors α_Qi and α_qi having a value of 1.00 and the proposed increased factors, with different values for each lane.

No.	Standard design load				Proposed design load
	αQi	TS	αqi	UDL	αQi	TS	αqi	UDL
	[−]	[kN]	[−]	[kN/m2]	[−]	[kN]	[−]	[kN/m2]
1	1.00	2 × 300	1.00	9.0	1.00	2 × 300	1.33	12.0
2	1.00	2 × 200	1.00	2.5	1.00	2 × 200	2.40	6.0
3	1.00	2 × 100	1.00	2.5	1.00	2 × 100	1.20	3.0
>3		—	1.00	2.5		—	1.20	3.0

Table 3.

Characteristic load values in model 1 with standard factors and proposed increased factors.

In characterizing the proposed design load, it can be seen that the values of the adjustment factors for concentrated forces are as recommended in the standard – for all lanes, they have a value of α_Qi = 1.00, while the values of the adjustment factors for uniformly distributed load have higher values than the standard – for the following lanes, the values are respectively: α_q1 = 1.33, α_q2 = 2.40 and α_q > 2 = 1.20.

3.1 Assumptions made for the analysis

1. It was assumed that the comparison of the standard design load with the proposed design load (Model 1 with different values of adjustment factors) is based on the comparison of internal forces – bending moments and transverse forces – in the load-bearing member of the bridge span structure.

2. Bridges with the static scheme of a simply supported beam were assumed. In spans with this static scheme, the internal forces in the load-bearing members of the structure that determine the strength of the section are usually bending moments and transverse forces.

3. Spans up to l = 200 m were assumed, as this span was determined by calibration of the basic design load – Model 1 (Section 4.1(1)).

4. Due to the analyzed spans up to 200 m, beam span cross-sections (without walkways) and the number of girders equal to: n = 2, 4, 6 and 12 were assumed.

5. It was assumed that the analyzed load-bearing element of the bridge span structure is the outermost girder, as it follows from the analysis of the transverse load distribution of the beam span and from the design practice that under standard loading, the outermost girder is usually the most overloaded load-bearing element in the span (the mutual location of the axis of this girder and the edge of the roadway determines this).

6. The usable width of the roadway has been assumed to be b = 6.00, 9.00 and 12.00 m, as, according to Table 4.1. in the standard, the width of the conventional lane is 3.00 m – that is, a two-lane, three-lane and four-lane roadway has been assumed, respectively.

7. The axial spacing of the outermost girders “s” has been assumed such that each girder has a roadway lane of the same width (e.g., if n = 2 and b = 6.00 m, then s = 3.00, and if n = 6 and b = 9.00 m, then s = 7.50 m).

8. In order to compare the internal forces in the extreme girder of the span structure, the forces arising in a simply supported beam loaded as individual lanes were multiplied by the ordinates of the line of influence.

9. The values of the ordinates of the lines of influence of the transverse load distribution in the structures were calculated using the Courbon method [13]. This is a method that assumes that there is a transverse beam with infinite stiffness at the center of the span. It is assumed that with a span-to-width ratio of the bridge span of not less than 2, the results obtained are correct. On the other hand, it should be noted that in this chapter, the loads are compared and in this case, the values of the ordinates of the transverse section are not so relevant.

10. As the horizontal forces (braking and acceleration) “should be calculated as a fraction of the total maximum vertical loads corresponding to Load Model 1” (4.4.1(2)), only the vertical loads were analyzed.

11. The loads given in the standard are expressed in kilonewtons (kN). However, due to the stipulation in Article 2 of the Council Directive [14] that a ton means the weight due to a load of one ton corresponding to 9.8 kilonewtons and the expression in this Directive in tons of the permissible mass of the vehicle per axle, in the following analysis the mass per axle is expressed in tons and denoted by the letter “t”.

3.2 Analysis of standard design load and proposed increased load

For the analysis, the standard design load – Model 1 – was adopted with adjustment factors:

• those recommended in the standard: both the value of the factor for concentrated forces α_Qi and the factor for uniformly distributed load α_qi is 1.00;

• proposed for adoption by European countries (already adopted by Germany and Poland): the value of the factor for concentrated forces α_Qi is 1.00, and the value of the factor for uniformly distributed load α_qi is 1.33 for the first lane, 2.40 for the second lane and 1.20 for the third and other lanes.

Table 4 gives the ratio of the internal forces in the extreme girder of the span at the proposed increased design load (with adjustment factors of a higher value than the standard), compared to the design load with standard factors. The force values were compared for spans equal to l = 50, 100, 150 and 200 m. The force ratios apply to both bending moments and transverse forces and to spans with different numbers of beams (n = 2, 4, 6 and 12).

l	Width of bridge deck
[m]	2 × 3 m = 6 m	3 × 3 m = 9 m	3 × 3 m = 12 m
50	1.19–1.20	1.21–1.24	1.23–1.24
100	1.25–1.26	1.28–1.32	1.30–1.33
150	1.27–1.28	1.32–1.36	1.34–1.37
200	1.29–1.30	1.34–1.39	1.37–1.40

Table 4.

Ratio of internal forces under loading proposed and standard.

Figure 2 shows the values of the internal forces in the outermost girder of the bridge span, with roadway widths of b = 6, 9 and 12 m and with the number of beams n = 2. The internal forces are bending moments (as unit forces, i.e., bending moments divided by the span). In Figure 2, the design load with factors α_Qi = α_qi = 1.00 recommended in the European standard is denoted “EU” and the design load with factors proposed to be uniformly acceptable in European countries is denoted “PL” (this is an acronym for “proposed load”).

Figure 2.

Values of bending moments in the outermost girder of a bridge span with roadway widths b = 6, 9 and 12 m and number of beams n = 2, under standard and proposed design loads.

Analyzing the data summarized in Table 4 and Figure 2, it can be concluded that the ratio of loads practically does not depend:

• on the type of internal force, that is, in a beam span, the ratio of bending moments is the same as the ratio of transverse forces, induced by these loads;

• on the number of girders in the cross-section of the span, that is, the ratio of bending moments in a 2-girder span is the same as that in a 12-girder span induced by these loads.

Moreover, analyzing the data summarized in Table 4, it can be concluded that:

• the larger the span is, the greater is the impact of the increased value of the factors, that is, increasing the span from l = 50 to l = 200 m, with a two-lane roadway, the value of the internal force increases from 19–30%, and with a four-lane roadway from 23–40%;

• the greater the number of lanes on the roadway, the greater the impact of the increased value of the factors, that is, for example, with a span of l = 100 m, the value of the internal force with a 2-lane roadway increases by about 25%, and with a 4-lane roadway by about 30%.

Since – under the assumptions made – the load ratio practically depends neither on the type of internal forces adopted nor on the number of girders, in further analysis, only the lateral forces in a 2-girder structure and – solely for the purpose of checking the calculations – a 12-girder bridge span structure were compared.

4. Characteristics of lorries according to the council directive and according to the European standard

The Council Directive [14] specifies the maximum weights and axle loads of vehicles allowed in national and international traffic in Europe. Vehicle weights depend on the number of axles. The maximum vehicle weight is:

• 26 t, if the vehicle has 3 axles;

• 32 t, if the vehicle has 4 axles;

• 40 t, if the vehicle has 5 axles.

The axle loads of a vehicle depend on the type of axle (powered or non-powered) and its wheelbase. The maximum axle load of a vehicle is:

• 11.5 t if the axle is driven and single;

• 19 t (2 × 9.5), if the axle is drive and double, or if one axle is drive and the other is non-drive, and the wheelbase is not less than 1.3 m;

• 10 t if the axle is non-drive and single;

• 24 t (3 × 8), if the axle is non-powered and triple, and the wheelbase is greater than 1.3 m;

• 21 t (3 × 7), if the axle is non-driven and triple, and the wheelbase is no greater than 1.3 m.

Table 5 summarizes the parameters of lorries that meet the requirements of the Council Directive [14]. The vehicles in this table are given under the numbers 2, 4, 6 and 7. In the column where the weight of the vehicle is summarized, the number of axles of the vehicle is also given with the letter “R”, indicating the real vehicle.

Table 5.

Summary of lorry parameters actual and standard.

In the European standard, the fatigue load model gives sets of “standard lorries which together produce effects equivalent to those of typical traffic on European roads” (Section 4.6.5).

Table 5 summarizes the parameters of the lorries given in Table 4.7 of the standard. The standard gives the parameters of two five-axle vehicles: an articulated vehicle, consisting of a motor vehicle and a semi-trailer, and a combination of vehicles, consisting of a motor vehicle and a trailer. Due to the fact that an articulated vehicle has a higher weight and shorter wheelbase than a combination of vehicles, the standard articulated vehicle (marked with the letter “A”) is characterized in Table 5. The standard vehicles in Table 5 are given under numbers 1, 3 and 5 (these vehicles are marked with the letter “N”, indicating the standard vehicle).

4.1 Comparison of vehicle weights and axle loads according to the council directive and the European standard

Table 5 summarizes the parameters of heavy-duty vehicles that meet the requirements of the Council Directive (Nos. 2, 4, 6 and 7) and the European standard (Nos. 1, 3 and 5).

Analyzing the characteristics of the vehicles given in the standard, it can be concluded that their parameters are inconsistent with those given in the Council Directive [14]:

• vehicle No. 1 – three-axle, has a mass of 31.6 t, which is 22% greater than the maximum mass of 26 t, and a double axle load (one drive) of 24.4 t (12.2 t × 2), which is 28% greater than the maximum load of 19 t, while its extreme axle spacing of 5.50 m is greater than that of some actual three-axle vehicles (e.g., vehicle No. 2 has an extreme axle spacing of 4.50 m);

• vehicle No. 3 – a four-axle vehicle, has a weight of 39.8 t, which is 24% greater than the maximum weight of 32 t, a single axle load of 14.3 t, which is 24% greater than the maximum load of 11.5 t, while the extreme axle spacing of 11.20 m is greater than actual four-axle vehicles (e.g., vehicle No. 4 has an extreme axle spacing of 5.55 m);

• vehicle No. 5 – a five-axle vehicle, has a weight of 50 t, which is 25% greater than the maximum weight of 40 t, a single axle load of 15.3 t, which is 33% greater than the maximum load of 11.5 t, and a non-driving triple axle load of 27.6 t (9.2 t × 3), which is 31% greater than the maximum load of 21 t.

Taking into account the fact that the parameters of vehicles given as standard in the standard have parameters that are incompatible with those of vehicles allowed on European roads under the Council Directive [14], real vehicles that are in traffic on European roads were adopted for further analysis.

4.2 Comparison of single lane loading with actual vehicles arranged in a column of 5.0 m intervals

In different European countries, the appendix to the European standard adopts different distances between the extreme axle of a given standard vehicle and other vehicles. For example, in the UK, 5 m was adopted [3], and in France – not less than 10 m [6]. The distance proposed in the British appendix to the standard of 5 m was adopted for further analysis.

In order to compare the loads of real vehicles of different weights, the internal forces arising in a simply supported beam loaded as if it were a 3 m wide lane were compared. Adopting the static scheme of a simply supported beam for analysis, and determining the internal forces arising when it is loaded, is in accordance with the rules given in the NATO STANAG 2021 standardization agreement [15] for determining the military load class of bridges.

Figure 3 shows the values of internal forces in the beam when one lane is loaded with real vehicles located in a column at 5 m intervals between the front axle of a vehicle and the rear axle of the next vehicle. Real vehicles weighing 26 t, 32 t, 40 t and A40t were analyzed. These are vehicles with the parameters summarized in Table 5 under the numbers, respectively, 2, 4, 6 and 7.

Figure 3.

Values of internal forces in the beam when one lane is loaded with real vehicles at 5 m intervals.

Table 6 gives the ratio of the values of internal forces in the beam when loaded with real vehicles of different weights, relative to the loading of 40 t articulated vehicles (marked “A40t” in Figure 3). The force values were compared for spans equal to l = 50, 100, 150 and 200 m.

l	Weight of the vehicle
[m]	26 t	32 t	40 t
50	1.14	1.27	1.38
100	1.15	1.28	1.39
150	1.16	1.28	1.40
200	1.16	1.28	1.40

Table 6.

The ratio of internal force values when loaded with real vehicles and articulated vehicles weighing 40 t.

Comparing the load on a column of real vehicles used primarily to transport construction materials (hereafter referred to as “construction vehicles”), with the load on a column of articulated vehicles weighing 40 t (3.33 t/m), it should be noted that:

• a column of vehicles weighing 26 t (5.78 t/m) induces 14–16% higher internal forces than a column of articulated vehicles;

• a column of 32 t (5.77 t/m) vehicles induces 27–28% greater internal forces than a column of articulated vehicles;

• a column of 40 t (5.63 t/m) vehicles induces 38–40% greater internal forces than a column of articulated vehicles.

The higher value of internal forces in the beam, when loaded with vehicles of lower weights (26 and 32 t), is due to the fact that the unit load of these vehicles (vehicle weight divided by wheelbase), given in Table 5 and above in parentheses, is greater than the unit load of an articulated vehicle. In order for the mass of a vehicle to determine the value of internal forces induced by the load on that vehicle (the greater the mass, the greater the value of internal forces in the structure) in the European Council Directive [14], one of the parameters characterizing the vehicle should be the maximum unit load.

5. Comparison of the standard and proposed increased design load with the actual 40 t vehicle load

Assumptions made for the analysis.

1. The standard does not specify either the distance between the standard vehicles in a lane or the way they are seated in each lane. However, the distance between a vehicle and another load is given. It is 25 m (A.3(6)). This chapter assumes that this is the distance between the extreme axles of adjacent vehicles in traffic.

2. Lane 1 was loaded with real vehicles weighing 40 tons in two ways:

   • vehicles are located in a column, and the distance between the extreme axles of adjacent vehicles is 5 m (as in [3]). Such loading is in accordance with the standard’s stipulation that Load Model 1 takes into account “traffic jam situations with a high percentage of heavy lorries” (Section 4.3.2(1b)). Due to the short distance between vehicles, the load was not increased by a dynamic factor. The load is indicated by the symbol “40 t/5 m” or “A40t/5 m”;

   • vehicles traffic in a column and the distance between the extreme axles of adjacent vehicles is 25 m. This distance between the extreme axle and another load is recommended by the standard (Section A.3(6)). Such a load will be treated as occurring in traffic “with a high percentage of heavy lorries”. In this case, the load was increased by a dynamic factor. The load is indicated by the symbol “40 t/25 m” or “A40t/25 m”.

3. Lanes other than lane No. 1 were loaded with real vehicles of 40 t moving in a column, and the distance between the extreme axles of adjacent vehicles is 50 m. Such a load will be treated as occurring in traffic on lanes loaded with lorries, but to a lesser extent than lane No. 1. In this case, the load was increased by a dynamic factor.

4. The dynamic factor recommended in the standard (p. A.3(5)) with a value calculated according to the formula was adopted for vehicle models that move at a speed equal to 70 km/h:

$$f=1,40-\frac{L}{500}$$E1

in which the following designations are adopted:

f – dynamic surplus (dynamic factor),

L – influence length (in this chapter, it is the span) [m].

1. Taking into account the conclusions of the previous analysis, the transverse forces were compared in a 2-girder structure and – only to check the calculations – a 12-girder span structure.

Table 7 gives the ratio of the internal forces on the roadway with widths b = 6, 9 and 12 m, with a standard load (marked “EU”) and the internal forces induced by the loading of a column of articulated vehicles weighing 40 t (marked “A40”) or a column of construction vehicles weighing 40 t (marked “40”):

• on the first lane located every 5 m or every 25 m;

• on the remaining lanes located every 50 m.

l	UE/A40/25 m	UE/A40/5 m	UE/40/25 m	UE/40/5 m	PL/UE
[m]	b = 6 m
50	2.40	1.97	2.06	1.43	1.20
100	2.44	1.62	2.16	1.16	1.26
150	2.62	1.49	2.27	1.07	1.28
200	2.82	1.42	2.46	1.02	1.30
	b = 9 m
50	2.05	1.76	1.81	1.36	1.24
100	2.09	1.50	1.87	1.13	1.32
150	2.23	1.41	1.96	1.04	1.36
200	2.40	1.36	2.12	1.00	1.39
	b = 12 m
50	1.86	1.64	1.66	1.31	1.24
100	1.91	1.45	1.73	1.11	1.33
150	2.05	1.38	1.82	1.04	1.37
200	2.22	1.35	1.97	1.01	1.40

Table 7.

The ratio of the values of internal forces under load standard and a column of vehicles weighing 40 t.

The ratio of internal forces from these loads is given for spans equal to l = 50, 100, 150 and 200 m.

It should be noted that Figure 4 shows the values of internal forces in the extreme girder of the bridge span, with a roadway width of 6 m. Obviously, these values are higher at roadway widths of 9 and 12 m, but the ratio of force values at the proposed and standard loads is similar, regardless of the roadway width (you can compare the values in the last column of Table 7). For this reason, the values of forces at a different span width than 6 m are not shown.

Figure 4.

Values of internal forces in the outermost girder of a bridge span with a roadway width of b = 6 m when loaded with actual 40 t vehicles and with the standard and proposed design loads.

Comparing the values of internal forces shown in Figure 4 and the ratio of internal forces under the standard load and 40 t vehicles, it should be noted that the load according to the European standard induces internal forces:

• 35–97% greater than the forces induced by a column of articulated vehicles weighing 40 t and located every 5 m;

• by 0–43% greater than the forces induced by a column of construction vehicles weighing 40 t and located every 5 m.

It should be noted that the larger the span is, the difference in internal forces from the standard load and vehicles weighing 40 t (at 5 m intervals), is smaller. For example, with a roadway width of 6 m and a span of 50 m, the internal forces from the standard load are higher by 43%, and with a span of 200 m only by 2%. However, with a roadway width of 9 m and a span equal to 50 m, the standard forces are 36% greater, and with a span equal to 200 m, equal to a vehicle load of 40 t. In addition, with a span of up to 150 m, the greater the width of the roadway, the smaller the difference in internal forces.

Annex B of the standard, which deals with fatigue life assessment of road bridges, states “a factor 1.4 for the load levels are recommended”. This value can be taken as a minimum when evaluating the value of internal forces induced by the design load relative to internal forces induced by the service load. In addition, since “the characteristic loads associated with special vehicles should be taken as nominal values” (Section A.2 (3)), design and vehicle loads should be compared without using design factors.

In summary, it can be said that the load, according to the European standard, is close to that of real 40 t construction vehicles located in one lane every 5 m, and in the other lanes every 50 m (Table 7 shows in gray the ratio of forces less than 1.4). Moreover, the ratio of these forces is less than 1.4, even for 40 t articulated vehicles. Keeping in mind the expected service life of bridges according to the European standard [1], certainly, such a design load, which can be replaced by 40 t real vehicle loads, cannot guarantee the use of structures for 100 years.

Table 7 also gives the ratio of internal forces on roadways with widths b = 6, 9 and 12 m, under the proposed load (with increased adjustment factors for uniformly distributed load, marked “PL”) and the standard load (marked “EU”); these values are also given in Table 4. The proposed standard load induces at least 33% higher internal forces (with a 6 m roadway and a span of 200 m) compared to the load of a 40 t construction vehicle column located at 5 m intervals.

6. Proposal for adoption of equal service load by European countries

Thousands of bridges designed according to already outdated standards have been built in the tracts of the trans-European road network. Practically every European country operates bridges designed according to several load normatives, with different ones used in different countries. Eurocode 1 [2] is the first normative (a fact that needs improvement) that is used for bridge design in European countries that are European Committee for Standardization members. The treatment of structures in service depends on the load for which they were designed and, perhaps before all, on the service load to be assumed when checking its service load capacity. The service load should be expressed in terms of the maximum weight of the vehicle permitted to travel over the structure without restriction and how the permitted vehicles are located on the roadway.

The establishment of a uniform service load on the trans-European road network is necessary because of the need to create a uniform level of traffic safety just as there are unequivocally defined maximum parameters of vehicles allowed on European roads, so the service load scheme of bridges located on the routes of the trans-European road network should be unequivocally defined.

Verification of the bridge’s service load capacity should be carried out using a service load. In this load, the standard vehicle is a five-axle articulated vehicle (consisting of a tractor and semi-trailer) with a weight of 40 tons and a wheelbase of 12.0 meters. These are the parameters of the vehicles most commonly found on the road. The article [16] gives the differences in traffic with lorries in Bavaria in 1984 and 2005. A German study shows that among lorries, articulated vehicles accounted for the largest number – 50% - in 2005, and their growth over the period was 2.5 times. It is currently difficult to give the percentage of articulated vehicles among all lorries on the road, since lorries include all vehicles weighing more than 3.5 tons. It would be necessary to amend the provisions of the Council Directive […], which recognizes that a heavy vehicle is a vehicle with a permissible total weight of more than 3.5 t, which is, for example, 4 t, and the permissible total weight of a two-axle vehicle is 18 t.

For bridges with a two-lane roadway, the following standard vehicle load should be applied:

• on the first lane – vehicles located every 5 m (without dynamic factor);

• on the second lane – vehicles located every 25 m (with dynamic factor).

For bridges with a roadway of three or more lanes, the following standard vehicle load should be used:

• on the first and second lanes – as above;

• on the third and subsequent lanes – vehicles located every 50 m (with dynamic factor).

The load on the third and subsequent lanes is derived from the submission that, regardless of the traffic situation in other lanes, a standard vehicle should be able to pass over a given lane. The service load diagram for bridges with standard vehicles is shown in Figure 5.

Figure 5.

Service load diagram of bridges with standard vehicles.

Vehicles should be located in the axis of the real lane. Only a full set of vehicles should be considered for loads, and loads should be positioned at the unfavorable point of the influence surface. The order of lane positioning on the roadway should be such that the effects induced by the loads are the most unfavorable.

When checking the service load capacity, the road infrastructure manager may exchange standard vehicles for vehicles with higher unit loads in the service load scheme, but the distance between vehicles in the lanes should not be changed. In addition, it is permissible to load each lane with one standard vehicle with the parameters specified in Table 5, No. 7, and replace the remaining vehicles with their unit load – in the case of a standard vehicle equal to 3.33 t/m (with a distance between vehicles of 25 m, the error is no more than 2%).

Checking the load-carrying capacity is not a form of structure design. When designing a given structural element (e.g., the outermost girder), the load of individual lanes is set at the most unfavorable point of the area of influence for this element (the area of influence for another element, in a multi-girder structure, will be different). On the other hand, when checking the service load capacity, the entire bridge structure, all its structural elements, carry the load that will be placed on the roadway (for example, for a two-lane roadway, one lane is usually loaded separately and two lanes together). Thus, the design of bridges should not be confused with checking their load-carrying capacity. In addition, it should be remembered that the proposed service load will primarily apply to bridges designed according to outdated load standards.

7. Conclusions

The European standard Eurocode 1 [2] includes variable design load models for road bridges. The basic load model is Model 1. The standard allows the use of parameters defined at the national level. Such parameters are the adjustment factors for the load in Model 1. The adoption by different European countries of different values for the adjustment factors in Model 1 makes it practically impossible to design bridges in Europe that would carry European traffic with the same level of safety. It is necessary to agree at the European level on the values of adjustment factors, that is, the same design load for bridges.

Equal loading for bridges designed along the trans-European road network can be achieved by adopting equal values for the adjustment factors in Model 1.

Taking into account that:

• the standard adopts design load models appropriate for “the actual traffic in the year 2000 in European countries” (Section 4.2.1(1)), and a quarter of a century has passed since then and traffic has changed fundamentally;

• the load according to the European standard is close to that of a column of real construction vehicles of 40 t;

• such design load, which can be replaced by the load of real vehicles of 40 t, does not guarantee the use of structures for 100 years.

adaptation factors with higher values than those recommended in the standard should be adopted in all European countries. Such values of factors have already been adopted in Germany and Poland (when applying these factors, for example, increasing the span from l = 50 to l = 200 m, with a 2-lane roadway, the value of internal force increases from 19–30%).

Thousands of bridges have been built on the trans-European road network, designed according to different and already outdated load norms. The handling of these in-service bridges depends on which service load is adopted when checking their service load capacity.

Determining a uniform service load on the tracts of the trans-European road network is necessary because of the need to create a uniform level of traffic safety. In the service load, the standard vehicle is an articulated vehicle with a weight of 40 t and a wheelbase of 12.0 m (these are the parameters of the vehicles most commonly found in road traffic).

For bridges with a roadway with any number of lanes, the following standard vehicle load should be used:

• on the first lane – vehicles located every 5 m;

• on the second lane – vehicles located every 25 m;

• on the third and other lanes – vehicles located every 50 m.

Equally safe and durable bridges over the trans-European transport network are structures designed for the same design load and then loaded with the same service load. The Polcevera viaduct in Genoa, Italy, collapsed when 3 lorries were caught in the support abutment [17]. On this viaduct, the same problem that is currently occurring throughout Europe – structures of excessive dimensions are being built – occurred in an extreme form. Bridges currently being built generally have large spans and the high proportion of Heavy Goods Vehicles in traffic will create large, previously unheard of service loads. One easy way to maintain the correct relationship between design and service loads is to build bridges with as small a span as possible.

The Council Directive [14] gives the maximum weights and axle weights of vehicles permitted in national and international traffic in Europe. However, taking into account the real parameters of vehicles, it turned out that vehicles with lower weights can cause greater internal forces than vehicles with higher weights (vehicles with a weight of 26 or 32 t cause greater internal forces than vehicles with a weight of 40 t). This is due to the fact that the unit load of vehicles with lower total weight (weight per wheelbase) is higher.

Such a situation is ambiguous for bridge management administrations, because until now, if a bridge could not safely carry the load of vehicles with the maximum weight allowed on public roads, traffic of vehicles with a lower total weight was usually allowed over it. And this may not ensure less strain on the structure.

Another vehicle-related parameter, the vehicle unit load, should be included in the Council Directive [14]. The maximum unit load of a vehicle should be established in Europe, similar to the maximum weight or maximum axle load. This is necessary in the context of the amendment under discussion in Europe, allowing vehicles up to 60 t in weight, up to 25.25 m in length, to operate on selected European routes. In addition, the Council Directive [14] should place other limits on which a vehicle can be considered a heavy vehicle. The existing definition of a heavy vehicle as a vehicle weighing more than 3.5 t is technically unreasonable if a two-axle vehicle can weigh 18 t. The weight of a heavy vehicle would have to be linked to the permissible axle load, for example.