EURO CODES FOR DESIGN OF PRE-STRESSED CONCRETE HIGHWAY BRIDGES-A REVIEW FOR HIGHWAY LOADINGS

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1 Special Session on Sustainable Design, 4 th International Conference on Structural Engineering and Construction Management 2013, Kandy, Sri Lanka, 13 th, 14 th & 15 th December 2013 SECM/13/264 Abstract EURO CODES FOR DESIGN OF PRE-STRESSED CONCRETE HIGHWAY BRIDGES-A REVIEW FOR HIGHWAY LOADINGS S. Seyanthan 1 and M.T.R. Jayasinghe 2 1 Department of Civil Engineering, University of Moratuwa, Sri Lanka Seyantyhan1990@hmail.com 2 Department of Civil Engineering, University of Moratuwa, Sri Lanka thishan@uom.lk Sri Lanka is a country with relatively high road density and also with tropical climatic conditions that cause high rainfall intensity. Therefore, there are so many bridges of various spans in these roads. With many bridges constructed or rehabilitated, there is a need to be precise with the design concepts to achieve a significant economy when constructing and maintaining the bridges. With the adoption of Euro codes as the design standard in many countries, there is a need to adopt such standards instead of British standards that are currently used since BS standards will not be updated in future. This paper describes some important findings that would be of high significance for the bridge engineers in their change over to Euro codes with the types of decks that are commonly used in Sri Lanka and also probable structural forms that can be adopted in future. In this research paper, the attention has been on the adoption of various loading conditions and the desirable modifications that could be recommended for various classes of roads as for the classification adopted in Sri Lanka. Key words: Highway loading, Euro code, M Beams, Y Beams 1.0 Introduction The structural design of highway bridges has been carried out in accordance with BS5400 guidelines and for the loading that is specified in BS5400: Part2: Since the initial issue of BS5400, many changes have been effected to take account of the changing nature of traffic condition and loads. However, some of these changes have not been adopted in Sri Lanka considering the type of traffic that is operated on a road network. For example, the number of units of HB considered has been kept at 30 for all the main roads. This led to the design of expressway bridges also for 30 units instead of more appropriate 45 units. Hence, there is a need to have a detailed study when the impact of adopting various clauses of Euro code guidelines with respect to material strength guidelines and loading that has to be adopted. Hence, a detailed study has been undertaken as a review the guidelines to select and adopt the appropriate values for the loading recommended for highway bridges. This paper present such, a study on applicable for different classes of roads available in Sri Lanka as for the highway classification adopted in Sri Lanka. 2.0 The objective 67

2 The main objective of this research is to determine the implications of adopting Euro standards for the structural design of highway bridge decks with special reference to the recommended highway loading intensities. 3.0 The methodology The following methodology was adopted: 1. A detailed literature review was undertaken for highway bridge loading and to determine the basis for the recommended values. 2. The effect of different types of highway bridge loading as for British and new Euro standards have been evaluated using typical bridge spans and using mathematical models prepared for computer analysis. 3. The results are presented graphically for comparative purposes. 4. Appropriate loading intensities have been selected for Sri Lankan scenario and recommendations have been made. 4.0 Comparison between different standards Two different standards BS 5400 and Euro codes give different methods to calculate the Notional lane width. According to the BS standard, the Notional lane width is limited within m [1]. It will vary according to the carriage width. However, in the case of Euro codes, it limits the Notional lane width as 3.0 m for all the carriage way width, except when the carriage way width is within 5.4 to 6 m [2]. The primary traffic loads specified in the BS standard, HA & HB are represented by the following Load models as specified in Euro code. That is given in Table1 below: Table 1: Traffic loading comparison values BS Standard[1] HA Loading (UDL+KEL) UDL-30kN/m/Notional lane KEL-120kN/Notional lane HB loading (Abnormal vehicle loads)-point load (30, 37.5, 45units etc.) One unit of HB is equal to 10kN per axle Euro Code Standard LM1(Load model 1) LM1(UDL+TS) LM3 ( Abnormal vehicle loads)-point loads Types of SV/SOV vehicle SV80, SV100, Sv196 etc. Table2: Load model 1: characteristic Location Tandem system(ts) Axle Loads Q ik (kn) Lane No Lane No Lane No Other lanes Remaining Area (q rk ) UDL system q ik (kn/m 2 ) 68

3 Figure 1: Lateral wheel arrangement for all SV axles [3] When HA and HB loading acting together as in combination1, the partial load factor used at ULS condition is 1.3. Similarly LM1 and LM3 combined and specified as group5 in Euro code and used in the design with the partial factor of 1.35 at ULS. Most of the bridges are designed to carry HB loadings with HA loadings when BS standards are used. Similarly the Euro code designs are carried out to LM1 with LM3. The Table3 describes the way HB and LM3 could be applied for the design purposes of different classes of roads. In order to have comparison, the bridge decks with precast beam have been modelled as torsion less grillages. The type of beam could be M or Y shapes. The loads have been applied on a grillage in a way to get maximum bending moment (BM) and maximum shear force (SF) to the design purpose. Figure 2 and 3 show the way that the combination of traffic loads can be applied on a grillage to get the maximum bending moment for British and Euro standards. Table 3: HB loading and SV model comparison Class of Roads Carried by Structure Number of Unit of types HB loading [4] SV models [5] Motor way and trunk roads 45 unit SV80, SV100, SV196 Principle roads 37.5 unit SV80, SV100 Public roads 30 unit SV80 69

4 Figure 2: Combination of HA and HB [1] [2] (Applied on a grillage to get maximum BM) Figure 3: combination of LM1 and LM3 (Applied on a grillage to get Maximum BM) NB: To get maximum shear, HB & KEL and LM3&TS will be applied near to the support 5.0 Applications with y-beams and m-beams Mathematical models have been created for Y-beams and M-beams separately for same span length and the critical bending moment & shear force values are compared for both systems. This study was carried out to check whether Y-beam and M-beam types could have any significant variation in behaviour. Figures 4 and 5 show that the critical bending moments and shear forces would be almost the same for M and Y shape beam for a given type of loading. These values have been obtained by using a mathematical model prepared using SAP 2000 [6]. This is an expected result since both types of beam decks are modelled as torsion-less system. Therefore, it can be stated with confidence that the further findings of this research will be applicable for any type of precast beam bridge with an in-situ cast top slab where the bridge will be modelled as a torsion-less system for the determination of critical values for design purposes. Y beams also cover the existing M beam ranges M2-M10 [7]. Y beams are heavier than M beams for a comparable section modulus due to their higher web thickness. Y beams can be introduced to Sri Lanka in future due to the following advantages over the other types of beams [8]: 1. Y beams allow easy access and inspection due to narrower bottom flange. 2. Depth of Bottom flange of the M beam (290mm) is not enough to meet the increased prestress forces and also it is very difficult to fit reinforcement with small depth of bottom flange. As a solution Y beams can be used with deeper bottom flange (380mm) and it allows higher covers for links. 3. With more steeply sloping top surface of lower flange, Y beams could be kept clean from debris and allow it to be cast without air bubbles under the shuttering being formed thus giving desirable finish to the top surface of the bottom flange. 70

5 4. Figure 4: Y beam and M beam comparison for maximum bending moment Figure 5: Y beam and M beam comparison for maximum shear forces 6. Results for different loading standards Grillage analysis is carried out for different span lengths and for each case the maximum bending moment and maximum shear forces have been obtained due to traffic loads using SAP 2000 [6]. This 71

6 analysis was carried out for both standards separately and Span vs Bending Moment & Shear Force graphs were plotted for the critical values. The results are given in Figures 6 and 7. Figure 6: Bending moment values for different standards Figure 7: Shear force values for different standards The results show that Euro code could give somewhat higher values for maximum bending moment and maximum shear compared to BS standard. It implies that the bridge decks could be designed for higher applied loads when Euro code is adopted. That means that designs to the Euro codes are capable of carrying higher loads than design to the BS standard. Also the result show the BS standard values are generally around 60-67% of Euro code values. So designing according to euro codes to Sri Lankan practice will lead to over design if the road traffic will remain in the current level with the type of vehicles being not very heavy. When the current situation in Sri Lanka is considered, it can be stated with confidence that the highway bridges currently at service are functioning satisfactorily even though they have been designed according to BS standards and also for the initial recommendations of BS 5400: Part 1: 1978 as envisaged in the design manual of RDA [9]. Therefore, another study has been carried out with grillage analysis with 70% of the Euro standard recommended loads for beam slab composite decks that could be analysed as torsionless systems. The results of this study with 70% of Euro code values are also shown in Figures 6 and 7 (with dotted-dot line). These comparisons indicate that there is a strong possibility to adopt 70% of the loads recommended in Euro codes as the desirable highway loading in Sri Lanka since it is difficult to expect a serious increase in the highway loading due to a change in the type of vehicles used in vehicles for all B, C, D and E class roads. For A class roads that will carry all types of traffic and also for expressways, it may be prudent to adopt either Euro 72

7 code recommendations or about 85% of the Euro code recommendations to make such bridges even more robust and versatile. Another structural form that is commonly used is pseudo slabs [10]. In this system, inverted T type beams are located at 0.5 m centres and the beam is formed to look like a solid slab by using in-situ cast concrete. Hence, the bridge will have better load sharing capabilities. Therefore, the mathematical models prepared for analysis using grillages will have the full torsion capacity of the slabs after allowing for any portion that will not behave as fully connected such as the bottom flange of adjacent precast prestressed concrete beams. These bridges have also been modelled as solid slabs for analysis under this research study. The results are presented in Figures 8 and 9. These figures indicate that a significant difference would not exist between the critical bending moments and shear forces resulting for both torsion-less systems and those analysed as being capable of carrying torsion. However, it can be seen that the critical values are much higher for the Euro code recommendations with these figures as well. Therefore, it is recommended to consider the adoption of 70% of Euro code values for B, C, D and E class roads while keeping 85% or full Euro code recommendations for A class roads and the expressways to be constructed in the future. Another alternative is to consider 85% for A class roads and to adopt full Euro code loads for the expressways designed for a speed of 100 km/hour and also could allow any type of vehicle while having the possibility of becoming an Asian highway network in a future date. Figure 8:- Comparison between systems that can resist torsion and also behave as torsion-less for critical bending moments 73

8 Figure 9: Comparison between systems that can resist torsion and also behave as torsion-less for critical shear forces 7. Conclusion It is very import to adopt Euro codes in future since there will not be any more updating of BS codes in further. However, the adoption of Euro codes should not result in a drastic change in the way that the bridges are designed and should not lead to significant cost enhancements when the conditions prevailing in Sri Lanka are considered. In this context, the adoption of appropriate loading will be very important. The detailed study undertaken to compare the loadings of two different standards have indicated that the direct adoption of Euro code recommendations could result in a significant increase in the critical bending moment and shear force values for typical beam slab bridges that will be constructed with standard Y or M beams. Therefore, it is recommended to have only about 70% of the Euro code based values adopted for the highway bridges in B, C, D and E class roads. For the more important A class roads, the adoption of about 85% of the Euro code values could be seriously considered if the recommended 70% limit would be too low. For the proposed expressways, it would be prudent to adopt the Euro code recommended values since such roads could become part of the envisaged Asian Highway network one day in the future. It is also shown that the above recommendations will be valid for the bridge decks that will behave as pseudo slabs as would result from the use of inverted T-type beams. All the above recommendations could be subjected to further discussion and more rigorous studies prior to the adoption in a future Bridge Designers Manual of Road Development Authority of Sri Lanka. 74

9 References [1] BS5400/2: 1978, Code of practice for design of concrete bridge, British standard institution, London. [2] BS EN :2003 Euro code 1: Action on structures, Part 2 Traffic loads on Bridges [3] NA to BS EN :2003, UK national annex to Euro code 1: actions on structures, part 2 traffic loads on Bridges [4] Design manual for roads and Bridges BD37/01 Chapter 4 Available: [5] The highway agency document IAN 124/11 Available: [6] Structural Analysis Package 2000 (SAP2000) Manual, CSI Analysis Reference Manual. [7] Final Technical Manual Edition 2: Pre-stressed Concrete Beams, UK [8] H.P.J. Taylor, L.A. Clark, C.C. Banks, The Y-beam: a replacement for the M-beam in beam and slab bridge decks, Available: [9] RDA bridge design manual, Road Development Authority, Sri Lanka, [10] Clark, L.A., Concrete bridge design to BS 5400, Construction press, 186 p,