comparative study of highway loads on bridges …

39th Conference on Our World in Concrete & Structures

20-22 August 2014, Singapore

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COMPARATIVE STUDY OF HIGHWAY LOADS ON BRIDGES ACCORDING TO DIFFERENT CODES

Medhat Kamal Abdullah*

Associate Professor, Civil Engineering Department, Faculty of Engineering,

Helwan University 21 Gamal Dwidar, Nasr city Zone 8, Cairo, Egypt

Email: [email protected]

Keywords: Highway loads, Highway codes, AASHTO, EC 45-1993, EC 201-2012, BD 37/01, EN-1991-2: 2003.

Abstract: A comparative study between different codes for highway loads on bridges is introduced. These codes are : (a) Egyptian code (EC45:1993), [1], (b) Egyptian Code (EC201-2012), [2], (c) The British standard (BD 37/01), [3], (d) The American code AASHTO-Standard, [4], (e) The American code AASHTO- LRFD, [5] and (f) The European standard ( Euro code 1), [6]. The comparison is made based on a detailed analysis of a typical slab-girder bridge deck, which has been used widely all over the world, with different spans and different spacing between the man girders. Comparison tables are produced to show the relations between the different codes and both the Egyptian code45-1993 and the American code, AASHTO LRFD. Major observations on the results of each of these codes are discussed. Charts are developed illustrating the relationships between the different codes relative to the American code, AASHTO- LRFD. These charts are important to support the bridge designers with a necessary design aid.

INTRODUCTION One of the most important steps to design a bridge is to specify the traffic load that move over the

bridge. The traffic loads vary from a country to another country and may vary from a location to another location in the same country depending on many parameters such as: the degree of civilization, the location of the bridge inside the country, the volume of traffic, the nature of the expected major traffic passing over the bridge, i.e. military, commercial…etc.

P. Djahani and E. Ibrahim, [7], introduced a preliminary comparison between AASHTO- LFD, [8], AASHTO- LRFD and BS5400. They applied their comparison to actual truck using UAE bridges. They recommend the use the AASHTO-LRFD with a multiplier factor of 2. N.S. Abou Elhagag, [9], considered five different codes [1], [3], [4], [5] and [6]. One of the main conclusions of this study is that despite big differences in the absolute values of the different forces specified by different codes, these differences decrease when these loads are combined according to the load combinations of each code. M.M. Bakhoum, [10], compared the traffic loading specified by different codes. One of the main conclusions is that the differences in main actions are more than the differences in section resistance when calculated by different codes. The methodologies of these studies are different than the one here.

In this paper, six major international highway codes are chosen for the comparison as mentioned in the abstract. The details of each code are shown in the references. These six codes are used in Egypt and all of the Arab countries in the design of bridges.

One of the main objectives of this comparative study is that there are many bridges in the Middle East have been designed and reviewed by different consultants who may be located in different countries. These consultants may be more familiar with the codes of their own country than the other

Medhat Kamal Abdullah

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codes, so it is very essential to support those consultants with necessary aids to convert easily from the different codes of design to the code they are familiar with

Another important reason for this study is to evaluate the performance of the bridges designed by different codes. For example, it was noticed that the bridges designed based on the highway traffic loads of the American standards AASHTO, are not sufficient to withstand the serviceability conditions as will be shown later in this research.

The highway traffic loads specified in these six codes are applied on a typical bridge deck, which

has been used very frequently with almost the same configurations in many huge highway projects, to compare the effect of the imposed load on the bridge deck. Different spans of the deck (L=20, 25, 30,35,40,45 & 50 meters) and different spacing between the girders (S=2.00, 2.50, 3.20 & 4.00 meters) have been used in this study. The grillage analysis method is adopted to model the bridge, [11].

The results of this comparison are discussed and simple charts are introduced as design aid for the bridges’ designers.

DESCRIPTION OF THE BRIDGE DECK USED IN THE CASE STUDY

The bridge deck under consideration is very common in the bridge industry allover the world. It has been used in many projects in Egypt. Some of these projects are: Al Maryoteia new link connecting both the eastern and western parts of Cairo ring road, Saft Al Laban, Al Meadia Bridge across Edko Lake, and other bridges all over Egypt. The same bridge configuration is very common worldwide. Pictures 1 to 4 illustrate some of these bridges.

Picture1 : Al Meadia Bridge

Picture 2: Al Marioteia Bridge


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Pictures 3 Abu Keir Bridge During Construction

Picture 4: Edko Bridge

The bridge deck is composed of simple spans, 30-meter long. Each span is composed of

reinforced concrete slab, 250mm thick over 4 post-tensioned girders spacing at 3.2 m. There is a 350 x 1750 mm reinforced concrete cross girder at mid-span of each bay. The main girders are connected together at each end by a 850x1750 mm cross girder. The characteristic strength of the concrete after 28 days (fcu) is 450 kg/cm2.The main components of this bridge deck are shown in figure 1.

Analysis of the Bridge Deck

The grillage modeling technique is used in this study. Both the longitudinal main girders are modeled with the properties of a T-section. The slab is modeled as lumped transversal beams at appropriate spacing. Figure 2 illustrates the grillage modeling technique; the details of this technique are illustrated in reference [11].

The highway traffic loads specified by the different codes mentioned above are applied on the bridge deck.


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1.60m S S S VARIES

1.75

m

0.25m

0.40m

13.20m

0.25

m

1.60m

L

L/2 L/2

SS

S13

.20m

1.60

mVA

RI

SECTION A-A

A -- PLAN

X-GI

RDER

(0.3

50x1

.750

)

X-GI

RDER

(0.9

00x1

.750

)B4 (0.400X1.750)

B3 (0.400X1.750)

B2 (0.400X1.750)

B1 (0.400X1.750)

SIDEWALK SIDEWALK

X-GI

RDER

(0.9

00x1

.750

)

0.250m THICK SLAB

B1 B2 B3 B4

XG(0.900X1.750)

Figure 1: Details of the main components of the bridge deck considered in this study

Figure 2: Grillage Modeling Technique

In order to make this study more general, the straining actions due to the highway loads specified by each code are determined for different spans of the deck, (L=20, 25, 30,35,40,45 & 50 meters) and for different spacing between the girders (S=2.00, 2.50, 3.20 & 4.00 meters).


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COMPARISON BETWEEN THE DIFFERENT CODES

The straining actions obtained by applying the different codes to the typical bridge deck are compared. The comparison is made for three different cases:

• UF= Unfactored values • SLS= Service limit state= γ(SLS) x UF

• ULS= Ultimate limit state= γ(ULS) x UF

The loading factors, γ(SLS) and γ(ULS) change according to the code. The values of these factors are shown in table 1. The comparisons are shown for the edge girder B1, as a sample for the results.

CODE γSLS γULS

EC-45:1993 1.00 1.60 EC-201:2012 1.00 1.35 EN-1992-2:2003 0.75 TS+0.40 UDL 1.35

BD 37/01 1.20 (HA)

1.10 (HA+45HB)

1.50 (HA)

1.30 (HA+45HB)

AASHTO Standard 17th Ed, 2002 1.00 2.17 AASHTO LRFD 2007 1.00 1.75

Table 1: Highway Traffic Load Factors in Different Codes.

A detailed comparison between the Egyptian Code EC45:1993 and AASHTO LRFD Bridge Design Specifications (4th edition – 2007) is shown in the following parts. The same comparison is made between the Egyptian Code EC45:1993 and the other codes mentioned in this research. The results of this comparative study are summarized in simple tables and design aid to enable the design engineers in the design of the girders of the bridge deck.

Comparison between the Egyptian Code 45:1993 and AASHTO LRFD:

Tables 2 and 3 show the values of the maximum bending moment, BM, and the maximum shearing forces, SF, due to Egyptian Code EC45:1993 and AASHTO LRFD (4th edition: 2007). And Table 4 shows the ratios of these values for AASHTO-LRFD compared to EC45:1993, for different spans when the spacing S is 2 m, for the three cases; UF, SLS and ULS respectively.

Span (m) UF SLS ULS

LRFD EC45-93 LRFD EC45-93 LRFD EC45-93

20 110 171 110 171 192.5 273.6 25 145 222 145 222 253.75 355.2 30 179 272 179 272 313.25 435.2 35 213 321 213 321 372.75 513.6 40 248 372 248 372 434 595.2 45 284 421 284 421 497 673.6 50 319 475 319 475 558.25 760

Table 2: BM Due to Highway Traffic Load According to AASHTO- LRFD and EC45 (Ton, m)


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Span (m) UF SLS ULS

LRFD EC45-93 LRFD EC45-93 LRFD EC45-93

20 27.5 37 27.5 37 48.125 59.2 25 30.5 40 30.5 40 53.375 64 30 33.1 42 33.1 42 57.925 67.2 35 35.6 44 35.6 44 62.3 70.4 40 38 46 38 46 66.5 73.6 45 40 47 40 47 70 75.2 50 42 49 42 49 73.5 78.4

Table 3: SF Due to Highway Traffic Load According to AASHTO LRFD and EC45(Ton)

Span (m)

RBM RSF

UF SLS ULS UF SLS ULS 20 0.64 0.64 0.70 0.74 0.74 0.81 25 0.65 0.65 0.71 0.76 0.76 0.83 30 0.66 0.66 0.72 0.79 0.79 0.86 35 0.66 0.66 0.73 0.81 0.81 0.88 40 0.67 0.67 0.73 0.83 0.83 0.90 45 0.67 0.67 0.74 0.85 0.85 0.93 50 0.67 0.67 0.73 0.86 0.86 0.94

Table 4: The Ratios RBM=BM (AASHTO LRFD)/BM (EC45:1993) and RSF= SF (AASHTO LRFD)/SF (EC45:1993) {S=2.00m}

Table 5 shows the average ratios obtained by repeating the above mentioned comparisons for the spacing between the girders, S, which ranges between 2m up to 4m.

Span (m)

RBM. Avrg RSF. Avrg


Table 5: The Average Ratios for RBM=BM (AASHTO LRFD)/BM (EC45:1993) and for RSF= SF (AASHTO LRFD)/SF (EC45:1993) , {S=2, 2.5, 3.2 and 4m}

Similar procedure was conducted for all of the other codes mentioned in this study. Tables 6 to 9 are similar to table 5 but for the other codes. The same tables are reproduced but using the AASHTO-LRFD as a common code for comparison as shown in tables 10 to 14


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Span (m)

RBM. Avrg RSF. Avrg

UF SLS ULS UF SLS ULS 20 1.23 1.23 1.04 1.20 1.20 1.01

25 1.27 1.27 1.07 1.24 1.24 1.05

30 1.31 1.31 1.11 1.29 1.29 1.08

35 1.35 1.35 1.14 1.33 1.33 1.12

40 1.38 1.38 1.16 1.38 1.38 1.17

45 1.42 1.42 1.20 1.42 1.42 1.20

50 1.45 1.45 1.22 1.47 1.47 1.24

Table 6: The Average Ratios for RBM=BM (EC201:2012)/BM (EC45:1993) and for RSF= SF (EC201:2012)/SF (EC45:1993) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg


Table 7: The Average Ratios for RBM=BM (BD 37/01)/BM (EC45:1993) and for RSF= SF (BD 37/01)/SF (EC45:1993) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg


Table 8: The Average Ratios for RBM=BM (EN-1991-2: 2003)/BM (EC45:1993) and for RSF= SF (EN-1991-2: 2003)/SF (EC45:1993) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg



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Table 9: The Average Ratios for RBM=BM (AASHTO Standard)/BM (EC45:1993) and for RSF= SF (AASHTO Standard )/SF (EC45:1993) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg


Table 10: The Average Ratios RBM=BM (EC45:1993)/BM (AASHTO LRFD) and RSF= SF (EC45:1993)/SF (AASHTO LRFD) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg


Table 11: The Average Ratios RBM=BM (EC201:2012)/BM (LRFD) and RSF= SF (EC201:2012)/SF (LRFD) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg


Table 12: The Average Ratios RBM=BM (BD 37/01)/BM (LRFD) and RSF= SF (BD 37/01)/SF (LRFD) , {S=2, 2.5, 3.2 and 4m}


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Span (m)

RBM. Avrg RSF. Avrg


Table 13: The Average Ratios RBM=BM (EN-1991-2: 2003)/BM (LRFD) and RSF= SF (EN-1991-2: 2003)/SF (LRFD) , {S=2, 2.5, 3.2 and 4m}

Span (m)

RBM. Avrg RSF. Avrg


Table 14: The Average Ratios RBM=BM (AASHTO Standard)/BM (LRFD) and RSF= SF (AASHTO Standard)/SF (LRFD) , {S=2, 2.5, 3.2 and 4m}

• The maximum average ratio Rf which relates the results of the different codes to the

AASHTO-LRFD is shown in table 15 for both the bending moment and the shear force considering spans from 20m to 50 m and spacings from 2m to 4m. The numbers shown may be used for approximate conversion between the different codes.

Code UF RBM SLS RBM ULS RBM UF RSF SLS RSF ULS RSF

EC201:2012 2.20 2.20 1.70 1.74 1.74 1.34 BD 37/01 2.05 2.26 1.56 1.61 1.77 1.19 EN 1991-2:2003 2.20 1.24 1.70 1.74 1.01 1.34 AASHTO Standard:2002 0.61 0.61 0.75 0.81 0.81 1.01 ECP-93 1.60 1.60 1.46 1.37 1.37 1.25

Table 15 Maximum Ratio Rf=f(codei)/f(LRFD)

DISCUSSION OF THE RESULTS:

From the above mentioned results, it is observed that:

• The results obtained by the new Egyptian code, EC201:2012, is usually higher than the old code, EC45:1993. The average ratio is 1.45 for BM in case of UF and SLS. This ratio decreases in the case of ULS to be 1.22. Similarly, for the shear force, these ratios are 1.47, 1.47 and 1.24 for UF, SLS and ULS respectively. These ratios increase by increasing the span due to the decrease in the impact factor used in EC45:1993, while the impact factor in EC201:2012 is included in the values of live loads.


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• The results obtained by BD 37/01 are higher than the results of EC45-93 in most of the cases. The ratio RBM ranges between 1.18 to 1.35, 1.30 to 1.48 and 0.96 to 1.12 for the UF, SLS and ULS bending moments respectively. On the other hand, the ratio RSF for the shear forces ranges between 1.17 to 1.23, 1.29 to 1.35 and 0.95 to 1.00 for the UF, SLS and ULS shear forces respectively. In general, the British Standard, BD37/01, develops larger service straining actions than the Egyptian code due to live loads, while they are almost equal for ultimate straining actions. (N.B: the comparison is made using 45 units of HB loading specified in the BD37/01)

• The results obtained by applying the European Standard (EN 1991-2:2003) are usually higher than the Egyptian code: EC45:1993 for the cases UF and ULS load cases. On the other hand, these results are lower for the case of SLS. The ratio RBM ranges between 1.23 to 1.45, 0.77 to 0.81 and 1.04 to 1.22 for the UF, SLS and ULS bending moments respectively. On the other hand, the ratio RSF for the shear forces ranges between 1.2 to 1.47, 0.74 to 0.81 and 1.01 to 1.24 for the UF, SLS and ULS shear forces respectively. It is worthy to be mentioned that the results obtained by both the Egyptian Code EC-201:2012 and the European Code EN-1991-2:2003 are identical and they are higher than EC45:1993.

• The results obtained by AASHTO Standard (17th edition: 2002) are usually lower than the Egyptian code: EC45:1993. The ratios range between 0.36 to 0.40 for both UF and SLS bending moment and between 0.49 to 0.54 for the ULS bending moment. For shear forces, the ratios range between 0.59 to 0.66 for UF and SLS, and between 0.81 to 0.60 for ULS shear force.

• The results obtained by AASHTO LRFD (4th Edition: 2007) are usually lower than the Egyptian code: EC45:1993. The average ratio ranges between 0.63 to 0.66 for the UF and SLS bending moment; and 0.68 to 0.72 for the ULS bending moment. For the shear forces, the ratio ranges between 0.73 to 0.85. For UF and SLS shear force; and 0.80 to 0.93 for the ULS shear force. The ratio increases by increasing the span due to the decrease in the impact factor used in EC45:1993, while in AASHTO LRFD, the impact factor is constant.

• As shown above, the straining actions caused by the traffic loads of both the AASHTO STANDARD and AASHTO LRFD are much lower than those obtained by any of the other mentioned highway codes used in this study for the three load cases described before UF, SLS and ULS. This remarkable difference may be attributed to the weight of the trucks specified by the AASHTO codes. In many countries these codes are adopted after increasing the value of the truck by a certain multiplier. In some projects, especially in Dubai and Saudi Arabia, this multiplier reaches a value of 2.

• These remarkable low results of the American codes explains why these codes when used in the gulf countries in the eighties, before using this multiplier, there were many damages in the bridges.

The results shown in tables 10 to 14 are illustrated in figures 3 to 6 for the UF and ULS values.

These curves are extremely useful to correlate the results of the different codes to each other

SUMMARY AND CONCLUSIONS:

A comparative study between the traffic loads specified by six major international codes is presented. The main conclusions of this study may be summarized as follows: 1- The highway traffic loads specified by both the new Egyptian Code EC-201and the European

Code EN-1991 are identical and they give higher values than the old Egyptian code EC45 which is still used in the design of the bridges in Egypt.

2- The British Standard, BD37/01, develops larger service straining actions than the EC45-1993, while they give marginally higher results for ultimate straining actions.

3- The straining actions caused by the highway traffic loads of both AASHTO STANDARD and AASHTO LRFD are much lower than those obtained by the other codes. This remarkable difference may be the main reason for the damages caused in the bridges designed in the eighties in the gulf countries using these codes.

4- The maximum average ratio for the different codes relative to AASHTO-LRFD is given for fast correlation between the different codes.

5- Calibration curves related to AASHTO-LRFD are developed to convert between different codes in the design stage

6- There is a need for a unified bridge design code in our countries which may be adjusted and fine tuned according to the nature of traffic in each country.


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REFERENCES

[1] Egyptian Code for Loads and Forces on Structural and Masonry works (EC45:1993). [2] Egyptian Code for Loads and Forces on Structural and Masonry works (EC201-2012). [3] The British standard (BD 37/01 Loads for Highway Bridges). [4] The American specifications AASHTO Standard for Highway – Bridges 17th edition: 2002. [5] The American specifications AASHTO LRFD Bridge Design – 4th edition: 2007. [6] The European standard (Eurocode 1: Actions on structures -part 2- Traffic loads on bridges EN-

1991-2:2003). [7] P. Djahani and E. Ibrahim, “ On development of A Bridge Design Code for GCC countries”, 23rd

Arab Engineering Conference, 14-15 March 2005, Bahrain [8] AASHTO-LFD Bridge Design Specifications, 17th Edition, 2002. [9] N.S.Abou Elhagag,“ Comparison Between Bridge International Codes Applied on Short-Span

Reinforced Concrete Bridges”, M.Sc.– Cairo University, 2012. [10] M.M. Bakhoum, “Traffic Actions for Design of Long and Medium bridges. A Comparison of

International Codes”, IABSE, Basis of Design and Actions of Structures Background and Application of Eurocode, 27-29 March, 1996, Delft, The Netherlands

[11] E. C. Hambly, “Bridge deck behavior”, 2nd edition, 1991.


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Figu

re 3

: The

ave

rage

Rat

io U

F R

BM=

UF

BM

(CO

DE

i)/ U

F B

M (L

RFD

)


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Figu

re 4

: The

ave

rage

Rat

io U

LS R

BM=

ULS

BM

(CO

DE

i)/ U

LS B

M (L

RFD

)


14

Figu

re 5

: The

ave

rage

Rat

io U

F R

SF=

UF

SF (C

OD

E i)/

UF

SF (L

RFD

)


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Figu

re 6

: The

ave

rage

Rat

io U

LS R

SF=

ULS

SF

(CO

DE

i)/ U

LS S

F (L

RFD

)

comparative study of highway loads on bridges …

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