Steel Composite Girder Flexural Capacity: AASHTO vs Eurocode

Written by midasBridge Team | November 25, 2021

Table of Contents

1. Introduction

2. Flexural Resistance as per AASHTO LRFD

3. Flexural Resistance as per Eurocode

4. Comparison of AASHTO vs Eurocode Flexural Resistance

5. Conclusion

1. Introduction

Steel composite bridges are widely used due to the proper utilization of tensile strength of steel girder and the compressive strength of concrete deck, thereby bending resistance is greatly enhanced, making it more efficient and economical. Multi-girder system or ladder deck forms of steel composite bridges are the main forms of construction having simply supported or continuous girders.

In this article, design provisions for ultimate flexural resistance as per AASHTO LRFD and Eurocode are detailed. Flowcharts are shown for better understanding.

2. Flexural Resistance as per AASHTO LRFD

The procedure of calculating flexural capacity of steel composite I section is outlined in Figure 1, where four cases are established.

Case 1: Flexural Resistance of Positive Flexure Moment in Compact Section.

Case 2: Flexural Resistance of Positive Flexure Moment in Non-Compact Section.

Case 3: Flexural Resistance of Negative Flexure Moment.

Case 4: Flexural Resistance of Negative Flexure Moment by using Appendix A6.

Figure 1. Flowchart for calculation of Positive Moment Flexural Capacity as per AASHTO LRFD

Figure 2. Flowchart of the flexural resistance of Positive Flexure Moment in Compact Section

Figure 3. Flowchart of the flexural resistance of Positive Flexure Moment in Noncompact Section

Figure 4. Flowchart of the flexural resistance of Negative Flexure Moment

Figure 5. Flowchart of the flexural resistance of Negative Flexure Moment by using Appendix A6

3. Flexural Resistance as per Eurocode

Bending resistance, M_Rd, can be calculated as follows based on its class.

Class 1 or 2 cross‐sections can be checked by using the plastic or elastic bending resistance.

Class 3 cross‐sections are checked with the elastic bending resistance, or possibly reclassified as effective Class 2 cross‐section and then checked with the plastic bending resistance.

Class 4 cross‐sections are also checked with the elastic bending resistance but by using the effective cross‐section, reduced to take account of buckling.

Figure 6. Flowchart of flexural resistance as per Eurocode

4. Comparison of AAHSTO vs Eurocode Flexural Resistance

Let's take a 2-Span Steel Composite I girder Curved bridge for comparison.

Number of main girder: Four, Steel Composite I girder

Curvature radius: 4.318 m

Construction Stage Analysis: Yes

The bridge is modeled in midas Civil as shown below:

Figure 7. Two-span Steel Composite I girder curved bridge model

Keeping the materials, sections, and loading the same, the positive bending region was designed as per AASHTO LRFD-17 and Eurocode (EN1994-2). The results are illustrated in figure 8 where the flexural resistance checks are satisfied as per the codes.

	AASHTO LRFD	Eurocode
Section
Materials	Steel fsk= 355.000 MPa Es=210000.000 MPa Concrete fck= 30.000 MPa Ecm = 33000.000 MPa Reinforcement fyk = 400.000 MPa Er = 210000.000 MPa	Steel fsk= 355.000 MPa Es=210000.000 MPa Concrete fck= 30.000 MPa Ecm = 33000.000 MPa Reinforcement fyk = 400.000 MPa Er = 210000.000 MPa
Region	Positive Bending	Positive Bending
Demand Forces	Steel only moment M_D1 =2462 KNm Long-term moment M_D2 =1045 KNm Short-term moment M_D3 =1457 KNm	Before Composite Moment M_a,Ed =2462 KNm After Composite Moment M_c,Ed =2502 KNm M_Ed = 4964 KNm
Section Classification	Non-compact section for Curved Bridge	Class 1
Flexural Resistance	▪ Check Flexural Resistance of Composite noncompact section (AASHTO LRFD Bridge, 2018, 6.10.7.2) i. Check compression flange F_nc = R_b · R_h · F_yc = 355.000 MPa F_bu = 67.858 MPa ≤ Ф_f · F_nc = 355.000 MPa HENCE OK ii. Check tension flange F_nt = R_h · F_yt = 355.000 MPa f_bu + (1/3) f_l = 99.115 MPa ≤ Ф_f· F_nt = 355.000 MPa HENCE OK in which : R_{b = 1.000} R_{h = 1.000} Ф_{f = 1.000}	▪ Check Flexural Resistance (EN 1994-2:2005) - Plastic resistance moment, M_{pl, Rd} Plastic NA = 1812.6 mm N_slab = 11221.020 kN N_g,top = 5348.875 kN (Upper side of PNA) N_g,bot =16569.895 kN (Lower side of PNA) M_pl,Rd = 22842.225 kNㆍm x_pl = 330.327 mm M_Rd= βM_pl,Rd= 22842.2kNㆍm here, β =1.000 M_Rd = 22842.22 kNㆍm >M_Ed = 4965.21 kNㆍm HENCE OK

Figure 8. Flexural resistance comparison between AASHTO LRFD and Eurocode

5. Conclusion

The bending resistance of a steel composite girder can be calculated using the plastic stress distribution method. AASHTO LRFD uses load and resistance factor design where resistance factors(Ф_f) are multiplied with the actual bending resistance, whereas Eurocode uses partial safety factors for materials(γ) and reduction factors(β). The reduction factor β is applied only when high-strength steels of F_yk = 420 and 460 MPa are used.

In both the codes, based on section classification the moment capacity will be affected by the demand moments. So designer must prepare the load combinations in midas Civil carefully. AASHTO LRFD suggests moment resistance checks for compact sections in positive moment regions and for non-compact and negative bending regions, a stress check approach is performed. Whereas Eurocode suggests a moment resistance check approach only.

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