Function
- Specify the fatigue parameters required for fatigue limit state.
Fatigue resistance is verified as per chapter 7 of the Manual for bridge evaluation, third edition, when AASHTO-LRFR19 is selected as the Rating Design Code.
Call
From the main menu, select [Rating] tab > [Type : Steel Bridge] > [Steel Bridge : AASHTO] > [Rating Input Data] group > [Rating Parameters] > [Fatigue Parameters]
Input
Firstly, select the members in the model and enter the following data:
Fatigue Parameter Detail
AASHTO-LRFR19
Num. of Lanes, nL : Number of stripped lanes.
(ADTT)Present : Present average number of trucks per day for all directions of truck traffic including all lanes on the bridge.
(ADTTSL)Present : Average number of trucks per day in a single lane in the present year.
n(cycles) : Number of stress-range cycles per truck passage estimated according to article 7.2.5.2
Check Position : Select the locations in the member where the fatigue rating is to be checked.
Top of Top Flange
Bot. of Top Flange
Top of Bot. Flange
Bot. of Bot. Flange
Category : Select the detail-category to obtain constant as given in LRFD Design Table 6.6.1.2.5-1
Longitudinal Warping Stress Range : For the verification of fatigue, flexure stress is calculated as the summation of Longitudinal Bending Stress and Longitudinal Warping Stress Range. Longitudinal warping stress range for the section with box flange can be considered in two ways:
Auto Calculation : Fatigue vertical bending moment is increased by 10% for the longitudinal warping stress.
User Input : Input the manually calculated longitudinal warping stress range for top and bottom flange.
Click Apply to assign the shear connectors.
Longitudinal warping stress range can be manually calculated as per BEF (Beam on Elastic Foundation) analogy presented by Wright and Abdel-Samad. The designer guide to Box Girder by Bethlehem Steel Corporation also presents this method. Detailed calculations can be seen in Design Example 5: Three Span Continuous Curved Composite Tub-Girder Bridge (page 85-94).
Fatigue calculations in midas CIvil do not account for Transverse Bending Stress due to Distorsion. Therefore, transverse bending stress range at the top or bottom corners of the tub/box section needs to be manually checked with the nominal fatigue resistance.