Fatigue Parameter
Specify the fatigue parameters required for fatigue check.
Note
Fatigue resistance of steel composite beams is verified as per Article 6.10.5.1 of AASHTO-LRFD 2012. Fracture toughness requirements mentioned in Article 6.10.5.2 are not checked in midas Civil.
From the Main Menu select Design > Composite Design > Fatigue Parameter
Firstly, select the members in the model and enter the following data:
Fatigue Parameter Detail
Category: Category defined by 75yr-(ADTT)SL equivalent to Infinite Life (Table 6.6.1.2.3-2)
(ADTT)SL: Number of trucks per day in a single-lane averaged over the design life (3.6.1.4.2). (ADTT)SL can be manually
calculated as per 3.6.1.4.2-1.
N: Number of stress range cycles per truck passage. Value can be taken from Table 6.6.1.2.5-2
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.
Note
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).
Note
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.