- Specify the fatigue parameters required for fatigue check.
Fatigue resistance of steel composite beams is verified as per Article 220.127.116.11 of AASHTO-LRFD 2012. Fracture toughness requirements mentioned in Article 18.104.22.168 are not checked in midas Civil.
From the main menu, select [Design] tab > [Type : Steel Composite Design] > [Design Input Data] group > [Design Parameters] > [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 22.214.171.124.3-2)
(ADTT)SL: Number of trucks per day in a single-lane averaged over the design life (126.96.36.199.2). (ADTT)SL can be manually
calculated as per 188.8.131.52.2-1.
N: Number of stress range cycles per truck passage. Value can be taken from Table 184.108.40.206.5-2
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.
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.