LBS V22.00.02.19
CS0035224

Service Case 1233802: [CS0035224] The fatigue stress range calculation doesn’t follow as per AASHTO Table 6.6.1.2.3-1, Section 7.2

Product Backlog Item 1233801: (VM):LBS:V22:The fatigue stress range calculation doesn’t follow as per AASHTO Table 6.6.1.2.3-1, Section 7.2

Child Service Case 1377226: [CS0165774] fatigue check for cross-frame

Child Service Case 1624094: [CS0165774] fatigue check for cross-frame

USER INFORMATION :

Brenden Good, E.I.T.

Structure Engineering

SAI Consulting Engineers, Inc.

Two Lemoyne Drive, Suite 101

Lemoyne, PA 17043

Voice: 717.763.5011

Fax: 717.763.0920

bgood@saiengr.com | www.saiengr.com

USER ISSUE :

#1. The ‘X-Frame Conn Bott.’ fatigue check calculates the E’ fatigue stress range at each diaphragm to stiffener connection.

I found on the Help Forum, you posted an article on this topic on 4/24/2023 titled “QUESTION: How is the program calculating ?f -- Fatigue stress range and the related Performance Ratio?”. Is the methodology explained in the post for ‘Frame’ struts, as ‘truss’ struts should have moments released at the ends of the members and my diaphragms are ‘truss’ struts? If the post is based on ‘frame’ struts’, may you provide the methodology of ‘truss’ structs? May you also explain why the fatigue stress range calculation doesn’t follow with AASHTO Table 6.6.1.2.3-1, Section 7.2, that the fatigue stress range shall be calculated based on the effective area of the member, Ae= U*Ag. Does the program take into account any type of weld length at each diaphragm connection, as that has a significant impact on the fatigue stress range?

VINAY's FINDINGS/REPLY TO USER :

I checked the product development team and according to them for Category E' we are not following the procedure in the table 6.6.1.2.3-1,

Section 7.2 to calculate the Fatigue Stress Range.

We need some additional information related to Fatigue Stress Range calculation for Section 7.2.

SUMANTA's COMMENTS & FEEDBACK :

The fatigue stress range used in the check, how area of the cross frame is considered. I clarified, the stress range is obtained from bending stress caused by girder vertical moment at that POI location under different non-composite and composite states including LL. We have a plan to implement fatigue design for cross frame individual members. The whole fatigue check for cross frame will be corrected in near future. For the time being they are going to ignore the fatigue check for CF and failures arising from it.

SUMANTA's WORKAROUND TO THE USER.

You can make use of the Cross Frame Detailed Forces report to extract the axial forces generated in the individual members for deal load and live load. As far as live load forces are concerned, you will maximum tension (positive LL) and maximum compression (negative LL). You need to factor these forces and get the stress range.

There is a cross frame design report where you can generate design report for cross frame individual members for all load combinations including fatigue. We are going to implement proper fatigue design in near future. At that point, we will keep in mind the requirement to obtain fatigue stress range of cross frame member.

USER RECOMMENDATION:

1. Ahmad pointed out, code requirement (most probably 6.6.1.2.1) to get the maximum stress range in diaphragm. This states to get the stress range keeping the fatigue truck on a single lane. We cannot make use of maximum positive stress arising from fatigue truck on one lane and maximum negative stress arising from another lane. This requires some work in the live load generator as far as I can see now.