Nathan Rick's Activities

Verifying Fatigue Stresses in Leap Bridge Steel

Nathan Rick — Thu, 10 Sep 2020 16:31:23 GMT

Hello,

I have a bridge model that is consistently telling me there are fatigue stress issues with my plate girder section.

I have exported the live load range that the model has created for a given girder and while comparing it to the section modulus at the locations reported to have issues I am coming up with a stress that is significantly less that what is reported.

When I use just the bottom fiber Sx for the areas, I compute the stress for point 42.18 as follows:

When I add in the load factor of 1.75 x 2 ksi I get the stress to be 3.52 ksi at point 42.18 for example. The fatigue report is telling me this stress is being computed for the 12 ksi allowed is 1.75 x 6.089 = 10.66 ksi. This leads to the the report of 88% of capacity being returned.

I am not sure how to reconcile these stresses, the model I have is a bridge that is skewed in excess of 20 degrees, so perhaps there is a deflection base later flange stress being considered, but the reports for this in the analysis do not return a lateral bending moment in the flange. If anyone can provide some direction it would be greatly appreciated, thank you in advance.

Line Girder Distribution Factors

Nathan Rick — Tue, 02 Aug 2022 22:29:02 GMT

Hello,

I am coming across apparent errors in the computation of live load distribution factors for our models when I check them under the line girder option for analysis. Before I get to far in to this, has their been an update to the software and where can I check that I am running the current version? I am not noticing a Build Number on this site for Leap Bridge steel, nor do I see a place to check the version I am using in the LBS interface.

The problem that I am noticing is that something in the computation for the Distribution Factors seems to change when you have multiple spans, where for the positive moment is seems to be correctly determining the values per AASHTO in the tail spans, but after the negative moment region is entered the calculation seems to blow up and leave with a result that is 30 to 40% lower than what is should be until the final span is reached.

I have checked the file against other live load generating software and hand computations and I believe the issue is present within LBS. I didn't see any recent posts regarding this issue, but this could compromise designs and ratings if there is indeed a problem present. The values shown in the load rating line girder analysis are 0.27 for g1 (single lane) and 0.41 for gm (multi lane). These appear to be incorrectly computed.

Our independent computations for these values are here, at point 131.5 g1 should be between .43 and .41 based on which side of the span you are on, and gm should be between .63 and .61 approximately.

Any information you can share would be appreciated, time is of the essence as this is a major issue in our modeling.

Nathan

Update to AASHTO Ninth Edition is causing an error I do not understand in previously valid models

Nathan Rick — Fri, 21 May 2021 18:59:01 GMT

Hello,

We recently deployed the current build of LEAP Steel for the AASHTO LRFD Ninth edition update. The design summary check is producing performance ratios that are above 1.0 in places that were never flagged in the previous builds of the software. This particular error does not seem to make much sense to me because it is instantaneous and leaves as it arrives, typically flexural failures show progressive failures for several POIs leading up to the worst case.

It also is not clear to me since this is a continuously braced flange due to composite action that issues would be present here. Can you provide some insight to how this check is being performed and what "FSLAB" is regarding this failure?

Centrifugal Forces

Nathan Rick — Wed, 14 Oct 2020 21:58:14 GMT

Hello,

I have a curved steel bridge that I am selecting LEAP steel to consider centrifugal forces for a 55 mph design speed. I was anticipating that when I checked to include the force, that the reactions for the bearings would show lateral components, but they do not.

Is there another case I need to add to get this to work?

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