Leap Bridge Steel - Diaphragm Reports

RE: Leap Bridge Steel - Diaphragm Reports

Vinay mysore — Tue, 27 Jul 2021 07:33:41 GMT

Diaphragms are pinned elements at both ends .

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RE: Leap Bridge Steel - Diaphragm Reports

Vinay mysore — Tue, 27 Jul 2021 07:30:58 GMT

For FEM analysis :

We release the moment DOF at two ends for diaphragm, not the shear. If there is any shear force developed, it will be reported. We can offer options to make the ends are fixed (/welded). FEM analysis considers the Cross frames as primary members.

In the Grillage Model if the diaphragm flanges are not connected (typical), the moments really can’t be generated Also the grillage analysis does not consider the cross frames as primary members.

RE: Leap Bridge Steel - Diaphragm Reports

Peter Molnar — Fri, 25 Jun 2021 11:25:45 GMT

I have a similar situation and the client is questioning the "fit condition for the diaphragms." When looking at staged analysis for steel only, then construction loadings (including deck concrete placement, finishing machine, overhang concrete, etc.) and then final condition, when are the diaphragms considered fully connected to the beams.

Also, based on the discussion above, the FEM models the diaphragms as pinned, but how does the Grillage analysis treat them?

RE: Leap Bridge Steel - Diaphragm Reports

Kevin Wood — Thu, 04 Feb 2021 21:51:34 GMT

I had a conversation with National Steel Bridge Alliance about the diaphragm forces being generated. Because my channel diaphragms did not have the flanges connected, they offered advice that the moments really couldn't be generated and the connection could be assumed to be a pin. I ended up using both a grillage and FEM model. I took the axial forces from FEM, and the shear forces from the grillage and used the resultant to design my connection. Vinay can confirm, but I had a follow up question for Bentley and they confirmed that the diaphragms are modeled as pinned in the FEM modeling routine. Here are a couple of references provided to me by NSBA. Take a look in Appendix B (can't recall which one) for a summary of modeling comparisons that were studied over the past decade or so:

FHWA Manual for Refined Analysis in Bridge Design - https://www.fhwa.dot.gov/bridge/pubs/hif18046.pdf

AASHTO/NSBA Guidelines for Steel Girder Bridge Analysis - https://www.aisc.org/globalassets/nsba/aashto-nsba-collab-docs/g-13.1-2019-guidelines-for-steel-girder-bridge-analysis.pdf

RE: Leap Bridge Steel - Diaphragm Reports

Joshua Morales — Thu, 04 Feb 2021 16:13:00 GMT

Would Bentley be able to provide reasoning behind their forces of cross frame detailed forces as it pertains to Wide flange diaphragms and if they are relevant or not? As an axial force, perhaps it is a PL/AE type equation. Would be used in conjunction with the cross frame forces (moments and shears)?

RE: Leap Bridge Steel - Diaphragm Reports

Vinay mysore — Mon, 30 Nov 2020 05:56:51 GMT

Please email me the LBS file and the Cross frame detailed report showing the extremely large axial forces .

My email is Vinay.Mysore@Bentley.com