I'm working on a heavily skewed parallel girder bridge which requires the diaphragms to be considered primary members. A grillage analysis is being used. Under the "Cross Frame Forces" for the analysis, diaphragm end shears and moments are reported for the various dead and live load results. These results make sense to me.
I'm a little confused with the "Cross Frame Detailed Forces" report which is reporting some extremely large axial forces (over 1,000 kips under service live load with a cured deck). Service dead load axial forces are a little less than half of this. Under the old V-Load analysis of cross frames, axial loads were not part of the loads applied to the cross frame system (my references are back at the office so I'm not sure if diaphragms were addressed). Any insight to how LBS is generating the diaphragm axial forces would be appreciated. Thanks!
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
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)?
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
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?
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.
Answer Verified By: MarcosBeier
Diaphragms are pinned elements at both ends .
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