I am modelling a very long lintel beam carrying a low roof, masonry, and a higher roof. Roof structure is composed of open-web joists. The low roof joists will be seated on the bottom flange. To model torsion induced by the slight eccentricity of the joist seats I am using a very short plate of the flange thickness, offset by rigid links to the elevation of the bottom beam flange. All of this works great and I get good, reasonable results with expected torsional moments, vertical and horizontal deflections. My problem arises when I use the very same W-shape in either A992-50 or A572-50 steel. The A992 shape results in a 2% overstress (102% usage), the A572-50 shows a max usage of 54%, with torsion governing the result. Even A-36 has a max usage of 91%.
I am using the materials supplied with the program, not custom materials. The only rational explanation I can find for the result is the Poisson ratio for A992 is shown at 0.3, and 0.26 for the other steels. Is this accurate?
Elements. I've replied via email and uploaded the files.
There are some custom sections in your model that I don't have. There is also an instability when I try to run it in version 16.03.00.1010. What version are you using?
I replaced the sections and restrained one end of the beam in torsion in order to get results. After doing that I get the same design/capacity ratio using A992 or A572. The variation in Poisson's ratio could account for some slightly different deflections, but I would not expect it to affect the torsional demand or design for your model.
It does appear that 0.26 would be the more accurate value of Poisson's ratio.
I am running 16.2.0.88. The custom sections are simply ¾” thick plate 6 wide. I am not restraining the ends to model a pinned end for torsion; the beam will be in masonry however not restrained so I think that is safest, and closest to the available nomographs for torsion, however there was no instability in the runs I had.
The Poisson’s was the only variable I could observe, and therefore assumed with the resulting variation in deflections eccentricity was changing and resulting in more torsional stress.
I have uploaded the custom section and a screenshot of it. Clearly this lintel is carrying a lot of load and I want to be very sure, if you would please take another look with the above information.
Make a new A992 material in your version (see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/3569/creating-custom-sections-materials-etc-in-ram-elements-tn ) and use 0.26 for Poisson's ratio to rule that out as a factor.
Otherwise update to v16.03. I see at least one issue that was fixed for torsion design in the release notes.
Ok, planning to do so, but want to make sure .26 is correct for A992. I don't want "it probably is", I need certainty here. I will also update, but again, want certainty. I am also uploading output files for the three versions, please take a look at them.
The A992 using .26 for Poisson works, however steel is typically .3 for Poisson. Since the other steels are indicated as .26 for Poisson in the standard materials database, I'm concerned regarding past output.
So I updated to V16 and got similar results, until I fixed one end for torsion and released the other for X-translation. Some deflection in Axis 2, NONE in axis 3, which also doesn't feel realistic. I will have to go back to my version, because our office still runs Win 7 and the latest connection client is not compatible, and will not let me license. I am very disappointed Bentley gobbled up Ram, the lack of attention to detail is noticeable and Support, while trying to be helpful, usually burns more of my valuable time. Don't bother anymore, I'll figure out a path
There are other ways to stabilize it if you don't want to fix one end (a spring for example).
Pure torsion on a doubly-symmetric section should not result in any minor axis deflection.
You can use the latest version of the program and revert an older version of the Connection client if you have to. One other thing that can fix problems with Connection Client is to Navigate to %localappdata%\Bentley and delete MySELECT.DB.
Sorry if you feel the support is not as good as it was 16 years ago before the Bentley acquisition, I might be getting too old for this :)
Yep, thanks Seth - not directed at you, but Bentley as a whole. I've noticed minor language issues in the manuals that didn't exist, and the occasional coding miss, such as 0.3 Poisson for steel in some materials and 0.26 in others... overall a frustration with a large company that seems to be constantly changing the rules of licensure and trying to upsell me products/services I don't need. Plus the bureaucracy to navigate when I need to address something.
You have been very helpful and appreciate your assistance. I'm going with the next size up just to be sure.