I am using RAM Concrete beam to design pan joists, and am having issues with two beam lines that are identical in loads, spans, tributary widths, etc. The only difference is that at the end supports of one beam line, the supports are concrete (gravity) columns, and the adjacent beam line has concrete spandrel beams as the end supports.
For case 1 (end supports as columns), the negative moment at the columns is very very high compared to the negative moment at the spandrel beams (case 2). What are the possible explanations for this? Is it a modeling issue that I am not comprehending? I would expect to see maybe a slightly higher negative moment since a column would be stiffer than a spandrel beam in this case, but I can't see why the difference would be so large. The first image is for case 1 that shows a negative moment higher than 350 kip-ft, and the second image is for case 2 with a smaller moment of around 60 kip-ft. Any insight would be greatly appreciated!
The negative moment at the end of the beam is associated with the rotational stiffness of the supporting element. When the support is a column, its flexural stiffness in that direction provides the rotational restraint. When the support is a beam, the torsional stiffness of the beam provides the rotational restraint. The flexural stiffness of the column is much larger than the torsional stiffness of the beam (especially after cracked factors for torsion are considered) and a higher negative moment for the column supported beam should be expected. Also, the deflection of the supporting element will affect the member forces. With a column support, the deflection at the beam end will be near 0. With a beam end, the deflection at the beam end is not 0 and can be significant depending on the span and stiffness of the beam member.
Thanks, Karl. I'm still skeptical that the negative moment would still be so high. Per ACI 318-14's simplified method, the moment should be in the ballpark of w*l^2/16. Taking into account the stiffness/rigidity of the column I can see that number being higher, but not to the point where the negative moment becomes w*l^2 over a very small denominator.
Would it be helpful to submit a copy of the model I'm using?
The Beam Line with the column at one end is supported by a beam on the opposite end. That beam has a lower rotational stiffness and deflects vertically just as the beam supports discussed in the earlier reply. This will affect the moments at the end of the beam supported by the column. As the vertical deflection of the interior beam supports increases, the negative moment at those supports will be smaller and smaller. This may be a cause were the ACI simplified method does not fit the behavior of the beam span that you are reviewing. One way to confirm is to gradually increase beam stiffness in a copy of the model and test how the negative moments change.
Please share your model using the Secure File Upload link at the bottom of the page and post the file name here after it is uploaded, if you want use to review your model.
Karl - I see what you mean. I've shared the model via the Secure File Upload link. It is titled "2020-01-28 West Tower RAM Model to Bentley Secure Upload". Thanks!