Hello,
I'm a new starter on RAM concept and have some questions regarding the fa/fcr plan and Ieff/Ig span plot under the Load Histroy Defelctions tab.
When I was comparing these two results, in the initial load step (self-weight considered only), the fa/fcr plot showed most places with a ratio smaller than 1, which means the section is untracked. But in the Ieff/Ig plot, most locations have a ratio smaller than 1, which means the section is cracked. Could you tell me why there is an inconsistency?
PDFPDF
please also find attached model for more information.
drive.google.com/.../view
Fa/Fcr is the ratio of the calculated axial stress due to the applied loading and induced creep and shrinkage strains divided by the modulus of rupture. As you have noted, a section is considered cracked when the ratio is greater than or equal to 1.
Ieff/Ig is a measure for the element stiffness reduction is determined using the curvature calculations performed on each cross section rather than cracked or uncracked transformed moment of inertia calculations. It is not a cracked moment of inertia to gross moment ratio as the plan label implies.
The plotted values of Ieff/Ig are roughly equal to the ratio of the mean curvature (average curvature interpolated between the uncracked transformed curvature and the cracked curvature using the tension stiffening model) and the gross curvature caused by externally applied loads and post-tensioning using gross section properties. Ieff/Ig will be greater than 1 when the mean transformed curvature exceeds the gross curvature. That could occur when:
You may also find the tabulated parameters outlined on the web page below useful for studying and understanding the behavior.
(+) RAM Concept Load History Deflection Analysis Results Table - RAM | STAAD | ADINA Wiki - RAM | STAAD | ADINA - Bentley Communities
Thank you Karl. Appreciate your response.
Another issue I found in the model is there was very high axial force in the element, which I did not expect to occur. In the model I previously attached I have reduced the axial force stiffness factors for all the elements so the high axial forces are eliminated.
I am not sure if this is the correct way and there is no obvious increase in the bending moment plot after reducing the axial stiffnesses. I had thought if the axial stiffness factors are reduced then the high axial force originally in the elements would transfer to the bending moment.
Could you explain why this would occur and what's the best approach to consider for the high axial force?
Thank you.
non-reduced axial force stiffness factor model attacheddrive.google.com/.../view
The axial forces are induced from the vertical elevation offset between the beams and the slabs. The web page below should explain in more detail.
(+) RAM Concept T-Beams and Axial Forces [TN] - RAM | STAAD | ADINA Wiki - RAM | STAAD | ADINA - Bentley Communities
I recommend against using axial stiffness reductions to reduce the axial forces. One significant risk is ignoring tension forces in middle strips adjacent to column beam strips, which could lead to an unconservative design.
You may want to use a simple example like the one presented on the web page to better understand how axial stiffness reductions can change both the axial/flexure demand and the reinforcement design.