Dear Officer:
I am currently using RAM Concept to analyze a reinforced concrete slab (RC slab) with stepped beams (slab elevation varies at different locations). I have the following questions:
1. following ACI-318, when calculating the concrete slab and beam strength, the phi for moment is 0.9, for shear is 0.75 etc. If I want to analyze a RC slab with phi=1 for moment, shear, torsion, may I ask if there is an option to do this setting?
2. it seems like RAM can report slab strip moment, torsion, shear at each column strip and middle strip. Based on the description of 40.12.3 in the manual, it seems like the torsion is calculated by integrating the vertical shear of the section to its geometry centroid. If this is true, if I want to check the slab or beam under torsion, may I ask if it is proper to use "torsion" or "twist" for our torsion design force?
3. may I ask if RAM considers any bending, torsional or other stiffness modifiers during the demand calculation?
Thank you for your help.
1. No, you cannot alter the phi factors used by Ram Concept.
2. Check this: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/26611/ram-concept-torsion-design-tn
3. There are a variety of stiffness reduction factors that can be applied to the beams and slabs on the mesh input standard plan. This can affect the total demand on the strip.
Thank you, Seth. May I have some follow up questions?
1. regarding the answer to question 2, if I just want to get the torsion demand on beams and slabs, and check the section DCR myself, should I export the "twist" and "torsion" from RAM and add these two together as my cross section torsion demand? I am curious that since vertical shear stress in the cross section may be caused by shear or twist, if RAM distinguish these two components when integrating the vertical shear to get the torsion.
2. for a typical RC slab, a good amount (>50%) of the moment is resisted by the column strip. For example, the column strip width should be like the yellow highlighted region, but if I define the column strip width as the beam width (hatched by blue) and assign the yellow region to middle strips, I think the moments reported for the middle strip include the contribution from the yellow region. May I confirm that if we just look at one middle strip, the moment in the yellow highlighted region is larger than the un-highlighted region since the yellow highlighted region is more close to the columns? May I ask if there is some corresponding information in the manual?
3. if I have a slab setup as shown below, the grey areas represent slab, dark blue represents columns, light blue represents beams. May I ask if it is possible to do some settings in RAM, to make Beam 1 and Beam 2 serve as supports of the two adjacent slabs, and use Beam 1 and Beam 2 to transfer the slab load to the vertical beams? The red lines represent the supports of the slabs to the beams.
1. I would not typically advise users to design based on the twist forces. Check the program manual (search for "twist") to see more.
2. Your image did not show, but yes you can report the forces in the column strip, in the middle strip, and if desired force the column strip to be designed for the resultant of the two.
3. I don't see how those secondary beams can be the supports, but you could place a line support, line spring or a wall below in those locations to force that behavior.
Hello, Seth,
Thank you for your response and suggestions. Regarding the 3rd question, let me explain it a bit clear. Assuming the dark grey slab and light grey slab have different elevation height. So the two slabs have discontinuity across the Beams 1, 2 (Beams 1, 2 are step beams connecting the two slabs).
It seems like when RAM analyze the slab, it assumes the dark grey and light grey slabs are continuous. May I ask if we can release slab edge (red line indicated) moment at the beam support location? This is not to set a support at the beam, but only to tell RAM that the two slabs are supported on beams and are not continuous in the longitudinal direction.
There is no way to hinge or release the slab per se. You can add a sliver of slab with modified behavior to accomplish something similar. See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/19966/ram-concept-creating-a-hinge-in-the-slab
Thank you, Seth.
Hi, Seth,
May I have a follow up question? I saw in RAM you have the flexibility of setting beams and slabs as one-way slab or two-way slab. If I assign Beam 1 and Beam 2 as one-way slab along their length direction, may I ask how RAM will treat the relationship or continuity of the dark grey and light grey slabs?
Thank you.
Continuity is still enforced. Check the program manual, section "55.3 Orthotropic behavior" for further details.
Thank you, Seth. May I ask if setting beam1 and beam2 as one-way slab may be similar to what is suggested in this document? https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/19966/ram-concept-creating-a-hinge-in-the-slab
Since setting beam 1 and beam 2 as one-way slab, then the bending stiffness in the normal to beam direction is set to very small, which is similar to the 3rd option in the above link.
Yes, the one way slab option is similar since the weak axis stiffness is reduced.