After reading the article on transfer slabs (https://communities.bentley.com/other/old_site_member_blogs/bentley_employees/b/josh_taylors_blog/archive/2010/01/27/transfer-slabs-in-ram-structural-system.aspx) I've got some confusion such as:
1. In case 4 it is mentioned that in order to overcome problems with shear wall design limitations we have to assign transfer columns as lateral elements. If this is the case, why can’t we always assign all vertical elements as “Lateral” and include them in lateral analysis (like it is done in all FEA software)? Why then we need “concrete” and “steel” module at all? Why not doing analysis of the members from the “Frame module” based on their real life work (and, if necessary, provide pins for the columns so they are actually treated as gravity elements)?
2. Can "Concrete" module be used with Flat transfer slabs at all? If this module analyses the structure as floor by floor it wouldn't take into account cumulative deflection of the transfer slab in redistribution of the forces between core and columns at the top of the building.
3. I am working with buildings with irregular transfer slabs (transfer columns AND walls) which combine few situations mentioned above. What is the limitation of this software for buildings with irregular column grids and transfer slabs? It seems I can not follow one or the other case mentioned above as my building is the combination of a few cases.
4. Can RAM SS be used for medium to high rise buildings or STAAD PRO is the way to go?
1. At the beginning of the blog post linked in your question, there is the following note:
" **NOTE: Version r14.04.00.00 of RAM Structural System, released July 2011, gives the user the option to consider gravity members in the RAM Frame analysis. This greatly simplifies many of the scenarios discussed in this article. Please read the r14.04.00.00 release notes, available at http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/5398.aspx, to learn more about this enhancement to the program, which is particularly important for two-way slab systems."
The option to include gravity members when analyzing gravity load cases for two-way slabs, is found in RAM Frame - Criteria - Diaphragm. Using this option will overcome the problems discussed with Case 4.
Modeling all vertical elements as frame members is also an option. Even if columns are pinned, a portion of the overturning moment due to lateral load may be resisted by the axial forces in the columns in the form of tension-compression force couples.
In many cases, users wish to fix the gravity columns to the two-way slab so that the there is moment transfer for gravity load cases, but wish to ignore this fixity when analyzing lateral load cases. If the gravity columns are included as fixed, lateral elements, then the columns will participate in the lateral force resisting system. If the columns are included as pinned, lateral elements, then the fixity will be ignored for when analyzing gravity load cases and the gravity load distribution may be different.
2. You are correct. RAM Concrete will not account for this effect since it only analyzes one floor at a time. RAM Frame will account for this effect since it is a 3D finite element analysis. If the redistribution effect is a concern, then you should model the columns as lateral members and use the gravity force results from RAM Frame.
3. The significant limitation is the item that you mention in Step 2. If this is a concern, then you should include the columns as frame members and use the forces from RAM Frame to design the columns.
4. RAM Structural system can be used for both medium and high rise buildings.
Thanks kjgullerud,
you wrote: "The option to include gravity members when analyzing gravity load cases for two-way slabs, is found in RAM Frame - Criteria - Diaphragm. Using this option will overcome the problems discussed with Case 4."
So what you are saying is that with the release 14.04.00.00 transfer columns assigned as gravity members are actually included in the analysis and transfer the load to the slab and hence to shear walls/frame columns under the transfer slab? If this is the case and gravity elements are actually not deleted in frame analysis, then what is the use of the feature to assign gravity members as vertical springs?
Prior to v14.04, the only option for including gravity members with two-way decks was with vertical springs. In v14.04, a new option was added to include the actual gravity members in the analysis. Rather than removing the option to include vertical springs, which could affect models built in previous versions and analyzed in the newer versions, we chose to leave that option in the program. Including the gravity members is the better option, especially for transfer conditions. As you suggest, this allows the transfer columns to transfer the load directly to the slab which eliminates the limitations discussed in Case 4 of the blog post. I recommend using that option rather than using vertical springs.
Answer Verified By: VBWGE
Hi VBWGE and kjgullerud,
I mentioned this in the comment section of the blog itself, but just wanted to reiterate here that I need to update the information in that blog for the current functionality of the software. Those recommendations are dated and quite honestly confusing when applied to the current version. Things are far simpler now. Many of these recommendations were made in order to overcome limitations in the software than no longer exist. I'll let you know when the post is updated.
Hi Josh,
Thanks for your comments in the blog. Could you please advise when this topic will be updated. I would be very interested to read.
thanks kjgullerud, now it's clear