Transfer Slabs in RAM Structural System

by Josh Taylor

 

Solutions Discussed: Analysis of concrete buildings

Products Discussed: RAM Structural System (SELECTseries1, r14.00.04.01)

 **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.

 

If you are using RAM Structural System to model a building that has a transfer slab, it is critical that the structure is modeled with due consideration for the analytical assumptions the software makes. This is necessary to ensure the vertical load path is complete, and all gravity loads are getting accounted for in the structure. By transfer slab, I mean a level at which a column or wall bears on a beam or bare slab, directly below which there is no column or wall. In this post I discuss some of the more common transfer scenarios we are seeing from our clients, how these situations should be handled, and any program limitations that come into play.

 

 

 Case 1 - Gravity column supported by gravity beam, all one-way slabs

 

 

 

All floors in this model utilize a one-way slab supported by gravity beams. The slab uses a rigid diaphragm property in RAM Frame. Thus the gravity floor loads will get distributed to the beams (and walls) using a beam network and a rule-based approach, just as if all the beams were steel. The RAM Frame analysis will include only the lateral-resisting walls, utilizing rigid diaphragm behavior for in-plane response. The beam end reactions from the gravity analysis will be applied to the walls for the RAM Frame analysis and will serve as the basis of the gravity load cases in RAM Frame. The RAM Concrete analysis will calculate the gravity forces in all members.

 

Case 1 in summary:

- Gravity forces in gravity members will come from the RAM Concrete analysis

- Lateral forces in the walls will come from the RAM Frame analysis

- Although gravity forces in the walls will be calculated in both RAM Frame and RAM Concrete, the RAM Concrete shear wall design module will use the gravity forces from RAM Frame.

Note: If the columns supporting the transfer level are assinged as lateral instead of gravity, they will be included in the RAM Frame analysis. When designing columns in the RAM Concrete Column module, the user will then have the option of using either the RAM Frame or RAM Concrete analysis as the source of gravity forces on the columns.

 

 

Case 2 - Gravity column supported by gravity beam, one-way slabs above with two-way transfer slab

 

 

 

The case 1 scenario has now been changed so that a two-way slab is used at the transfer level instead of a one-way slab. In addition, the transfer level now uses a semirigid diaphragm property rather than rigid. A rule-based approach is still used in calculating the gravity loads at the roof, 4th, and 3rd levels. At the transfer level, the slab is meshed as shell finite elements and is now included in the RAM Frame analysis. Both the membrane and flexural stiffness of the slab will be accounted for in RAM Frame. Gravity floor loads at the transfer level will be resolved as point loads applied to the individual nodes of the shell elements.

Since the gravity columns bearing on the transfer level are supported by a gravity beam, the reaction from each column will be applied directly to the supporting beam, with none of the column reaction being taken by the slab. Since the transfer level slab will be included in the RAM Frame analysis, but the gravity columns supporting the slab will not, it is important to select the option to use a spring in place of the gravity columns in RAM Frame, so that the transfer slab is supported at the gravity columns. The resulting analytical model produced by RAM Frame will look as shown in the figure below.

 

 

 Case 2 in summary:

- Gravity forces in gravity members will come from the RAM Concrete analysis.

- Lateral forces in the walls will come from the RAM Frame analysis.

- Although gravity forces in the walls will be calculated in both RAM Frame and RAM Concrete, the RAM Concrete shear wall design module will use the gravity forces from RAM Frame.

- Option in RAM Frame to consider gravity columns as vertical springs should be turned on. Otherwise, the transfer slab will be unsupported at the gravity columns.

Note: If the columns supporting the transfer level are assigned as lateral instead of gravity, they will be included in the RAM Frame analysis. When designing columns in the RAM Concrete Column module, the user will have the option of using either the RAM Frame or RAM Concrete analysis as the source of gravity forces on the lateral columns.

 

 

Case 3 - Gravity column supported by bare slab, one-way slabs above with two-way transfer slab

 

 

This is the same scenario as case 2, except that the beam supporting the transfer column has been removed, and the column bears directly on the two-way slab. The cumulative gravity load at the transfer column will now be applied directly to the meshed slab as a point load. This is the case in both the RAM Frame and the RAM Concrete analysis.

 

Case 3 in summary:

- Gravity forces in gravity members will come from the RAM Concrete analysis.

- Lateral forces in the walls will come from the RAM Frame analysis.

- Although gravity forces in the walls will be calculated in both RAM Frame and RAM Concrete, the RAM Concrete shear wall design module will use the gravity forces from RAM Frame.

- Option in RAM Frame to consider gravity columns as vertical springs should be turned on. Otherwise, the transfer slab will be unsupported at the gravity columns.

Note: If the columns supporting the transfer level are assigned as lateral instead of gravity, they will be included in the RAM Frame analysis. When designing columns in the RAM Concrete Column module, the user will have the option of using either the RAM Frame or RAM Concrete analysis as the source of gravity forces on the lateral columns.

 

Case 4 - Gravity column supported by bare slab, all two-way slabs

WARNING: This configuration will result in inaccurate design forces in the RAM Concrete Shear Wall Design module.

 

 

All floors in this model utilize a two-way slab without beams, with a semi-rigid diaphragm property. Thus, all slabs are meshed as shell finite elements and are included in the RAM Frame analysis. Both the membrane and flexural stiffness of the slab will be accounted for in RAM Frame. The RAM Frame analysis will include only the lateral-resisting walls and floor slabs. Since the floor slabs will be included in the RAM Frame analysis, but the gravity columns supporting the slab will not, it is important to select the option to use a spring in place of the gravity columns in RAM Frame.

 

 

Note that since the gravity columns in the RAM Frame model have been replaced with springs, the transfer forces from the columns above do not get applied to the transfer slab and thus the wall below the transfer level sees no transfer load. This is not the case in the RAM Concrete analysis. In the RAM Concrete analysis the transfer loads will get carried down through the structure correctly as all members are considered in this analysis.

However, the RAM Concrete Shear Wall Design module always obtains its design forces from RAM Frame only. There is no option in this module to use RAM Concrete as the source of gravity forces on lateral members. As a result, the configuration shown here will not correctly account for the gravity design forces for shear walls.

The shear wall design limitation illustrated here can be overcome by using lateral-resisting columns in the model instead of gravity columns. This way the columns will be considered in the RAM Frame analysis and gravity load path will be accounted for by the finite element analysis.

 

Case 4 in summary:

- Gravity forces in gravity members will come from the RAM Concrete analysis.

- Lateral forces in lateral members will come from the RAM Frame analysis.

- Option in RAM Frame to consider gravity columns as vertical springs should be turned on. Otherwise, the floor slabs will be unsupported at the gravity columns.

- Design gravity forces for shear walls will not be accurately accounted for with this configuration. An alternate means of modeling will be necessary if designing walls with RAM Concrete. One sufficient alternate approach is to assign all transfer columns as lateral members.

Note: Consider the scenario in which the columns supporting the transfer level are assigned as lateral instead of gravity, in which case they will be included in the RAM Frame analysis. When designing columns in the RAM Concrete Column module, the user will have the option of using either the RAM Frame or RAM Concrete analysis as the source of gravity forces on the lateral columns. To correctly account for the gravity forces on these columns, the user must select RAM Concrete as the source of the gravity forces on lateral members. This is because the RAM Frame analysis replaces the gravity columns with springs and hence the load from these columns will not get transferred down through the structure.

 

Case 5 - Lateral wall supported by bare slab, all two-way slabs

 

 

All floors in this model utilize a two-way slab without beams, with a semi-rigid diaphragm property. Thus, all slabs are meshed as shell finite elements and are included in the RAM Frame analysis. Both the membrane and flexural stiffness of the slab will be accounted for in RAM Frame. The RAM Frame analysis will include only the lateral-resisting walls and floor slabs. Since the floor slabs will be included in the RAM Frame analysis, but the gravity columns supporting the slab will not, it is important to select the option to use a spring in place of the gravity columns in RAM Frame.

Since the gravity columns are continuous all the way down the building, and all the wall members are lateral-resisting, the full gravity load will be tracked all the way down the structure for both the RAM Concrete analysis and the RAM Frame analysis (unlike case 4).

 

Case 5 in summary:

- Gravity forces in gravity members will come from the RAM Concrete analysis.

- Lateral forces in the walls will come from the RAM Frame analysis.

- Option in RAM Frame to consider gravity columns as vertical springs should be turned on. Otherwise, the floor slabs will be unsupported at the gravity columns.

- Although gravity forces in the walls will be calculated in both RAM Frame and RAM Concrete, the RAM Concrete shear wall design module will use the gravity forces from RAM Frame.

Anonymous
  • Hello VBWGE. First, let me say that this post was written years ago and is in bad need of updating. These how-to's are now much simpler with advancements made in RAM since the original posting. We will work on updating this for the current functionality of the software. To answer your specific questions:

    1) You absolutely can assign all elements as "lateral" and consider all members effective for all load cases, providing releases to minimize their contributions in resisting lateral forces. When a framing system is especially complex, this is in fact the procedure I would recommend for a concrete-framed system.

    2) The vertical loads are accumulated in columns and walls in each story going downward as the analysis is carried out. So if a column or wall stops at a level, all the cumulative load from above in that member will be applied to the slab.

    3) We will update this wiki for present day and see if this clarifies things.

    4) RAM Structural System is a great product for buildings of all sizes.

  • 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?

  • can you give me documents about tranfer slab, please? thanks so much,please sent to lecuongkta@gmail.com