| Applies To | |||
| Product(s): | RAM Concept, RAM Structural System | ||
| Version(s): | Various | ||
| Environment: | N/A | ||
| Area: | Modeling; Analysis | ||
| Original Author: | Bentley Technical Support Group |
Overview
Podium slabs are concrete transfer slabs supporting multiple levels of wood or steel framed superstructures above. This popular framing system can be analyzed and designed using RAM Structural System and RAM Concept. The purpose of this tech note is to discuss issues and common questions relating to the modeling and design of this system using these programs.
Modeling Options
Two common options for designing this type of structure include:
1. Use a two-stage analysis in which the upper portion and lower portion are designed as separate structures in RAM Structural System. See ASCE7 12.2.3.2 for details.
2. Model the entire structure in one model in RAM Structural System.
Regretfully, there is not an automated two-stage analysis feature in RAM Structural System. This means that loading from one model needs to be manually entered in the second. One possible work around:
- Model and analyze the upper portion in RAM Structural System with a mat foundation.
- Import the mat foundation into RAM Concept.
- Add columns to convert the model to an elevated slab.
- Amplify imported loads to satisfy ASCE7 12.2.3.2.
- Add lateral and additional gravity loads on lower portion in Concept.
- Analyze and design.
With this approach, foundations will need to be designed manually using the analysis results from RAM Concept. There is not a way to import them into RAM Foundation for design. Also, the lateral analysis for the lower floor is done in RAM Concept. This will not work well if there are multiple concrete levels below the upper portion.
RAM Structural System Tips
In RAM Modeler, live load reduction percentages should be modeled manually for columns and walls supporting two-way decks. See the following web page for more information on using two-way decks in RAM Structural System:
For any transfer slab the out-of-plane stiffness should be considered when analyzing lateral load cases. For details refer to How does the diaphragm out-of-plane stiffness affect behavior?
It is important that all walls fit on the podium slab. If any wall extends outside the podium slab edge, unintended foundation supports can be added in the model. The reaction at the foundation support would not be transferred to the podium slab. This problem most often occurs at walls that are skewed to the global x or y axes and the slab edge segment is modeled continuous past the wall end. To prevent this from occurring it is best to snap slab edge segments directly to walls ends near the slab boundary. Displaying reactions in RAM Frame (Process – Results – Reactions) is a good way to check for unintended foundation supports. See reaction points circled in the screenshot below for an example of an unintended foundation support.
In RAM Frame, pseudo-flexible or semirigid diaphragms are the best options for models at wood framed levels. If walls are skewed relative to the global x and y axes in plan, then semirigid diaphragms should be used. See the following web pages for more discussion on these diaphragm types.
RAM SS Pseudo-Flexible Diaphragms
In RAM Frame, the in-plane flexural stiffness of lateral walls above the podium level is always considered. As a result, the walls will be meshed with the podium slab and act as a stiffening element. This can produce significant moments at the base of the transfer walls. In most cases, loads are assumed to transfer directly from the wall to the slab, ignoring the wall stiffness above.
In RAM Concrete, there is an option to ignore the stiffness of walls above the slab. This option is located in the Analysis Criteria dialog in RAM Concrete – Concrete Analysis mode (Criteria –Analysis). Check the box for “Ignore Wall Stiffness on Above Story” to ignore the stiffness of the walls above.
More discussion on walls in RAM Structural System can be found on the following web page;
In RAM Concrete, skip live load cases can significantly increase analysis time. Many models with podium slabs containlevels with many “other” type beam members. If the “Skip-Load the Live Load on Non-Beam Line Beams” is checked in RAM Concrete Analysis – Analysis – Criteria, the program will skip the live load on all of the “other” beams. Typically, this is not intended and the box can be unchecked.
RAM Concept Tips
RAM Concept offers three options for import of gravity forces: forces from RAM Steel, forces from RAM Concrete, and forces from RAM Frame. Generally, it will be best to use the RAM Steel forces as the first preference when importing. In RAM Steel, load is distributed to members based on a simple tributary area. The wall stiffness is considered in both RAM Concrete and RAM Frame. The transfer wall moments may be significantly higher if RAM Concrete or RAM Frame forces are selected with a higher priority than the RAM Steel forces.
When importing gravity forces from RAM Steel, line and point loads are converted to an equivalent load at the base of the wall assuming the wall is a rigid element. Point loads that are eccentric to the center of the wall will translate to moments and a non-uniform load at the base of the wall. Segmentation of the wall will affect the transfer wall load that is imported into RAM Concept. See example below:
Each wall is 20 ft long. The wall on the left is modeled with one wall segment. The wall on the right is modeled with two wall segments; there is a 1-ft segment near the end of the wall. Each wallhas a 10 k/ft live load and a 10 k point load located 1-ft at the wall end (same point where the right wall is segmented).
The following line loads are imported into Concept:
The line load on the left has a magnitude of 11.8 k/ft at one end and 9.15 k/ft on the other. These loads are calculated from:
10 k/ft + (10 k)/(20 ft) +/- (10 k)*[(20 ft/2) - 1 ft)]/[(20 ft)^2*6] = 11.85 k/ft and 9.15 k/ft
The line load on the right is divided into two segments, representing the 1-ft wall segment and the 19-ft walls segment.
The 1-ft wall segment has a magnitude of -0.008333 k/ft on one end and 29.99 /ft on the other. These are calculated from:
10 k/ft + (10 k/2)/(1 ft) +/- (10 k/2)*[(1 ft/2)]/[(1 ft)^2*6] = 30 k/ft and 0 k/ft
The 19-ft wall segment has a magnitude of 11.05 k/ft on one end and 9.473 k/ft on the other end. These loads are calculated from:
10 k/ft + (10 k/2)/(19 ft) +/- (10 k/2)*[(19 ft/2)]/[(19 ft)^2*6] = 11.05 k/ft and 9.473 k/ft
In this example, modeling the wall in one segment will result in more accurate loads in RAM Concept.
For elevated slabs, live loads are always imported as unreduced live loads. If you would like to account for live load reduction you can either reduce the loads manually after importing or manually assign live load reduction parameters in the Live Load Reduction tab of the Span Segment Properties dialog. See below:
For mat foundations, reducible live loads are reduced when they are imported into RAM Concept. If you are using a two-stage analysis, set the Code for Live Load Reduction to “None” after importing to ensure the loads are not reduced twice.
See Also
RAM SS Pseudo-Flexible Diaphragms