| Product(s): | RAM Structural System | ||
| Version(s): | 14.00.00.00 or later | ||
| Environment: | N/A | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group |
Use of Two way decks in RAM Structural System
The table below clarifies how One way and Two way decks can be used in RAM SS v14.00 or later:
- One way deck always requires a complete nodal network, a network of supported beams or walls such that a tributary for every member is defined and encompasses the entire one way deck area.
- If you have a semi-rigid diaphragm with a one way deck that is not properly supported by a network of beams, RAM Frame or RAM Concrete may run without warning, but loads will be zero.
- When Two way deck is used, only two modules can give results. RAM Frame with a semi rigid diaphragm option (2-way rigid diaphragms are also allowed starting in v14.03 and were altered in 14.07 see Criteria - Diaphragms for details), or RAM Concrete. Furthermore, the user must specify the deck effective E value, thickness and Poisson's ratio for those modules to work. The diaphragm will always be meshed, and out-of-plane stiffness will always be assumed. Hence the beams (if there are any) will resist less force compared to a one way system. Any such beams have a centroid alignment to the center of the slab.
- No automatic Live Load reduction calculation is performed for members carrying loads from two-way slabs. The reduction to be applied to the Live Load on such members must be assigned to the member in the Modeler (e.g. Layout - Columns - LL Reduction).
- RAM Concrete typically considers skip loading for live loads on the beam lines lying under one way decking, if desired. A beam line lying under a two way deck can have skip loading cases only if line and point live loads are applied directly on it. Currently, the surface loading applied to two way decks does not generate any skip loading cases.
Other notable warnings:
RAM Frame, using a Two way deck without using a semi-rigid diaphragm (only applies to versions 14.00 to 14.02):
RAM Frame: Two-way Deck Found Inside Diaphragm 1 of Story 2. The Diaphragm Type is not Semirigid. Gravity Loads on the Diaphragm Disregarded for the Analysis. Do you want to continue?
RAM Steel - using Two way decks always gives an error of some sort, example:
Warning: Failed to Create Slab Edge Load Polygons for diaphragm 1 on Layout Type Roof. Slab edge loads will not be applied to any beams around the perimeter of that diaphragm. Disregard this warning if the slab edge is Two-way deck.
Hybrid Decks
For Hybrid Deck levels, those that include areas of both one way deck and two way deck, the rules for nodal networks still apply to the one-way decked area. If the network is not complete various framing tables errors can occur.
Furthermore, when the level is meshed in Ram Frame or Ram Concrete you will see that the mesh covers the entire floor so that the diaphragm is continuous. This can cause some unexpected behavior in the one-way regions. Specifically the meshed slab can help in resisting some of the applied loads, effectively holding up the beams.
There is an option in the Concrete Analysis mode, under Criteria - Analysis to alleviate this effect.
By not checking the option to "Include Out-of-Plane Stiffness for One-Way Decks in Hybrid Slabs" you are telling the program to use a near zero stiffness element in the one-way deck areas so that the beams have to do the work.
For these reasons, mixing one and two way decks in the same diaphragm is not generally recommended.
Concrete Column design with Two-Way Slabs
In Ram Concrete, the column K factor is determined based on the relative stiffness of the beams to the columns. The stiffness of 2-way slabs is not considered in this calculation, so the user should manually assign the proper K factor for columns supporting 2-way slabs.
As noted above, Live Load reduction percentages also need to be manually assigned.
Semi-rigid Diaphragms for Two-Way Slabs
Out-of-plane stiffness is assumed not only for gravity loads, but also for lateral loads when the deck is two-way and semi-rigid only. When the diaphragm is set to rigid, out of plane stiffness is ignored in the lateral load cases. Prior to version 14.07, rigid diaphragm two-way decks considered the out-of-plane stiffness for all loads. This was a major change in version 14.07. Consequently if you have a 14.06 or earlier model where your are relying on slab out of plane stiffness to tie your columns together and provide frame action, you will now must change the diaphragm setting from rigid to semi-rigid to get similar behavior out of the new version. The effect of this is most notable when transferring lateral Self Equilibrium result to Ram Concept.
There are some general concerns in RAM Frame for these diaphragms. The distribution of gravity loads is determined by meshing the diaphragm and then the program calculates the gravity load that is tributary to each node. Gravity columns/walls are ignored in the Frame finite element analysis. If you have gravity columns and/or walls, the gravity forces on the lateral members will be inaccurate unless you utilize the consider gravity columns/walls as springs options. However, utilizing this option will have an impact on the lateral analysis as well. The following concepts apply to one way decks with out-of-plane stiffness considered, too.
Consider a five story shear wall building with two way 8 inch concrete slabs. Here is the typical plan.
The moments at the base of the walls for a lateral load case in the X direction are 13555 k-ft and the shears are 242 kips.
If you run the same load case and include the gravity columns as springs the moments drop to 11163 k-ft as the gravity column spring form couples that resist much of the overturning moment. The wall major axis shear forces stay the same. You would get similar results if you modeled all of the columns as pinned lateral columns.
As the out-of-plane stiffness of the diaphragm and axial stiffness of the columns increase the moments in the walls decrease. Conversely, when there is negligible out of plane stiffness to the diaphragm, the moments in the walls would not be affected. (Using version 14.07 or later with rigid two-way decks transforms this behavior so that the slab out-of-plane stiffness is only considered for gravity load cases. See Criteria - Diaphragms for details.)
In the RAM Concrete Shear Wall Module all of the design forces, including gravity load results, come from the RAM Frame analysis. For the design of shear walls it is important to understand the impact gravity columns have on the forces in the walls.
Transfer Forces
A column or wall may set directly upon a 2-way deck without the need for a beam on the story below (using version 14 or later). The force from the vertical member will transfer through the meshed slab to the supports below. Since this requires a finite element analysis of a meshed two way slab, it has the same limitations in the table above, i.e. it only works using Ram Frame or Ram Concrete analysis.
Generally we recommend that the columns or walls that offset should be modeled as lateral members so that the analysis in RAM Frame will consider those members in the analysis. That way the program can display or report important information like axial member forces and nodal displacements.
We also recommend modeling a transfer beam in addition to the slab when reasonable to do so.
There are some special considerations when using RAM Structural System in conjunction with Ram Concept for transfer slabs. See these topics for further details:
See Also
RAM Frame - Criteria - Diaphragms
RAMSS Common Framing Table Errors
Structural Product TechNotes And FAQs