| Product(s): | RAM Structural System; Ram Frame; Ram Concrete | ||
| Version(s): | 14.00.00.00 or later | ||
| Area: | Analysis |
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 under gravity loads. 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. 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 for 2-way decks is always considered for gravity loads. It may also be considered for lateral loads. For details on the significance of out-of-plane stiffness in lateral load case analysis refer to How does the diaphragm out-of-plane stiffness affect behavior?
There are some additional 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 and walls are ignored in the Frame finite element analysis when one-way slab are used. For two-way slabs, there are two options for including gravity columns and walls when analyzing gravity load cases in Criteria - Diaphragm (see below).
In general, it is best to use the "Include Gravity Members" option. The vertical spring option does not account for rotational stiffness and will not account for load transfer to supporting slabs when a column or wall is supported by a beam or slab with no column or wall below. Gravity columns and walls are only including when analyzing gravity load cases and the Eigenvalue analysis; they are always ignored when analyzing lateral load cases.
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. (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). This condition will be flagged as a DataCheck warning in RAM Modeler, but that warning can be ignored. 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