RAMSS Two Way Decks [TN]

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 or later:

1. 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.
2. 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.
3. 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), 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.
4. 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). 
5. 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:

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.

For levels that include areas of one way deck and two way decks the rules for nodal networks still apply to the one-way decked area. 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 slab can help hold up the beams, indirectly resisting some of the applied loads. For this reason, mixing one and two way decks on one level is not generally recommended.

Semi-rigid Diaphragms for Two Way Slabs

Out-of-plane stiffness is assumed when the deck is two way. 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 but the shears stay the same. The rotation of the building is being resisted by the gravity column springs and out-of-plane diaphragm stiffness. 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 would decrease. Conversely, if there was negligible diaphragm stiffness the moments would not change. In the RAM Concrete Shear Wall Module all of the forces, including gravity forces, come from RAM Frame. If you intend to design the shear walls in RAM Concrete it is important to recognize how the Frame results will be impacted when the diaphragm is semi-rigid and out-of-plane stiffness is being considered.

Transfer Forces

A column or wall may now set directly upon a 2-way deck without the need for a beam on the story below. The force from the vertical member will transfer through the meshed slab to the framing below. Since this requires a finite element analysis of a meshed two way slab, it has the same limitations in the table above.

Generally we recommend that the columns or walls that transfer should be modeled as lateral members so that the analysis in RAM Frame will consider those members in the analysis.  Thus the program can display or report important information like axial member forces and nodal displacements.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

RAM SS Analysis Types

RAMSS Common Framing Table Errors

RAM SS Semirigid Diaphragms

Transfer Slabs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

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