Analysis with Two-way Slabs: Out-of-Plane Stiffness (Bending)
Diaphragm slabs can be defined either one-way or two-way slabs. A combination of these two types is also allowed within the same diaphragm.
With one-way slab system, gravity loads are processed based on tributary area concept: gravity loads are projected onto lateral members based on deck orientation. This part is the same as if gravity loads on rigid diaphragms.
On the other hand, loads on two way deck areas are directly applied to diaphragms. To do this, two-way decks are meshed first and then, gravity loads (line, surface and point loads) are applied directly to the meshed nodes.
An example is given in the following figure: one way deck and two way deck are defined within the same diaphragm. As shown in the figure after, only two-way deck is directly loaded with gravity loads and the loads on one-way deck are handled with tributary area approach.
- Gravity loads (including dead, live, roof and snow loads) are applied to two-way slabs
- User specified live load reductions for members are used for these members in two-way slab areas.
- Decks can be defined as a two-way system in the Modeler. Note that a value of bending thickness is needed for considering out-of-plane action (bending or plate action) in two-way load distribution.
- A diaphragm is directly meshed if it includes a two-way deck.
- Out of plane stiffness for shells in a two-way deck is always included in analysis.
- Gravity loads on two-way decks are handled in a similar way as explained for mass loads. See Loads on Diaphragms (Physical and Mass Loads)
- Applied story forces command can be used to visually see gravity loads on two-way decks.
- Gravity loads report shows calculated gravity loads on two-way decks.
- Gravity columns and walls under a two-way deck can be also represented either with vertical springs or with including these gravity members directly in the analysis (see Diaphragm Stiffness Section in Diaphragm Out-of-Plane Stiffness ).
- If applicable, the program applies crack factor to out-of-plane bending stiffness.
- If a transfer gravity column is sitting on a one-way deck, the load in the column is projected to nearest lateral beams or walls based on the deck orientation.
- If a transfer gravity column is sitting a on two-way deck, the load in the column, is applied to the deck directly.
- Lateral Columns that are supported on one-way slabs are not permitted and should not be modeled as such. The engineer should always apply a supporting lateral beam under this column to appropriately distribute the loads (the beam can run in the direction of the slab to get the desired behavior). If this is not done, the results of the analysis may result in instability, large inappropriate displacement of the column through the one-way slab if the slab is considered semirigid, or distribution of the load through two-way action in the one-way slab if out-of-plane stiffness is considered on one-way slabs. In this last case there could be up to a doubling of the transfer load considered for gravity load cases due to transfer point loads on one-way slabs also being distributed through the framing process and applied to the supporting lateral beams and walls as a point load.