Diaphragm Out-of-Plane Stiffness

The Story Diaphragm dialog includes several options related to out-of-plane stiffness (bending) for a meshed diaphragm. If a diaphragm includes a two-way deck, its out-of-plane stiffness is automatically included for gravity load cases in the program. For other cases, the program offers a variety of options for different structural configurations in terms of including in-plane and out-of-plane diaphragm stiffness. In the following table, all possible cases for diaphragms are addressed, default conditions enforced by the program are identified and three options for diaphragm stiffness are indicated. Option selections are available in Story Diaphragm dialog.

Span	Diaphragm	Load Case	Include In-Plane Stiffness	Include Out-of-Plane Stiffness	Include Gravity Member for Gravity Cases
One-way	Flexible / None	Gravity	Never	Never	Never
	Flexible / None	Lateral	Never	Never	—
	Pseudo-Rigid	Gravity	Never	Never	Never
	Pseudo-Rigid	Lateral	Never	Never	—
	Semi-Rigid	Gravity	Always	Option 1	Never
	Semi-Rigid	Lateral	Always	Option 1	—
	Rigid	Gravity	Always*	Never	Never
	Rigid	Lateral	Always*	Never	—
Two-way	Flexible / None	Gravity	Never	Always	Always
	Flexible / None	Lateral	Never	Never	—
	Pseudo-Rigid	Gravity	Never	Always	Always
	Pseudo-Rigid	Lateral	Never	Never	—
	Semi-Rigid	Gravity	Always	Always	Always
	Semi-Rigid	Lateral	Always	Option 2	—
	Rigid	Gravity	Always*	Always	Always
	Rigid	Lateral	Always*	Option 3	—

Option 1: Include Out-of-Plane Stiffness for Semi-rigid Diaphragms with One-Way Decks for Lateral and Gravity Load Cases
Option 2: Include Out-of-Plane Stiffness for Semi-rigid Diaphragms with Two-Way Decks for Lateral Load Cases
Option 3: Include Out-of-Plane Stiffness for Rigid Diaphragms with Two-Way Decks for Lateral Load Cases
(*): Rigid Diaphragm, rather than the properties of the diaphragm, is used.

Additional flexibility provided for two-way deck systems. You can include gravity columns and walls, which are under a two-way deck, as vertical springs.

Setting	Description
Include Gravity Columns as Vertical Springs	If this option is selected, the program inserts vertical springs at the locations of all gravity columns inside the meshed diaphragm. A stiffness value of AE/L is assigned to these springs.
Include Gravity Walls as Vertical Springs	If this option is selected, the program inserts vertical springs at the locations of all gravity walls inside a meshed diaphragm.

The Model Data report tabulates calculated spring stiffness values for these gravity columns and walls. In addition, the Spring Forces report (available after the analysis run) shows how much loads are carried by these springs.

In addition, for gravity members (columns and walls) that support a two-way deck, you can include these gravity members in analysis. This can be carried out by selecting the Include Gravity Members option in the dialog. It should be noted that this is only applicable for gravity members that are under a two-way deck. The followings are applied if the option is selected:

Only those gravity columns and walls under a two-way deck are included in analysis.
These members are only included for gravity load cases (dead and live load cases). One can see member forces calculated for these members. In addition, the program graphically shows them on screen but there is no report that prints outs members forces for the gravity members.
These members are excluded when a seismic or wind load case is run. Hence, they do not provide any stiffness for lateral load cases.
For Eigenvalue analysis (and also for dynamic-response spectra- load cases), the program includes these members in Eigenvalue calculation.
Deformed shape on screen includes these members.
Calculated reactions also include reactions at these gravity members.
The program does not calculate any rigid end zone for gravity columns that support a two-way deck.
It is not possible to assign any fixity to these members in the program (member end fixity conditions can be defined in the Modeler).
In Center of Rigidity calculation for diaphragms, gravity members are never used.

For newly created models, the Include Gravity Members option is the default choice by the program. In addition, it is the engineer’s responsibility to assign sizes to gravity columns prior to run the model if this option is selected.