| In-Plane Stiffness |
Models can be analyzed as having Rigid, Semirigid, Pseudo-flexible, and Flexible\None (none-rigid) diaphragms and combinations of these choices can be also used in the same model. If the Flexible\None option is chosen for all the levels, the model will be analyzed using the Nodal Load Cases assigned in RAM Modeler.
If the Rigid diaphragm option is chosen with at least one of the levels having a diaphragm, the model will be analyzed using both story Load Cases and Nodal Load case. This is also correct for Pseudo-Flexible and Semirigid Diaphragms. See the RAM Frame manual for further discussion on Rigid, Semirigid and Pseudo-Flexible Diaphragms.
Note: Semirigid diaphragms are always meshed. Rigid diaphragms are also meshed if they contain any two-way decks.
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| Disconnecting Nodes |
RAM Frame automatically disconnects certain nodes from a rigid diaphragm. Click Disconnect to change these settings as described in Disconnect Nodes. The engineer is be able to indicate which nodes to automatically disconnect from the 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 |
| Lateral |
Never |
Never |
— |
| Pseudo-Rigid |
Gravity |
Never |
Never |
Never |
| Lateral |
Never |
Never |
— |
| Semi-Rigid |
Gravity |
Always |
Option 1 |
Never |
| Lateral |
Always |
Option 1 |
— |
| Rigid |
Gravity |
Always* |
Never |
Never |
| Lateral |
Always* |
Never |
— |
| Two-way |
Flexible / None |
Gravity |
Never |
Always |
Always |
| Lateral |
Never |
Never |
— |
| Pseudo-Rigid |
Gravity |
Never |
Always |
Always |
| Lateral |
Never |
Never |
— |
| Semi-Rigid |
Gravity |
Always |
Always |
Always |
| Lateral |
Always |
Option 2 |
— |
| Rigid |
Gravity |
Always* |
Always |
Always |
| 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.
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| Diaphragm Mesh |
| Setting | Description |
|---|
| Analytical Model group |
| Setting | Description |
|---|
| Merge Node Tolerance |
Once meshing process for walls and semirigid diaphragms is completed, the program generates a finite element model for analysis, which is basically composed of nodes and elements (columns, beams, braces and shells in walls and meshed diaphragms). During this process, it is possible that several nodes are very close. To have a good (and valid) analytical model, these nodes are merged to a single node and elements connected to this node are adjusted accordingly. The Merge Node Tolerance is used in such a way that if distance between any 2 (or more) nodes are found to be smaller than the Merge Node Tolerance, then they are treated as a single node (i.e., these close nodes are merged together). |
| Max. Distance between Nodes on Mesh Line |
it allows the user define the maximum distance between nodes on wall edges and nodes on semirigid diaphragm edges. Note that the program may generate additional nodes closer than user entered value, but it is never allowed to be larger than that value. The user can generate coarse or fine mesh by setting Max. Distance between Nodes on Mesh Line. Usually, a coarse mesh gives conservative results since the walls and diaphragms are stiffer. A finer mesh leads to more flexible walls and diaphragms, which leads better results but it may significantly increase analysis time. A good balance should be provided before starting analysis. Note that it does not only affect meshes for walls, but also it affects generated meshes for diaphragms. |
| Geometry Tolerance |
it is mostly used in geometry calculation as a threshold tolerance value. This should not be confused with a merge (close) node tolerance. Examples are as follows: it is used as a tolerance to check a node is on a line or to check a point is inside a polygon, etc. |
| Hard Node Density Factory |
This factor is used to determine mesh density around hard nodes which are always located inside semirigid diaphragms. A hard node is defined as a node where a column or a wall is attached. For most cases, a value of 1.0 is a good estimation to obtain relatively good mesh density around hard nodes. |
Note: Making changes to any of the above criteria will necessitate a re-analysis of the structure.
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| Diaphragm Boundary group |
It determines the extension of meshed area of the diaphragm. If Use Slab Edges for Exterior Boundary is chosen, the program uses slab edges as an exterior boundary of the meshed diaphragm. It is important to note that the area between the most exterior beams and slab edges are also meshed. If Use Beams for Exterior Boundary is chosen, the program uses the most exterior beams as exterior boundary of the meshed diaphragm. Note that there should be a closed beam loop constructed with exterior beams. Otherwise, the program fails to mesh in the absence of a closed boundary if this option is chosen, and then it automatically switches to the other option. These two options are provided because:
- For some configurations, slab edge offset distance may be too small (only a few inches) and there are many beams running parallel to slab edges. If the first option is chosen, then it is very likely that very small distorted elements are created between exterior beams and slab edges. Thus, the user is advised to use the second option, which leads to a more balanced meshed configuration.
- For slabs with no beams (like concrete flat slabs) or no closed beam loop exists, then user needs to use the first option.
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| Diaphragm Mass group |
It determines how diaphragm mass should be processed in analysis. If Use Calculated is selected, the program applies all surface, line, and points mass loads (as well as deck mass) directly to meshed diaphragm (member masses for beams, columns and walls are directly applied to these members). In contrast to rigid diaphragm analogy where diaphragm mass is lumped at diaphragm mass center, this procedure considers a spatial mass distribution over diaphragms. If Uniformly Distribute Total Mass Over Diaphragm is selected, the total mass calculated for the diaphragm is distributed to diaphragm shell elements based on shell's effective areas. This method can be used if a truly uniform mass distribution over the diaphragm exists. Also, it is provided as a alternative method if the Use Calculated method fails to resolve mass polygons over the diaphragm due to some numerical difficulties. |
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