Revision 7 posted to RAM | STAAD Wiki by Seth Guthrie on 5/4/2017 8:31:30 PM
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| | Product(s): | RAM Structural System; Ram Frame | |
| | Version(s): | All | |
| | Environment: | N/A | |
| | Area: | Analysis | |
| | Original Author: | Bentley Technical Support Group | |
What diaphragm types are available?
The diaphragm options in RAM Frame include: Rigid, Flexible/None, Pseudo-Flexible, and Semirigid. Technical notes on the diaphragm types can be found in Section 6.12 of the RAM Frame Manual.
Rigid
All nodes connected to the diaphragm are assumed to translate and rotate as a rigid membrane. Diaphragm forces for lateral load cases are applied as a nodal load at one point on the diaphragm. These lateral forces are directed into the frames based on relative stiffness: the stiffer the frame or wall, the larger the force directed into the frame. If the load is not applied through the center of rigidity, then there will be a torsional moment on the diaphragm.
Most frame beams have only two nodes and both nodes are connected to the diaphragm by default. The rigid diaphragm in this case will prevent any axial strain in the beam and no beam axial force will occur. Some users will disconnect one node or the other from the diaphragm to force lateral loads through the beam before reaching the braces or columns below, but the selection of which node to disconnect can impact the amount of load, or even the sign of the force, so it is often preferred to check frame beams in rigid diaphragms manually for the axial compression they might realistically take. The frame story shear report gives the total forces transferred form the diaphragm to the frames to help.
Flexible/None
Nodes within the diaphragm are assumed to displace independently. Program generated story forces are not calculated. Nodal lateral loads acting on the frames should be calculated outside the program and modeled as nodal loads in Elevation mode of RAM Modeler. Flexible/None diaphragms are assumed to have no stiffness and cannot transfer torsional moment.
Pseudo-Flexible
Behavior is similar to Flexible/None diaphragms. Unlike Flexible/None diaphragms, a story force is calculated at each level. Based on frame numbers and percentages entered by the user, this story force is divided into nodal loads on each frame. Like Flexible/None diaphragms, Pseudo/Flexible diaphragms do not transfer torsional moment.
Semirigid
The diaphragm is included in the model as meshed shells, using the effective thickness, Elastic Modulus, and Poisson’s Ratio defined with the deck properties in RAM Modeler. For wind load cases, the loads are placed as a series of nodal loads on the windward and leeward edges of the diaphragm. For seismic load cases, the loads are placed as point loads at all finite element nodes. An infinitely stiff semirigid diaphragm will behave like a rigid diaphragm. As the stiffness of the semirigid diaphragm approaches zero, the behavior will approach the behavior of a flexible diaphragm.
What diaphragm type should I use?
ASCE 7-10 Section 12.3 discusses diaphragm flexibility. Section 12.3.1 states that diaphragms should be analyzed as semirigid unless they can be idealized as flexible or rigid. The sections that follow list the requirements for idealizing the diaphragm as flexible or rigid. User Note: The ASCE requirements are modified slightly in IBC (see IBC 2012 Section 202, for example).
ASCE 7-05 Section 12.3.1.1 states that untopped metal decks and wood diaphragms should be considered flexible unless you have moment frames. For moment frames, Section 12.3.1.3 permits a flexible diaphragm if the maximum in-plane deflection of the diaphragm is more than 2x the average story drift.
ASCE 7-10 12.3.1.2 states that concrete slabs and concrete topped metal decks can be considered rigid if the span-to-depth ratio is less than 3 and there are no horizontal irregularities (see ASCE 7-10 Table 12.3-1).
RAM Frame does not automatically determine if the in-plane deflection exceeds 2x the average story drift or if a horizontal irregularity exists. Displacements at any location can be reviewed using RAM Frame Analysis - Process - Results - Drift at a Point (or Drift at Control Points) can be used to look at rigid diaphragm displacements at locations other than the center of mass. The displacements at the center of mass are shown in the RAM Frame Analysis - Reports - Story Displacements.
Since nodal loads for Flexible/None diaphragms need to be calculated outside the program and placed at the appropriate locations by the user, it is generally easier to use Pseudo-Flexible diaphragms for untopped metal decks and wood diaphragms. Pseudo-Flexible diaphragms are also useful for checking that moment frames in dual systems are capable of resisting 25% of the design seismic force (see ASCE7-10 12.2.5.1).
Flexible/None diaphragms are useful for small diaphragms that do not have are connected to few or no frame members. The mass of these diaphragms can be combined to diaphragm at other levels in RAM Frame Analysis – Loads – Masses and the exposure of these diaphragms can be set to None in RAM Frame Analysis – Loads – Exposure, so the seismic and wind forces associated with these diaphragms are collected into the adjacent diaphragms and not ignored in the analysis.
Semirigid diaphragms are useful for models with several or large slab openings, large overhangs, and structures with horizontal irregularities.
How does the diaphragm type affect two-way decks?
Two-way decks are included in the RAM Frame analysis as meshed shell elements for all diaphragm types.
Under gravity load, out-of-plane stiffness of the two-way deck is always included. Since two-way decks are supported by both gravity and lateral members for gravity load cases, it is not appropriate to exclude gravity members as is done when analyzing one-way decks in RAM Frame. There are two options for including gravity members supporting two-way deck in RAM Frame: as vertical springs and as framing members. When vertical springs are used, the program places a spring with a stiffness of AE/L at the support. Note that the spring has axial stiffness only and no flexural stiffness. When gravity members are included, the member is included in the analysis and axial and flexural stiffness of the member is included. Note that fixity of columns is assumed fixed and fixity cannot be assigned to gravity members in RAM Modeler.
In version 14.06.02 and earlier, the out-of-plane stiffness of two-way decks defined as rigid diaphragms was always included in the analysis when analyzing lateral load cases. Including the out-of-plane stiffness of the slab can have a significant and often unintended effect on the lateral force resisting system, especially for thick two-way slabs that are meshed with walls. The slab acts like the beam in a moment frame, coupling the vertical elements together and resulting in a broad, stiff lateral system.
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In versions 14.07 through 15.01, the out-of-plane stiffness of two-way decks defined as rigid diaphragms was ignored for lateral load cases. This was accomplished internally by setting both the in-plane and out-of-plane stiffness of the shell to a very small number when analyzing lateral load cases only. This meant that the overturning moment in the example above would have been resisted entirely by the wall (C) with no axial forces in the pinned columns (A,B).
Stiffness of the two-way decks could still be considered in those versions by defining the diaphragm as semirigid.
Starting with version 15.02 we expanded the options in Criteria - Diaphragm so that he user is in control. When the out-of-plane stiffness of the diaphragm is needed in resisting lateral load drift or overturning force, check the option to include out-of-plane stiffness for rigid and semi-rigid diaphragms. When you want the lateral walls (or braces) to resist the overturning completely, uncheck the options.
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Out-of-plane stiffness of two-way decks is ignored for lateral load cases when the diaphragm is Flexible/None and Pseudo-Flexible in all versions.
How does sloped framing impact diaphragm behavior?
Any of the 4 diaphragm types can be sloped by raising and lowering the columns and walls of the model. Below is a summary of the common side effects that might happen as a result.
Rigid
A rigid diaphragm constrains the plan (X, Y) coordinates of the frame nodes, but it does not limit relative vertical (Z direction) displacement. For flat slabs, this means that beams under gravity load can bend without the diaphragm taking any of the force. In sloped, rigid diaphragms, however the rigid diaphragm might absorb some axial thrust and limit the forces in the beams. We do not recommend rigid diaphragms when the sloped level includes trusses, bent frames, etc.
It's also worth mentioning that the story height used in calculating wind loads or seismic loads is based on the original story datum. Furthermore, the applied load elevation is also at this datum which determines the net overturning moments on the structure. For this reason, it is generally recommended to model the story heights near the mean height of sloped levels.
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Semirigid
In order for a semirigid diaphragm to properly mesh, the deck polygons of any sloped level need to fall in a single plane. If the column and wall elevations lead to a warped surface, then the mesh will not be properly connected to the frame members. See RAM Frame Meshing and Segmentation for details.
If wind or seismic loads are applied to a sloped, semi rigid diaphragm, then some component of those loads will be out-of-plane relative to the slab elements and will therefore cause out of plane bending. If there is not enough stiffness in the elements or the supporting lateral framing, then out of plane deflections will be large and instabilities are likely to occur, or there will be problems solving the eigensolution or using P-Delta.
Pseudo Flexible
Since the diaphragm here has no stiffness, the only significance of sloped framing is that it might cause a thrust that will topple frames over if there are no braces or column fixity to prevent it.
Program generated loads will still be based on the original story height as noted above.
How can I report the drift for Flexible type diaphragms?
When the diaphragm is Flexible or Pseudo Flexible then the various lateral frames and walls move independently and there is no accurate story drift to report. It is best to review nodal displacements to evaluate code compliance for drift in these cases.
See Also
RAMSS Two Way Decks
RAM Frame - Pseudo Flexible Diaphragms
RAM Frame - Semirigid Diaphragms
RAM Frame - Building and Frame Story Shear
Tags: Rigid, Axial Force, Flexible, pseudo-flexible, RAM Frame, RAM Structural System, Diaphragm, SELECTsupport, semirigid
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