What floor-to-floor height should I enter in the story data?

Whatever story height you enter into RAM Modeler, that is where the centerline of the frame beams will fall in the finite element model and vertical braces always connect to a work point at the beam and column centerlines. This is done for simplicity in the finite element analysis.

For drift sensitive structures, using a first story height that is equal to the distance from the ground level (or foundation level) up to the top of steel - average frame beam depth / 2 is probably the most accurate modeling (see "Alternate Story El." below). But using a distance from ground level to beam top of steel (a.k.a. deck bearing) is more common practice and is conservative in most aspects (see "Common Story El." below).

The common story approach is also used when the RAM SS 3D model is exported to ISM. In the ISM model the beam locations are established relative to the story datum based on the following rules:

• Non composite deck – top of beam, deck bearing at story datum
• Composite deck – top of beam, deck bearing at story datum
• Concrete slab – top of slab at story datum

Keep in mind, story height can also affect the following calculations:

• Calculated wind exposure (and Kz factors)
• "h" in the vertical distribution of seismic loads calculation (wi*hi/Sum (wi*hi))
• Overturning moment (related to lateral force times story height)
• Unbraced length for columns
• Slope angle and length of braces
• Material takeoff quantities
• And the elevation of the beams shown in the 3D view, or exported out to Revit, ISM, or dxf, which are all artificially adjusted to show all beams with top of steel at the story height.

How can I model a continuous beam?

In Ram Structural System, the framing must all be determinate, so multi-span indeterminate framing is not directly possible. There are two approaches to modeling and designing continuous beams. 

The first is to model each span as a lateral beam. use the same size for each span and be sure to assign the ends to be fixed. The supporting columns also need to be lateral, but they may be pinned (in the plane of the framing). To see the accurate member forces or steel design of the beams, use Ram Frame analysis and the Steel Standard Provisions respectively.

Alternatively, for those that do not have Ram Frame, the system can be approximated using a cantilever and suspended span approach. In other words, model one span normally and add a cantilever extension into the second bay. Then add a suspended span from the end of the cantilever to the third support (or add a cantilever beam in every other bay for continuous beams more then 2 spans long). The length of the cantilever is important here since it dictates the inflection point or point of zero moment.

Can a beam cantilever directly from a support with no back-span?

Yes, the option to create a stub cantilever or beam with a single support was added in version 14.02.  The following applies to prior versions:

In the RAM Modeler, a cantilever can only be created by extending a simple-span beam over its support. When the building requires a stub cantilever which cantilevers directly from the column, then a special modeling trick can be employed.

In order to get an accurate analysis and design of that cantilever beam, requires the use of RAM Frame. Start by modeling the column which will support this cantilever. Make that column lateral and make sure that it is supported by other lateral columns if this is not the lowest level. Next add a dummy column at the point where the cantilever stops. Make this dummy column out of “Other” material and assign it a very small “E” value (e.g. 1). Then assign that column a section size under Property table – Other column sections that is very small (e.g. 1” square).

Next model in the cantilever as a simple, lateral beam from the real column to the dummy column. At the end where this beam frames to the real column be sure to assign a fixed end condition so that moments can develop. You will also need to make sure that the column is capable of taking moment at the top.

The finished section of the model should look something like the figure below. Note – the analysis results from RAM Frame will provide an accurate design, but the gravity beam and column modules will not recognize that this dummy column is too weak to take load. Only the finite element analysis of RAM Frame is capable of that.

How can I create a sloping floor or roof?

There are some basic limitations to what you can model with RAM Structural System, so it may not be possible to model some structures perfectly, but you can usually get close. The following guidelines should help

• Every beam must have exactly two supports, never 1 or 3, and those supports must be on the same level type. So you can't directly model a bent beam forming a gable unless there is a support at the peak.
• You must be able to model the structure as a flat (wedding cake) type structure first, then create the slopes by changing column (and wall) elevations.
• Any time you have a step (two beams framing into the same support at different elevations), two levels types and two stories are required and the higher beam must be on the higher story.
• You can raise or lower a column (or wall) using the Layout - Column - Modify elevation command and thus slope beams that connect to it. If you want to lower the column more than the story height, then you must also lower the same column on the next lower story. Think of the column like a string with beads on it at each story. As you modify the elevation you are lowering the bead, but you cannot cross another bead. In the end, the beads must be at least 0.1’ apart (more separation is preferred).

Other things to note:

• You can have a rigid diaphragm that is sloped, but in RAM Frame this is treated as a horizontal diaphragm. We don't analyze sloped rigid diaphragms. Furthermore, if your structure is subject to trust, diaphragms should be turned off at least while investigating gravity loads. Using a sloped semi-rigid diaphragm is an alternative.
• The gravity steel and concrete beam design will not include any effects of axial forces.

I modeled a roof with multiple slopes, but in the 3D view the deck is a single sloped plane shooting up.  How do I fix this?

Starting in version 14.0, the program will display the surface of the deck in 3D.

For a sloping deck to function correctly in this regard, the deck must be input using a separate polygon for each sloping surface. In other words, for a gable roof, two deck polygons should be defined, one for each side or slope of the roof.  A whole-floor application of the deck is no longer sufficient.

It is suggested to snap to the beam end points when defining these decks, since the beam end points have exact elevations which are derived from the supporting column or wall elevations. Here's a better way to lay out the deck polygons for the model illustrated above:

Even slight imprecision in the support elevations (i.e. warping of the deck) can cause this anomaly in 3D.

The analysis results will only be affected if the warped deck is part of a semirigid diaphragm in RAM Frame or is defined as a two-way deck.

How can I model a 2 story brace, or one that skips a level?

When a brace needs to skip a level use Layout - brace - Add Special and follow the prompts at the bottom left.

For details on how these braces effect frame story shear reporting, please reference [[RAM Frame - Building and Frame Story Shear]].

I have a brace in 3D that does not appear in any elevation view. How can I delete it?

There are two ways to remove rogue braces that are no longer in an elevation view that can be selected.

1. If the braces do not connect to any frame members at either end, then an integrated data check can be performed and the following message will appear: "Some Braces do not attach to Nodes. Do you want them deleted?". Click "Yes" to have them removed. Note – if the brace is connected to a lateral member at one end, but not the other, this message will not appear, so you may need to make some supporting beams or column gravity temporarily.
2. Delete and re-enter the story data for this particular level. This will remove all braces at that level.
    

How do I model a transfer girder, or a column setting on a beam?

On the upper story level model the column as a standard column (not a hanger).

On the lower level model the beam passing through the location of the column above. This could also be a beam cantilever.

Use Reference layout types (under the options menu) or construction grids to aid in the alignment of the column and beam below.

I have a column that supports the roof, but does not support anything at the lower level. Do I have to model the column on both layouts in RAM Modeler?

Think of the RAM SS Layout type like a section through the whole structure. Any columns that are cut by the section should be modeled on that layout typically. Don't worry, the program will design the column for the correct, longer unbraced length so long as there are no beams (or optionally decks) at the intermediate story to brace the column.

There is one alternative, however. If the column is only modeled on the Roof, and not modeled on the lower level at all, then you can add a foundation under that column and lower the foundation to stretch the column to the full height. This approach can be helpful in cases where the column is sloped and the story heights are not constant making it hard to determine the exact bottom offset to keep the two-story column in one straight line.

Why am I unable to copy information or import a DXF into a layout?

The Copy and Import from DXF features (RAM Modeler - Layout - Type menu) are only active for layout types that contain no information. These commands are deactivated in RAM Modeler even if the layout contains only grids and no other objects. To use either command, create a new layout and then use the copy or import features before any other information is defined on the layout.

How Do I Model Grade Beams?

Although there is currently no direct way to model grade beams in RAM Structural System, they can be simulated by modeling concrete beams and designed in RAM Concrete. These basic steps produce satisfactory results for most configurations.

1. Model a grade level that contains the concrete grade beams supported by columns. The top of the columns should be pinned and the bottom fixed.
2. Model a slab edge on the grade beam level and assign a noncomposite deck with no selfweight. This is necessary so that there is a diaphragm present on the level in RAM Frame.
3. Add the grade level to your story data.
4. In RAM Frame, set the ground level to the grade level.
5. Run the RAM Frame analysis and RAM Concrete Beam design.

By creating a rigid diaphragm at the grade level with the nodes at the grade beam - column intersections to it, translation is restrained when the ground level is set to the grade level. Therefore, no shear will exist in the column stubs. Since the top of the column stubs are pinned, no moment will be developed in the column stub below the grade beams. Spread footings or pile caps can be modeled at the column stub locations. The foundation loads will only be vertical forces.

There are a couple of important things to note. First, automatic calculation of effective length factors may be inaccurate for this procedure. No boundary condition is assumed at the lower node of the column above the grade beam level. Therefore, the G value for the lower node is a function of the column and grade beam stiffness in the direction being considered. If this is not an accurate assumption, the effective length factor should be explicitly defined. Second, don't specify a story height on the grade beam level that is too small. Using an extremely small story height is not necessary because there will be no translation of the grade beam level and only vertical forces in the stub columns. The only ramification of using a larger story height is an increased selfweight for the stub columns. If you use too small of a story height you might produce a poor mesh if lateral walls are modeled on the grade beam level.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

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