Hi everyone,

How to define this section in Ram Connection?

Thanks!

Hello, I want to include earthquake case in a stress simulation, but I don't know how to calculate load as multiples of gravity, please.

Best Practices Documents

Modeling and Node Generation Best Practices

STAAD.Pro Loads and ISM 

ISM Revit Plug-in

[[ISM Import into Tekla Structures]]

Installation

[[How can I find the ISM installer?]]

[[Can't Install the 32-bit Version on a 64-bit Operating System]]

[[Synchronizer Won't Install due to Prerequisites]]

ISM Toolbar Buttons Grayed out

Solutions

[[Boundary Is Not Valid Error]]

[[ISM Section mapping and STAAD.pro]]

[[ProStructures ISM Section and Material Mapping]]

[[RAM Manager Crashes when using ISM - New From Repository]] 

[[ISM - IFC Import Details]]

Where is the option to import an IFC File?

The IFC import action can be started from the Windows Start menu - All Programs - Bentley Engineering - Structural Synchronizer V8i - Import IFC

The dialog box that comes up allows you to select the ifc file and choose a name for the new ism.dgn repository that will be created.

Click Settings to make changes in the default mapping of Section and Materials

Click Import to start the process.

A progress dialog will then appear along with any warnings.

If the IFC file has materials that have not already been mapped, then a Material Mappings dialog box will appear where you can map the IFC file materials to ISM material types.

If the option to show the Structural Synchronizer is turned on, then the process ends with a view of the imported data in the Viewer. Click Close to finish the process.

What IFC entity types can be imported?

The following is a summary of the various IFC entity types that can be imported, For each, the mapped ISM entity type and properties are listed.

Supported IFC Entities:

1. IfcBeam -> IsmCurveMember (Use – Beam)
2. IfcColumn ->  IsmCurveMember (Use – Column)
3. IfcPile -> IsmCurveMember (Use – Pile)
4. IfcMember -> IsmCurveMember (Use – HorizontalBrace/ VerticalBrace)
5. IfcWall -> IsmSurfaceMember (Use – Wall)
6. IfcWallStandardCase -> -> IsmSurfaceMember (Use – Wall)
7. IfcFooting -> IsmSurfaceMember (Use – Footing)
8. IfcSlab -> IsmSurfaceMember (Use – Slab) (true)
9. IfcPlate -> IsmSurfaceMember (Use – Footing)
10. IfcDiscreteAccessory -> IsmSurfaceMember (Use – Unset)
11. IfcFeatureElementAddition -> IsmFeatureAddition
12. IfcOpeningElement -> IsmSurfaceMemberOpening/IsmFeatureAddition (depending on situation) 

Supported IFC representations:

• IfcExtrudedAreaSolid
• IfcRevolvedAreaSolid
• IfcSweptDiskSolid
• IfcSurfaceCurveSweptAreaSolid

 Supported IFC profile definitions:

• IfcArbitraryClosedProfileDef
• IfcArbitraryProfileDefWithVoids
• IfcDerivedProfileDef
• IfcCompositeProfileDef
• IfcIShapeProfileDef
• IfcCircleProfileDef
• IfcRectangleProfileDef 

Why is there a warning, "IcfFacetedBrep is not supported"

The ISM API has some surface limitations. Consequently Boundary representation (BREP) surfaces are not imported at this time. 

Description

The installer for the ISM applications is called "Structural Synchronizer" and must be installed to utilize any ISM link from any application. It can be downloaded from My SELECT CD, or the Fulfillment Center like any other product. 

1. Using My SELECT CD :

Search based on:

• Product Line - Structural Design and analysis,
• Files Posted within last - All 

Then scan the list of downloads for "Structural Synchronizer". The most recent release will typically be first in the list.

Other products like the ISM Revit plugin can be found using the same search criteria.

2. Using Fulfillment Center : 

Type part or all of the word "Synchronizer" in the search field and select "Structural Synchronizer V8i".

Once found, expand the All Downloads list and select the appropriate version:

Note, starting with SELECT series 5, version 8.11.11.46 Synchronizer comes in a 32 or 64 bit variety and the version installed must match the computer operating system.

Also note, is the US English spelling here, "z" rather than "s"

3. For non-SELECT customers, the Synchronizer may also be downloaded from the iware downloads.

See Also

Can't Install the 32-bit Version on a 64-bit Operating System

Information on using ISM Revit Plug-in

ISM Revit Plug-in Best Practices

Best Practices

ISM Revit Plug-in Tech Notes

Tech Notes

ISM Revit Plug-in Tips 

 Tips

See Also

Integraged Structural Model Home

I often use the compression only function of the foundation elastic mat with a specified subgrade modulus. However, I have found over and over again that Staad is automatically unchecking this box and reverting back to allowing uplift reactions if any change is made to the model. For instance, if I run an analysis with compression only then want to modify a load, I go back to model and modify the load and run the analysis however the compression only portion is removed.

Has anyone else had this issue? Is there a solution to this?

Thanks in advance.

Ryan

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.

When using the cantilever approach one side effect is that the supporting columns will assume zero eccentricity in the design.

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.  Prior to that version a dummy column of near zero stiffness and a lateral beam with one end fixed was required.

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. If you want to raise the column more than the story height of the level above, then you must also raise the same column on the level above. 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.

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 self-weight. 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 self-weight 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.

How can I change a Beam’s material property from Steel to Concrete?

In the RAM Modeler first change the material to “Concrete”. Then go to Layout>Beam>Change Material. This process also applies to columns and braces.

See Also

RAM SS - Using DXF as a Reference Layout

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

Tell me, please. Analysis showed the very large variations in the Fy
reaction
loads
the supports (point 202, 586, 870, 1078). Especially the difference in points (506 and 202) Indeed own weight of the structure can be so hard
shift for a given wind load in different directions?(Please visit the site to view this file)

Can I assign a section such as a HSS or channel to a beam?

Currently, only I-shaped (wide flange) sections can be assigned to beams in RAM Connection. It is not possible to assign other section types, like HSS or channels, to beam members, though they can be used as columns or braces in many connection templates.

What is the difference between Basic Connections and Smart Connections?

The RAM Connection Manual defines these connections as follows:

Basic Connection:  A connection template that can automatically adjust the geometry (position or dimensions) of the connection pieces to fit the connection members. It does not calculate the quantity or dimensions of the connecting pieces (bolts, plates, etc) to resist the applied forces.

Smart Connection: A connection template that can automatically calculate the quantity and dimensions of the connecting pieces (bolts, welds, plate sizes etc) to resist the applied forces.

When basic connections are designed, the program searches through a list of predefined connection templates and selects the first connection in the list that satisfies the design requirements.

When smart connections are designed, the program optimizes the connection parameters. See the RAM Connection Manual for a list of parameters that are optimized for each connection type. If a parameter is not optimized, the program uses a default value that be modified in the Connection Pad as needed.

Some complex connection templates like gusset pate or base plates only have a smart variety. 

Where are the abbreviations used for joint types and connections defined?

The abbreviations are defined in the RAM Connection Manual (available from the help ? or as a pdf from the Windows Start menu). The naming conventions for both joints and connections are listed in Chapter 2, The Connection Database - Database organization. Here is a list of the joint types from that section:

1. Beam – Column Flange (BCF)
2. Beam – Column Web (BCW)
3. Beam – Girder (BG)
4. Beam Splice (BS)
5. Column Splice (CS)
6. Continuous beam over column (CC)
7. Column, beams and braces (CBB)
8. Chevron braces (CVR)
9. Vertical X braces (VXB)
10. Column – Base (CB)
11. Column – Base – Braces (CB)

How can I change the design code (AISC 360 or BS 5950) or the design method (ASD or LRFD)?

RAM Connection Standalone:

1. Click on the Design menu tab at the top of the program window.
2. Find the Assignment toolbar.
3. Double click on the small square box with arrow pointing to the lower right corner to open the Customize Connection Design dialog.
4. Edit the design code (or design method in version 8).

Note, in Ram Connection Stand-alone version 9.0, changing the design code does NOT retroactively alter the assigned code for the existing joints in the file. This was done intentionally so that the user can have some joints designed to one code and other joints designed to another code within a single file. Consequently, if the design code for existing joints needs to be changed, the code should first be changed, then reassign connections to the joints.

RAM Connection for RAM Structural System:

1. Click on the Design menu tab at the top of the program window.
2. Find the Assignment toolbar.
3. Double click on the small square box with arrow pointing to the lower right corner to open the Customize Connection Design dialog.
4. Edit the design code (or design method).

RAM Connection for Elements:

The design code and design method is controlled by the code selected for design when performing a design in the RAM Elements model. To change the design code or design method, redesign the model and choose the desired design code.

Changing the design code will not automatically update generated load combinations. After changing the design method, delete and regenerate the load combinations.

When designing a base plate connection, the ACI 318 Appendix D checks are not completed.

Since the ACI Appendix D checks are based on ultimate limit state design, RAM Connection will only complete the ACI Appendix D checks if LRFD is selected for the design method. See frequently asked question above for information on changing the design method.

Information that is modified in the Connection Pad is not saved after clicking the Save button and exiting the dialog.

Any item that has an icon with a red arrow* to the left of it (see figure below) is defined in a dialog outside the Connection Pad. These parameters can be edited in the Connection Pad, but the information will be lost after closing the dialog. To change the parameters permanently, modify the values in the dialog where the information is initially defined. Edit the Joint to modify loads, section, materials, etc. Edit the seismic provision options in the Customize Connection design dialog.

 * In Ram Connection 9.0 this arrow is blue.

Can I design a Gusset Connection using a Pipe Column?

No, currently in Column-Beam-Brace joints (CBB) only Wide flange (W) and Square or Rectangular Tube (HSS-rect) shaped sections can be used for columns. Circular shapes can be used for braces and as columns in some joints, but not the gusset type. A change request for pipe columns in these joints has been logged.

Why is the controlling load condition reported as a single load case?

RAM Connection completes a design check for all load conditions, including individual load cases and load combinations. For some connection types, such as a base plate connection with wind uplift, the design for an individual load case may control the design. The single load cases can be removed from consideration as follows:

RAM Connection Standalone or Ram Connection for RAM Structural System :

1. Click on the Design menu tab at the top of the program window.
2. Find the Assignment toolbar.
3. Double click on the small square box with arrow pointing to the lower right corner to open the Customize Connection Design dialog.
4. To have only the load combinations considered click the button, "Select all load combinations", or manually check the desired conditions.

Please note that this will not permanently delete the load case results from the worksheet. See frequently asked question above for details.

RAM Connection for Elements:

1. Click on the Modules ribbon menu.
2. Double click on the small square box with arrow pointing to the lower right corner to open the Customize Connection Design dialog.
3. To have only the load combinations considered click the button, "Select all load combinations", or manually check the desired conditions.

I'm designing a connection with seismic provisions, but the Ry and Rt values don't look right, what's wrong?

Ry (Yield strength ratio) and Rt (Tensile strength ratio) are properties of the material in Ram Connection. The can be reviewed using Home - Databases - Materials - Edit.

To add your own materials with different values, refer to the wiki Creating custom elements in RAM Elements which also applies to Ram Connection.

Note, imported materials from RAM Structural System or STAAD.pro may not have the expected values for Ry and Rt since those are not directly supplied by either of those applications. For Ram Elements users with imported RAM SS files, edit the imported RAM SS materials as shown below (or reassign different steel material to the members):

For STAAD users, be careful to define the proper values when using the RAM Materials dialog box within the connection mode.

For further details refer to Tips for Using RAM Connection within STAAD.Pro [TN]. and How to Customize a RAM Connection Template in STAAD.Pro 

See Also

Troubleshooting Errors when Assigning Connections

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!

How is the "model soil structure interaction" in the 'springs' module suppose to be used? Is this for single footings only? Or can this be applied to several nodes to create a Matt foundation?

Thanks

We are investigating some post-tensioned beams that are failing the torsion checks (due to ACI 318-11 Eqn 11-18).  Under the torsion design options, there is 'Beam', 'As Shear', 'As Bending', and 'Wood Armer'.  

In the manual, you are clear on not using Wood-Armer for torsion design for beams.  However, we would like to know if the 'As Shear' and 'As Bending' options are applicable to beams.  Additionally, do you have any references that discuss this conversion of torsion for beam design?

Thanks,

Problem Description

When printing the screen from any of the 3D modules (Ram Steel Column, Ram Frame or Ram Concrete), the print preview and print to paper shows an image which is too small as if zoomed out. 

Explanation

When using Print Screen or Print Preview, the program converts the display image into a white background image and then scales it to fit the paper. It does not automatically trim blank space from the plot so any blank space on the sides of the image will remain on the sides of the plot. When printing a tall aspect ratio image on a wide screen monitor to a portrait paper the problem is most noticeable.

Screen image of a tall plan view:

Print Preview:

Solution

Rather than using a full screen window, adjust the module widow to better fit the image of what you want plotted before using Print Preview.

Zooming in and using landscape paper orientation can also help.

The table below lists the features available in ISM 4.0 and indicates the Bentley applications or plug-ins that Import and Export or only Export each feature. For a list of features in ISM 3.0, click here.  

The versions referenced below are from Q1 2014 or earlier. Some features are not supported as a result of these elements not being required data in the client application.

ISM 4.0 should not be used in conjunction with:

• SACS V8i SELECT series 3 (05.06.01.07)
• AECOsim Building Designer V8i (08.11.09.593) (the information in the table below is for ABD 8.11.09.735 or later)
• STAAD(X) V8i (08.02.02.75)

Future releases of these products will be compatible.

I/E         Import and Export

I            Import only

E           Export only

NO        Not Implemented

1. Curve Members applies to straight and curved members, though truly curved members are not supported in all applications.
2. Some sections not supported in all applications.
3. Concept approximates non-parametric sections with a rectangle, and always exports a parametric section.
4. Elements only supports web tapered sections, not flange tapering.
5. Revit imports tapered members only. Manual mapping to Revit family type required.
6. Elements slab openings limited to square or rectangular shapes.
7. RAM SS circular or rectangular beam web openings and stiffener plates only. Stiffeners only exported.
8. Revit material physical properties ignored.
9. Revit only supports "Linear Stiffener-Plate" and "Linear Stiffener-Plate_M" families.
10. Revit joists export by semi-manual ISM Section procedure in mappings.
11. Revit imports pile caps as foundation slab.
12. ProStructures imports cladding as Panel and pile caps as pad footings.
13. ProStructures imports horizontal and vertical braces as beams.
14. Applications that do not explicitly support substructures always synchronize the full repository.

Multiframe Advanced 16.0 can export and ISM repository but does not currently support import or update workflows. Curve members (linear), surface members (slabs), sections, materials and supports are included at this time.

SACS (Precede Modeler) 5.4 and later can import or export an ISM repository once, but the update workflows are not supported. SACS 5.4 - 5.6 should only be used with ISM 3.0. SACS 5.7 (Precede V8i SELECTseries 4 Version 6.1.7.1) can be used with ISM 4.0.

Contact Tekla for details on what object types the Tekla ISM plugin supports.

I am working on the analysis of an existing waffle slab structure.  I am using a RAM Concept model (started from RSS).  I have uploaded the model as Waffle_TDL.zip

I'm not sure I've got it set up right, an am looking for some guidance on a variety of subjects.  I have read the previous posts regarding waffle slabs (here and here) but am still left scratching my head.

-Analysis takes a very long time (2+ hours) even with a generous mesh size (6 feet).  Looking at my resource monitor, it seems that concept never goes over about 12%  of my CPU and only 1.mb RAM (out of 32 total), though I'm using the 64bit version.  I'm using the Pardiso solver.  Is there anything else I can do to speed up the analysis time? 

-I only have design strips and assigned rebar in the portion of the building that I'm interested in - there are some one way slabs outside the scope of my current study, but in the model. I'm also only running a validation analysis (not designing new bars)

-I'm not sure I've set up my priorities right, slab and drop cap as same priority, ribs as beams. Should the ribs also have priority assigned?

-Errors with shear reinforcing/shear cores.  I've modeled the shear bars as the existing shows, but have had my analysis fail with "overlapping shear bars" or something to that effect.

Any and all guidance is welcome.

Thanks,

Defect Description

When performing Process - Design All in the Ram Steel Column module in version 14.06.02.00 (x32 or x64) a crash may occur part way through the process:

Steps to Reproduce

The problem occurs in the base plate design process for columns that set upon the slab with no beam or column or wall below. It only happens in version 14.06.02.00 not prior versions. The defect is being investigated and a fix should be released in version 14.07.00.00.

Avoidance

Add a beam below the columns that do not align to columns or walls below.

Tip - Use the Options - Reference Layouts Types so that you can see the column above while modeling beams on the level below.

The TechNotes and FAQs in this section cover various topics regarding the Frame module Steel design including Standard and Seismic Provisions in RAM Structural System. Use the navigation tree at the left to browse.

RAM Frame AISC 360 Stability Analysis and Design

For a broader explanation of the Direct Analysis methods in RAM Frame, including a step-by-step procedure, please refer to the following article:

AISC 360 Direct Analysis method in RAM Structural System

General

Chapter C of the AISC specification (2005 and later) covers stability analysis and design.  This document discusses how the requirements are met in RAM Frame.  For additional information, refer to the technical notes of the manual pdf file, RAMFrameSteelPostprocessor.pdf, starting with section 3.2.1. (To access the manual go to Start - Programs - Bentley Engineering - RAM Structural System - Manuals or click Help - Manual within the Steel Standard Provisions mode of RAM Frame).

Effective Length Factor

The effective length factor in the steel post processor is assumed to be 1.0 for beams and columns. Brace effective length factors default to 1.0 but can be assigned a different value (ie K = 0.5 for X bracing). Due to this assumption, only analysis and design methods that are not a function of the effective length (K = 1.0) can be used in RAM Frame. The Direct Analysis Method allows this assumption and is the preferred method because there are no restrictions on when it can be used.

Direct Analysis Method

AISC Appendix 7 covers the Direct Analysis Method. The AISC commentary is very informative and it is clear that this is the preferred method. The design-analysis constraints have four basic requirements:

• 2nd order analysis (such as an iterative 2nd order analysis or amplified 1st order analysis), including both P-Δ and P-δ
• Notional loads
• Reduced flexural stiffness
• Reduced axial stiffness

Most of the analysis parameters for the Direct Analysis Method are set in RAM Frame Analysis – Criteria – General. In this dialog box, you can toggle the parameter for reduced stiffness (A-7-2 and A-7-3) and select whether or not you want P-Delta included in the analysis. The general requirements of the appendix require that all component and connection deformations contribute to the lateral displacement of the structure. For this reason, we recommend ignoring rigid end zones in the analysis.

To meet the second order analysis requirements, the amplified first-order elastic analysis can be used if B1 and B2 are selected in RAM Frame Steel – Criteria – B1 and B2 Factors. B2 accounts for big P-Delta effects. In lieu of using B2, big P-Delta effects can be considered in the analysis if P-Delta is enabled in RAM Frame Analysis– Criteria – General. Please note, P-Delta in the analysis does not consider small P-delta effects. Therefore, B1 should always be selected in the steel post-processor when required by the specification. You are never required to use P-Delta in the analysis and B2 in the steel post-processor simultaneously. These two different options are for the same requirement.

Reduced flexural and axial stiffness should be selected in all instances when using the Direct Analysis Method. The parameter tau_b is a function of the member required compressive capacity and yield strength. The program does not automatically calculate tau_b and must be specified. Tau_b is permitted to be 1.0 when alphaPr/Py is less than or equal to 0.5. Otherwise a smaller value is required. In lieu of using a smaller value of tau_b when required, a value of 1.0 can be specified but the notional load factors must then be increased from 0.002 to 0.003. Since the value of alphaPr/Py is not known until after an analysis is performed – and may require iterative analyses to converge on the value – it is recommended that initially tau_b be set to 1.0 and that the lower value (0.002) be used for the notional loads. Generally this combination will be acceptable, but if not, the AISC 360 Direct Analysis Validation report will give a warning that either the notional loads must be increased (using 0.003) or a smaller value of tau_b should be specified. A third option may be to slightly increase the size of those columns for which the initially calculated value of alphaPr/Py is greater than 0.5 such that the calculated value of becomes less than 0.5, which then eliminates the need to either use a smaller value of tau_b or larger notional loads. If increased notional loads are used they should be analyzed and included in the load combinations in the steel-post processor per Appendix 7.

There are some limitations of Direct Analysis Method in RAM Frame. In particular, some building configurations cause problems for P-Delta in the analysis and/or for the calculation of B2. Problems with P-Delta in the analysis generally present themselves as instabilities in the finite element analysis. This is discussed further in the P-Delta technote (See Also section below). Occasionally with B2 enabled, members for a particular diaphragm(s) will fail with a Pnt>Pe2 error in the steel post-processor. It is possible that this is a real failure and there are excessive displacements, but often the error is thrown for load combinations where the displacements are reasonable. The underlying problem stems from the calculation of Pe2. Some load combinations produce displacements and small, non-zero shears. When such a condition occurs, Pe2 will also be a small non-zero number leading to negative B2 value which triggers the Pnt>Pe2 error. When this occurs, turn off B2 and make sure there is adequate strength in your members. Make sure your displacements are reasonable. Often the combination creating the error would not control the design of the member. If you are able to make this determination, you might be able to bypass the error by unselecting the problem combination in your load combinations. Please keep in mind that this would exclude the combination for all members. If both P-Delta in the analysis and B2 cause problems, you may need to account for second order effects by hand.

Other Methods

Any method that requires the effective length of the member to exceed 1.0 is not permitted while using the AISC 360 code in Ram Frame. Also, AISC 360 C2.2 requires that the Direct Analysis Method be used when the ratio of second-order drift to first-order drift exceeds 1.5. C2.2a and C2.2b can be used in the steel post-processor under certain conditions.

AISC C2.2a(4) permits K = 1.0 for braced frames or when the ratio of second-order to first-order drift is less than or equal to 1.1. Second order effects (B1 plus B2 or P-Delta) and notional loads based on a factor of .002 must be included but the reduced stiffness requirements in the Direct Analysis method need not be followed.  AISC2.2a may be the prefered methodology for braced frames.

AISC C2.2a(2) requires the analysis to be carried out under 1.6 times the ASD load combinations and divided by 1.6 to obtain the required results. This presumes that an iterative method of P-delta is used. However, RAM Frame uses the Geometric Stiffness method for the P-delta analysis which does not require that the analysis be carried out this way for ASD. However, to accurately capture the P-delta effects the P-delta analysis should be performed using ultimate (factored) masses (using an ultimate Scale Factor) if the Mass Loads are used or by using ultimate gravity loads (using ultimate Scale Factors on the Dead and Live Loads) if Gravity Loads are used, as specified in the RAM Frame – Analysis General Criteria, even if ASD is going to be specified as the design code.

AISC C2.2b permits members to be designed for the first-order required strengths when all members satisfy Eq. C2-7. If you are permitted to follow this method, P-Delta (or B2) is not required to be considered. In addition the reduced flexural/axial stiffness requirements are not necessary. However, you must still consider notional loads and B1. The factor for notional loads will exceed the options in the AISC generator and you will need to create user defined notional load cases with the appropriate factor per Eq. C2-8.

See Also

Structural Product TechNotes And FAQs

AISC 360 Direct Analysis method in RAM Structural System

RAM Frame P-Delta

Structural eSeminars

British Sections designed, in RAM Frame, in accordance with BS EN 1993-1-1:2005, can, depending on the initial section size assigned, be classified as Class 4 in accordance with section 5.5 of this standard. As a reminder, the role of cross section classification is to identify the extent to which the resistance and rotation capacity of cross-sections is limited by its local buckling resistance.  

Four classes of cross section are defined in BS EN 1993-1-1:2005 as follows:

Class 1 -  cross-sections are those which can form a plastic hinge with the rotational capacity required from plastic analysis without reduction of the resistance.

Class 2 - cross-sections are those which can develop their plastic moment resistance, but have limited rotational capacity because of local buckling.

Class 3 - cross-sections are those in which the stress in the extreme compression fiber of the steel member assuming an elastic distribution of stresses can reach the yield strength, but local buckling is liable to prevent development of the plastic moment resistance.

Class 4 - cross-sections are those in which local buckling will occur before the attainment of yield stress in one or more parts of the cross-section.

For a member subject to both bending and axial compression as shown below, the critical section checks may occur at the beam ends or at the point of contra flexure where the beam is subject to compression only:

The above continuous beam comprises two spans, one 6.0m and one 4.0m. The 6.0m span will be considered for this example.

It should be noted that all UB sections which are classified as Class 4 when subject to a compressive force only, that the section classification is controlled by the web dimensions. This is confirmed in pages B – 62 and B – 63 of SCI Publication P 363.

RAM Frame does not check the design capacity of Class 4 sections in accordance with the requirements of BS EN 1993-1-1:2005 and this article outlines how users can perform a hand calculation to verify the adequacy of a Class 4 section.

Assume the section being designed is a 457x191x67 UB, its properties for section classification are:

See Also

RAMSS Beams [FAQ]

Structural Product TechNotes And FAQs

How can I assign a member to be tension only?

Any vertical or horizontal brace member may be assigned to be tension-only. The assignment is made in RAM Frame using the Assign menu - Braces - Tension-Only command. Assigning a brace to be tension only does not automatically release the brace for moments or torsion, however. That is controlled through the Assign menu - Braces - Fixity command.

If no assignment is made, then the braces will resist tension and compression elastically. Beams, columns and walls are always capable of resisting compression.

How can I resolve an error, "tension-only member in compression"?

In the steel design mode, you might see the warning "T-O in comp" on the View/Update dialog box or "Member with Tension Only Shape is in Compression" on the design report. There are some cross sections that the RAM Frame Steel postprocessor can only check when they are in tension. If a single angle, rod or flat bar goes into compression, then the design will always give these warnings. You can manually check the design of such using the member force envelope results.

The same error message, "T-O in comp", can also occur for a member that is defined as a tension-only brace if there are custom load combinations utilizing negative terms on lateral load cases. It's important for models with any tension-only braces that the lateral loads be applied in all directions so that the load combos can use positive terms exclusively.

Why is my tension-only brace failing in compression?

If a brace that was specifically assigned to be tension only is going into compression, that usually indicates a problem with the load combinations. For models with tension only members, the load combinations should never include negative terms in any load combinations (generated or custom). If there is a combo with a negative term, the resulting brace force could be compression and the warnings above will occur.

Consequently, any model with tension only members requires one lateral load case in the positive direction and another load case in the opposite direction if all the braces are to be correctly evaluated. For program generated loads, there is a check box for "Generate Additional Load Cases for Analysis with Tension-Only Members" that can be selected to do this automatically, but for user defined story forces or nodal loads, an additional case for the reverse direction must be manually input.

How is a tension only brace considered during dynamic analysis?

For any dynamic, response spectrum load case, all braces are assumed to be 50% effective. This generally accounts for an appropriate amount of stiffness for a symmetrically braced frame, but does not prevent the braces from showing compression results. This approach is also used for the eigensolution.

Why am I getting large deflections under gravity loads?

In the RAM Frame analysis, braces that are designated to be tension-only are completely ignored during the analysis of the gravity load cases (dead, live, roof and snow). For the evaluation of lateral loads the program uses an iterative approach to determine which braces are in tension under each lateral load case. Then the program superimposes the results based on the defined combinations in order to code check the braces.

In structures where all the braces are tension-only, a global instability can often result for gravity loads. If you get an instability warning or if the deflections are significantly large under gravity load cases there are several things you can do.

1. Simply ignore the gravity load deflections. For the purpose of evaluating deflections, use the lateral load cases alone (uncombined). So long as there are no significant shears or moments developing in the other lateral members due to the drift, there is really no harm in this.
2. Provide a secondary system to give the structure some lateral stiffness even when the tension-only braces are ignored. Typically, this is a matter of fixing the columns all the way to the base. If a fixed base condition is too stiff, try adding a rotational foundation spring at the base of the columns to reduce the lateral stiffness.
3. Remove the tension-only assignment from the braces altogether. In this case, the braces can typically be hand-designed for tension equal to two times the maximum (or minimum) axial force from any combination (obtained from a member force envelope report). This approach can alter the load path affecting column and foundation design however.  

An enhancement was added in RAM Structural System v14.06, which should resolve the problem with large displacements and instability errors. Beginning with this version, P-Delta effects will not be included when analyzing gravity load cases with the special tension-only analysis. This is a reasonable implementation since in virtually all cases the P-delta moments due to lateral sway caused by gravity loads is extremely small. P-Delta effects will still be considered with the lateral load cases. More on this can be found in the v14.06 release notes, which are located here: V14.6 Release Notes.

See Also

RAM Instability In Finite Element Analysis

Product TechNotes and FAQs

Structural Product TechNotes And FAQs