Specifying and Designing CoreBrace Buckling Restrained Braces in the RAM Structural System


Comprehensive analysis and design of CoreBrace Buckling Restrained Braces are now available in the RAM Structural System. This document describes the use of this feature.

 

Modeling

Create the model, including the braces in the braced frames:

 

 

In Elevation View in the Modeler assign preliminary sizes to the braces. In the Layout – Braces – Assign Size command select CoreBrace from the list of Shapes, select the desired brace size, and select the connection type (Bolted, Welded, Pinned or Splice Plate).

 

 

Note that sizes can also be assigned in RAM Frame using the Assign – Braces – Size command.

In the 3D view the brace will be represented as a crucifix core with a rectangular case for a portion of the length, with the CoreBrace logo. The logo icon will also indicate the connection type assigned to the brace.

 

 

When a CoreBrace size is assigned to a brace, the program will automatically recognize that brace as a Buckling Restrained brace.

 

Criteria

In RAM Frame the Criteria for these braces can be specified using the Criteria – Buckling Restrained – CoreBrace command.

 

 

Unless the seismic R factor is governed by a different seismic system, Cd for Buckling Restrained Braces is 5.0 in ASCE 7-10.

 

For CoreBrace braces the Yield Strength for the core generally ranges from 38 to 46 ksi.

 

The Minimum Clearance refers to the distance between the column and beam flanges and the end of the brace. This dimension varies according to connection type, but should generally not be less than the default values used by the program; otherwise it will be difficult, if not impossible, to assemble the connections.

 

A powerful feature of the program is that it can automatically calculate the Stiffness Modifier. Note that these calculated values are only applicable to CoreBrace braces. In lieu of the calculated values, a value can be specified.

 

The Criteria applies to all CoreBrace Buckling Restrained braces. It is possible, however, to assign specific values to specific braces, on a brace-by-brace basis, using the Assign – Braces – Buckling Restrained – CoreBrace command.

 


 

The Use Global option is merely for the purpose of clearing any specific previous assignments and resetting the brace to use the values set in the Criteria command. Note that it is not necessary to invoke this Assign command if all of the braces are to use the values specified in the Criteria – Buckling Restrained – CoreBrace command, which are considered to be the global values; they will automatically use the Criteria values unless something different is assigned with this command.

 

AISC 341-10 Section F4.3 states “Buckling Restrained Braced Frames shall not be considered as resisting gravity forces.” To accommodate this, an option has been added to Criteria – General to allow the user to specify that the braces are not to be included in the analysis of the gravity load cases.

Seismic Loads

In ASCE 7-10 the value of R given for Buckling Restrained Braced Frames is 8. Furthermore, in the equation for the Approximate Fundamental Period, Ta (Eq. 12.8-7), Buckling Restrained Braced Frames get the more favorable value of Ct of 0.030.  These are specified in Loads – Load Cases when using the program’s seismic load generation features.

 

 

Analysis

The View – Members command can be used to view what has been assigned to each brace for the Stiffness Modifier, whether it is a calculated value or an assigned value. Select the Brace tab and select the Buckling Restrained labels option. If the program is using a calculated value the label will show “Calc KF” followed by the calculated value in parentheses. If the program is using a factored calculated value the label will show “Calc KF-Fact” followed by the factored calculated value in parentheses. If the program is using a user-specified value the label will show “Use KF” followed by the assigned value.

 

 

When the Analysis is performed, the Stiffness Modifiers are applied accordingly. The Member Forces report has been enhanced to include pertinent CoreBrace brace information, including the Stiffness Modifier. When the Calculated value is used, it is listed; when a User-specified value has been assigned, both the Calculated and the User-specified values are listed, along with the ratio of the User-specified to Calculated values.

 

 

 

Steel – Standard Provisions

When the program performs a basic AISC 360 Specification code check on these braces it recognizes that since the braces are restrained against buckling, the capacity in either tension or compression is that given by the capacity in tension.

 

 

Steel – Seismic Provisions

In the Steel – Seismic Provisions mode the seismic provision requirements for AISC 341-05 and AISC 341-10 ASD and LRFD have been implemented for buckling restrained braces. Select one of those codes in the Criteria – Codes command. In that dialog there are also options on how the required strength of columns is to be calculated: Based on Adjusted Brace Strength, Amplified Seismic Forces with Omega_0, or Larger of the two. When the option to use the Adjusted Brace Strength is selected the program calculates the force on the column assuming the forces in the braces correspond to their adjusted strength, using the Beta and Omega factors that were either calculated by the program or assigned by the user. It appears that the first option is what is required by the Specification, but in practice some engineers also consider the Omega_0 level forces, to be conservative. Be cautious with the second option, however, because it isn't necessarily conservative; if it is desired to consider the Omega_0 level forces it is generally better to select the third option rather than the second option.

 

In the generation of the Load Combinations, ASCE 7 indicates that the value of Omega_o should be 2.5 for buckling restrained braces.

 

It is necessary to assign a frame type to each member, used by the program to determine which seismic provisions are pertinent. This is done using the Assign – Frame Type command. Select either Buckling Restrained Brace Frame or Buckling Restrained Brace Frame - Chevron as appropriate.

 

 

The Frame Type assignment on each member can be displayed using the View – Members command by selecting the Frame Type labels option on the Frame Beam, Frame Column and Brace tabs.

 

 

The text should show either BRB or BRB-V.

 

Select the Process – Member Code Check to have the code check performed on all of the members and select the Process – Member View/Update command to select and view the results for an individual member.

 

 

The Seismic Provisions Member Code Check report shows the results of the checks as well as pertinent information necessary for the design and detailing of these braces.

 


Before the Process – Member Code Check command is performed the View – Members command can be used to show whether the Beta and Omega values are to be calculated or have been user-assigned; if to be calculated it will show “Calc”, if user-assigned it will show the values assigned. After the Process – Member Code Check command has been performed the View – Members command can be used to show the actual values used.

 

                                      

The CoreBrace Brace Summary report, available from the Reports menu, shows a summary of all of the braces, with information pertinent to their design and detailing.

 


 

Note: Designs using the CoreBrace Buckling Restrained Brace should be reviewed and verified by CoreBrace. To assist in this process, upon completion of preliminary and/or final lateral designs using CoreBrace braces, users should send their RAM model (the “.rss” file) to CoreBrace (801-280-0701) at brandt.saxey@corebrace.com for verification and validation of the computer model. Upon completion of model review, CoreBrace will provide structural drawings (notes and details), including stamped/signed calculations for the specific project.