Specifying and Designing Star Seismic Buckling Restrained Braces in the RAM Structural System
Comprehensive analysis and design of Star Seismic 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 Star Seismic from the list of Shapes, select the desired brace size, and specify whether the brace is to be a POWERCAT (Pinned) or WILDCAT (Welded/Bolted). Note that WILDCAT braces are generally more economical, although the option to use those is limited to sizes StarBRB_19.0 and smaller.
Note that sizes can also be assigned in RAM Frame using the Assign – Braces – Size command.
When the brace sizes are displayed onscreen the connection type is also indicated, with "(P)" for Pinned and "(W)" for welded.
In the 3D view the brace will be represented as a core with a rectangular or round core for a portion of the length, with the Star Seismic logo.
When a Star Seismic 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 – Star Seismic 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. Ie is the Seismic Importance Factor. Enter Cd/Ie.
For Star Seismic braces the Yield Strength for the core generally ranges from 39 to 46 ksi.
The Minimum Clearance refers to the distance between the column and beam flanges and the end of the brace. This dimension is generally 4 inches, but should never be less than 2 inches; otherwise it will be difficult, if not impossible, to make the connection welds.
A powerful feature of the program is that it can automatically calculate the Stiffness Modifier. Note that these calculated values are only applicable to Star Seismic braces. In lieu of the calculated values, a value can be specified.
The Criteria applies to all Star Seismic 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 – Star Seismic command.
The Use Global option is merely for the purpose of clearing any specific 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 – Star Seismic command, which are considered to be the global values; they will automatically use the Criteria values unless something different is assigned with this command.
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” followed by the calculated value in parentheses.
When the Analysis is performed, the Stiffness Modifiers are applied accordingly. The Member Forces report has been enhanced to include pertinent Star Seismic brace information, including the Stiffness Modifier. When the Calculated value is use, 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 ASD and LRFD have been implemented for buckling restrained braces. Select one of those codes in the Criteria – Codes command.
In the generation of the Load Combinations, the value of Omega should be set to 1.0.
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.
The Star Seismic 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 Star Seismic Buckling Restrained Brace should be reviewed and verified by Star Seismic. To assist in this process, upon completion of preliminary and/or final lateral designs using Star Seismic braces, users should send their RAM model (e.g. “xxx.rss” file), including any customized beam, column or deck tables, to Star Seismic (435-940-9222) at engineering@starseismic.net for verification and validation of the computer model. Upon completion of model review, Star Seismic will provide structural drawings (notes and details), including stamped/signed calculations for the specific project.