Star Seismic buckling restrained brace shapes are now available in the RAM Structural System. This feature allows the engineer to conveniently assign Star Seismic shapes to buckling restrained braces in the RAM Structural System model.
To model, analyze and design buckling restrained braces using Star Seismic shapes, do the following:
A Master Steel table, BRB_STARAISC.TAB, is now available containing the Star Seismic Buckling Restrained brace shapes. This table is a duplicate of the RAMAISC.TAB table that contains all of the AISC shapes for beams, columns and braces, with the addition of the Star Seismic Buckling Restrained brace shapes. To select this Master Steel table select the Criteria - Master Steel Table command and select BRB_STARAISC from the list of tables. Note that this table can be edited and updated if necessary as explained in the RAM Manager manual.
Create the model in the Modeler. Braces are added in Elevation view using the Layout - Braces - Add Standard command. Typically Fy for the braces is specified as the minimum Fy value, and is recommended to be not less than 38 ksi. Engineers at Star Seismic can assist the engineer in determining the appropriate value to use.
Brace sizes can be assigned either with the Layout - Braces - Assign Size command in Elevation View in the Modeler or with the Assign - Braces - Size command in RAM Frame.
The Star Seismic shapes are listed under the Flat Bar shapes. Select the desired size from the list and assign it to the braces in the model.
Braces are listed by steel core area. As an initial assignment select a brace with a core area greater than the expected axial load, Pu, divided by 0.9Fy-min. Sizes can subsequently be easily changed as necessary to meet strength and drift requirements.
In the 3D View Star Seismic shapes are drawn as square or rectangular braces, scaled to the size of the typical casing for that brace size. If necessary, contact Star Seismic for the exact size of the casing that will be used for the specified brace size for the actual frame geometry (that information is not necessary for the analysis performed in the RAM Structural System).
In RAM Frame use the Assign - Braces - Buckling Restrained command to specify the braces to be BRB's; at that time an Axial Stiffness Multiplier, as determined by the engineer in cooperation with Star Seismic, is assigned to each brace.
In the analysis performed by RAM Frame the braces' lengths are based on centerline dimensions; the purpose of the Axial Stiffness Multiplier is to take into account the increased stiffness resulting from the true joint dimensions, the buckling restrained brace stiffness and the size of the connection. By increasing the brace stiffness with this Multiplier, a more realistic analysis is obtained. For preliminary design a value of 1.5 may be satisfactory; Star Seismic should be contacted to determine the appropriate values to use in the final design.
In RAM Frame, analyze the model using the Process - Analyze command.
In the RAM Frame - Steel Standard Provisions mode, select the desired steel design code (e.g., AISC 360-05 ASD or LRFD), generate the corresponding load combinations (either automatically or manually) and then perform a code check using the Process - Member Code Check command. A code-check is performed per the Standard provisions of the specified Code for the load combinations that have been generated.
With the View/Update command the user can select any brace of interest, view the results including the interaction equation, and can select and investigate other Star Seismic sizes.
If the currently assigned size is not acceptable, different sizes can be investigated, and when the desired Star Seismic size is determined, it can be saved back to the brace using the Update Database command. In this way the engineer can quickly arrive at the optimum sizes. Of course, if the Axial Stiffness Multiplier is different for the new brace size, the user would have to reassign that value.
The design results can be viewed:
Note that since it is a Buckling Restrained Brace, the axial capacity is based on the tensile capacity of the section, regardless of whether the brace is in tension or compression.
In addition to the standard provisions of the steel specification, there are additional seismic requirements per AISC 341, but those provisions are not currently checked by the program for Buckling Restrained Braces. The engineer should perform these checks independently.