June 28, 2016
Calculation of Forces in Seismic Brace Connections
Seismic brace connections are a bit different from other connection types in RISAConnection. This is because the brace and connection elements must be designed for both tension and compression loading.
Brace Design Tension Load
The AISC Seismic Design Manual requires that all seismic brace connections are designed for the minimum of the required tensile strength of the connection based on the seismic load combinations and the expected yield strength of the brace in tension. Therefore, RISAConnection takes the design tensile load as:
For the stand alone version of RISAConnection "T_input" would be the user-input Brace Tensile Load. For the RISA-3D integrated version, this will be imported as the governing axial tension value in the brace per the RISA-3D solved load combinations.
For more information on how to calculate "T_expected", see AISC 341-10 sections F1.6a(1) and F2.6c(1)(a).
Brace Design Compression Load
The AISC Seismic Design Manual chapter F requires that OCBF brace connections are designed for the minimum of the required compressive strength of the connection based on the seismic load combinations and the expected brace strength in compression. For SCBF, the Manual requires that you consider two cases of compression: the first where all braces are assumed to resist their expected strength in compression, and the second where all braces are assumes to have buckled.
Therefore, RISAConnection takes the design compression load as:
For the stand alone version of RISAConnection "C_input" would be the user-input Brace Compressive Load. For the RISA-3D integrated version, this will be imported as the governing axial compressive value in the brace per the RISA-3D solved load combinations.
For more information on how to calculate "C_expected", see AISC 341-10 sections F1.6a(2) and F2.3.
Loading Direction Cases
Even when integrated with RISA-3D, RISAConnection does not always have all the information about the possible load combinations and resulting forces. This is because seismic brace connections may be designed to their expected strengths, which is different fundamentally from RISA-3D’s analysis.
Therefore, if a connection has two braces, such as a Chevron brace or a Diagonal brace with braces above and below, the program will consider multiple combinations of brace load in order to account for reversible lateral load directions. RISAConnection considers the following load directions.
Load Distribution Calculations
RISAConnection will then calculate the load distribution to each interface sub-connection for each applicable loading direction shown above.
For example, if you have selected the “Zero Moment (Uniform Force Method)” analysis for your SCBF diagonal brace connection, RISAConnection will calculate the axial and shear forces at each interface sub-connection (such as the “Gusset to Beam” connection) for each of the four possible SCBF loading directions (S1-S4). Each of these calculations is shown in detail on the “Seismic” tab of the results report.
The maximum/governing values are then reported in the “Load Distribution (Governing)” section and on each of the subsequent result report tabs.