BBCGEN,<panel>,<ShearModel>,[fc],[fy],[Pn],[Nu],[bre],[bys],[CrackForceLevel]

The BBCGEN command automatically generate the backbone curve(s) for a single panel or all panels using the computed ultimate shear strength values that are identical with those obtained using the SHEAR command. The user can run first the SHEAR and get the computed shear strength estimates based on ACI 318-08, Wood, Barda and Gulec-Whittaker empirical equations. Before using the BBCGEN command, the user needs to decide which of the four shear strength models described for the SHEAR command would like to use for the nonlinear SSI analysis via Option NON.

The BBCGEN command always generates a 22 point BBC curves, the first point being the cracking point of the BBC curve. The next 20 points will be equidistantly spaced along the strain axis until the yield point is reached. The final shear failure point for all BBC curves will be defined by default at (shear strain = 2% and shear force = 1.02 x ultimate shear force value).

All of the shear strength model parameters specified by the user in the BBCGEN command are optional. If the user leaves a command parameter blank, the command will use 0 for that variable. This command assumes the panel references a non-empty group with a uniform material across all elements. It user responsibility to check that all elements within a panel have same thickness and elastic material properties.

• panel - panel number. If panel = 0 BBCs will be generated for all panels defined by the user.
• ShearModel - Numeric code for which shear model will be used to generate ultimate shear of the backbone curve. The shear model equations can be found in the description of the SHEAR command. The upper or lower bound will be used for the BBC if the calculated shear for ACI 318-08 or Wood 1990 fall outside the bounds specified in the model equations.
  • 1 - ACI 318-08
  • 2 - Wood (1990)
  • 3 - Barda (1977)
  • 4 - Gulec-Whittaker(2009)
• fc - compressive strength (ksi or kN/m²)
• fy - reinforcement yield strength (ksi or kN/m²)
• Pn - web reinforcement ratio
• Nu - axial force (kips or kN)
• bre - boundary element reinforcement area (ft² or m²)
• bys - boundary element reinforcement yield strength (ksi or kN/m²)
• CrackForceLevel - floating point scalar used to determine the cracking point of the backbone curve. Any scale that falls outside of the range and isn't 0 will cause an error that will be reported to the Command History screen.
  • 0 - Use the equation [3*sqrt(fc)*cross-sectional area] as the cracking shear.(default) NOTE: equation was [3.5*sqrt(fc)*cross-sectional area] in User Interface V1.0.1
  • [0.1-0.5] - Use [CrackForceLevel* utimate shear] as the cracking shear.