Unfortunately from my research, a capacity based design module for BRBs in ETABs does not exist. See snip below.
Given this glaring missing feature, I figured I would outline how I have "tricked" ETABs into doing capacity based design, the poor man's pushover approach to capacity based design for BRBs. The example images presented above are from the AISC Seismic Manual and will serve as the reference for this post. AISC and ETABs own all rights to their documentation.
The Approach
To tackle this problem initially, I went down the route of wanting to apply manually calculated expected strength loads to the BRBF frame. This approach falls short in that load redistributes and goes where it wants given the elastic nature of the model that is built. Trying to tell a specified BRB in the ETABS model to be at x kips is not doable with a traditional force based approach.
With this in mind, I went to a pushover, nonlinear analysis route. This would allow me to specify an exact force at which the braces should yield in both tension and compression(BRBs have slightly higher compressive strength than tensile strength) so I thought this might be the ticket.
Model Setup
Model setup is nothing special, I built the (4) story example model and modelled pinned based braces and columns, with continuous columns (more on this later).
I have uploaded this model to github if you want to play with the model and review.
The Fun Parts
The cool part of the analysis was faking ETABs into doing a capacity based design of the beams and columns, this was done through a pushover load case and non-linear hinge definitions for the axial deformation and load in the BRBs.
Defining a pushover load case is pretty in ETABs, see the screenshots below:
Above, I asked ETABs to move the upper most level 4 node over 36 inches so that I can make each BRB yield in the frame system.
I also defined a specific BRB material, a BRB frame section, and a BRB nonlinear hinge for each BRB.
Above is the material definition, I am not sure if ETABs ever uses this non-linear material definition, but for completeness, I put this in.
To define the frame sections used in the AISC example, I made the (4) differing core sizes, sample shown below.
To define the frame hinge, I input the specified w and beta factors that were noted in the AISC worked example. See blow:
with all this in place, I crossed my fingers and hoped the results would match.
Pushover Results
The results pretty much exactly matched the AISC worked example which is great, the poorman's capacity based design approach seems to work and capture the design effects that AISC asks designers to consider.
Below you see each brace getting pretty close to the specified yield force, with the tension values being less than the compression values as expected.
It's fun to step through the pushover as well and see how the frame reacts as more and more displacement is induced. You can see when each BRB begins to yield and redistribute load down to the next frame. Below is the forces about 20 steps in the forced displacement. You can see the upper most braces have not yet yielded.
The capacity based design value compression and tension values for columns and beams matches the design example as well.
With these correctly determined capacity based design values, you can feed this load case into the design combos of ETABs and verify you beams and columns can adequately resist the capacity based demands.
Words of Caution
One should pay attention to continuity of columns from floor to floor. None of the examples in the AISC manual seem to consider this, but ETABs does. I am inclined to ignore these moments and just design for the capacity based axial demands, but it might be worth considering these moments if you are concerned about them. Below you can seem some really large moments in the columns due to the large displacements. Given that the columns are elastic, they will continue to develop bending moment as they do not know when to yield and give up.
You can resolve these moments and maybe conservative design values in ETABs by pinning your columns as shown below:
The new bending moments look like this, that's much better!
How do other Structural Softwares Handle Capacity Based Design of BRBs?
Given that I just outlined this process, I figured I would research how other structural design software handles capacity based design of BRBs.
RISA has a cool, "semi" finite element approach where they essentially remove the brace, apply the specified compression and tension yielding values, apply artificial frame restraints and then do the "semi" finite element analysis on the reduced structure. I thought this was a really cool approach, but I could see a few areas where this could possibly break down. I would have to test it out more to see if this works in all cases.
From my reading, it sounds like RAM structural does this as well.
RAM does not provide a nice write up like RISA on how it is determining these values for beams and columns, but appears to be doing some form of capacity based design based on D1.4a which is good.
All of this is to say, ETABs and CSI, let's step it up to make engineers lives easier. The process I outlined above can work to accurately design beams and columns for capacity based BRBs, but it's a bit convoluted, albeit cool.
NOTE: I am no expert in this and I hope the process I have outlined captures the intent of the code. If you see any flaws in my approach, please comment/message me. I want to make sure my beams and columns don't fail.
