In this video we will talk about secondary members. According to EC8 Section 4.2.2, Secondary Seismic Members are those not part of the Seismic Action Resisting System of the building.
To summarize EC8 requirements: secondary members should not take seismic forces away from the seismic resisting system and need not be designed for these forces, however they need to keep providing vertical load support while subject to seismic displacement.
- A. Strength and stiffness of secondary members against seismic actions are to be neglected
- B. Secondary members need be designed for gravity loads only
- C. While subject to seismic displacement, secondary members need to be capable of supporting the gravity portion of seismic load combinations.
How to accomplish all this is rather complex. Here is an example with a possible approach, starting from a SAP2000 model.
In the example we will tweak the analysis a little, adopting the one-step stage construction option (already available at the SAP entry level). The intent is to assign hinges at the end of all secondary beams during the seismic situation. This will allow us to meet provision A.
First, let us create groups identifying primary and secondary members. In this example we choose to treat the interior beams as secondary members. Everything else we’ll consider primary.
Next, let us define a property set, where moments are released at both ends of a member.
Then, let us set up a stage construction case, where that property set is assigned to all secondary members. We are also imposing reduced inertia to all members. The gravity analysis cases should start from the zero condition and the modal analysis case should start after the staged construction; the response spectrum analysis needs to be based on this modal analysis.
Now let us take care of provision C. Run the analysis.
Look at the rotations of the joints of secondary beams from the response spectrum case in the Y direction. They are all pretty much around .00045 radians. These rotations need be multiplied by the structure behavior factor q, in our case 2.64. The result is a rotation of 0.0012 radians.
Let us isolate a single secondary beam and apply that rotation at the two ends. The resulting moment is 30.9 KN-m. Now go back to the original model and create a load pattern where that moment is applied at the ends of all secondary beams. Let us make a load case where that pattern is applied at the end of the stage construction case. In the load combinations, add this to the response spectrum case in the Y direction with both the plus and minus signs.
Basically, we are making sure that the secondary beams, in addition to the vertical loads, can take the forces deriving from the seismic displacement. This rather conservative approach is suggested by Section 5.7 of EC8.
Finally, let us run the analysis and start VIS, and we’ll take care of provision B.
Simply select the groups inherited from the SAP model and tell VIS which members are to be considered primary or secondary.
VIS automatically excludes from design of secondary members all capacity design requirements. Only strength design and general construction detailing requirements are applied. Secondary members will still be subject to seismic load combinations, but with the limitations above.
Run the VIS Wizard and look at the results. These are from our analysis with secondary members. These are from a regular analysis (that is, one with no secondary members). As you can see, while the longitudinal reinforcing remains pretty much the same, the transverse reinforcing is lower when secondary members are considered.
That is a nice benefit. The real kick, however, comes from the impact on primary members. Since secondary members do not take part in the seismic action, they do not affect capacity design of the primary members they are connected to. In our case columns and joint panels. As a result, the weak direction reinforcing of these primary members gets a substantial reduction.
This concludes our presentation.
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