I assume your question relates to the design of structural connections under the requirements of AISC 341 as well as those of ASCE 7. AISC 341 requires design of certain connections for the lesser of the design seismic forces (computed using the specified R value), amplified by the Omega-0 coefficient, or, the maximum force that can develop in the connection as limited by the strength of the connected members. There are several reasons for this:
1- Earthquake ground motions are extremely uncertain, and can vary substantially from the design spectrum, even when site specific analysis is used to develop the spectra.
2- In reality we are designing structures to resist collapse in MCE shaking, which is 1.5 times more intense than DE shaking.
For the example you cited, use of an R=1 for connection design would satisfy the code requirements, as Omega-0 has a value of 2.5 and R/2.5 is 2, so you would be designing for twice the required strength. However, this would not be particularly economical. Further, if you also designed the members for R=1, your structure would nto be very ductile as inelastic behavior of the strutcure could be limited by connection faiure, rather than member yielding, as intended.
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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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Original Message:
Sent: 01-17-2024 10:53 AM
From: Anonymous Member
Subject: Use of overstrength factor for connections design with actions from a Response Spectrum Analysis with R=1
This message was posted by a user wishing to remain anonymous
For connections design in seismic resistance systems, the static EQ is 5 times larger than the Response Spectrum for R=5. If we consider an R = 1 for RS, the value will be equivalent to the static EQ with R=5, shall we still use an overstrength factor for connections design?
#EarthquakeandEarthquakeLoads