Section 126.96.36.199 of ASCE 7-05 (or -10) deals with overstrength (Ωo) load combinations and allows a 1.2 increase in allowable stress when using these combinations. We received a question from a customer last week asking if the 20% increase applies to Simpson Strong-Tie connectors. The simple answer is yes. When demand loads are based on amplified seismic forces, connector allowable loads may be increased by 1.2 per Section 188.8.131.52.
Since the increase may be combined with the duration of load increases permitted in the NDS, you would apply the 1.2 increase to connector allowable loads at a load duration of 1.6, which makes the overstrength factor a little less terrible.
The question got me thinking a little more about overstrength load combinations, so I wanted to discuss what they are used for. It also made me think about a sales meeting several years ago where one of our engineers was addressing a question about an application that required a design using amplified seismic forces. A salesperson asked why the forces needed to be amplified and he said, “Well, there’s this Omega subzero factor…” Never speak in Greek letters to salespeople. They call him Omega Subzero to this day.
So why does the code have amplified forces?
ASCE 7 and other model building codes acknowledge that structures will be loaded beyond their elastic range during seismic events. Damping and ductile yielding make it unnecessary to design for the full inelastic design force, so the code divides the seismic response by the R-factor to get a lower elastic design force or base shear. Higher R-factors represent more ductile systems and, therefore, yield a lower seismic design force. Deflections are multiplied by the Deflection Amplification Factor, Cd, to obtain the expected inelastic deflections. Similarly, the System Overstrength Factor, Ωo, is an amplification factor that is applied to the elastic design forces to estimate the maximum expected force that will develop.
ASCE 7 Section 12.3.3 addresses limitations and additional design requirements for structural systems with irregularities. Tables 12.3-1 and 12.3-2 define horizontal and vertical structural irregularities and reference the code requirements applicable to each type. In some cases, the irregularities are simply prohibited for high seismic areas.
Many of the irregularities are allowed, albeit with additional design requirements that make use of the load combinations with the overstrength factor. The purpose of applying the overstrength load combinations to irregularities is to prevent non-ductile failures in the structural system.
Designing for amplified forces can be a real challenge, but the alternative would be the building code not allowing structural irregularities at all, which would not be realistic. I have always thought of the overstrength factor, Ωo, as being a sensible compromise.
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