The Omega Factor

Section 12.4.3.3 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 12.4.3.3.

Image credit: ASCE 7-05.
Image credit: ASCE 7-05.

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.

Image credit: Select Seismic Design Coefficients from ASCE 7-05 Table 12.2-1.
Image credit: Select Seismic Design Coefficients from ASCE 7-05 Table 12.2-1.

 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.

Image credit: ASCE 7 Section 12.3.3.1.
Image credit: ASCE 7 Section 12.3.3.1.

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. 

Image credit: ASCE 7 Section 12.3.3.2 thru 12.3.3.4.
Image credit: ASCE 7 Section 12.3.3.2 thru 12.3.3.4.

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.

What are your thoughts? Let me know by posting a comment.

– Paul

Paul McEntee

Author: Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day:

Q.   What is your mom/dad's job?
A.   Goes and gets coffee and sits at his desk

Q.   What does your mom/dad actually do at work?
A.   Walks in the test lab and checks things

When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems.

I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.