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

  • SR_Eng

    Why do they allow a stress increase for allowable combinations? Seems unconservative for steel now that they have essentially balanced the ASD capacity with LRFD.

  • DAVID K

    The code has become to complex. Most buildings in USA can have a simple design load, and the penalty would be 5% for the structure, and less than .005% for the cost of the building. What is wrong wiih haveing extra reserve strength in buildings. The final cost is not much.
    The way real building are designed is with simple loads simple concepts. Who knows how a computer designs a building. We ignor parts of the Code
    We do not know what the loads are. WE know how to design so it does not fall. That is great . Spend the extra 0.005%

    • Tim Gudgel

      Thankyou David K …..the absolute truth in regards to all the errors that can and are made with the application of the code now does not make it worth the risk with so little savings(???) to the owner and liability to the SE.

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  • Phil Brazil, PE, SE

    There is a change to the provisions of ASCE 7-05 noted in the blog post on the overstrength factor that are now found in ASCE 7-10. In Sec. 12.4.3.3, the NDS load duration factor is expanded to all NDS adjustment factors. Also, there are also requirements In Sec. 12.2.5.2 and 12.10.2.1 of ASCE 7-05 and 7-10, requiring use of the overstrength factor.

  • C Jones, PE

    Thank you so much for this brief and straight-forward explanation! I have been trying to sort this out for a while now and it’s all perfectly clear. They really ought to reference back to these passages from the Section 12.4.3.2 as they are very easy to overlook…

    The overstrength factor used as a “safety factor” of sorts to account, indirectly, for irregularities. Boom.

  • tirumala naidu serudla

    whether we have use this amplified load combinations for designing Foundations