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IBC 2003 1605.4 Special Seismic Combinations

IBC 2003 1605.4 Special Seismic Combinations

IBC 2003 1605.4 Special Seismic Combinations

Paraphrase of IBC 2003 Section 1620.2.9:

Columns subject to vertical reactions from discontinuous walls or frames...shall resist special seismic combinations of 1605.4.

My question is:

Can someone show me a graphical representation (or a link to this subject) of which columns are subject to this requirement?

Here is an example of a bracing line:


There is a picture in the code (Figure 1620.2.9) that shows a couple of examples but I am dealing with an industrial structure that has vertical bracing in some bays and not others and where to apply these special combinations is not clear from the code.

RE: IBC 2003 1605.4 Special Seismic Combinations

Looks to me you probably meet one of the conditions in Table 1616.5.1.2.  In any case its' probably easier to check the combinations in 1605.4 than to try to determine from the tables 1616.5.1.1 & .2 whether 1605.4 applies.

RE: IBC 2003 1605.4 Special Seismic Combinations

It is easier to check the combinations but that results in columns increasing a couple of sizes. I don't want to be overly conservative in interpreting the code if I don't need to be. The omega factor here is 3, so tripling the seismic loads in these columns makes a large difference in weight.

RE: IBC 2003 1605.4 Special Seismic Combinations

I don't have my code with me, but in the '97 UBC, IBC may or may not have this, there is a reduction of the omega factor allowed.  The structures I typically work on have an omega factor of 2.8.  This factor reduces to 1.73 due to the following factors:

2.8*1.4(strength)/1.7/1.33 = 1.73

RE: IBC 2003 1605.4 Special Seismic Combinations


The 1.7 factor you noted does not apply to this section of the IBC 2003.  You might be considering the AISC Seismic Provisions.


Yes, designing ALL the columns for this load is the easy way.  But why take that route?  We should have an understanding of why the code requires this increased load & how to apply it effectively.  In a large braced frame like Dave shows in the link, the extra load could significantly increase the material costs.

Let me carry Dave's question one step farther...does anyone know of any papers or research that backs up the code requirements for using the seismic load Em (= omega*Qe + 0.2*Sds) rather than just E.  I have not been able to find any even though it has been in the code a long time.


RE: IBC 2003 1605.4 Special Seismic Combinations

Where do you think the discontinuity in your frame exists?

If one of your vertical elements is not continuous then certainly omega comes into play. Your bracing is not continuous but that might be a good thing for seismic response.

If the columns in the frame line you posted get the overstrength factor then I have been misreading the code's intent!

RE: IBC 2003 1605.4 Special Seismic Combinations

The vertical bracing shifts inward below the 2nd level creating the discontinuity, therefore the columns below the discontinuity are subject to the special seismic combinations (if I am interpreting the code correctly).

IBC 2003 1620.2.9 states:

"Columns or other elements subject to vertical reactions from discontinuous walls or frames of structures having....vertical irregularity Type 4....shall have the design strength to resist special seismic load combinations of Section 1605.4"

RE: IBC 2003 1605.4 Special Seismic Combinations

Your offsets are NOT out of plane. Your columns ARE continuous.

I don't see why these columns would be crucial and need the overstrength factor; however, where your bracing jumps from one bay to another the collector elements between may require the additional strength.  

RE: IBC 2003 1605.4 Special Seismic Combinations

Example 7 of the 2000 IBC Structural/Seismic Design Manual Volume 1 has a similar example.  In this particular example they design the columns below the discontinuity for Omega and the collector beams along that line for omega as well.

RE: IBC 2003 1605.4 Special Seismic Combinations

Not really answering your question, but if your columns are heavily loaded, they may need to be designed for omega loads just for being part of the lateral system . . . See Section 8.3 in Part 1 of AISC 341-02.

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