Eccentric Braced Frame EBF or Not EBF (IBC 2000) 2

You are absolutely correct. The EBF "R" values do not apply when links are in the columns. Braces framing eccentrically into columns create kinks that can lead to collapse. That is why K-bracing is forbidden in most codes.

Taro - that was my initial thought - that the eccentricity on the COLUMN means it is a "K" brace (outlawed) while the eccentricity on the beam is an EBF....right? So there really isn't any R value for a "knee" brace in that it is simply a K brace and not allowed.

Just my opinion....

Eccentricity in the brace beam connection should effect the structural damping. The deliberate working of the structural system is done in the beam, being less critical to failure than the column.

If the diagonal were eccentrically connected to a beam above and below, it would still be an EBF. Or if diagonals were specifically designed to load each other like the diagonal beam connection, it too would be an EBF.

I think the system if it were designed to "work" the columns, would not be allowed.

Forgive my ignorance, as I am currently working outside the US. What juridictions use the IBC? What jurisdictions use the UBC? Are they the same?

Regards

VOD

In the last 20 years or more, there were three model building codes in the US, each tended to be the predominant code in these regions:

Uniform Building Code (UBC) - Everything west of the Mississippi River
Building Officials Code Association (BOCA) - Northeast US
Standard Building Code (SBC) - Southeast US

These three merged in recent years to form the ICC (International Code Council) which produced the International Codes (IBC, IPC, etc.) This was an attempt to create a singular model code for all of the US.

Unfortunately, the NFPA (National Fire Protection Association) did not like certain portions of the International Codes that pertained to their areas. They decided to create their own code and this is the NFPA building codes.

Right now, most of the US is going to the IBC but some areas (such as the State of California - not necessarily the cities) have adopted the NFPA - but its all a very political struggle.

Hi JAE,

Thanks for your response. In this thread I have noticed from Taro and yourself that K frames or chevrons are now "outlawed" in most American codes. Is this true?

I had noticed that the UBC '94 allowed EBF K frames (eccentric on the top horizontal beam only) for seismic loads as can be seen at under Steel Tips.

Regards

VOD

I guess it is the k brace which is eccentric to the column only that are "outlawed".

Regards

VOD

VOD - I believe that they are outlawed in high seismic zones only.

In particular seismic zones 3 and 4. See Section 2210, 2.3 UBC 97 (2-241). EBF are allowed if used with moment frames. K-bracing is only allowed on a limited basis see 9.4.b and 9.5 of section 2210.

I've done some homework lately. Thanks gentlemen.

EBF's in the IBC aren't restricted except without dual system moment frame detailing in high seismic areas. Even then most buildings are still allowed.

Neither Chevron nor K braces are EBF's.

EBF's introduce bending in the link beam by offsetting the connection from the beam column joint.

My IBC refers to a section 2212 for seismic rules, but there is no 2212 in my book!! AISC is referenced. LRFD is referenced.

So it's obvious, High seismic zones can't adopt the IBC. Gotta stay with the UBC.

And K braces are specifically outlawed there.

The AISC has a free publication for Seismic design dated 2002 on their website. It looks to me as though they will assume leadership in this area. It's a whole new steel code book several hundred pages in length. They discuss bracing options and incredibly in depth steel seismic issues.

IBC can be and is adopted in high-seismic regions. IBC Section 2212 basically just references the AISC Seismic Provisions.

The AISC Seismic Provisions aren't really new. They were first published in 1990 and have been updated about 5 times since then.

AlohaBob - Taro is correct - I have an IBC and there is a 2212 section in it - it is one of the first editions, too!

Comparing 1988 UBC (Steel Ch 27 60 pages) 1997 UBC (Steel CH 22 35 pages) to 2000 IBC (Steel Ch 22 6 Pages).

The code treatment of steel is altogether different. IBC basically says see AISC. The UBC on the other hand assumed to codify the design standards especially in seismic issues. The AISC was just not the adapted reference standard for seismic issues. However, the UBC published basic steel design criteria, but truthfully, I used the AISC reference first for basic steel design.

I'm glad to see AISC take a better practical role in seismic issues, but I still see the UBC just too vested in their knowledge and responsible administration of seismic design to see it yet anytime soon relinquish authority to a lesser responsible authority. And the IBC assumes no authority whatsoever in this steel design issue.

Keep in mind that the UBC and the IBC have the same organization behind them (UBC has ICBO, which is a member of ICC which publishes the IBC)

The UBC is now a code that will not be renewed as the IBC is intended to replace it in the future. The intent of the ICC committees was to begin to develop model codes that would (properly in my mind) reference OUT to more established bodies such as the ACI, AISC, etc. instead of manhandling all the provisions coming out of those specialized material bodies.

So the UBC (ICBO)has essentially "relinquished their authority" already by signing on to create the IBC.