8/7/2019 DisadvantagesofBRBs

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BRBF (Buckling Restrained Braced Frame), originally known as UBB (UnBonded Brace) - These are a type ofenergy dissipation product being offered by several (three) manufacturers in both Japan and the USA. Nippon Steelfrom Japan, and CoreBrace / Star Seismic in the USA are the manufacturers/suppliers. In all cases, the BRBFdevice is comprised of two items: a small diameter steel bar with an outer concrete/steel casing. The purpose of theouter concrete/steel casing is to keep the thin steel bar from buckling in compression. Between the thin bar and the

outer concrete/steel casing is some type of lubrication to keep the two parts from sticking together, thereby making it"unbonded" or "de-laminated". The idea is to allow the inner steel bar to yield in both compression and tension todissipate energy, while the outer casing prevents buckling.

The advantages of BRBFs are:  1. Yielding steel is a good energy dissipater, and is predictable.2. BRBFs yield in both tension and compression, which is better than regular bracing, only yielding in tension.

The disadvantages of BRBFs are: 1. You can use any type of steel bracing to yield for energy dissipation. Eccentric bracing can be better than BRBF.2. BRBFs are often high cost compared to FVDs, and certainly more expensive than typical bracing.3. BRBFs add stiffness to the structure, which is counter-productive when adding damping.4. BRBFs yield and after an earthquake occurs, must be assessed/replaced before aftershocks occur.5. There is potential for corrosion to occur between the BRBF interfaces, thereby "bonding" the brace.6. When testing, the BRBF becomes yielded, which degrades earthquake performance (yielding causes damage).7. BRBF goes from elastic brace to yielding brace, only providing energy dissipation at the yield level of the steel.8. BRBFs may have local buckling issues. When pulled in tension, the thin bar is extended out of the casing.Compression could then cause buckling of this exposed, unsupported portion of the bar (local buckling).9. BRBF cannot provide energy dissipation for low level earthquakes, nor can they adjust for higher levelearthquakes like an FVD with F=CV.

California Engineers:Engineers in California that have used these devices, typically just use the BRBF for added stiffness, and plan foruseful energy dissipation in the event of an earthquake (this is a very expensive way to add stiffness and a poor wayto "hope" for some energy dissipation).

Japanese Engineers:  The Japanese do not like to use BRBFs “alone” in a building structure because the braces yield. After a piece ofsteel yields, the leftover strength is unknown. It may be about to fail, or it may still have some reserve strength left.The Japanese would not want to depend on something that has these unknown characteristics. They would replaceall the BRBF's after any earthquake. In more modern designs, the Japanese are using BRBF with fluid viscousdampers. In these projects, the FVDs provide the energy dissipation, and the BRBF is used for added stiffness. Inthe case of an extra-large earthquake, the BRBF may yield and the Japanese engineers hope this will help thebuilding performance. The various manufacturers’ websites are listed below. Not much available from Nippon Steel (nsc website). Thedifferences between the manufacturers are generally in the way they connect the ends of the brace (pin connection,rigid connection, etc.).

http://www.starseismic.net http://www.corebrace.com http://www0.nsc.co.jp/shinnihon_english/jigyo/index.html