What testing is required for a typical project?

Typically no testing is required. Most jurisdictions are starting to follow the SEAOC/AISC Recommended Provisions for BRBFs or the 2005 AISC Seismic Provisions for Structural Steel Buildings, which call for testing in the event that existing test data applicable to the project in question is not available from the manufacturer. Nippon Steel Engineering Co.'s test data covers a range of brace sizes from about 50 kips to 1,200 kips.

Why do UBBs use bolted connections?

The bolted configuration allows the core plate to be continuous from end-to-end (cut from a single plate) eliminating the presence of welds in load path. In addition, a slip-critical bolted connection provides redundancy in the connection and improves out-of-plane stability of the brace and gusset plate connection. It has been shown through experimental testing that bolted connections will easily accommodate end rotations corresponding to story drift ratios exceeding 3%.

Can UBBs use pinned connections?

Yes. Nippon Steel Engineering, Co. recommends bolted connections, but pinned connections can and have been used in US projects.

What jurisdictions/regulatory agencies have approved UBBs?

UBBs have been used in California, Oregon, Utah and Washington, and have been approved by nearly twenty different entities. These have included the Federal General Services Administration (GSA), the Office of Statewide Health Planning and Development (OSHPD) and the Division of the State Architect (DSA) of the State of California, two counties, and numerous cities.

Is peer review required?

Peer review is not required as part of the design and approval process for BRBFs. Peer review was common for the early US projects, given the lack of experience with UBBs, and the lack of building code provisions at the time. A major development since the initial projects has been the completion of the SEAOC/AISC Recommended Provisions for BRBFs,and the inclusion of buckling-restrained braces in the 2005 AISC Seismic Provisions for Structural Steel Buildings.

What is Nippon Steel Engineering Co.'s patent position for the UBB?

Nippon Steel Engineering Co. currently has a US patent for the Unbonded Brace (No. 6,826,874).

What is the maximum deformation possible for a UBB?

The maximum deformation for a UBB is a function of the length of the brace which is subjected to yielding. The deformation limits are a function of both the maximum deformation and the cumulative plastic demand (number of cycles of yielding) to which the brace is subjected. Design ranges of plastic strain are about 1.5-2.0% for a DBE-type seismic demand, and 2.25-3.0% for a MCE-type demand. For brace geometries in buildings with typical floor-to-floor and column spacing dimensions, these strain ranges correspond to story drifts of about 1.5% for the DBE and 2.5-3.5% for the MCE condition.

Have UBBs been used in hospitals?

Yes. The Unbonded Braces has been used for 7 hospitals in the US.

Are UBBs suitable for use in retrofit?

Yes. UBBs have been used for dozens of retrofits in the US, of both steel and reinforced-concrete buildings. UBBs are particularly attractive structural elements for seismic retrofit because of their reliable hysteretic behavior, which results in clearly definable forces from the braces for capacity design of existing critical portions of the structure.

What is the Core Plate of a UBB?

The "core plate" of an Unbonded Brace is that portion of the brace which resists axial force. The core plate is placed inside the confining tube, and extends beyond the length of the tube to facilitate connection at the ends. The core plate is cut from a single piece of steel and does not contain any welds; connection to the ends is typically achieved by bolting.

What type of steel is used for the UBB core plate?

Nippon Steel Corporation uses several different grades of steel for the core plate, depending on the designer's objectives for brace yield force and stiffness. These steels all have very good toughness and elongation properties. The steels most commonly used for US projects are ASTM A36 steel and Japanese Industrial Standard SN400B (yield stress approximately similar to A36, but with lower and upper limits for yield and tensile stress).

Have UBBs been used in schools and colleges?

Yes. In California, schools (K-12) and community college design and construction falls under the jurisdiction of the Division of the State Architect (DSA). Unbonded Braces are being used for multiple buildings as part of the campus replacement project at San Bernardino Valley College. These are the first projects using buckling-restrained braces to receive DSA approval.

Are there any recommended provisions for the use of UBB?

Yes. From late 1999 to 2001 an AISC and SEAOC joint task group developed a document called Recommended Provisions for Buckling-Restrained Braced Frames. The Recommended Provisions were subsequently updated in July 2003. Since this development, buckling-restrained braces have been included in Section 8.6 of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 450), and in Section 16 of the 2005 AISC Seicmic Provisions for Structural Steel Buildings. These documents provide guidelines for the design of buckling-restrained brace elements, connections, and makes recommendations for brace testing, when it is required.