A buckling-restrained braced-frame system solves seismic upgrade project

The University of Utah Marriott Library had the highest earthquake hazard risk of all the state-owned buildings, if not all buildings, in Utah. The Wasatch Fault, located a mere 200 feet to the east, made it highly probable that such an event could occur. Four thousand students accessing the building daily made the project crucial to complete.

The Federal Emergency Management Administration (FEMA) also recognized this precarious situation and granted $3 million of aid. Approaching the zero hour of the FEMA fund withdrawal date, the Utah state legislators came to the rescue and approved the remaining funds (the majority of the project budget) necessary for the seismic upgrade project.

Planning and design

Reaveley Engineers & Associates, headquartered in Salt Lake City, first performed a study of the library in 2001. The structure had five stories totaling 536,800 square feet. It was built circa early 1960s and utilized the trendy "lift slab" approach. Although the concrete waffle slabs were actually cast in place, the support detailing was identical to the lift-slab technology. Angles welded to built-up steel-plate box columns and concrete shearwalls supported the floor slabs.

Reaveley determined that a 5.0 magnitude earthquake could cause the building to drift; resulting in failure of the angle welds and the floor slabs dropping on top of each other, according to Michael Buehner, P.E., the project engineer. The existing concrete shear walls were also deemed deficient. Based on these discoveries, it was determined that the building required major upgrades, and Reaveley was hired to complete the seismic retrofit design in 2002.

The firm also assisted the library administration in preparing extensive documents for submission to FEMA to obtain support funding for the project. The work was worth the effort and the project received the largest grant from FEMA of any other project in 2004.

Reaveley, with the peer review firm Degenkolb Engineers (headquartered in San Francisco) selected FEMA 356 as the design standard. Two design objectives were chosen. The structure should remain fairly stiff and mostly elastic during moderate earthquakes. Then, under the maximum credible earthquake (2 percent in 50 years recurrence probability), major seismic dissipation would be utilized.

Reaveley and Degenkolb Engineers considered every possible structural framing system available. These were narrowed down to three viable options: interior concrete shearwalls, base isolation, or exterior-braced frames. Since the University of Utah needed to maintain operation of the library during construction, the concrete shear walls (along with cost issues) were ruled out. Base isolation was the most expensive option; therefore it was not desirable with the limited funding. Reaveley had previously designed buildings with buckling-restrained braced frames (BRBF)—including a retrofit project—and was well aware of the high performance the system could deliver within a tight budget. Other braced-frame systems were determined to not have the same ability to dissipate the earthquake forces or the ability to minimize structural damage and were not as cost effective. Eventually, the BRBF system prevailed. It was also decided to jacket the existing columns with concrete wraps to provide redundant floor support.

What is a BRB?

A buckling-restrained brace (BRB) is basically a brace that does not buckle. It is comprised of a load-carrying steel core that passes through an outer casing that only supports the steel core and prevents it from buckling. The steel core and casing behave bifurcatly. Essentially, the steel core yields in compression as it is guided by the outer casing and then stretches in tension with the casing—going along for the ride only, the casing takes no load. It is simply there to control global and local buckling. It is not difficult to design with BRBs. It is just another steel member in the structural frame model. The area of the brace is sized to provide the required capacity at the full-yield stress of the material and to provide the desired stiffness.

The BRBF system is now recognized by current building codes. Provisions for design are found in the AISC/ANSI 341-05 Seismic Provisions for Structural Steel Buildings, ASCE 7-05, and the 2006 IBC. A design guide titled Design of Buckling-Restrained Braced Frames can be downloaded at www.steeltips.org.

BRBs have most likely been tested more than any other structural system. Testing is required per the AISC seismic provisions to prove that the BRBs will perform as desired. Testing has shown that BRBFs have the potential to provide superior seismic performance compared with most structural framing systems.

BRBF acquisition

Once the design was complete and funding approval was imminent, the project was sent to purchasing. Salt Lake City-based Okland Construction, the general contractor, and the project team chose the structural retrofit work via a quality-based selection (QBS) process. Since the project budget was limited, they required all three of the preapproved subcontractors to include value engineering (VE) concepts and cost credits.

After the QBS proposals were reviewed, each subcontractor was interviewed. CoreBrace, a BRB supplier in West Jordan, Utah, in partnership with SME Steel Contractors, offered a VE alternative method for connecting the BRBs and framing. CoreBrace’s connection design was similar to the base bid design. Rather than a single pin connecting the BRBs to the gussets, conventional slip-critical bolts were utilized. SME provided a more than $500,000 credit to utilize CoreBrace’s design.

Several factors contributed to the costs savings. Higher-grade gusset material (GR65) was used to keep the gussets smaller and thinner, reducing the size and weight of the gusset and CJP welds dramatically. This was achieved because bearing typically governs pin design; the gussets need to be thick and even built-up with repads to control bearing demands. The framing was detailed in a manner that allowed most of the welding to be done in the shop. This significantly reduced field-welding costs and sped up erection. Also, the fabrication detailing permitted the beam and two braces to be erected simultaneously with only one pick of the crane.

Cost was not the only criteria in the QBS process. Capability, schedule, quality control, and BRB test results were other factors. CoreBrace had previously tested a series of BRBs with the proposed VE connection and was already in the process of testing two larger CoreBraces similar to the project’s BRB capacities. Since the project BRBs were nearly twice the capacity of those previously tested, the two new CoreBrace tests had to meet the project requirements prior to use. With these considerations, the project team selected CoreBrace as the best value for the project, in spite of some potential risk. CoreBrace’s tests were witnessed by Reaveley, and they exceeded the project requirements.

Fabrication and erection

Fabrication of the CoreBraces and steel was completed ahead of schedule. The frames and braces all fit perfectly to the surveys provided by Okland. Tolerance for erection was provided by oversizing the holes in the gusset and using standard holes in the CoreBraces. With the oversized holes in the inner ply only, the full slip-critical capacity of the bolts could be maintained, minimizing the number of bolts required and keeping the gusset connections compact. Since this was the first time a job was done in this manner, the bottom gussets were shipped loose to provide more tolerance. Subsequently, on most CoreBrace jobs the gussets are completely shop-installed with plenty of erection tolerance and additional cost savings.

High performance, low cost

BRBFs do not rely on the beam deforming in bending or shear, like moment-resisting frames and eccentrically braced frames. Conventional-braced frames rely on the brace and the connection to buckle, which results in a system that loses capacity quickly. These issues can lead to costly repairs. Alternatively, deformation in a BRBF is mostly limited to the yielding steel core, keeping the column, beam, and connection behavior more elastic and minimizing potential damage. Extensive BRB testing has proven the technology to be very reliable and capable of sustaining multiple strong earthquakes.

The over-strength requirements for BRBFs are orders of magnitude less than conventional-braced frames. BRBFs have more favorable seismic design factors and they are less stiff, both of which significantly reduce seismic demands, resulting in smaller connections, columns, and beams. Furthermore, if the frame is detailed to minimize field welding and to permit a mostly bolted-up frame, the erection speed is dramatically increased. Bolting conventional-braced frames is typically not a viable option. Reduced foundation and diaphragm requirements are a result, as well. If V-type frame bay configurations are required, the unbalanced force on the beam is insignificant, avoiding the massive beams required in other braced-frame systems.

Cost studies have indicated that BRBFs are more economical than conventional steel-braced frames and steel moment frames that are designed to AISC Seismic Provisions. Thus, CoreBraces can deliver better performance at less cost.

Andrew Hinchman, P.E., and Mark Daniels, P.E., are responsible for engineering, design, fabrication and erection, and sales for CoreBrace. They can be reached at andyh@corebrace.com and markd@corebrace.com, respectively.