The Crosby Group has been a leader in the field of evaluating the seismic performance of new and existing structures through the use of dynamic and non-linear analysis, involvement with existing building code development, as well as scale testing of structural elements. Having participated in numerous earthquake site investigations, including following earthquakes in Kobe, Japan; Guam; Northridge, California; Loma Prieta, California; and Landers, California, we bring this knowledge of seismic building performance to determining the ‘real’ capacity of a building to withstand strong ground motions.
Structural Modeling Capabilities
Since its inception, Crosby Group has maintained a strong portfolio of structural modeling programs. These programs allow us to capture the dynamic and/or the non-linear response of building structures. More importantly, based on our extensive modeling experience, we feel we have a thorough understanding of the inherent weakness in structural modeling, as improper assumptions can lead to drastic errors in determining a buildings ‘true’ strengths and weaknesses. Our programs include; SAP2000, IDARC, DRAIN 2DX., ETABS, NONLINPRO and others. With these programs, we have modeled buildings with base isolation, passive dampers, non-linear elements and degrading material strengths.
Building Code Developments
Building codes can be used to guide an engineer to provide a structure that meets a minimum life safety standard. Whether the codes were developed by NEHRP, ATC, UBC, FEMA or the like, the objective is the same. However dangerous designs can arise from the misuse of the ‘intent’ of the code. To fully appreciate the effectiveness of a building code, an engineer should fully understand the reasoning behind it. The Crosby Group has been actively involved in the development of portions of these codes, having participated in Chapter Nine of FEMA 273/274 and the Passive Damping Committee of SEAOC. Recently the codes (ex: FEMA 273/274) have introduced a concept long due in the field of structural engineering; performance based design. With this as a guide, one can more effectively design a structural system to conform to the particular needs of an owner. The Crosby Group has pioneered the use of these codes before they were formally adopted, when we used them to guide us in the seismic retrofit design of buildings utilizing passive dampers and non-linear elements. We have performed evaluations and designs for seismic retrofit projects using these codes; FEMA 273/274, ATC-40, UBC/CBC, HBC, UCBC.
Coupled with the tools stated above, The Crosby Group has performed numerous tests on new and existing elements to determine their likely performance in the event of an earthquake. These include:
- press-welded steel boundary element for concrete
- visco-elastic and viscous passive dampers at UC Berkeley Richmond Field Station
- glass fiber composites on CMU for out-of-plane loading and shear transfer at SUNY Buffalo
- glass fiber composites on quarried sandstone blocks for shear transfer at UC San Diego
- buckling restrained braces (yielding braces) at UC Berkeley Mare Island Field Station
- special moment resisting frame bolted connections at UC San Diego
- wind tunnel testing
Innovators in New Technologies
The Crosby Group has gained a reputation for providing innovative solutions to difficult seismic upgrade projects utilizing new technologies. From being the first firm in the United States to incorporate a passive damping system in a building structure to the development of a new ‘yielding’ brace to upgrade Frank Lloyd Wright’s only Municipal Building, we continue to incorporate new proven technologies. The key to better seismic performance is the ability to dissipate energy induced in the building from strong ground shaking. Traditionally structural engineers have detailed elements in the building to have a controlled failure of that element thus dissipating energy. Recently new technologies have emerged to help the engineer mitigate damage to a building with non-sacrificial elements. The Crosby Group’s experience with these elements includes:
- Passive Damping: Since the first use of passive damping in the United States by The Crosby Group, we have completed five projects utilizing these energy dissipating devices. FEMA 273 was used as the governing code.
- Glass/Carbon Composites: As one of the early proponents of strengthening through the use of composites The Crosby Group has completed four projects and is currently in the design phase for three other seismic retrofits which utilize the confining and tensile strength enhancements of composites.
- Base Isolation: The Crosby Group designed the new base isolated Public Safety Building for the City of Berkeley. This structure, designed to remain operational following a seismic event, is the first new concrete structure to be isolated in the United States.
- Yielding Brace: As ‘yielding’ braces had not yet been written into Building Codes, the Crosby Group provided full scale testing of the ‘yielding’ braces utilized in the seismic retrofit of the Frank Lloyd Wright-designed Marin County Hall of Justice. The intent of developing this brace is to take advantage of the cost effectiveness of a traditional chevron concentric brace while capturing the large energy dissipating characteristics of an eccentrically braced frame.