RenewWrap® FRP System: Seismic Retrofit
Northgate Terrace, an 11-story concrete structure in Oakland, CA, built in 1970, now functions as a senior retirement and assisted living facility. To comply with current seismic codes, a seismic upgrade was required to increase the shear, flexural and tensile capacity of some of the concrete shear walls, beams and interior columns.
This facility was the first—and possibly the only—building to be constructed using a patented precast concrete construction technique called Foldcrete. The Foldcrete process is an on-site, pre-casting structural technique that allows the flat casting of the reinforced concrete elements that are lifted in place in a building block manner design to reduce the construction schedule.
The retrofit had to be completed while the building was fully occupied, so a minimally invasive upgrade technique was chosen for the project that could be completed in a relatively short timeframe. Residents on the floor to be retrofit were relocated for a two week period during construction. The repairs included the seismic upgrades as well as a complete interior renovation of each room.
The RenewWrap fiber-reinforced polymer (FRP) strengthening system is designed to strengthen and enhance the performance as well as restore the strength of deteriorated and compromised structures. It is easier and faster to install than traditional retrofit techniques, limiting disruption and downtime, so these characteristics made the RenewWrap system a prime choice for the structural upgrades to Northgate Terrace. The RenewWrap system was used to address the following conditions:
- Strengthen the tensile connection between the exterior concrete wall panels separating the stories of the building
- Increase the vertical tensile force transfer between exterior and interior corridor concrete wall elements
- Increase in-plane shear strength of concrete wall panel
- Providing collector and drag capacities of the wall panels
- Enhance vertical shear transfer between the wall panels
- Beam and column shear strengthening
This was a challenging project regarding not only scheduling but also logistics, as a lot of the work involved FRP connector and anchorage ties between stories and adjacent rooms. The lightweight properties of the RenewWrap FRP system, however, made it easy to transport from one story of the building to the next. The type of upgrade and limited width of many of the concrete elements required the strengthening to be done predominantly with one to three plies of the RenewWrap ESR CF600 system ranging from 6” to 12” in width.
Milliken Infrastructure’s proprietary EZ-Slit™ system enabled the contractor to easily and quickly slit fabrics to the desired width with minimal waste. The elimination of fraying fabric edges allowed for a convenient utilization of a saturator, which can be challenging with traditional fabrics. In addition to its standard 2 x 12,” 3” x 8” and 4” x 6” EZ-Slit configurations, Milliken Infrastructure supplied custom rolls configured with 6”/8”/10” EZ Slit ply widths.
The RenewWrap FRP system contributed to a rapid retrofit that did not increase the overall dimensions of the upgraded concrete elements. The use of this product with its minimal aesthetic impact only required a simple application of finishes to match the new room renovations.