Construction Projects - Seismic Retrofit of Terraces
There are two elevated seating terraces on the North and South side of the venue. The seating terraces are steel-framed grandstand structures designed in 1982. The terrace structures consist of a series of narrow concrete slabs on corrugated metal decking, supported by steel framing. Resistance to lateral loads generated through seismic events are provided by steel columns acting as cantilever poles, placed in drilled shafts embedded in bedrock. The decks are fully exposed to the weather and have suffered significant deterioration caused by leaching of acidic beverages into the unprotected concrete over the years. The Department upon taking control of the facility in 2016, issued a contract to design and construct additional steel reinforcing to support all areas of deteriorated deck with the full understanding that the terraces would require eventual replacement and seismic renovation in the near future.
Over the past 18 months, staff issued contracts for seismic and soil analysis to determine the degree of renovation required for compliance to current building codes. The investigation was primarily focused on determining the seismic behavior of the terraces in their current configuration, and on developing a viable concept for improvement of life safety during a severe earthquake. All available architectural and structural documents were reviewed and were verified by site visits. In order to develop a clear understanding of the site, a geotechnical study was performed by Wood Consulting to determine the depth of bedrock beneath the terraces and the capacity of the fill above the terraces to resist lateral loads. This data was then used by Miyamoto to create a SAP2000 model of the north terrace, which was analyzed for seismic forces based on the criteria of ASCE 41.
This analysis revealed that the terraces, due to their unusual shape and extreme variations in column length, suffer from highly irregular distribution of seismic forces between columns, with nearly all seismic loading resisted by a small number of columns located at one end of the structure. This leads to large torsional movements and potential failure of the most heavily-loaded columns. The structures do not meet the life-safety standards of ASCE 41. A conceptual seismic retrofit solution was developed which consists of three primary elements, as follows: 1. Installation of a small number of Buckling Restrained Braces 2. Introduction of sliding joints at the columns currently resisting the highest seismic load, and 3. Addition of grade beams linking several columns. This combination of elements was found to significantly reduce overall lateral movement, reduce torsional effects, and provide a much more uniform distribution of seismic load between columns. The proposed system will result in much-improved seismic behavior and will bring the structures into conformity with the life-safety criteria of ASCE 41.
Due to the complexities of this work and the likelihood of it impacting at best a partial loss of a concert season, staff was determining how best to schedule this work in the near future. However, with the arrival of the Covid-19 Pandemic and ultimate closure of the Greek Theatre, the time best to complete this work is now. The work will be completed under the supervision and Project Management staff from the Bureau of Engineering. It is envisioned the majority of the construction will be handled by existing RAP on-call contractors. The work requires an expediting schedule for the anticipated reopening for the 2021 season on April 1. It is critical the work proceed expeditiously to prevent any further revenue loss.
The following work pertains to the North and South Terraces (Note: There will be no change in the number of seats):
• Demolition of seats, concrete decking, secondary structural supports, electrical, hvac ductwork, emergency generator, fire sprinklers, concrete walkways, ADA Ramp and wood framed enclosure under north terrace
• New seismic bracing, grade beams and new concrete decks
• Replace approximately 1500 seats
• Replace non-compliant ADA ramp at upper entrance to North Terrace
• Replace electrical infrastructure and HVAC ductwork at underside of both terraces
• Replace emergency generator
• New concrete waterproofing over new concrete decks