KTLA Channel 5 recently relocated to Stage 6 at Sunset Bronson Studios in Hollywood, Calif. Sunset Bronson Studios is listed as a Los Angeles Historic-Cultural Landmark. KTLA had broadcasted from Stages 7/8 since 1958 when the studio lot was called Warner Bros. Studios.
To house the news channel, Stage 6 went through a major upgrade designed by Bastien and Associates and Structural Focus. This $12 million tenant improvement project included renovation of Stage 6 and a portion of Building 16. The 75-year-old buildings presented many unique challenges for the design team. KTLA’s new 15,000-square-foot facility includes the station’s main news set, a secondary area for production, news center, control room, support facilities, and corporate offices.
Stage 6 is a 42-foot-tall, one-story wood building originally constructed in 1928. This building is a steel-frame structure approximately three stories in height and rectangular in plan with a flattened gambrel roof. Exterior walls are clad in stucco. The gravity system of the building consists of wood joists spanning to steel trusses. The steel trusses are supported by steel columns. The ground floor of the building is supported on interior wood posts and beams and cripple stud walls at the perimeter. The lateral system of the building consists of a wood roof diaphragm spanning to exterior wood walls. Original steel X-braces between steel columns provide resistance of lateral loads in the north-south direction. On the north and south elevations, there is a wood-framed lean-to structure of one bay in width.
The structural scope for the improvements of Stage 6 included two new “box-in-box” studios, a voluntary seismic upgrade, and removal of an existing mezzanine at the south end of the building.
The biggest project challenge was construction of two new box-in-box live broadcast studios within the existing building. These provide acoustical separation from other spaces within the building and from the exterior. Working closely with the City of Los Angeles, the team developed complex yet efficient and innovative solutions to minimize demolition of the floor and extensive foundation excavations during construction of the new studios.
The original design concept introduced new reinforced concrete stem walls with strip footings under the raised ground floor below the new studio bearing/shear walls. This scheme resulted in several high-cost construction items, including removal of existing raised floor framing below all the perimeter walls of the new studios, deep soil excavation due to the depth of native soil at the site, and undermining adjacent existing pier footings.
To limit the impact on the existing structure and to lower the construction cost, the studios were designed and constructed using cold-formed steel ceiling and shear wall framing, supported by steel beams and columns. The wall framing, serving as shear walls, was vertically supported from the ceiling steel beams using continuous light gage steel saddle track to avoid adding gravity loads on the existing raised ground floor framing. The steel columns were typically located at the corners of the studios (ends of the shear walls) and were supported on new concrete pile foundations.
The lateral force-resisting system consisted of plywood-sheathed diaphragm spanning between the plywood-sheathed “inverted” shear walls on four sides of each studio. The inverted shear walls were designed with steel collector beams at the top, transferring the diaphragm shear loads to the inverted plywood-sheathed shear wall. The overturning forces were resisted by the steel columns located strategically at the ends of the inverted shear walls. At the base of the inverted shear wall, the shear load was transferred from the wall panels to the slip track at the base of the wall. The shear load was then transferred from the slip track to a continuous steel collector plate that connected to the top of the pile foundations at each end of the shear wall.
The shear capacity of normal cast-in-place anchor bolts at the top of the piles would have been insufficient due to the minimal edge distance and constructability considerations. Also, in view of the limitations on removal of the existing floor framing, the design team was required to come up with another innovative solution for connection of the inverted shear wall end columns to the top of the pile foundations.
At each column/pile location, a smaller steel HSS tube (connector tube) with headed studs welded on all four faces was cast at the top of the pile and extended 12 inches above the top of the pile. The larger steel column with a steel baseplate, that had a hole slightly larger than the connector tube cut in the center of it, slid over the connector tube.
The cavity in between the column and the connector tube was filled with a fluid non-shrink grout and two through bolts were installed to connect the steel column to the connector tube. The through bolts transferred the vertical loads from the steel column to the connector tube. The collector shear plates were welded to the steel column base plate and the shear load was transferred by the direct bearing of the base plate against the concrete to the connector tube and the pile foundation.
As a historic site, the 2010 California Historical Building Code (CHBC) was used as the basis for the design of the voluntary seismic strengthening in Stage 6. The voluntary seismic upgrade introduced new shear walls at the north and south ends of the building. The new shear walls were positioned on the same line as the high roof-to-low roof transition at both ends of the building to allow direct force transfer from each roof level to the new main building shear wall.
At the south end, new plywood sheathing was installed over the existing wood-framed wall, and at the north end, a new wood-framed shear wall was constructed. Similar to the foundation system at the new studios, new pile foundations were added at the ends of each wall and were tied together with a concrete pile cap at the raised floor elevation. For the connection of wall end posts to the pile cap at the south end of the building, the existing wood posts were steel jacketed; threaded rods were bolted through the steel-jacketed wood post and extended on either side of it and were cast in the concrete pile cap. The shear at the base of the walls was transferred to collector steel angles installed on both sides of the walls. The steel collector angles were anchored into the pile cap at each end of the wall and served as tie beams between the concrete piles as well.
The renovation of Stage 6 also included removal of the existing mezzanine at the south end of the building. The existing wood columns that supported the roof structure were cut off at the mezzanine level in a previous renovation, therefore, they needed to be extended and strengthened for both continuity and ability to support the same load with a longer unbraced length. The columns were extended using similar-sized wood posts and were strengthened by the addition of continuous full-height steel channel members on both sides of the extended wood columns. The new wood columns landed on and were tied to the original column foundations.
KTLA, one of the largest independent television stations in Los Angeles, has called Sunset Bronson Studios home for more than 50 years. The newly renovated broadcasting facility and office space ensure that this successful partnership will continue. The renovation of Stage 6 not only provides a high-performance state-of-the-art facility, but also protects the important history of this Los Angeles landmark.
Melineh Zomorrodian, S.E., is a project engineer at Structural Focus (www.structuralfocus.com). She has worked on seismic upgrades and tenant improvements including the Wilshire Boulevard Temple, Red Bull North American Headquarters, and KTLA Channel 5.
David W. Cocke, S.E., F.SEI, F.ASCE, is the founder and president of Structural Focus. He is an alternate member of the California Historical Building Safety Board and sits on the LA Earthquake Technical Task Force; he is also the current vice president of the Earthquake Engineering Research Institute and is the president-elect of the Structural Engineering Institute of ASCE.