Figure 8: The finished lobby space without the existing Column #60.
Figure 1: Column #60 (center) was removed between the first and the second floor.
Figure 2: The transfer girder was designed as back-to-back built-up channels. The individual channel members were delivered into the building through a window on Level 2.475 Fifth Avenue is an existing building near Bryant Park at the intersection of 41st Street and Fifth Avenue in New York City. The 24-story, 1920s-era building had a major renovation that required structural engineering to support the building’s architectural and mechanical changes. The structural scope for the project included strengthening the flooring system of the second floor for retail loading, framing a new mechanical penthouse, framing new floor and wall openings to accommodate MEP ductwork and louvers, framing a new canopy, framing a new stair crossover, and closing abandoned shafts and openings. One challenging aspect of the project was to remove an existing column between Level 1 and Level 2 that was in the middle of the lobby (Column #60 in Figure 1). Its removal is the primary focus of this article.
Figure 3: The transfer girder channel members were connected using stiffener plates and cross bracing at the top and bottom.
Figure 4: Details of the transfer girder and preloading approach to column removal.
Figure 5: Finite element analyses were performed to check stresses and susceptibility to buckling of the transfer girder and supporting columns.
Figure 6: Conventional and preloading approaches to removing existing columns.
Figure 7: Two pairs of brackets were welded onto Column #60 — a pair of bearing brackets to transfer the load from the column onto the transfer girder and a pair of brackets used for jacking.The building’s structural system is a steel moment frame. The gravity system consists of cinder concrete topping slab over a wire meshreinforced concrete slab supported by steel beams spanning between girders. The girders are moment connected to the columns in both directions to resist lateral loads. All existing shear and moment connections are rivet connected, as is typical for these buildings. The original structural drawings for the building were unavailable so all information necessary for design had to be obtained in the field.
Since no structural drawings existed, DeSimone Consulting Engineers carried out exhaustive exploratory work, including structural probes, walkthroughs, and field measurements. Structural steel coupon tests revealed the steel to be weldable with a yield strength close to that of ASTM A36 steel. Concrete density tests determined the weight of the existing floor system was 110 pounds per cubic foot.
Transfer system analysis and design
Figure 1 shows the portion of the frame where the column is to be removed between the first and the second floor (labeled Column #60). The bay spacing between columns was approximately 18 feet. A transfer girder was designed to span 36 feet to support the weight of the 23 floors above Column #60 and transfer it over to the adjacent supporting columns (numbered #52 and #68).
Based on the exploratory work, service gravity loading carried by each of the columns was estimated. To maintain the tall floor-to-floor height at the ground floor lobby, the transfer girder was designed to be supported at Level 2. For ease of delivery and rigging, the transfer girder was designed as back-to-back built-up channels. The individual channel members were delivered into the building through a window on Level 2 (Figure 2). They were connected using stiffener plates and cross bracing at the top and bottom (Figures 3 and 4).
The supporting columns were cover plated to increase their capacity to support the additional load. Bearing and supporting brackets were designed to be welded directly to the columns to transfer the load to the girder. Existing footings (concrete piers) for Columns #52 and #68 were found to have enough reserve capacity to support the additional load increase; however, lean concrete was poured around the top of the piers to increase confinement of the existing concrete at the point of load transfer. Finite element analyses, including Eigen value buckling analysis, were performed to check stresses and susceptibility to buckling of transfer girder and supporting columns (Figure 5).
Preloading approach to column removal
In a conventional column removal, a transfer girder is installed and connected to the supporting columns. Then the column to be removed is attached to the transfer girder and cut below the girder. As illustrated in Figure 6, the transfer girder is loaded instantaneously from an undeformed state. This causes the girder and its supported levels to undergo deflection instantaneously, leading to potential serviceability issues such as cracks in the concrete floor, finishes, partitions, etc. To counteract potential problems with the conventional approach, DeSimone employed an innovative preloading method to transfer the column load. The transfer girder was preloaded using hydraulic jacks to a load value equal to the service dead load supported by the column to-be-removed (Column #60). This imposes an initial dead load deflection on the girder.
Details of this approach are shown in Figure 4. First, two pairs of brackets are welded onto Column #60 — a pair of bearing brackets that will transfer the load from the column onto the transfer girder and a pair of brackets used for jacking (Figure 7). A small gap was left directly below the bearing brackets to ensure that all the load during the preloading operation goes to the jacking brackets and bearing brackets are not engaged.
The existing column splice for Column #60 between Levels 1 and 2 was removed and strain gages were installed at the splice to monitor the vertical movement of the column during the jacking operation. The transfer girder was gradually preloaded to the service dead load value or until vertical movement at the splice was detected by the strain gages.
The hydraulic jacks were then locked and shim plates were introduced to close the gap between the bearing brackets and the transfer girder. Once the shim plates were in place, the hydraulic jacks were removed, thereby disengaging the jacking brackets and transferring the load to bearing brackets. This finally allowed for Column #60 to be cut between Level 1 and 2.
This preloading approach to column removal has several advantages over the conventional approach. The primary advantage is that only the transfer girder undergoes deflection and the supported floors are unaffected, causing no serviceability issues on the supported levels. DeSimone successfully removed the column from the ground floor of an existing 24-story building using this innovative preloading approach, which minimized serviceability problems that are common in such projects. The finished lobby space without the existing Column #60 is shown in Figure 8.
PRATIK SHAH, P.E., is a project manager in DeSimone Consulting Engineers’ (www.de-simone.com) New York City office. He has more than nine years of experience in various aspects of structural engineering, with a focus on high-rise buildings, structural analysis and design, value engineering, and peer reviews.