Proposed building section illustrating new timber levels
Proposed interior with interconnecting stair
DPR Construction to conduct an extensive study of the material, re- viewing case studies and reaching out to local code officials to start a dialogue about the material’s use in DC. After a thorough analysis of cost and the coordination between the structure and building system’s infrastructure that proved feasibility for commercial development, the team set out to put their knowledge to the test. The Opportunity Columbia Property Trust felt it could create incremental value for its investors by adding floors to 80 M Street SE, an existing DC of- fice building with unused floor area ratio (FAR) square footage. In partnership with Davis Construction and Arup, Hickok Cole pitched a three-story CLT addition reasoning that the incremental increase in construction costs could be offset by a higher per-square-foot lease rate. The concept design – which features an all wood interior visible from the street through a glass curtain wall – differentiated the space so effectively within the competitive DC market that a tenant expressed interest before design development even began. The Contextual Challenges In Washington, DC, adding floors to an existing building is common, especially when the zoning regulations translate to underutilized leas- able area. One of the key issues is the capacity of the existing building to accommodate the additional weight. In the case of 80 M Street, one of the advantages of the CLT structure was its light weight which mini- mized existing column reinforcement and resulted in no foundation reinforcement. Another consideration was the code regulation which allows for mass timber structures to reach 85’-0” feet in height. The 2021 code will allow mass timber structures to reach 270’-0” feet, but will require additional fire protection measures to reach these heights, which has prompted some code officials to accommodate CLT through code modifications. In the case of 80 M Street, the design team sought to build wood over 85’-0” in height as the proposed addition would add a 3 story Type IV wood structure on top a seven story Type 1A concrete building. One of the aesthetic goals for these floors was to showcase the wood by leaving it exposed which was made possible by the build- ing having access to all four sides for fighting access and additional fire safety measures including increased fire resistance ratings, an
improved fire water supply, addition of a redundant fire pump and an upgraded sprinkler system, to mitigate most of the concerns regarding fire suppression. The Design The 3 additional stories of CLT structure are comprised of CLT slabs supported on Glulam columns and beams. The column grid in the addi- tion matches the existing column grid of 30’-0”x30’-0” below. Because timber is lightweight, larger grids can result in vibration performance issues. To improve the floor vibration performance of the timber struc- ture, stiffer, deeper beams were used, which were accommodated by additional floor height. This design challenge led to taller floor to floor heights that provide unparalleled open space in the DC market. The additional height also allowed the design team to wrap the building in floor to ceiling glazing, enhancing its biophilic nature and provide increased access to natural light for its prospective tenants. Construction of the mass timber vertical expansion at 80 M Street is expected began in 2020. HOLLY LENNIHAN, RA, LEED AP is a Director of Sustainable Design and Senior Associate at Hickok Cole with over two decades of industry experience working on a variety of project types including, historic preservation and adaptive reuse, corporate interiors, multifamily housing, and master planning studies. Currently, Holly spearheads Hickok Cole’s High Performance Building practice and is the project manager for the American Geophysical Union’s headquarters renova- tion to net zero energy. Under Holly’s leadership, Hickok Cole has won a series of state and federal grants focused on sustainability, resilience and urban ecological systems. Her team was awarded a United States Forest Service Wood Innovation Grant to design the Kingman Island Ranger Station out of cross laminated timber, resulting in a more comprehensive understanding of how to incorporate mass timber into the firm’s holistic design offerings. THOMAS CORRADO, LEED AP is a Senior Associate and architect at Hickok Cole, where he works for the firm’s Commercial Architecture practice. His expertise in urban studies, place making, digital fabrication, and sustainable design has been applied to projects ranging from large scale commercial interiors to build to suit. Of late, Tom has been an integral member of the design team for 80 M Street, Washington, DC’s first mass timber commercial office renovation. His noteworthy projects include the Center for Strategic and International Studies headquarters and Anthem Row.
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april 2020
csengineermag.com
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