Proposal
Executive Summary
The Wren D.C. is a mixed use 592,000 SF, 110’ high, ten story building situated in Northwest Washington D.C. The program of the Wren includes three levels of underground parking; a Whole Foods Grocery store to go on the first floor; and mezzanine level that take up most of the site’s foot print. The remaining eight stories will be composed of residential units totaling 428 apartments. Amenities for the residents are distributed between the second floor, third floor, and roof. The roof has outdoor amenities such as a dog park, pool, garden, and a 107’ long bridge connecting two sides of the building. With a score of 59, the Wren meets LEED Silver requirements to reduce the environmental impact of the building as well as improve how the building interacts with the community and public transportation.
The structural system of the Wren is primarily reinforced concrete, with post tensioning in certain areas. Two-way flat plate concrete slabs are utilized on all levels of the structure to carry gravity loads to the columns and act as a diaphragm to carry the lateral loads to the shear walls. The slabs for the residential levels of the building are 8” thick and post tensioned, increasing floor spans and keeping the floor-to-floor heights lower. From the second floor down into the parking levels the floor slabs are not post tensioned: these levels have thicker slabs and have drop panels at all column and slab connections. Typical columns for the structure are 32”x16” or 32”x14” sections reinforced with either 6 or 12 #9 rebar. The building sits up a spread footing foundation system: in the typical conditions the columns are terminated at a square concentrically loaded footing that reduces the pressure on the soil below 10,000 psf. The lateral force resisting system of the Wren utilizes shear walls, of which there are five that travel through the whole height of the building. There are also two masonry walls on the face of the building next to adjacent building that go from grade to the roof that provide additional lateral stiffness. The concrete used for the structure has compressive strength of either 3000, 4000, or 5000 psi depending on what application it is used for.
Depth Topic
The existing design the Wren implements a cast in place concrete super structure, while concrete can be a versatile and effective material it has draw backs by having a large amount of embodied energy, requires days to cure and requires thousands of square feet of formwork and shoring. An alternative to this construction type is to use cross laminated timber panels for floors and walls to construct the residential levels of the Wren. This mass timber construction type will allow for a considerably faster erection time, minutes per panel, that allows the follow behind trades to move into a floor immediately after the erectors are done instead of waiting days for the concrete to cure. Another advantage of the mass timber option is that structure will sequester carbon keeping tons of C02 out of the atmosphere and storing it indefinitely. In addition to speeding up construction, decreasing the complexity of the construction process, and reducing the man power required to build the structure mass timber provides a sustainable and environmentally beneficial structure.
The mass timber design of the residential levels be adhere to the Oregon Statewide Alternate Method No. 18-01 Tall Wood Buildings code addendum and will consist of load bearing walls and floor panels, the thickness of the walls to meet strength will be 4 1/8" and the typical floor panel thickness will be 8 5/8". Changing the gravity system of the residential levels does affect other aspects of the structure, special considerations will be given to the lateral system and retail transfer slab. The existing lateral system of moment frames will not be able to the integrate with the new design; therefore, for the lateral force resisting system will be comprised of shear walls made of concrete or possibly CLT. The subgrade levels and the retail level will remain concrete, to transfer load to the existing column grid for the retail level a post tensioned concrete transfer slab will be designed to transfer the forces from the load bearing walls to the columns. The preliminary thickness of this slab will be 20".
Washington DC has very strict height restrictions for buildings because of this developers try to minimize floor to floor heights to get another level of leasable space on a project. This drive for shallow structural systems has lead to the prevalence of flat plate post tensioned concrete structures in Washington DC. The proposed system of CLT floor panels bearing on CLT walls can maintain a similar structural depth at 10 5/8" to the 8" flat plate post tensioned concrete. Also the CLT floor panels can span far enough to allow the existing exterior and partition walls to be used as bearing walls meaning that the current floor plans will only need minimal modification to accommodate the mass timber system.
Breadth Topic
Breadth topics that would address problems potentially raised by changing the structural system to utilize mass timber would be acoustics and fire protection. The acoustical performance between dwelling units can be changed based on concrete topping, CLT thickness, and the ceiling assembly; these factors will be manipulated to meet and/or exceed code required sound isolation. With the structure consisting of a combustible material fire protection must be revised and improved to ensure resident safety. The wall assemblies, egress paths, and impact of adding a sprinkler system to all the residential units will be analyzed and redesigned if required to meet code.
Structural Existing Conditions Report and Proposal