The Crow’s Nest Residence at Sugar Bowl Ski ResortApr 08 2014 · 0 comments · Award-Winning Project, NISHKIAN MONKS, Residential, Seismic ·0
Located in the Crow’s Nest development at Sugar Bowl Ski Resort in Tahoe, California this 5,600-square-foot, three-story, unique ski-in/ski-out chalet is truly a one-of-a-kind custom home. Nishkian Monks PLLC participated in the project as the structural engineer of record, working directly with San Francisco-based architectural firm Baldauf Catton Von Eckartsberg/BCV Architects, and general contractor Mt. Lincoln Construction of Truckee, California. Situated in the Sierra Nevada mountain range where maximum expected design snow depth is 16 feet – equating to 380 pounds per sq.ft. of snow weight, construction of this luxury residential building posed challenges due to the site and program constraints. Additionally, the site is located at one of Sugar Bowl’s highest reaches – higher than many of the resort’s ski lifts, and situated in a region of high seismicity. BCV Architects challenged Nishkian Monks with designing a multi-folded, double sloping plane roof with oversized overhangs out of wood framing that could support the extreme roof snow loads. Through numerous design iterations and collaboration with BCV, Nishkian Monks successfully achieved a structural design for BCV’s striking exposed wood purlin roof. The roof purlins were arranged in such a fashion so as to emanate from the center of the chalet when viewing the house from any side.
Our engineers incorporated a steel frame system into the design of the chalet to be utilized to resist extreme seismic forces and to expedite construction in a region where snow accumulates most of the year. The Crow’s Nest Residence is constructed of a combination of reinforced concrete, steel frames, conventional light wood framing, and heavy timber construction. The main level is set 13-feet above finish grade to provide protection from the annual snow pack, and is constructed of board formed concrete walls and concrete topped metal deck. Integral with the main floor is a large outdoor concrete deck cantilevered 15-feet beyond the lower level walls. The upper level of the chalet is constructed of conventional light wood framing. The roof is uniquely designed with multi-folded, double sloping planes from front to back of the chalet. The roof planes overhang 10.5-feet at the front of the chalet, and 5-feet at the sides and the rear of the chalet. The structural members of the roof were an architectural focal point of the chalet design. BCV Architects designed a roof purlin layout in three radial arrays; one array for each sloping roof plane. BCV challenged the Bozeman firm to engineer a wood roof structure to achieve their goal of spectacular sloping purlin arrays and large overhangs. A difficult roof structure was made even more difficult with a design snow load of 380 pounds per sq.ft. (psf). As a comparison, most roof structures in the State of Montana are designed for a roof snow load of 30psf; less than a tenth of the snow load for the chalet!
Nishkian Monks developed a three-dimensional computer model of the roof framing to help facilitate designing of the wood purlins. Multiple design iterations were completed with the architect to achieve the aesthetically pleasing look and resist the tremendous snow loads. In the end, 6.75-inch wide x 19.5-inch tall, high-strength (specially fabricated) glue laminated wood purlins were utilized to support the roof. The purlins radiated outward from three different origins to form a 4ft-6inch spacing at the overhangs. Our engineers were able to achieve all wood framing for the exposed beams as desired by the architect, except at the valleys. At those locations, steel beams were utilized and wrapped in wood to conform to the rest of the roof.
The remainder of the framing for the roof assembly was a special design of wood rafters in a composite sandwich panel, overlaid with vented cold roof framing. The rafters were a combination of high-strength engineered lumber and 2×8 Fir framing at a spacing of 8-inches and 16-inches depending on span. The sheathing for the cold roof had to be designed for the extreme snow loads as well, and required ¾-inch thick plywood sheathing spanning to 2×4 wood sleepers at 16”o.c.
Special connections for the roof purlins had to be developed and fabricated for this project. On two sides of the roof, 18 roof purlins were originating from a common location. 8 of the purlins were ended before reaching the origin of the array; transferring their loads to the remaining 10 members. The purlins were connected to a steel bracket at the origin of the array. The bracket was designed to minimize appearance and express the wood.
The roof structure was supported with a steel frame. The steel frame allowed for numerous large and small fenestrations around the perimeter of the chalet, and aided in the speed of construction (necessary to avoid significant delays with and costly snow removal). The steel frames also served as part of the lateral force resisting system for the structure. The large snow load coupled with the potential for high seismic ground accelerations resulted in the need for steel moment frames to resist lateral forces.
The exposed cantilevered concrete deck presented additional challenges. The large front roof overhang was determined to not afford shelter for snow accumulation on the deck. The deck was therefore designed for the same 380 psf snow load as was used for the roof. To support the heavy self-weight of the structure and the snow load, steel beams and braces encased in concrete were used. The encased steel frames were tied back to interior concrete shear walls on the lower level to resist overturning forces from the cantilevered deck.
A final design challenge for this project was engineering a foundation to resist the concentrated accumulation of snow loads and self-weight of the structure. Low soil bearing capacities resulted in large, unconventional sized (as compared to similar sized structures) continuous footings.
The challenges Nishkian Monks overcame in designing this architecturally expressed structure with some of the largest snow loads in the United States and steep, rugged terrain was an opportunity that the team relished. Over the years our engineers collaborate diligently with home owners and design teams to ensure the desired aesthetic environment is attained utilizing cost effective creative structural solutions. If you need assistance with your custom home design or facing another construction challenge, contact us to schedule a meeting with one of our team members.
Photo Credit: Bruce Damonte