Project Spotlight: Sugar Bowl ResidenceApr 08 2014 · 0 comments · NISHKIAN MONKS, Residential, Seismic ·0
Nishkian Monks was proud to be the structural engineer on a unique and challenging ski-in/out chalet, located in Sugar Bowl Resort in Tahoe. The three-story, 5,600 sq.ft. residence chalet was designed by San Francisco-based architectural firm Baldauf Catton Von Eckartsberg / BCV Architects. The general contractor for the project was Mt. Lincoln Construction, from Truckee, California. The chalet is located in the Sierra Nevada mountain range where maximum expected design snow depth is 16ft – equating to 380 pounds per sq.ft. of snow weight! Additionally the site is located in a region of high seismicity. The chalet is constructed of a combination of reinforced concrete, steel frames, conventional light wood framing, and heavy timber construction. 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.
Nishkian Monks 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. Intensive collaboration, careful considerations for constructability of a complex structure in an adverse weather climate, and engineering ingenuity resulted in a successful project. The chalet is stunning architecture with an explicit appreciation of the structural system, and will inspire others and spark interest in structures for future generations to come. The chalet is constructed of a medley of concrete, steel, 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 Nishkian Monks 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. Nishkian Monks was 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 challenges for Nishkian Monks. 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 overcome by Nishkian Monks in designing an architecturally expressed structure with some of the largest snow loads in the United States, will hopefully inspire other structural engineers, architects, and the public; to realize that a structure in high snow regions can be unique, visually appealing, functional, and create interest in how forms are achieved.