A Case Study in Integrated Design: Modeling for High-Performance ...

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(a) (b) Figure 2 (a) Full building section cut through NW and SE curtainwalls. (b) Plan for typical office floor. Site constraints formed a large, rectangular building plate with bays that average 140 ft (42.67 m) by 184 ft (56 m) around a central core. This leaves 63 ft (19.20 m) deep office bays on the northwest and southeast, with narrower bays of 34 ft (10.36 m) depth on the southwest and northeast. Recognizing site height restrictions, the floor-to-floor dimension is 13 ft-9 in. (4.2 m) with a 9 ft-10 in. (3.0 m) floor-to-ceiling height to accommodate deep beams for the long spans (Figures 2a and 2b). The client requested a sustainable building and intends to apply for U.S. Green Building Council LEED TM certification at the Gold level. This goal established, from the beginning, a focus on energy efficiency, renewable energy, and interior environmental quality in the design. A high-performance curtainwall, including daylighting and sun control, was considered to address a number of LEED TM requirements (USGBC 2003). The roof of the building has been designated for photovoltaic arrays. This mid-rise office building offered an opportunity to explore an integrated design process in delivering a sustainable building design (Monroe 2002). The architects focused quickly on the curtainwall as the major design challenge and gathered together a design team that included structural, mechanical, and building performance consultants (the authors). Frequent meetings between team members, working design sessions, and increased communication was the primary management strategy for the integrated process. A critical method for making the integrated design process work on this project was to incorporate a wide range of modeling and simulation techniques early in the schematic design phase with followup detailed studies in the design development phase. These are the methods discussed in the sections below. THE CURTAINWALL AND THE DESIGN PROCESS The complexities of designing a high-performance façade range from coordination of disciplines (architectural, structural, mechanical, curtainwall, daylighting, electrical lighting, and space planning) to cost and value considerations in value engineering and specifications. While the design and evaluation of a curtainwall system in relation to single performance criteria (daylight, energy use, cost, lateral loads, etc.) are not necessarily difficult, resolving all performance criteria within the design process and timeframe poses significant challenges. The curtainwall itself performs a highly integrative role in overall building performance. As the primary building envelope in multi-story office buildings, the curtainwall is responsible for controlling the flow of heat, visible light, solar radiation, sound, water, and view between inside and out. It must resist gravity and lateral loads from wind and seismic activity. It must be designed to be washed regularly. In addition, the curtainwall creates the aesthetic image of the building; it determines what the building looks like and how it will fit visually into the site and city context. The design team is usually organized to address only a few of these issues in the early phases of design. In schematic design (SD), the design team typically determines the massing, orientation, plan, and section organization of the building, 2 Buildings IX
initial structural and mechanical systems, and a first pass at material selection. Most aspects of curtainwall performance, including glass specification, are addressed in the design development (DD) phase, when consultants are brought into the project to size and specify components and systems. Integrated Design Process In order to achieve a highly integrated envelope design and associated sustainable building performance, traditional design delivery must be rethought and reorganized. As Mendler and Odell (2000) note, “We should not expect to produce fundamentally new buildings using the same traditional design process.” Those disciplines that bring expertise in the performance issues need to be brought into the design team early in the schematic design phase in order to identify and exploit the opportunities that may arise as initial design decisions are being made. Putting this into practice is more difficult than it would seem. Standard practice is structured to exclude many relevant consultants until later in the design process, to the extent that the standard architectural contracts and documents describe consulting work in the early design phases as “special services” (AIA 1994). The architects need simultaneously to establish a design direction and to accept input from a wide range of disciplines, which is quite difficult. This integration of expertise in the design process can be somewhat cumbersome and unwieldy, with a large number of consultants sitting around the table before a design has enough direction to talk about. As the sustainable design community recognizes the need for an integrated design process, much of the literature suggests the strategy of a “design charette” as an initial means of pulling all these voices together, but it does not offer much additional advice or guidance. Sustainability and building performance consulting is a relatively young, emerging discipline, currently being defined through the efforts of those working in the field. Although there are recently published general guides for incorporating sustainable issues into practice (Mendler and Odell 2000; AEC/CEC 1999), there is at present no generally understood or agreed on set of services or methodologies recognized as standard practice. In many ways, this project offered an ideal set of conditions for developing an integrated design process. Because the building was designed virtually to fill out the available envelope, design issues were quickly focused on the curtainwall and exterior envelope. This is different from early schematic design phases on low-rise or suburban office buildings where the architects have more leeway in the massing and orientation decisions. It was easier to bring advanced modeling techniques to bear on the early design because the design team recognized that many of the performance questions and LEEDTM evaluations would focus on the curtainwall. Because of this, the design team was able to employ energy and daylight modeling to optimize the envelope at the schematic design stage. INITIAL THERMAL MODELING The thermal and energy modeling software DOE2.1e performs hour-by-hour calculations based on a building description and weather data. A powerful technique for modeling, the software is often used to model annual energy use once many of the building design decisions have been made. However, in this project, very early simulation was possible. The overall building envelope was unlikely to change shape or area due to project constraints and the downtown site provided extensive site information unaffected by design changes. The input files could be generated and then changed incrementally as various performance issues were addressed. The schematic design phase DOE2.1e runs used a California Energy Code Title 24 compliant envelope as the base case to help determine the objectives of the curtainwall design. From the beginning of the project, the design team was concerned that the large, canted northwest curtainwall exposed the building to excessive summer afternoon solar loads. A series of sun and shade projections for summer and winter conditions in a three-dimensional modeling program supported this concern. The other orientations received some shading from the surrounding city blocks and did not appear to be as much of a problem as the northwest façade. Parametric runs were used to generate heating and cooling loads that result from the performance of the building envelope (Ternoey et al. 1985). In these, the input model was rich in architectural detail; the size of each floor plate, the size and location of the core, the orientation of each façade, and the floor-to-floor height were known. Additionally, the surrounding site geometries and differentiation of glazed curtainwall and punched openings at the northeast and southeast corner bays were established very early in schematic design. The office floors were defined by nine thermal zones, each determined by a unique orientation and curtainwall conditions (Figure 3). The vertical elevation of each of the ten office floors was also specified and therefore took into account specific conditions of site shading. The hour-by-hour simulations used the San Francisco Typical Meteorological Year (TMY) climate data file and were run without lighting or mechanical systems to isolate the loads. Unexpected Results The heating loads (Figure 4) quantify the annual conditions on a typical floor. The northeast and northwest corners of the building (simulation zones 4 and 2 in Figure 3) are areas with the highest heat loss and the greatest need for heating during winter months. These results were as expected and supported the initial direction of the design team to have some concern for the perimeter zones and early morning building warmup during the cooler winter months. As indicated in Figure 5, the southeast façade (zone 8) has the highest cooling loads. Once identified by the parametric runs, this made sense because the full façade is without significant site shading and receives direct sun during warmer Buildings IX 3
Page 1: A Case Study in Integrated Design:
Page 5 and 6: Figure 5 Typical floor cooling load
Page 7 and 8: Figure 8 Comparative solar loads du
Page 9 and 10: Figure 12 Comparative solar loads o
Page 11 and 12: Figure 15 (a) Sketch section of dou
Page 13 and 14: a dueling spreadsheet to the financ
Page 15 and 16: Figure 20 THERM images of the two c
Page 17 and 18: Figure 22 PPD results for the NE co

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