Consulting-Specifying Engineer - December 2007 - (Page 42) Ability to influence cost Ability to influence cost decisions Construction Programming Schematic design Construction documents Design development Value engineered design development Occupancy Redoing: • Load calculations The cost of change Redoing: • Load calculations • Design drawings Redoing: • Load calculations • Design drawings • Demolition • Purchase of new equipment Redoing: • Load calculations • Design drawings • Demolition • Purchase of new equipment • Business disruption $500 Design Time $5,000 Construction documents $50,000 $500,000 Construction Post-occupancy Figure 2 Once construction has started, the ability to influence various decisions in the building’s engineering nearly has plateaued. Choices about a building’s MEP options should be made early in the process. Source: Arup, based on ASCE “Quality in the Constructed Project” Figure 3 Building owners must realize that once their facilities are occupied, it’s more costly to make changes to any of the engineered systems. Source: Rick Lasser/ Arup For this project, what would it have cost the team if the mechanical engineer hadn’t been allowed to ask just this one challenging question? The opportunity cost would have been the annual energy savings of $110,000/ year and the first cost savings on the order of $710,000 (based on a typical installed cost of $1000/ton and $7/cfm for air handlers). Just by challenging one key assumption, we learn about a 15% overage in refrigeration load. Moreover, in the typical design approach, the first thing the mechanical engineer does is to add 10% to his tonnage and his supply air fans just in case, because there is nothing so embarrassing as not having enough cooling. This bumps up the horsepower on almost every motor. Then the electrical engineer goes to the electrical code, which requires sizing for the installed loads as opposed to the running loads. Last but not least, just to allow for future expansion, the electrical engineer specifies an infrastructure with 10% more spare capacity. Not only would equipment be oversized and possibly run in an inefficient manner, but the owner also is paying for an excessive amount of equipment capacity that may never actually be needed. The only way to find such inherent assumptions is to have direct communication with the owner and the other engineers on the team. An early-phase decision to verify real densities and to hold them without safety factor through all disciplines could be a way to address this systemic bias in calculation procedures toward over-sizing. Based on his access to ASHRAE, American Society of Plumbing Engineers, and IEEE research data, the MEP engineer should be the first and most knowledgeable person to advise the owner about what assumptions should be used in relevant occupancies. The envelope, please The second critical area in which early communication can be invaluable is in the discussion surrounding building envelope and facades. The design of facades, their configuration, orientation, construction, and transparency are all contractually within the scope of the architectural team. While these elements present the building’s image portrayed to the world, a great deal of analysis by the MEP team is necessary to ensure that the building envelope is consistent with higher-performance buildings as defined by local energy codes. This is particularly true with the recent architectural emphasis on large quantities of glazing. There is a systematic tension in the design process between sunlight and thermal comfort in heavily glazed buildings. We all know this, and yet we spend many cycles performing involved calculations in order to prove it. A more proactive communication approach helps reduce the number of overly involved iterations for everyone. It’s a question of yet another early-phase challenge to the architectural team. Our firm has gathered data to help make the case, based on a 10 ft deep by 10 ft wide perimeter zone for three typical occupancy types and 12 different glazing configurations, assuming a 10 ft high wall exposure. If there is any hope to design a high-performance building with low-energy air conditioning, solar heat gain must be controlled right at the façade through either reduction of glazed area or a reduction of solar heat gain coefficient. With targeted sensitivity analyses on single rooms rotated through multiple facade orientations, it is fairly straightforward to test with the architect a number of shading/glazing configurations, and the MEP engineer becomes an analysis and design partner. Then the team can move into the use of energy modeling as a design tool for a low-energy building. Design convergence With each discipline’s decisions affecting the next moves of all other disciplines, the MEP engineer must not only state the current need, but also spin tales of alternate realities of how this might affect future decision-making. Communication skills are not necessarily the forte of those who excel in engineering, and most engineering curricula do not spend a great deal of time teaching students how to get the most out of a design interaction. But MEP engineers no longer have the luxury of staying in the background of building design. With 36% of all energy in the United States used by commercial and residential buildings, and with HVAC and lighting systems in a typical office using some 68% of the building energy, it is obvious that MEP engineers can have influence over a substantial portion of the nation’s energy use. 42 Consulting-Specifying Engineer • DECEMBER 2007
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