Structural Engineering Strategies Towards Sustainable Design SEAONC Sustainable Design Committee
preclude damage. The damage has a cost associated with it. If the damage is to be repaired, the repair effort uses resources in the form of labor, raw materials, and business resources that are suspended or redirected. If the damage is extensive enough that repair is not pursued, then resources are used to dismantle the building, and the building components are hauled away to landfill.
Performance considerations of a structural system can have different criteria, depending on the function and objectives set out for the building. Good performance can be one where damage to the building shell and contents is minimized if the intent of the design is to have the building as intact and functioning soon after a seismic event. For another type of building and set of objectives, a measure of acceptable performance may be more extensive damage, but the damage is controlled. Evaluating and choosing a structural system for either scenario involves the same process of evaluating and balancing considerations in a probabilistic risk and cost analysis that includes performance metrics (accelerations, drift, damage), performance for different seismic levels, initial first cost, cost of repair or reengagement of the building system.
There is a negative financial impact with damage. If there is damage to the building and its contents, then the owner can lose initial advantages of any sustainable design decisions if there is significant value of earthquake damage. Other losses are in the downtime of the business, as well as in the efforts afterwards to recoup damage, such as creating duplicate functions to cover the losses until the damaged portion is repaired. The recovery efforts have physical and time components: repair and recovery of the physical plant as well as recovery of the interruption of business.
Synergies Synergy refers to the acting together of building parts or systems to the benefit of the building as a whole. The concepts of heat radiation and absorption using thermal mass, light and controls, photo-voltaics (PVs), and green roofs are presented, along with the structural engineer’s participation in the synergistic design process.
In order to avoid these negative cost and time scenarios, structural systems can be selected using a performance based design method, as informed by interaction with site characteristics. Typical structural systems such as braced frames and shear walls result in stiff structures with low drift and high accelerations. Other systems such as moment frames result in more flexible structures with high drifts and low accelerations. Energy dissipating systems such as isolation systems can reduce accelerations, and viscous dampers with moment frames can reduce drifts.
NIGHT TIME HEAT PURGE Structure can work in synergy with a building’s cooling system by providing thermal mass for night heat purge, also referred to as nighttime cooling. In locations such as the west coast and desert regions of the United States, where the climate has daytime to nighttime (diurnal) temperature change of about 30 degrees F, mechanical engineers can design cooling systems that take advantage of radiating cool from the structure.
In the evaluation of different systems, the interaction of the site, building shell, and building contents should be considered. Life cycle cost implications should also be evaluated. The initial cost of implementing a structural system should be considered with respect to the value of the sustainable design components, the savings in operating costs, damage prevention, and the prolonged life of the building. Typically, the initial costs of a total new building consist of a structural system in the cost range of 10% - 20%, with the remaining 90% to 80% consisting of architectural, mechanical, electrical, and plumbing systems. Any slight premium in the small percentage of structural costs of the initial structure can be offset by the amplified benefits in the protection of the non-structural components.
The temperature people sense in a building comes from the temperature of the surrounding air and the heat or cool temperature radiating from surrounding walls, floors, and ceilings. Traditional mechanical cooling systems provide cool (conditioned) air blown down from air supply diffusers in the ceiling, with 100% of the cooling provided by air handling units that consume energy to cool and drive air into the mechanical duct work. Night time heat purging, on the other hand, utilizes the mass of the structure to absorb heat generated by the building occupants and/or by sunlight entering the building during the day that is then exhausted at night,
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