Rockin’ and Rollin’
Considerations when designing a building to withstand a seismic event.
August 1, 2014 by Rod Yeoh with Fadi Ghorayeb
As a mechanical engineer designing building systems, there are many different factors that have to be taken into account. One that is often not thought of by the general public is that in seismic zones, all mechanical systems must be designed to withstand seismic events. Mechanical components must be seismically restrained so that they do not become a hazard to the building occupants during a seismic event. This will be discussed in more detail later in the article, but I would like to start by discussing seismic design of buildings in general.
There are many things to consider when designing a building to resist seismic events. What forces do you design for? What level of damage is acceptable? How do you design a building to withstand these forces?
Buildings built to current code are designed for an earthquake return probability of two per cent in 50 years. This is equivalent to a probability of once every 2500 years. The design forces that this results in varies depending on location, and is prescribed in building codes. It should be noted that the restraint of non-structural components (largely mechanical and electrical equipment) is also designed to withstand these forces. These seismic design requirements are fairly new. Buildings built to previous codes were designed to withstand lower forces, or if they are more than 30-40 years old, they may not be able to withstand any seismic forces at all.
Generally, buildings are not designed to remain undamaged during a seismic event. They are expected to sustain some damage, but they must not collapse, the occupants must be kept safe and paths of egress must be kept clear. This is where the seismic restraint of non-structural components is essential. Many injuries and much of the damage that occurs during an earthquake is caused by falling pipe, duct, ceilings, or equipment, and fires following the event. These fires are usually caused by broken gas pipes and sparks from damaged electrical equipment. It is important that the “stuff” inside the building does not shake loose and cause damage, injuries, block the paths to exits, or worst case, set the building on fire. It does not matter how well the structure will stand up if the building catches fire.
There are some buildings that are designed not only to survive an earthquake, but also to remain functional after the event. These post disaster buildings are typically highly important, such as police stations, hospitals, and emergency services facilities; however, more and more different types of clients are now asking for their buildings to be designed to post disaster standards. Clients with data centres, head offices or other business critical facilities, now recognize that the additional cost to design and construct a building to post disaster requirements can be much lower than the cost of interruption to business operations after an event. This thinking also applies to buildings where the contents may have a very high value, such as art galleries, museums or specialized laboratories where the equipment may be very expensive. The extra cost of a post disaster building may be much less than the cost of damage to the building’s contents.
Another factor to consider is that structures are designed with seismic “fuses.” These structural fuses attract and dissipate energy created by seismic forces. They are designed to yield (but not break), to dissipate the energy and protect the rest of the structure. This is important to note in the design of non-structural seismic restraint because if non-structural components are attached to these fuse elements, this yielding must be taken into account.
The primary goal of seismic restraint of mechanical systems is to reduce damage and injuries caused by falling equipment and components. Typically, other than in post disaster buildings, there is no extra effort made to ensure that the equipment or systems will remain operational. However, even in buildings that are not designed to post disaster
standards, there are some systems that should be designed to remain operational as well. These include life safety systems such as fire sprinkler and standpipe systems, and emergency smoke exhaust and pressurization systems. Fires following the event cause much of the damage done during earthquakes. It is important to keep the fire and smoke control systems operational. Of course, every effort should be made to prevent the fire from starting in the first place, so it is important that all building gas services have seismic shut-off valves installed. Just as you would expect from the name, these valves are designed to shut-off the gas service to the building in the event of a seismic event.
In post-disaster facilities, it is also important to identify which building services (in addition to the life safety systems) are required to remain operational. Other than seismic restraint, it may be necessary to provide redundancy and back-up for these systems as well. Examples of this include: secondary water supplies for both fire fighting and plumbing operation if required; back-up fuel sources for emergency generators; back-up cooling systems for high importance data centres; and secondary electrical services.
There are numerous areas that are subject to seismic events within Canada and throughout the world. In these areas, it is important to ensure that the buildings we design and construct are done so to withstand the forces that these seismic events can generate. <>Rod Yeoh, P.Eng., P.E., is a LEED accredited professional and a principal, mechanical engineering, with DIALOG in Vancouver, BC. As a sought after thought-leader, Yeoh has recently presented on sustainable mechanical systems integration to the Building Owners and Managers Association, BC Hydro, Terasen, Light House Sustainable Building Centre, ASHRAE, APEGBC and at Buildex Vancouver. Fadi Ghorayeb is structural engineering principal at DIALOG’s Vancouver studio. He has extensive experience designing structures in various seismic zones, and in the seismic upgrading of existing buildings.