The rapid growth of advanced technology, and its quick and consuming integration into critical business operations, has created an increasing need for facilities designed to protect crucial equipment and business operations. The resolution of this need is the design and construction of so-called 'mission-critical' facilities. As a mission-critical facility must be operable at all times, under all conditions, the established design and engineering requirements vary greatly from more traditional structures.

What is a Mission-Critical Facility?

The purpose of any mission-critical facility is to create a stable, reliable and protective environment to house organizational operations and high-value information and equipment. This means the structure and its internal components must be designed and built to resist potentially damaging forces including nature, utility disruptions, equipment failures, and human error. This also means that initial site selection is crucial in the protection and operation of the facility.

While site selection is important in ensuring a secure facility, the primary method of addressing risk factors is the design and implementation of redundant systems. This includes redundant design of climate, information, electrical, security and fire-protection systems, as well as on-site reserve resources such as power generators and potable water supplies. The ability to conduct concurrent maintenance on non-functioning components without losing facility functionality is another important characteristic of redundant design. Without built-in redundancy, simple breaches such as roof leaks or water entry into floor areas might harm operational systems and cause disruption or termination of critical organizational processes.

Component Redundancy in Mission-Critical Facilities

Often times, the level of redundancy necessary in a facility varies depending on the operations housed within. The Uptime Institute(1) has established a tiered system addressing facility performance and reliability for ‘data centers’, one of the most common types of mission-critical facilities in the private sector:

  • Tier 1 Facility (Basic Site Infrastructure): Demonstrates a 99.67% availability rating and is potentially susceptible to damage due to severe weather, can sustain power outages of less than 10 minutes and has limited redundant components.
  • Tier 2 Facility (Redundant Capacity Components Site Infrastructure): Demonstrates a 99.75% availability rating and has redundant power and climate systems, can sustain a 24-hour power outage, and are designed so as to separate critical data rooms from other areas of the building. Concurrent maintenance on many components is not possible at this tier level.
  • Tier 3 Facility (Concurrently Maintainable Site Infrastructure): Demonstrates a 99.98% availability rating and has at least two utility paths, redundant power and climate systems, an increased fire rating and the ability to conduct concurrent maintenance on equipment without interruption to business processes.
  • Tier 4 Facility (Fault Tolerant Site Infrastructure): Demonstrates a 99.99% availability rating and has high physical security, 24/7 onsite maintenance staff, a minimum 2-hour fire rating, and highly redundant systems throughout.

While the building enclosure might only represent 15% of the total construction costs for a mission-critical facility, the protection it provides to the equipment and operations within the building is vital to maintaining continuous operability. If the building exterior fails and moisture enters critical areas, the ability of redundant interior components to ensure ongoing operability is drastically reduced.

Design of Mission-Critical Building Enclosures

The roof assembly is of great importance as it plays a key role in protecting the building from inclement weather. The concept of redundancy utilized throughout the building applies to the roof as well, with some buildings designed with two roofs, and in some cases, two roof decks. Typically, roof decks on mission-critical facilities consist of concrete decks which are insulated from above. As drainage is of particular concern, slope is created either within the insulation or with the deck assembly, and drainage occurs through scuppers or internal roof drains placed outside of critical areas.

Another key consideration is the maintenance of a constant interior environment to support sensitive equipment requirements. These facilities must be designed to control vapor flow. If the interior space cannot maintain constant temperature and humidity, critical systems may degrade and malfunction.

The design community has recognized that these essential facilities must maintain greater performance than normal buildings so as to remain operational at all times. This need for greater design performance is reflected in current design standards and codes, and is typically achieved by increasing variables within structural calculations such as wind speed, wind exposure and building importance factors.

Though not typically used as a primary design strategy for standard buildings, designing a structure to endure higher wind speeds will increase the performance of the structure in terms of its ability to withstand severe weather events and continue operating.

Likewise, increasing the wind exposure level of the building will increase performance and reliability. Wind exposure levels, categorized as ‘B’, ‘C’, and ‘D’, are determined by ground surface roughness, including natural topography, vegetation and adjacent buildings. Exposure ‘B’ is used for locations with a large number of obstructions, exposure level ‘C’ is used for scattered obstructions and exposure ‘D’ is utilized for areas without obstructions and sites facing open water. If a structure is located in an area with ‘D’ level exposure, it is not possible to design for a higher wind exposure to increase building performance.

In addition to increasing wind speed or exposure, the design criteria can also be modified by introducing an Importance (I) factor in the wind calculation. The Importance factor accounts for “the degree of hazard to human life and damage to property” where a standard value of 1.00 is assigned to all non-essential facilities such as office and residential buildings. When the Importance factor is increased, the ability to withstand damaging external forces is also increased. ASCE 7(2) places the minimum importance factor for essential facilities at I=1.15.

It is important to note that when using ASCE 7 to calculate design pressures, an increase in the design wind speed variable will have a more pronounced effect on the calculations than will an increase in the Importance factor. This result occurs due to the direct proportionality between wind pressure and the Importance factor. A 15% increase in the Importance factor from 1.00 to 1.15 will result in design pressures that are 15% larger whereas a 15% increase in design wind speed will result in wind pressures that are over 30% higher. For example, when calculating design loads for a structure with a mean roof height of 50 feet in an exposure ‘C’ location, a design wind speed of 100 MPH and an Importance factor of 1.00 is utilized resulting in a design wind pressure of 27.9 pounds per square foot (PSF). If the Importance factor is increased 15% to 1.15, a design wind pressure of 32.1 PSF is achieved, exactly 15% greater than the original pressure of 27.9 PSF. However, if the wind speed factor is increased by 15% from 100 MPH to 115 MPH, the result is a design pressure of 36.9 PSF, 32% larger than the original design pressure.

The design qualities applied throughout a facility, along with the operations housed inside, are the key factors in whether or not a facility can be accurately characterized as mission-critical. While there are several degrees of redundancy that can be applied to a facility to increase durability and reliability, a true mission-critical facility will remain operable even during the most severe of circumstances. Architects, engineers, contractors and building managers involved with the development and maintenance of mission-critical facilities should have a solid understanding of these design elements and the role they play in the classification of a facility as ‘mission-critical’.

March 2009
Issue One, Volume Four

This article by Colin Murphy.

Colin Murphy is a founder and managing partner of Trinity | ERD.

Trinity | ERD


"Pushing the Envelope: A Monthly Journal of Issues Concerning Building Design, Science, and Litigation" is a monthly publication of Trinity | ERD. This newsletter is intended as a thoughtful look into the issues of building construction.

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(1) The Uptime Institute:
(2) ASCE 7-05: Minimum Design Loads for Buildings and Other Structures; American Society of Civil Engineers, 2005.