When a building’s construction permit is issued, its construction type is officially recorded, effectively locking in the parameters and requirements for most of the building’s proposed architectural components set by the building code. Two important requirements are its maximum volume (of height, stories and area) and its minimum width of surrounding open space. The data center building’s total allowable square footage is connected to how many server towers can be deployed within its walls, so it’s easy to see the importance of starting in the initial planning stage to prevent the building code from overly limiting its allowable volume. Such a limit could force the quantity of servers to fall short of the amount defined in the project’s pro forma.
The primary building code (herein understood as the 2015 International Building Code) seems fairly restrictive of the initial height, stories and area allowed for any of the 10 standard use groups in most of the five basic construction types. But various conditions built into the code enable the volume’s initial restrictions to be increased (some exponentially) so that one way or another, the desired buildable volume (read: servers) is always possible.
Increasing Allowable Height
The 2015 IBC says, “The maximum height, in feet, of a building shall not exceed the limits specified in Table 504.3.” In its Section 2 definitions, it notes that when the topmost occupied floor is 75 feet above the grade plane of fire-department vehicle access, it kicks a building into the realm and requirements of a high-rise.
Other than Table 504.3’s height limits, the definition of a high-rise and some exceptions for uninhabited roof towers, the code says little more about height—but most in our industry tend to understand buildings as falling into three basic categories: low-rise, mid-rise and high-rise. Although not specifically spelled out in the IBC, the determining factor for a low-rise or mid-rise building’s height is most likely also measured from the grade plane of fire-department vehicle access. As in a high-rise, this measurement is also meant to correspond to a firefighter’s difficulty in gaining access into or onto a burning building, where the topmost rescue or fire-suppression requirement could start from a position at the lowest grade plane of a sloped site. Without providing further description, however, the IBC leaves it the reader to logically conclude that for most data centers, “the maximum height” listed must be the height of a building’s top edge (not its topmost occupied floor level). This top edge usually means its parapet. Additionally, zoning codes normally refer to height as the highest point of the roof over the highest occupied floor.
Notwithstanding that although most data centers aren’t in high-rise buildings and don’t sport tall towers, increasing the allowable height is nonetheless a challenge because the available methods are sparse. The 2015 IBC’s Table 504.3 grants the use of only one of two methods, and both require a sprinkler system for data centers’ typical use groups (S, B and sometimes H):
- Method 1. Sprinkler System: According to Table 504.3’s allowable heights, all construction types that are fully equipped with an automatic sprinkler system (designated in the table as “S”) are allowed to be 20 feet higher than those that are not (“NS”).
- Method 2. Unlimited Height: Per Table 504.3 (with a footnote to Section 903.2), Construction Type 1A (noncombustible, protected) can have unlimited height (UL) for all use groups (except R), but it requires the full building to have an automatic sprinkler system for Use Group S-1 moderate hazard (per 903.2.9) and Use Group H high hazard (per 903.2.5).
But while the methods available for increasing allowable height are limited, the good news is that even for the least-restrictive building shell (that being Type 5B’s unprotected, combustible construction) the code’s initial allowable height for Use Groups B, S and H is 40 feet. Since most one-story data centers are under that height, and unless additional floors are desired or needed, pursuing a height increase is seldom necessary.
Increasing Allowable Stories
Although most data centers are low-rise buildings—usually just one or two stories—the number of stories above the grade plane, even at that low limit, can nonetheless be a critical factor when trying to boost the volume allowed for a data center’s typical use groups (S, B and sometimes H).
Just like increasing allowable height, the determining factor for increasing a building’s allowable stories begins from the grade plane that surrounds the building. It too has a thin selection of available methods, given that the initial limit on allowable stories can increase through only one of following two ways:
- Method 1. Sprinkler: The IBC allows a building complete with an automatic sprinkler system to have a one-story increase in its basic stories allowed above the grade plane.
- Method 2. Unlimited Stories: IBC Table 504.4 grants buildings of Construction Type 1A (noncombustible protected) to have unlimited stories (UL) for most use groups, including S and B (but excluding H-1, H-5 and sometimes R).
And like the rare need to increase allowable height, few data center projects have a need to increase allowable stories. Yet a typically low story count nonetheless plays a major role when selecting a method for “increasing allowable area,” which is nearly always critical in data centers.
A side note regarding the “and sometimes H” use group noted above: employing UPS batteries without safety caps, improper mechanical ventilation or fuel-fired generators in the building can earn a data center a high-hazard “H” designation. And owing to Group H’s extensive additional requirements, that designation should be avoided.
Increasing Allowable Area
According to 2015 IBC’s Table 506.2, the most-limited initial area allowed for Use Group S-1 (being nonsprinklered, Type 5B unprotected combustible construction) is a mere 9,000 square feet, yet some data centers that I’ve worked on are as large as 550,000 square feet—a substantial difference to resolve.
Fortunately, while increasing allowable height and allowable stories both have a limited palette of methods, quite a few methods are available to increase the allowable area. Most beneficially, these methods can be added on each other. Keeping in mind that areas listed in the IBC tables represent a single floor plate rather than the entire building’s combined floor areas, any or all of the following methods can be employed (listed here from least to most useful), enabling any desired building area:
- Method 1. Fire Walls: A fire wall is defined as a three-hour-rated, self-supporting wall that under fire conditions will remain standing if either side collapses. Such requirements make it quite different from just a one-, two-, or three-hour fire-rated wall. Whereas a fire-rated wall creates a barrier to limit the spread of fire, a fire wall does more: because it remains self-supporting during a fire, even when the construction on either side collapses, it acts as a separation between buildings. Fire walls can thus be added to a design to divide a large building, whose square footage exceeds the allowable area, into “individual” buildings, each small enough to conform to the code’s allowable area limit. Note, however, that fire-wall requirements are often misunderstood. I’ve found over the years during many site inspections fire walls, in many different use groups and construction types, that were improperly designed or constructed yet apparently approved by the building’s architect, structural engineer, general contractor and local building inspector.
- Method 2. Sprinkler System: According to Table 506.2’s allowable areas, all construction types that are fully equipped with an automatic sprinkler system (designated in the table as “S1”) are allowed to be much larger than those that aren’t (“NS”).
- Method 3. Street Frontage: Because the 20-foot minimum distance between buildings typically reduces the spread of fire from one building to another, the code encourages the use of open frontage by permitting an increase of a building’s initial allowable area as its open frontage increases. The equation for the increase is a bit complicated, but the essence is that the code’s initial allowable area can increase by a multiplier of up to 75% of the building’s open frontage.
- Method 4. Unlimited-Area Buildings: Being a powerful method for data centers, “unlimited-area buildings” can achieve any area defined in a project’s pro forma. IBC Table 506.2 grants unlimited-area buildings (UL) for only the most restrictive building shell (Type 1A’s noncombustible and protected construction), for all use groups except Hazardous H-1 and H-2. But IBC Section 507 is an entire section devoted to unlimited-area buildings:
- Section 507.4 is of common interest to most data centers since it’s for Use Groups S and B (plus F and M, but not others such as H) that are in one-story buildings; it must, however, be fully sprinklered and surrounded by open yards not less than 60 feet wide.
- Section 507.5 is for Use Groups S and B (plus F and M, but not others such as H), placed in a fully sprinklered building no more than two stories above the grade plane, and surrounded by public ways not less than 60 feet wide. Per Section 507.2.1 the “not less than 60 feet wide” open space noted in 507.4 and 507.5 can be revised to be “not less than 40 feet wide” for up to 75% of the building perimeter, provided the exterior walls (and its opening protectives) facing the reduced width are fire rated for at least three hours.
- Method 5. Cumulative: The best benefit of all is not really a method; it’s that the IBC allows a combination of any available methods to increase area. The cumulative effect of adding the different methods effectively gives an exponential boost to the initial allowable area.
A typical data center design needs to prevent its maximum allowable volume (of height, stories and area) from being too limited by the building codes, and it also commonly employs a sprinkler system to protect the invaluable data servers. Therefore, the above-cited IBC requirements and the need to keep most options open lead me to believe the following is the best approach to the initial planning stage:
- First and foremost, establish the quantity of servers desired.
- Next, assess the area necessary for corresponding support spaces (electric, UPS and air-handler rooms, as well as offices and entry areas).
- Plan to provide an automatic sprinkler system throughout the building, not only to protect the inhabitants and data servers but also to get the needed increases in height, stories or area.
- Plan about 20 to 25feet for each story, with a one-story building plus parapet being around 25 feet tall, a two-story plus parapet being about 45 to 55 feet tall, and so on.
- Surround it with open spaces on all sides, each being at least 60 feet wide (or 40 feet wide per IBC 507.2.1).
- Keep the Use Group B office areas under 10% of the project’s total area to qualify it (per IBC 508.2.3) as an accessory occupancy to the larger Use Group S designation.
- Design for nonseparated mixed-use groups of S-1 (moderate-hazard) and B (business), but contact the local authority having jurisdiction (AHJ) early on to determine whether they deem data centers to be the less restrictive S-2 low-hazard.
- Avoid a Use Group H designation, owing to its many extra requirements.
- Avoid using the often improperly built fire walls.
- Last, employ any methods for increasing allowable area so the building is assessed at its lowest possible Construction Type, but then design and build it per the safer fire-resistant construction materials and the quicker construction methods of Type 1’s or Type 2’s noncombustible precast or tilt-up construction.
Leading article image courtesy of DVA Architects
About the Author
Dean Ventola, RA, NCARB, LEED AP BD+C, is the Director of Construction Administration at DVA Architects in Gaithersburg, MD, a nationally prominent mission-critical data center architect.