Seismic, wind and snow design (ASCE 7 and the IBC)

Natural hazards are the loads that most distinguish the structural design of a critical facility from that of an ordinary building. A warehouse that is damaged in an earthquake or a storm can be repaired; a data center that stops working during the disaster may take down hospitals, banks, emergency communications and cloud services used by millions of people. Codes recognize this through importance or risk categories that raise design loads for buildings whose functions matter most, and data center owners frequently go further than the code requires. This lesson explains how snow, wind and seismic loads are determined, how importance affects them, how equipment and racks are protected, and how a changing climate is affecting design data.

Risk categories

The International Building Code (Table 1604.5) and ASCE 7 assign every building a risk category based on the consequences of its failure:

A typical commercial data center is Risk Category II. Facilities that support emergency communications may be Risk Category IV, and owners often voluntarily design to Risk Category III or IV for resilience. In ASCE 7-22, Risk Category III and IV buildings east of the Rocky Mountains must also be designed for tornado loads where the mapped tornado speed exceeds 60 mph, and Risk Category IV structures in higher seismic design categories have stricter requirements for equipment needed to keep operating after an earthquake.

Snow loads

ASCE 7-22 specifies the flat roof snow load as:

where \(p_g\) is the ground snow load for the site from the risk-category-specific maps (or the ASCE Hazard Tool), \(C_e\) the exposure factor (0.8 to 1.2 depending on terrain and roof exposure) and \(C_t\) the thermal factor (1.0 for heated buildings, higher for unheated or well-insulated roofs). Sloped roof, drift and unbalanced loads are added for non-uniform conditions.

Large flat data center roofs collect snow evenly, but drifts form wherever the roof changes height or has obstructions: against parapets, at steps between roof levels, and around rooftop equipment and penthouses. Wind carries snow from the upwind roof and deposits it in the aerodynamic shadow, forming a roughly triangular drift whose height grows with the length of upwind roof (the fetch) and the ground snow load. Drift loads can be several times the uniform load and are the cause of many roof failures.

Wind loads

ASCE 7 calculates the velocity pressure at height \(z\) as:

where \(V\) is the basic wind speed from the risk-category-specific map (for example a 700-year return period for Risk Category II, 1,700 years for III and 3,000 years for IV), \(K_z\) the exposure coefficient for height and terrain, \(K_{zt}\) the topographic factor and \(K_e\) the ground elevation factor. Design pressures then apply gust effect factors and external and internal pressure coefficients for each surface.

Wind pressures are highest at roof corners and edges, where flow separates, often two to three times the pressure on the roof interior. For data centers, wind design covers the main structure, cladding and roofing (which must not peel off and expose equipment to rain), rooftop equipment and its anchorage, louvers and intake screens, and the effects of wind on air intakes and generator exhaust. In hurricane- and tornado-prone regions, owners specify impact-resistant envelopes, hardened rooms for critical equipment and protection for outdoor equipment.

Seismic design

ASCE 7 seismic provisions use risk-targeted design spectral accelerations \(S_{DS}\) and \(S_{D1}\) for the site, adjusted for the site class (ASCE 7-22 provides multi-period spectra and more site classes). For regular buildings, the equivalent lateral force procedure gives the base shear:

where \(T\) is the fundamental period, \(I_e\) the importance factor, \(W\) the effective seismic weight (dead load plus a portion of storage live load and of significant snow), and \(R\) the response modification coefficient of the lateral system (for example \(R = 3.25\) for ordinary steel concentrically braced frames, 6 for special ones). The force is distributed up the height in proportion to weight times height. The seismic design category (A to F), set by the spectral accelerations and risk category, determines which systems are permitted and which detailing is required.

import numpy as np

# equivalent static seismic base shear for a two-level braced steel building
# design spectrum (5% damping), short- and 1-second values for the site and site class
S_short, S_1 = 0.80, 0.30      # design spectral accelerations (g)
h_n = 14.0                     # building height (m)
W = 36000.0                    # seismic weight (kN): structure, roof equipment and part of floor load
R = 3.25                       # response modification / ductility factor for an ordinary braced frame (illustrative)

T = 0.0488 * h_n ** 0.75       # approximate period for braced frames, h in m
Sa = min(S_short, S_1 / T)
print(f"approximate period T = {T:.2f} s, spectral acceleration Sa(T) = {Sa:.2f} g")
for name, I in (("normal (I = 1.0)", 1.0), ("high (I = 1.25)", 1.25), ("post-disaster / essential (I = 1.5)", 1.5)):
    V = Sa * I / R * W
    print(f"{name:36s}: base shear V = {V:6.0f} kN ({V / W * 100:4.1f}% of weight)")

# distribute the I = 1.5 base shear to the two levels in proportion to w*h
w = np.array([20000.0, 16000.0]); h = np.array([7.0, 14.0])
V = Sa * 1.5 / R * W
F = V * w * h / (w * h).sum()
for lvl, f in zip(("level 2 (floor)", "roof"), F):
    print(f"   {lvl:16s}: {f:6.0f} kN")

approximate period T = 0.35 s, spectral acceleration Sa(T) = 0.80 g
normal (I = 1.0)                    : base shear V =   8862 kN (24.6% of weight)
high (I = 1.25)                     : base shear V =  11077 kN (30.8% of weight)
post-disaster / essential (I = 1.5) : base shear V =  13292 kN (36.9% of weight)
   level 2 (floor) :   5112 kN
   roof            :   8180 kN

In this illustrative case (design spectral acceleration 0.8 g at the building's 0.35 s period, a reduction factor of 3.25 for a moderately ductile braced frame), the base shear is 25% of the building weight for normal importance and 37% for the highest category. Because the roof carries 16,000 kN at twice the height of the floor, it receives 62% of the lateral force, which is why heavy rooftop equipment increases seismic demand on the whole structure.

Protecting equipment and racks

Most earthquake losses at data centers come not from structural collapse but from damage to non-structural components: unanchored equipment sliding or toppling, racks overturning, raised floors collapsing, pipes and sprinklers breaking and flooding equipment, and ceilings falling. Codes require non-structural components to be anchored and braced for seismic forces that increase with height in the building, because floor accelerations are amplified above the ground.

In ASCE 7-16, the lateral force on a component was:

where \(a_p\) is the component amplification factor, \(R_p\) the component response modification factor, \(I_p\) the component importance factor (1.5 for components needed for life safety or continued operation of essential facilities) and \(z/h\) the relative height of attachment. ASCE 7-22 revised this method, using building-specific amplification and component resonance factors, but the principle is the same. Equipment in Risk Category IV facilities with \(I_p = 1.5\) must be certified, by shake-table testing or analysis, to remain operational after the design earthquake.

For racks and IT equipment, options include anchoring racks to the slab (with braced, seismically rated racks), bracing rack tops together and to the structure, or mounting racks or whole rows on base isolation platforms that let the floor move beneath them while limiting the acceleration transmitted to servers. In the most seismically active regions, some data centers isolate the entire building on rubber or friction pendulum bearings, so the structure and its contents experience a fraction of the ground acceleration. Pipes, cable trays, busways and sprinkler lines require seismic bracing, and flexible connections where they cross seismic joints or connect to isolated equipment.

Climate change and design data

Snow, wind and rainfall design values in codes have traditionally been based on historical records, assuming the climate is stationary. With a warming climate, extreme precipitation is increasing in many regions, snow loads are changing (decreasing in some places, increasing in others with heavier wet snow), and the intensity and track of hurricanes and severe storms are shifting.

ASCE 7-22 introduced risk-category-specific snow loads, tornado loads, and updated wind and flood requirements, and work on the next edition is examining how to account for climate trends. Building codes are adopted by states and local jurisdictions, often several years after publication, so designers must check which editions apply at each site. Owners of long-lived critical facilities increasingly apply future climate projections, from sources such as federal climate assessments, beyond code minimums.

Because a data center's structure may last 30 to 50 years while its equipment is replaced several times, the structure should be designed with margins for future heavier equipment and more severe climate loads, which cost far less to include at the start than to retrofit.

Lesson 11 extends these hazards to resilience objectives beyond the code minimums, including floods, tornadoes, vehicles and blast.