Structural systems and roof equipment loads

The structure of a data centre has an unusual job. It must enclose very large, column-free or regularly gridded halls; carry heavy equipment on floors, roofs and from ceilings; resist wind, snow and earthquakes to a higher standard than most buildings; and be built fast, often in repeated phases, by owners racing to bring capacity online. This lesson covers the main structural systems used for data centres, how dead, live, equipment and hanging loads are combined, the design of roofs carrying mechanical equipment, the support of overhead services including liquid cooling piping, floor vibration, and the growing attention to embodied carbon in structural materials.

Structural systems

Most data centres use one of a few structural forms:

• Single-storey steel frame: steel columns on a grid (commonly 9 to 18 m), with steel joists or trusses and steel deck supporting the roof, and walls of insulated metal panels or precast concrete. Fast, economical and adaptable; the dominant form for large campuses with available land.
• Precast or tilt-up concrete: concrete wall panels cast on site (tilt-up) or in a factory (precast) form the exterior walls and often act as shear walls, with a steel roof structure. Robust against wind-borne debris, fire and security threats.
• Multi-storey concrete or steel: two to five or more storeys of data halls, with heavy suspended floor slabs (often post-tensioned concrete or composite steel deck) on a column grid. Used where land is scarce or expensive, in cities and around constrained power sites. Floor loads, vibration and service distribution between levels govern the design.
• Pre-engineered and modular buildings: factory-built frames or modules assembled on site (Lesson 9).

Clear heights inside data halls have grown to accommodate hot aisle containment, overhead busways, cable trays at several levels, sprinkler piping and liquid cooling distribution: 5 to 7 m to the underside of structure is common, with more in high-density AI halls. Grid spacing is coordinated with rack rows (a rack row pitch of 2.4 to 3.6 m fits neatly into 9 to 18 m bays), so columns do not interrupt rows.

Loads and load combinations

Structural design considers several categories of load:

• Dead load (D): the weight of the structure, cladding, roofing, fixed services and permanent equipment.
• Live load (L): occupancy and movable contents, including racks (Lesson 3).
• Equipment loads: often treated as dead load when permanent and precisely known, or as live load when they may change; owners usually specify allowances for future equipment.
• Hanging (collateral) loads: busways, cable trays, ductwork, sprinkler and cooling pipes, containment and lighting hung from the roof or floor above.
• Snow (S), wind (W) and earthquake (E) loads (Lesson 7).
• Rain and ponding loads on roofs.

These loads are combined using load factors that reflect their uncertainty and the low probability of several maximum loads occurring together:

Roof-mounted equipment

Many data centres place cooling equipment on the roof: air-cooled chillers, dry coolers, condensers, air handling units and exhaust fans. Each unit may weigh 5 to 40 tonnes in operation, with water-cooled units heavier when full. Roof-mounted equipment affects the structure in several ways:

• Concentrated gravity loads, carried by dunnage steel (secondary beams or frames) that spreads the load to primary girders and columns rather than to the light roof joists.
• Snow drifts that form against and around the equipment (Lesson 7), adding significant load.
• Wind loads on the equipment itself, and uplift and overturning that must be resisted by anchorage.
• Seismic forces on the equipment, which are amplified with height in the building and must be resisted by anchors and the supporting structure (Lesson 7).
• Vibration from fans and compressors, which requires isolators and stiff supports.
• Maintenance access, walkways, guardrails and crane access for replacement.

Placing equipment on grade beside the building avoids most of these issues and simplifies maintenance, at the cost of land; placing it on the roof saves land and shortens piping. Owners decide based on site size and density; many AI data centres now place heat rejection on grade or in dedicated mechanical yards because of the weight and size of the equipment.

Ponding and roof drainage

Flat roofs are especially vulnerable to ponding: rainwater that accumulates in a deflected area adds load, which increases deflection, which collects more water. If the roof is too flexible, this feedback leads to collapse. Designers prevent it by providing adequate slope (at least 2% is common), primary and secondary (overflow) drains or scuppers sized for the design rainfall, and members stiff enough that ponding stability is assured. The secondary drainage system must work if primary drains are blocked, because the rain load is calculated assuming the water rises to the level of the overflow. Large data centre roofs, sometimes over 30,000 m² on a single building, need careful layout of drains, crickets and slopes.

Supporting overhead services

In slab-on-grade data halls without raised floors, almost everything is hung from above: busways, several tiers of cable tray, containment ceilings, lighting, fire detection and sprinklers, and increasingly large liquid cooling pipes and manifolds. These collateral loads can reach 1.5 to 3 kPa averaged over the hall, with heavy concentrations along rack rows. Structural engineers coordinate a support system, often a grid of hanging steel (unistrut, angles or purpose-made frames) from the roof structure, and specify where heavy point loads may be attached. Liquid cooling piping is particularly heavy when full: a 300 mm steel pipe full of water weighs about 140 kg per metre.

Floor vibration in multi-storey data centres

Suspended floors in multi-storey data halls must be stiff enough that footfall, equipment and rack movement do not cause vibrations that disturb people or sensitive equipment. Vibration criteria are expressed through the floor's natural frequency and response to walking; heavy concrete floors with frequencies above about 8 to 10 Hz generally perform well. Long-span lightweight floors are more susceptible. Mechanical equipment on upper floors needs isolation and, where large, dedicated stiff structure.

Embodied carbon

Concrete and steel production are major sources of greenhouse gas emissions, and a large data centre contains tens of thousands of tonnes of concrete and many thousands of tonnes of steel. As operators pursue carbon goals (DCE-460), they increasingly track embodied carbon (the emissions from producing and building with materials) through environmental product declarations, and reduce it with:

• Lower-carbon concrete using supplementary cementitious materials (fly ash, slag, calcined clay) and Portland-limestone cement, and optimizing strength and quantity.
• Steel with high recycled content made in electric arc furnaces.
• Efficient structural design: right-sizing members, avoiding overly conservative loads, and reusing standard designs.
• Mass timber for some building elements, such as office and support areas, and increasingly for data hall structures, where fire and load requirements allow.

Lesson 6 develops the design of steel members, joists, diaphragms and connections, and Lesson 8 the concrete structures used in walls and multi-storey floors.