The heaviest thing in the room is chosen last

Steelwork goes up eighteen months before the first rack arrives, and the rack schedule is written by somebody who has not been appointed yet. The frame is therefore designed on an assumed load, and that assumption - not the grade and not the coating - decides whether the hall floor is a slab on grade or a suspended slab with beams at three metre centres.

What the assumption should be is not a mystery. It is arithmetic.

A rack is a point load wearing a floor area

A 42U rack is 600 mm wide and 1,200 mm deep. Filled with servers, power shelves and cabling it weighs between 1,000 and 1,500 kg. Over 0.72 square metres that is 1,390 to 2,080 kg per square metre, against a general office floor designed for a few hundred. The gap is not a percentage. It is a different structure.

The raised floor hides it. A 600 mm access floor grid carries the rack down through pedestals, and a rack standing across two tiles shares its load between six of them, which puts 170 to 250 kg on each - a number a pedestal takes comfortably, provided the slab beneath is flat and the pedestals are not standing on a screed nobody designed for that duty.

The average is the wrong number

Take a one megawatt hall at 8 kW a rack: 125 racks, about 150 tonnes of equipment, spread across 500 square metres. That averages 300 kg per square metre, which sounds like a warehouse and is not what the slab sees. The slab sees up to 2,000 kg under each rack and nothing under the aisle.

So the enquiry carries two numbers, a uniform load and a point load allowance, and the fabricator's first question is not the beam size but the spacing and the imposed load. Q345 plate and A36 plate both run 0.3 to 200 mm in 1000, 1250 and 1500 mm widths, and the base plates and stiffeners that spread a column into a slab come out of that stock, cut and drilled to a connection drawing.

The members above them come by the piece. A29M H-sections run 5 to 34 mm thick with flanges of 50 to 400 mm and webs of 100 to 900 mm, supplied in 6 to 12 m lengths, tolerance plus or minus 1 per cent, one tonne minimum. 50 x 150 sections cover the light end and IPE 450 the mid span, and the perimeter members are specified first because they carry the roof as well as the floor.

Serviceability sets the depth, not strength

A beam spanning 12 m clear in a hall rarely fails. It deflects, and a floor that moves under a 1,500 kg rack cracks tiles, racks cabinet frames and takes tape libraries out of alignment. A span over 360 is the usual limit for imposed load; over a data hall that limit, not the bending stress, picks the section, and a 900 mm deep member is often specified where 600 mm would pass on stress alone.

Vibration is the other half. Racks tolerate it, switchgear and UPS rooms do not, and a frame whose natural frequency sits near a footfall or a pump harmonic is cheap to correct on the drawing and expensive to correct in service.

None of that changes the order, but it changes what the shop does to the steel. Base plates and gussets get sheared and punched rather than cut on a machine that leaves a ragged edge, and the welds that join them are the ones an inspector will look at first - plate welding on 30 mm steel is a different procedure from welding a 6 mm stiffener.

The secondary steel is where the cable goes

The primary frame is a schedule of beams and columns. What carries the tray, the busway, the pipework and the containment is secondary: C and Z purlins, angle and circular pipe, all Q235B on the steel structure material list, plus runs of square tube in 1 to 12 m lengths, 10 x 10 to 100 x 100 mm, for supports and handrails. It is a small share of the tonnage and a large share of the site labour, because every bracket is measured, cut and drilled in place.

Hanging 200 kg of loaded tray from a purlin sized for roof load is the classic omission. The tray load and its spacing belong in the enquiry with the rest.

Wet rooms and dry rooms

Most of a data hall is dry, cooled and never sees condensation on a steel surface, and paint is enough there - the steel structure page describes exactly that choice, derust and then galvanize or paint. The plant room is the exception: chilled water, humidifiers, condensate and a floor that gets washed. That is where zinc earns its cost, in Z275 coil of 0.12 to 3 mm with coating weights from Z60 up, galvanized plate from 0.19 to 2 mm, and a hot dip galvanized H-beam for the supports standing in it.

The same logic runs outdoors: generator enclosure, fuel bund, cooling plant. All wet, all bought coated rather than painted on site.

The envelope and the boxes inside it

The hall envelope is a coated product rather than a structural one: TDC51D prepainted coil, 0.12 to 2 mm thick, in widths of 600 to 1250 mm and any RAL colour, or PPGL where the roof will see more sun than rain. Inside, the enclosure panels and containment covers are thinner and flatter - SECCN2 from 0.13 to 5.0 mm with an anti-fingerprint surface, and DC01 or ST12 from 0.12 to 3 mm where the part is formed and then painted.

Fire: the pipe that has to work once

Sprinkler and hydrant mains are usually ASTM A53, welded black or hot dip galvanized, from half an inch to 36 inch nominal bore with 3 to 12 mm wall, in schedules 40 through XXS, supplied in 3 to 18 m lengths with mill certificates to EN 10204/3.1B. Cold drawn pipe holds plus or minus 0.1 mm and cold rolled 0.05 mm, which is why threaded and grooved joints line up on site.

The specification worth arguing about is the certificate. A dry system tested once at handover and then left for a decade is only as good as the wall that was delivered.

What goes on the enquiry

Hall dimensions, clear height, column grid, uniform floor load and a point load allowance per rack row. Rack weight and footprint if they are known; if they are not, say so and state the assumption - a supplier told the assumption can flag it. Then the secondary loads with spacing, the coating for each zone, and the delivery sequence - a frame that arrives complete has to be stored, and a data hall site rarely has anywhere to put it.

Every other building is designed around a use that is already defined. A data hall is designed around a number nobody has yet, and getting that number into the enquiry is worth more than any grade decision taken after it.