Structural steel — sections, connections and why it must be fireproofed

The foundations topic put load into the ground and concrete covered the material that does it. Above ground in a process plant, most of what you see is steel — pipe racks, platforms, stairs, equipment supports, structures.

Why steel above ground

• Strength for its weight, so long spans need less material.
• Speed — members are fabricated in a shop while foundations are still being poured, then bolted together on site.
• Predictability — it is manufactured to tight tolerances, unlike concrete cast in a hole in the ground.
• Reversibility — it can be unbolted, modified and extended, which matters enormously on a plant that will be revamped repeatedly over forty years.

Its weaknesses are exactly two, and both are addressed by coatings: it rusts, and it loses strength in a fire.

Reading a section designation

UB 457 × 191 × 67
│    │     │     │
│    │     │     └── mass, kg per metre
│    │     └──────── nominal width, mm
│    └────────────── nominal depth, mm
└─────────────────── Universal Beam

Two sections of the same depth and width can have different masses, because the flange and web thicknesses differ. The mass is how you tell them apart, and it is what you are billed for.

Regional naming varies for the same thing: UB/UC in British practice, W-shapes in American (W18×40 — depth in inches, mass in lb/ft), IPE/HE in European, ISMB/ISHB in Indian.

Section	Typical designation	Best at	Poor at	Used for
Universal beam (I-section)	UB 457×191×67, W18×40, ISMB 450	Bending about its major axis	Twisting, and bending the weak way	Floor beams, pipe rack transverse beams, crane runways
Universal column (H-section)	UC 254×254×73, W14×90, ISHB 250	Axial compression — stocky in both directions	Long spans in bending	Building and pipe rack columns
Channel	PFC 200×90, C10×20, ISMC 200	Edge members, bracing, framing openings	Bending — it twists because the shear centre is off the web	Stair stringers, platform edges, trimmers
Angle	L 100×100×10	Tension members and light bracing	Compression over any length	Bracing, ladder frames, small trusses, cleats
Square/rectangular hollow (SHS/RHS)	SHS 150×150×8, RHS 200×100×8	Torsion, compression, and clean appearance	Bolted connections — you cannot reach inside	Architectural steelwork, handrails, light columns
Circular hollow (CHS)	CHS 168.3×8	Torsion, and low wind drag	Connections — every joint needs profiling	Offshore structures, flare stacks, exposed trusses
Plate girder	Fabricated from plate	Very long spans and very heavy loads	Cost — it is fabricated, not rolled	Long-span pipe bridges, heavy crane girders
Castellated / cellular beam	Fabricated from a UB	Deep span with services passing through the web	Concentrated point loads over an opening	Long spans where ducts and pipes must pass through

Sections are shaped to put metal where the stress is. An I-shape carries bending efficiently because the flanges are far from the neutral axis; a hollow section resists twisting because it is a closed loop. Choose the shape for the action, then size it.

Connections decide the cost

Steelwork cost is driven far more by connections than by tonnage. A simple beam is cheap; a heavily stiffened moment connection is not.

Bolted connections dominate site work — no power, no weather protection, no inspection delay, and immediately load-bearing. Two kinds:

• Bearing type. The bolt shank bears against the hole. Ordinary structural bolting, and the default.
• Slip-critical (HSFG). Bolts are tensioned to a defined high value so that friction between the plates carries the load. Used where slip cannot be tolerated — crane girders, vibrating machinery supports, fatigue-loaded joints. The faying surfaces must be prepared and left unpainted, and the tension has to be verified.

Bolt grades are marked on the head: 8.8 and 10.9 in metric practice, A325 and A490 in American.

Welded connections are stiffer and cleaner, and are mostly done in the shop where conditions are controlled and inspection is easy. Site welding is slower, needs weather protection and access, and carries all the procedure and qualification requirements covered in welding.

By restraint, connections are either:

• Simple (shear) — transfers vertical load, allows rotation. Cheap, and the norm.
• Moment — transfers bending too, so the frame itself resists sway. Necessary where bracing cannot be fitted, and considerably more expensive.

Corrosion protection

Steel in a humid, salt-laden or chemical atmosphere corrodes. ISO 12944 classifies environments from C1 (heated indoor) to CX (offshore, severe chemical), and the paint system follows from that class.

• Galvanising — hot dip zinc, excellent for handrails, gratings, small members. It is sacrificial, so minor damage still gets protected.
• Paint systems — typically a zinc-rich primer, an epoxy intermediate coat and a polyurethane topcoat. Most of the performance comes from surface preparation, not from the paint.
• Weathering steel — forms a stable protective rust layer, but only where it can wet and dry properly. Rarely suitable in a process plant.

Fireproofing: buying time, not immunity

Steel keeps most of its strength to about 300 °C. By 550 °C it has lost roughly half, and that is the temperature at which a loaded member starts to sag and fail.

In a hydrocarbon pool fire, an unprotected member can reach that in around ten minutes.

Method	Typical rating	Advantages	Drawbacks	Used for
Cementitious spray	60 – 240 minutes	Cheap per square metre, thick, robust against radiant heat	Rough finish, heavy, traps moisture and can hide corrosion	Pipe rack and structure legs in process areas
Intumescent coating	30 – 180 minutes	Thin, paint-like, keeps the steel profile visible	Costly, thickness must be verified, damaged easily before it swells	Exposed steel where appearance matters, retrofits
Concrete encasement	120 – 240 minutes	Very durable, needs no maintenance, resists impact	Heavy — it loads the foundations, and it is slow to build	Column bases, blast-exposed areas, vessel skirts
Board systems	30 – 240 minutes	Clean dry installation, consistent thickness	Awkward around complex connections and bracing	Indoor structures, switchrooms, escape routes
Flexible jacket	30 – 120 minutes	Removable for inspection, good on irregular shapes	Needs fixings maintained; can be left off after maintenance	Valves, actuators and equipment needing periodic access

Steel loses roughly half its strength by about 550 °C, and an unprotected member in a hydrocarbon fire can reach that in ten minutes. Fireproofing does not make steel fireproof — it buys time for people to escape and for the fire systems to work.

Fire ratings are quoted against a fire curve, and there are two very different ones:

• Cellulosic (ISO 834, BS 476) — an ordinary building fire of wood, paper and furnishings. It heats up relatively slowly.
• Hydrocarbon (UL 1709) — a pool fire, reaching over 1000 °C in around five minutes.

A product rated 120 minutes on the cellulosic curve does not give 120 minutes in a hydrocarbon fire. Specifying to the wrong curve is a serious and not-uncommon error.

What gets fireproofed is decided by a fire risk assessment: typically structural steel within a defined radius and height of a credible leak source, supports for vessels holding significant inventory, escape routes, and the supports of the fire and emergency systems themselves.

What civil needs from the other disciplines

Same story as foundations, one level up:

• Piping — rack loads, anchor and guide forces, where lines route through the structure, and future expansion allowances.
• Mechanical — equipment weights and maintenance access, lifting beams and laydown areas.
• Electrical and instrumentation — cable tray routes and loads, lighting and junction box supports.
• Process and safety — which areas require fireproofing, and to which curve.

What to take away

• Section designation is depth × width × mass per metre. Mass is what distinguishes similar sections.
• Shape follows the action: I-sections for bending, H for compression, hollow sections for torsion, angles for tension.
• Connections drive cost more than tonnage. Simple shear connections are the cheap default.
• Slip-critical bolting carries load by friction, so surface preparation and verified tension are the whole point.
• Steel has lost about half its strength by 550 °C and gets there in ten minutes in a hydrocarbon fire.
• Hydrocarbon and cellulosic fire curves are not interchangeable.
• Never site-drill, flame-cut or weld to existing steel without approval.