Last reviewed 30 Sept 2026 · 6 min read
Secondary components
These are called "secondary" because they do not form the main frame, but they are the parts the passenger touches and the parts that fail visibly if they are badly designed.
Staircases
- Live load about 5 kN/m² for public stairs (crowd), with the hand-rail designed for a horizontal load along the top (of the order of 1.5–3 kN/m as per the occupancy code).
- Structure: RCC waist slab stairs on beams or walls, or steel stringers with precast treads; landing slabs; width and rise to meet fire and evacuation requirements — the stair width is determined by the evacuation time for the peak crowd.
- Vibration: footfall on lightweight steel stairs must be checked; expansion joints where stairs meet the structure.
Escalators
An escalator is a steel truss carrying the steps, machinery and balustrades, supported at both ends on the station structure:
- Reactions: dead load of the truss and the machine, plus a live load of the passengers (of the order 5 kN/m² of plan area in the step area, or as specified by the manufacturer).
- Supports: one end fixed (pin) and the other on a sliding support to allow expansion and deflection — so the horizontal force from friction and the drive is taken at one end only; support seats are cast with embedded plates or pockets.
- Deflection limit of the truss about span/750 (a mechanical requirement); the trim around the openings needs framing.
- Pit at the lower end; fire and sound requirements.
Lifts
A lift shaft is an RCC core or a steel frame with pit and overrun; the design covers the guide-rail and buffer reactions (the buffer impact loads are large) and the machine-room floor and hoist beams. Tolerances of the shaft verticality are tight (a few millimetres).
Platform screen doors and edge
Platform screen doors (PSD) are anchored to the platform edge beam; loads include crowd push (a horizontal load on the door panels), wind or piston pressure from trains in the tunnel and the impact of a door. The platform edge and the edge line have strict tolerances relative to the track (moving-dimension clearance) — so the slab deflection and the settlement of the platform edge are limited.
Canopies, roof sheeting and façades
- Elevated stations have steel or aluminium roof systems on trusses; wind uplift, fatigue at the fixings and thermal movement are the governing effects.
- Cladding and glazing are designed for local wind pressure, thermal expansion and building movement (drift) in earthquakes; fixings are ductile and allow movement.
- Drainage of roofs and gutters, and cable and utility supports.
Durability — design for 100 years
A metro station has a design life of 100 years and can be repaired only with great disruption, so it is designed to be durable from the start.
- Exposure class: assessed from the environment — moderate to severe (chlorides in groundwater near the coast, sulphates in soil and water, industrial atmosphere, urban stray currents).
- Concrete mix: low water–cement ratio (about 0.40 or less), a high binder content with fly ash, slag or silica fume for low permeability, and a suitable grade (commonly M40–M50 or higher); cement type chosen for sulphate or chloride resistance.
- Cover to reinforcement: larger for exposure to soil, water and weather (typically 50 mm and above for external and soil faces; check the code for the exposure class).
- Crack width limit: 0.2 mm or smaller (and smaller where water-tightness is required); designed by bar spacing and stress; early-age cracking controlled by the thermal plan.
- Quality of construction: cover blocks, compaction, curing, and testing (RCPT — rapid chloride permeability, water penetration).
- Additional protection where needed: corrosion inhibitors, epoxy-coated or stainless steel bars in critical locations, and surface coatings for exposed concrete.
Chloride, sulphate and carbonation attack
- Chloride penetration corrodes the reinforcement: dense concrete and adequate cover delay the time to initiation.
- Sulphate in soil and water attacks the cement paste (expansion, spalling): sulphate-resisting cement or blended cements and low permeability.
- Carbonation lowers the pH of the concrete and starts corrosion; more important in the above-ground and ventilated parts.
Stray-current corrosion
Metro systems using DC traction return the current through the running rails; if the rails are not perfectly insulated, part of the current leaks into the soil, the station reinforcement and the piles, and where it leaves the steel it causes electrolytic corrosion at a rate far higher than ordinary corrosion. Protection includes insulating the rail fixings, electrical continuity of the reinforcement in the station and viaduct (a connected "stray-current collection system" that returns the current to the rails), monitoring, and keeping the structure electrically separated from utilities and piles where required.