Steel Beam Design Explained

Where steel beams are used, and why

Steel is used where a relatively long clear span is required with a shallow structural depth, or where a concentrated load has to be carried and redistributed. In housing that usually means removing a wall, forming an opening, replacing a chimney breast or supporting a loft conversion. In commercial and industrial buildings it means frames, mezzanines, plant support, crane runways and openings formed in existing structure.

Timber, reinforced concrete and masonry all remain valid options. Steel is chosen when the combination of span, load, available depth and speed of construction makes it the most economical answer, not because it is automatically the strongest material.

The load path: the first thing an engineer establishes

Before anything is calculated, the engineer establishes what the beam is being asked to carry and where that load ultimately goes. Load originates at roofs, floors and walls, travels through beams and columns, and must reach the ground through a continuous, adequate path. A beam design is only as good as the understanding of that path.

This is why the questions asked at the start of a project matter: which way do the floor joists span, is there a wall above the opening, is the roof spanning onto that wall, and what is underneath the proposed bearing positions. A beam correctly sized for the wrong load path is still the wrong beam.

The loads a beam is designed for

Loads are combined and factored to Eurocode rules. Strength checks use factored loads at the ultimate limit state; deflection checks use unfactored, serviceability-level loads. That is why a beam can pass on strength comfortably and still be governed by deflection.

• —Permanent actions — the self-weight of the structure, walls, floors, screeds, finishes, roof coverings and services above the beam
• —Imposed actions — people, furniture, storage, plant and equipment, taken from the relevant category in BS EN 1991-1-1 and its UK National Annex
• —Snow — for beams supporting roofs, including drifting against upstands and adjacent higher structures
• —Wind — normally relevant to roof beams, canopies and lightweight structures rather than internal floor beams
• —Point loads — a column, a truss, a padstone from another beam, a water tank or a piece of plant landing at one position
• —Accidental and construction actions — including the temporary condition while the beam is installed

Bending and shear

Bending is usually the first check. The applied bending moment from the design loads is compared with the bending resistance of the section, which depends on the section's plastic or elastic modulus and the steel grade — most commonly S275 or S355 in the UK.

Shear is checked at the supports and at any point load, where the vertical force is highest. For typical beams shear rarely governs, but it becomes important for short, heavily loaded spans, and where large point loads sit near a support.

Local effects at the supports and under point loads also matter. Web bearing and web buckling checks confirm that the web can carry the concentrated force without crushing or buckling; where they fail, stiffeners or a longer bearing length are required rather than a deeper beam.

Deflection and serviceability

Deflection is a serviceability issue, not a safety one, but it is what people notice. Excessive deflection cracks plaster, causes doors to bind, makes floors feel lively and can transfer unintended load to non-structural partitions and glazing.

UK practice commonly limits deflection under imposed load to span/360 for beams supporting brittle finishes, and often span/500 where masonry, large glazing or bi-fold doors sit on or under the beam. Total deflection limits such as span/250 are also applied. These are not code minima to be met marginally; they are the reason a beam is frequently one or two sizes larger than strength alone requires.

Precamber can be specified on longer spans so that the beam deflects towards level under permanent load. It is common in commercial work and rare in housing.

Lateral-torsional buckling and restraint

A steel beam bent about its major axis has a compression flange. If that flange is not restrained laterally, it can buckle sideways and twist long before the section reaches its full bending capacity. This is lateral-torsional buckling, and it is one of the most frequently misunderstood aspects of steel design.

Restraint comes from the structure connected to the beam — a concrete slab, timber joists correctly fixed, a masonry wall built tightly on top, or discrete restraints such as ties and secondary beams. The effective length between restraints, and where the load is applied relative to the shear centre, both change the answer significantly.

The practical consequence is that a beam left fully exposed in an open-plan space, with no wall or slab restraining its top flange, may need to be substantially larger than the same beam built into masonry. It is also why site changes that remove restraint — omitting the blockwork above, or notching joists rather than hanging them — are not neutral decisions.

Choosing a section: UB, UC, PFC and hollow sections

Section properties come from the SCI 'Blue Book' (P363). The engineer is balancing capacity, depth, weight, availability, connection practicality and how the beam will physically be brought into the building — a five-metre beam that cannot get through the house is not a solution.

• —Universal Beam (UB) — deep and efficient in bending; the default choice where depth is available
• —Universal Column (UC) — squarer, shallower for a given weight, useful where headroom is tight and for columns
• —Parallel Flange Channel (PFC) — used in pairs either side of a wall, at edges, and where a flat face is needed
• —Rectangular and square hollow sections (RHS, SHS) — torsionally stiff, good where the beam is exposed or laterally unrestrained
• —Circular hollow sections (CHS) — used where appearance or torsional performance governs
• —Fabricated plate girders and flitch beams — where standard rolled sections cannot achieve the span or depth required

Bearings, padstones and what sits below

The beam has to land on something. Bearing stress at each end is checked against the strength of the supporting masonry or concrete, and where it is exceeded a concrete padstone or spreader plate is specified to distribute the load.

Below the padstone, the pier, wall or column must carry the reaction down to a foundation capable of taking it. On alterations this is where problems most often arise: a new beam concentrates load that was previously spread along a wall, and the existing foundation beneath a narrow pier may never have been designed for it.

• —Minimum bearing lengths, typically 100 mm to 150 mm onto masonry
• —Padstone size and concrete strength, or a steel spreader plate
• —Capacity of the pier or column below, including slenderness
• —Foundation adequacy beneath the new reaction, and whether local underpinning or a new pad is required
• —Restraint and levelling — bedding mortar, grouting and packing all being properly executed

Connections

Where beams meet beams or columns, the connection is designed as well as the members. Simple connections transfer shear only; moment connections transfer bending and are used where frame stability depends on continuity.

In housing, connections are usually bolted cleats, end plates or welded fabrications made in the workshop and bolted on site. Fabrication tolerance, bolt access and the sequence of erection all influence what can realistically be built, which is why connection details are agreed rather than assumed.

Fire protection and corrosion protection

Unprotected steel loses strength as it heats, so structural steel usually requires fire protection appropriate to the required period of resistance — commonly 30 or 60 minutes in housing. Encasement in plasterboard, intumescent coatings and boarding systems are all used, and the specification is part of the structural design rather than an afterthought for the decorator.

Corrosion protection depends on exposure. Internal, dry, heated environments need little more than a shop primer; external, damp, coastal or industrial environments require a specified paint system to BS EN ISO 12944 or galvanising. Beams built into external walls or supporting balconies deserve particular attention.

Why two similar-looking beams can be very different

• —One carries a roof and a wall above; the other carries only a partition
• —One is restrained by a slab; the other is fully exposed and unrestrained
• —One supports brittle finishes or bi-fold doors, so a tighter deflection limit applies
• —One receives a point load from a second beam or a column at midspan
• —One has limited depth available and must be a UC section rather than a UB
• —One has poor masonry at the bearings, requiring padstones and pier strengthening

Common misconceptions

• —'It's only a small opening' — span is one variable among several; load and restraint often matter more
• —'The builder can pick an RSJ' — substitutions change capacity, restraint assumptions and bearing loads
• —'Bigger is always safer' — a heavier beam increases reactions and can overload the pier and foundation below
• —'Deflection doesn't matter' — it is the most common cause of cracked plaster and binding doors after alterations
• —'The beam is the whole job' — padstones, piers, foundations, restraint and temporary works are part of the same design

What your engineer will need from you

• —The proposed opening or span, with dimensions, and the intended finished arrangement
• —The age and construction of the building, and any previous alterations
• —What is above the opening — wall, floor, roof, and the direction of span
• —Photographs of the area, inside and out, and of the ceiling and floor above
• —Any drawings, previous calculations or Building Regulations records
• —Whether the beam will be exposed, encased, or built into masonry

Related guidance

• BES-TG-002Removing a Load-Bearing Wall: A Structural Engineer's GuideRead →
• BES-TG-011Structural Engineering for Mezzanine FloorsRead →
• BES-TG-005What Information Does a Structural Engineer Need?Read →
• FAQCan I remove a load-bearing wall?Read →
• FAQWhen do I need structural calculations?Read →
• FAQWhat is a steel connection design?Read →