What is a beam in construction?

A beam is a horizontal structural member that spans between supports (walls, columns, or posts) and carries loads from the structure above by bending. The loads - from floors, roofs, or walls bearing on the beam - cause the beam to bend: the top face is compressed and the bottom face is in tension. The beam transfers these forces to its end supports, which must be sized to carry the resulting concentrated load. Beams may be steel (universal beam/RSJ), solid or glulam timber, engineered timber (LVL or I-joist), or reinforced concrete.

What is the difference between a beam and a joist?

A joist is a smaller, closely spaced horizontal member that carries floor or ceiling loads directly - typically spaced at 400-600mm centres, spanning between walls or beams. A beam is a larger, more widely spaced member that carries the loads from multiple joists (or from a floor slab or roof) and transfers them to columns, walls, or other beams. In practice: joists span the short dimension of a floor bay; beams span the longer dimension and carry the joists. A 'binder' is a beam that directly carries floor joists.

What sizes of steel beam are used in house construction?

The most common steel beam sections in domestic construction are universal beams (UB) ranging from 152x89 UB16 (for short spans and light loads) up to 305x165 UB40 or larger for longer spans or heavy loads. The designation gives the approximate depth x width in millimetres and weight per metre in kg/m. For typical domestic spans of 3-5m, a 203x133 or 254x102 section is often sufficient. A structural engineer must specify the correct size based on the span, loading, and end bearing conditions - never guess a beam size.

Do steel beams need fire protection?

Unprotected structural steel loses strength rapidly at temperatures above 550 degrees C - well below the temperatures reached in a building fire. Approved Document B requires structural elements to achieve a specified fire resistance period (typically 30 or 60 minutes in domestic buildings). Steel beams can be protected with intumescent paint (which expands on heating to form an insulating char), fire-rated board encasement (plasterboard casing), or spray-applied mineral fibre. The required protection thickness depends on the section factor (surface area to volume ratio) of the beam and the required fire resistance period.

Structural noun

Beam

/ biːm /

Also known as: structural beam, steel beam, binder, universal beam, RSJ

Definition

A beam is a horizontal structural member that spans between supports (walls, columns, or posts) and carries loads from the structure above by bending. The top face of the beam is compressed; the bottom face is in tension. The resulting forces are transferred to the supports at each end, which must be adequately sized (padstones or bearing plates) to distribute the concentrated load into the supporting wall or column below. Beams may be steel (universal beam), solid or glulam timber, engineered timber (LVL, I-beam), or reinforced concrete.

In practice

In domestic construction, steel beams (universal beams, still commonly called RSJs) are used where a large opening is created in a load-bearing wall - for example, removing a chimney breast, forming a wide opening between rooms, or creating an open-plan kitchen. The beam is sized by a structural engineer based on the span, the loads it carries (dead load of the structure above plus imposed/live load), and the deflection limit (typically span/360 to avoid cracked plaster below).

Timber beams are used in traditional construction - solid oak or pine beams in period buildings, glulam (glued laminated timber) in modern exposed-beam designs, and LVL (laminated veneer lumber) where a slim profile is needed. Engineered timber I-beams are used as floor joists and rafters in modern housebuilding. All beams require adequate end bearing on a padstone or bearing plate to spread the concentrated load into the supporting masonry below - minimum 150mm bearing length for domestic steel beams.

Building Regulations

Relevant PartPart A - Structure

Design standardBS EN 1993 (Eurocode 3) - Steel structures

Timber standardBS EN 1995 (Eurocode 5) - Timber structures

Fire (Part B)Steel beams require fire protection to achieve 30 or 60 min FRR

Approved Document A requires all structural elements to be designed to carry the loads imposed on them with adequate safety margins against failure, overturning, and excessive deflection. Installing a beam in a load-bearing wall requires Building Regulations approval - a structural engineer's design must be submitted. The temporary propping arrangement during installation also requires engineering consideration, as improper propping during beam installation is a common cause of structural incidents on domestic projects.

Full Building Regulations guidance

Beam

Just so you know...