Structure and support · Frames & Load-Bearing Walls
Structural Steel Frame System
This entry shows how members, connections and the stability arrangement of a steel frame combine into one structure, and what has to be settled at its junctions with floors, envelope and substructure.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What steel frame is
A steel frame is assembled rather than formed. Members are cut, drilled and fitted with plates and cleats in a fabrication shop, arrive with their connection preparation already in them, and are bolted together in position. The geometry of the building is therefore decided at fabrication, and what happens on site is largely the consequence of decisions taken before anything was delivered.
The connections decide the behaviour. The same columns and beams joined so that the beam ends can rotate give a frame that leans on something else for stability; joined so that the beam and column hold their angle, they give a frame that resists sway itself. This is why a connection cannot be treated as a fixing detail and substituted for one that looks similar.
The nearest relative is portal-frame-system, and the distinction is visible in a building. A portal frame gets its stability in its own plane from a small number of rigid frames repeated along the building, and the deep haunches where rafter meets column give it away at the eaves. A general steel frame carries a separately identifiable braced bay or a solid core, and the diagonals or the core are what you can point at. The test is whether the frame itself resists sway or something else does.
A steel frame is also not a complete structure until its floors are on. The floor plates are what collect horizontal force and take it back to the bracing, so the frame standing during erection and the frame in service are different structures. Temporary stability is designed as its own arrangement rather than assumed from the permanent one.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- steel skeleton frame
- structural steelwork frame
- braced steel frame
- steel column and beam frame
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Carry load through a small number of columns so that the plan below is free of walls that have to line up.
- Transfer beam reactions into columns through connections designed for the behaviour the frame relies on.
- Resist sway through a defined arrangement of bracing, cores or rigid joints rather than through the enclosure.
- Deliver the whole structure into the substructure at a small number of concentrated positions.
- Provide the fixing arrangement from which floors, envelope and services are supported.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportColumns
The vertical members that gather beam reactions at each level and carry the accumulated load to the base. They are usually continuous through several storeys, with splices in agreed positions, so a column is a single structural line rather than a stack of separate pieces.
- Primary supportBeams
The horizontal members that carry the floor and pass its reaction into the columns. What a beam is asked to do depends on how its ends are held, which is why the same section can appear in two positions doing structurally different work.
- AttachmentBeam-to-column connections
The plates, cleats and bolts that join members. They decide whether a joint is intended to rotate or to hold its angle, and they are the point at which the fabricator's work and the erector's work meet, so tolerance is resolved here or nowhere.
- Primary supportBraced bays, cores and rigid joints
The elements that stop the frame leaning. Whichever of them is used, it occupies plan space and cannot be moved to suit a layout, because the frame around it was designed on the assumption that it was there.
- SubstrateBase plate, bedding and holding-down assembly
The arrangement that spreads column load into the substructure and holds it against uplift and overturning. It is where fabrication tolerance meets groundwork tolerance, so it is set out before the frame is ordered rather than adjusted when it arrives.
- Primary supportFloor bearing and shear connection
The interface at which the floor sits on or engages with the beams. Beyond carrying the floor, it is what allows the floor plate to act as a horizontal plane, which is what the bracing arrangement depends on to receive force.
- AttachmentEnvelope attachment provision
Cleats, plates and secondary steelwork provided for cladding, glazing and canopies. These are usually fabricated into the frame, so an envelope designed after the steelwork has been made has to fit the fixings that already exist.
- ProtectionApplied protective treatments
Coatings and applied systems put on the steel for corrosion or for protection in use. Most are applied before delivery and interrupted at every site connection, so the treated frame and the erected frame are not quite the same object.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The connection type propagates through the whole frame. A joint detailed to let the beam end rotate transfers vertical reaction but deliberately does not hold the beam square to the column, so the structure has to lean on bracing or a core. Replace that joint with a stiffer one, or the reverse, and the forces travel differently everywhere, not only at the joint that was changed.
Bracing only works if the floors can feed it. Horizontal force arrives across a whole facade and has to be collected by the floor plate and delivered to the braced bay, so a large stair opening, a riser or an atrium cut through that plate interrupts the delivery route. The bracing is unchanged and the frame behaves differently.
Everything gathers at the base. A whole column line arrives on a plate over a small footprint, where the bedding beneath transfers it and the holding-down assembly resists lift and turning. Because the frame is fabricated to fixed dimensions, a base set out inaccurately cannot be absorbed later, and the reconciliation of the two tolerances is a design matter rather than a site one.
The envelope and the frame meet at a bracket, and that bracket has several duties at once. It carries cladding load back into a structure that moves under wind and load, it passes through whatever is controlling water and air, and it is a metal path across the insulated line. Deciding it settles a structural question, a weathering question and a thermal question in the same detail.
Protective treatments and connections are in tension with one another. Shop-applied protection is broken wherever a bolt is made up, a plate is welded or a member is trimmed, so the positions that most need protection are the ones the shop could not reach. What is done at those positions is specified rather than left to the erection sequence.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
These are commonly encountered as the members of a steel frame, and they behave very differently when left exposed. Which belongs in a given frame, and what its exposure demands, is settled by the structural designer and the supplier's documentation.
Attachment
Connection components, secondary steelwork and fixings are commonly encountered in these forms. Selection depends on the exposure at the position and on contact with other metals, and is a determination for the designer rather than a property of the metal.
Substrate
These are commonly encountered where the floor engages with the frame and where column bases are bedded. What each contributes to the behaviour of the completed floor plate is a matter for the structural design, not for the material alone.
Protection
Applied treatments of these kinds are commonly encountered on structural steelwork. This entry states no performance for any of them; what a treatment achieves is a property of a tested arrangement, applied and maintained as its documentation requires.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Attachment
- Support Angle →The veneer's weight is delivered into the frame through the angle and its fixings, which the frame has to be arranged and positioned to receive.
- Cleat →The connection position at which a beam is joined to a column or to another beam.
- Base Plate →Transfers column load into the foundation and anchors the frame in position.
- Gusset →The connection position where converging members — bracing, truss members, frame members — are joined at a node.
- Haunch →A moment-transferring connection position, distinct from the simple connections used elsewhere in a frame.
- Shear Connection →Welded to the top of the frame's beams, making the floor above part of the beam's structural action.
- Splice →Joins fabricated lengths of column or beam so that the member continues as one through the joint.
- Stringer →In a steel flight the string is both the carrying member and the element the treads are attached to.
Edge and termination
Finish surface
Primary support
- Beam →Spans between columns and carries the floor or roof structure.
- Bracing →Braced bays provide the frame's resistance to sideways movement.
- Cantilever →Members continuing past a column line, where the back span and its hold-down are part of the frame.
- Cleat →The connection position at which a beam is joined to a column or to another beam.
- Column →Carries floor and roof loads to foundations, defining the frame's vertical load path.
- Base Plate →Transfers column load into the foundation and anchors the frame in position.
- Gusset →The connection position where converging members — bracing, truss members, frame members — are joined at a node.
- Haunch →A moment-transferring connection position, distinct from the simple connections used elsewhere in a frame.
- Shear Connection →Welded to the top of the frame's beams, making the floor above part of the beam's structural action.
- Splice →Joins fabricated lengths of column or beam so that the member continues as one through the joint.
- Landing →A framed landing platform carried by the steel structure, to which the flights above and below are connected.
- Stringer →In a steel flight the string is both the carrying member and the element the treads are attached to.
- Stiffener →Holds the shape of a section where concentrated force arrives, at supports and at connections that transfer turning.
- Bearing →Connections and seatings through which member loads pass into columns and foundations.
- Transfer Beam →Redistributes load where the column grid changes between levels.
- Chord →The booms of a steel truss or lattice girder, connected to the web members at each node.
- Web Member →The internal members of steel trusses and lattice girders, connected at nodes to the booms.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Floor engaged with the beams
A deck and topping working together with the beams gives a floor that is also the horizontal plane the bracing needs. Whether the two act together depends on the connection between them, which is settled with the frame design rather than with the floor finish.
Read about Composite Metal Deck Floor →Envelope hung from the frame
The facade is carried at brackets on the frame and has to accommodate the frame moving beneath and around it. Bracket positions, the movement allowance at each floor and the way the barrier lines pass the bracket are one coordinated detail.
Read about Curtain Walling →Column bases into the substructure
Load arrives at points rather than along lines, so the substructure follows the column grid. The setting out of holding-down assemblies has to match a frame that has already been fabricated, which makes accuracy here a programme risk as well as a structural one.
Read about Pad Foundation →Steelwork passing through the insulated line
Balconies, canopies and supports that continue outside the enclosure carry a metal path with them. Whether a break is introduced, and what it does to the connection it sits in, is a joint decision between the structural and the envelope designers.
Read about Structural Thermal Break →The frame within the building's stability arrangement
Where the frame is not itself resisting sway, a braced bay or a core is doing it, and that element governs the plan. This entry supplies the vertical path and the connections; the horizontal route from floor plate to ground belongs there.
Read about Lateral Stability →Added floors within an existing frame
An inserted floor loads columns that were designed for a particular arrangement and can also change how horizontal force is collected. It is a change to the frame rather than an addition inside it, however independent the new structure appears.
Read about Mezzanine Structure →Plank bearing onto the beams
An alternative floor bearing onto the beams, with a different arrangement for making the plate act as one plane.
Read about Precast Plank Floor →Light Steel Frame Wall
Where the primary frame is steel, the wall spans between beams whose deflection and fire protection both affect the junction. Fixing back to a protected steel member is a detail that involves the fire protection system as well as the wall.
Read about Light Steel Frame Wall →Precast Facade Panels
A steel frame moves differently and offers a different fixing opportunity, usually welded or bolted rather than cast in. It also deflects under load in ways that a rigid panel cannot follow, so the restraint arrangement is where that difference is settled.
Read about Precast Facade Panels →Siphonic Roof Drainage
Collector pipework is hung from the frame and transfers force into it when the circuit primes. What the frame is being asked to carry, and at which points, is information the structural engineer needs rather than something resolved on site.
Read about Siphonic Roof Drainage →Masonry Infill Wall
A steel frame deflects under the floors it carries and its members are set out to fabrication tolerances that a masonry trade does not work to. Both facts land at the same junction: the head gap has to absorb the deflection and the flank restraint has to take up the difference between where the column is drawn and where it is.
Read about Masonry Infill Wall →Sheeted Metal Wall
Where the wall is hung on a steel frame that is not a portal, the columns and the secondary steel define where the rails can go and how far the sheets span. The frame's own movement under load then has to be accommodated where the sheeting is fixed to it.
Read about Sheeted Metal Wall →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Balustrade Post to Stair Landing
Where a guarding post is fixed into the edge of a landing, delivering everything the guarding resists into a small platform.
Structural transfer · Movement · Buildability
Beam End to Column
Where a beam meets a column, and where the connection's stiffness determines how the whole frame behaves.
Structural transfer · Fire continuity · Buildability · Movement
Beam to Masonry Infill Panel
Where the top of an infill panel meets the frame member above it, which deflects toward the panel while the panel still has to be held against wind.
Movement · Structural transfer · Air leakage · Buildability
Bracing Member to Gusset Connection
Where a brace delivers the whole of the lateral force it carries into the plate at a node, which redirects it into the members continuing on.
Structural transfer · Movement · Fire continuity · Buildability
Brick Support Angle to Outer Leaf
Where a masonry veneer stops carrying itself and hands its weight back to the structure, at a line that also interrupts the cavity.
Structural transfer · Movement · Drainage · Thermal continuity · Buildability
Column Base to Foundation
Where a column's concentrated load is spread into a foundation and the frame is anchored in position.
Structural transfer · Buildability · Moisture · Movement
Column Bearing on a Transfer Beam
Where a column that is discontinued below lands on the length of a transfer member, so the vertical load path changes direction of travel at this point.
Structural transfer · Movement · Fire continuity · Buildability
Column to Masonry Infill Panel
Where the vertical edge of an infill panel meets the frame member beside it, which has to hold the panel sideways without being propped by it.
Movement · Structural transfer · Air leakage · Buildability
Column to Portal Frame Haunch
Where the leg of a portal frame meets the deepened rafter end at the eaves, and bending has to pass around the corner rather than stop at it.
Structural transfer · Buildability · Fire continuity · Movement
King Post to Truss Chord
Where the vertical centre member of a truss meets a chord, at a joint that in the usual arrangement is resisting a pull rather than a push.
Structural transfer · Movement · Fire continuity · Buildability
Stair Landing to Stair Stringer
Where a flight's edge member lands on the platform at its end, delivering everything the flight carries into a small structure of its own.
Structural transfer · Movement · Acoustic continuity · Buildability
Stair Landing to Trimmer
Where the platform of a stair bears on the member framing the floor opening the stair passes through.
Structural transfer · Movement · Fire continuity · Buildability
Stair Stringer to Stair Tread
Where the end of each tread is received by the edge member of a flight, which is the stair's primary load path.
Structural transfer · Movement · Acoustic continuity · Buildability
Truss Chord to Truss Web Member
Where an internal member of a truss meets a chord at a node away from the centre line, which is where force crosses the depth of the truss and where the chord's own force changes.
Structural transfer · Buildability · Movement · Fire continuity
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Fire
- Fire performance is a property of a tested arrangement, including whatever protection is applied and how it is maintained. This entry states none, and the tested system together with the relevant authority governs what is required for the building and its use.
- Durability
- The positions that decide how a frame ages are the ones the fabrication shop could not treat: site connections, trimmed members, contact with other metals and anywhere water can collect on a horizontal surface within the structure.
- Movement
- Steel changes length with temperature, and a frame is generally longer than the elements attached to it. Where movement is intended to be taken - at a joint in the frame, at a bracket, or in the envelope itself - is designed rather than left to be absorbed.
- Buildability
- The frame is made before it is erected, so the design has to be fixed early and access, craneage and delivery order become design constraints. A change agreed on site is a change to something already fabricated and possibly already coated.
- Interfaces
- Every attachment for cladding, services, guarding and equipment belongs in the fabrication rather than in later site work. Drilling or welding to a frame afterwards affects both the member and its protective treatment, so late requirements are disruptive out of proportion to their size.
- Thermal
- Any steel continuing from inside to outside carries a path with it, and the frame supplies many such opportunities at balconies, canopies and support brackets. What that means for the enclosure is assessed by a qualified professional for the whole build-up.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is stability coming from bracing, from a core, or from the connections themselves, and where does that element sit in plan?
- Which connections are intended to rotate and which are intended to hold their angle?
- How does each floor plate collect horizontal force, and do the openings in it interrupt that route?
- Where do columns land, and does the substructure arrangement follow that grid?
- What attachments does the envelope need, and are they being fabricated into the frame rather than added later?
- Where does steelwork continue past the insulated line, and what is being done about it?
- What is holding the frame during erection, and how does that differ from the permanent arrangement?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A steel frame is assumed to be inherently stable because it is made of steel, when its stability may depend entirely on a braced bay or core.
- Connections are read as fixings and treated as interchangeable, when the joint type is what defines the structure.
- Adding a mezzanine is described as independent, though it loads existing columns and can alter how force reaches the bracing.
- Shop-applied protection is taken as covering the whole frame, when every site connection interrupts it.
- A frame standing on site is assumed to be complete, though it may be relying on temporary arrangements until the floors are on.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is providing stability in each direction, and where must those elements remain?
- Which connections in this frame are moment-resisting, and which are not?
- How are floor openings affecting the way horizontal force reaches the bracing?
- What tolerance has been assumed between the fabricated frame and the substructure it lands on?
- Which fixings for cladding, services and equipment are being built into the fabrication?
- What protective treatment is specified, and what happens at site connections?
- What temporary stability is required during erection, and when may it be released?
What this page does not do
- Nothing here establishes whether a member may be cut, drilled, notched or removed; that is a determination for a structural engineer on the specific frame.
- No member size, connection arrangement, bolt schedule or dimension is stated in this entry, and none should be inferred.
- Fire protection requirements are set by the relevant authority for the building and its use, and this entry states no rating or period.
- Welding or drilling an existing frame affects both the structure and its protective treatment, and is not a maintenance activity.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Structural Fire Protection · Structure
- Superstructure · Structure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Primary Structural Frames and Load-Bearing Wall Systems
Complete primary load paths from roof to foundation in masonry, timber, steel and concrete, together with the stability system without which none of them stands up.
Browse all frames & load-bearing walls entries →