Fixed vs Pinned Supports in Structural Engineering: Simple Guide for UK Home Projects
When you put in a steel beam, remove a wall or build an extension, the way the ends of the beam are held makes a big difference to the design. Engineers talk about fixed vs pinned supports and about base stiffness. This guide explains those ideas in plain English and shows how they affect beams, columns and frames in real UK home projects.
What Are Fixed and Pinned Supports?
Simple definitions in plain English
A pinned support lets the end of a beam rotate but not move sideways or up and down. Think of it like a strong hinge.
A fixed support stops the end of a beam from moving and from rotating. The beam is clamped in.
Both types can carry heavy loads. The difference is how much the beam bends and where the bending happens.
How supports affect bending in a beam
With two pinned supports, the beam sags more in the middle and has higher bending in the span. With one or both ends fixed, the beam is stiffer. Some of the bending shifts toward the supports.
- Pinned at both ends → beam bends more.
- Fixed at one or both ends → beam bends less, but the supports have higher forces.
What “base stiffness” really means
Base stiffness describes how much rotation the support allows. A perfectly pinned end has almost no stiffness against rotation. A perfectly fixed end has very high stiffness. Real life sits between the two. A steel beam bolted to a column plate has some rotational stiffness, but not as much as a fully cast-in frame.
Why Support Conditions Change Beam Sizes
Fixed support example
Take a 4 m span beam carrying a uniform load from a typical domestic floor. If both ends are treated as fixed in the design, the maximum bending in the span is lower. The engineer may choose a smaller UB, because the fixed ends share more of the bending.
Pinned support example
If the same beam is treated as pinned at both ends, the bending in the middle is higher. The beam will need a heavier section to keep deflection and stresses within limits.
What happens if you assume the wrong support
If a beam is designed as fixed but actually behaves more like pinned on site, it will deflect more than expected. You may see:
- sagging ceilings
- cracked plaster above openings
- doors that stop closing properly
That is why the support detail, plate thickness, bolt layout and bearing must match the design, not be guessed on site.
Where Fixed and Pinned Supports Appear in UK Homes
RSJ for load bearing wall removal
When you carry out load bearing wall removal, the new RSJ is usually treated as simply supported, which is close to pinned at each end. The beam sits on padstones or posts, often on masonry piers.
Beams and frames in extensions
In many rear or wraparound extensions there is a goalpost frame: a beam on two steel columns. The base of each column may be:
- cast into a foundation block (stiffer base)
- fixed with base plates and anchor bolts (partial stiffness)
- sitting on masonry (often closer to pinned)
The way the base is detailed decides whether the frame is modelled with fixed or pinned bases in the calculations. See a full example in our home extension structural design article.
Loft conversion beams and ridge supports
In loft conversions, new steel beams support loft floors and the roof. Some are designed as simply supported on walls, others as part of a stiffer frame with ties and connections. Again, whether the ends are treated as fixed vs pinned affects beam sizes and deflection.
Base Stiffness of Columns and Frames (Explained Simply)
Flexible vs stiff bases
Think of a column base like the foot of a table leg:
- A loose leg is flexible and moves a lot.
- A leg that is bolted down firmly is stiff.
A flexible base behaves closer to a pin. A stiff base behaves closer to fixed.
When engineers treat a base as fixed
An engineer may treat a base as fixed if:
- the column is welded to a thick base plate
- the plate is anchored into a strong concrete foundation
- there is enough depth and reinforcement in the foundation block
The joint can resist rotation as well as shear and axial load. That means the frame can help brace the structure.
When a base behaves like pinned
If the column sits on a small padstone or brick pier without a proper moment connection, the base acts more like a pin. The column can rotate more under load, which increases bending in the beam above. In most domestic work, bases are treated as pinned unless the detail is clearly designed as a rigid frame.
What Building Control Expects to See
Support conditions in calculations
When you submit a steel beam calculation, the support conditions are part of the model. Building Control expect:
- a description of supports (pinned or fixed)
- clear reaction forces at each end
- deflection limits checked against the correct span
Bearing details and padstones
The drawings must show:
- bearing length (commonly 100 mm or more each end)
- padstone size and strength
- any columns or posts under the beam
If the design assumes a fixed end, the connection must be drawn in more detail.
Fire protection and load paths
Supports also need fire protection and a clear load path down to foundations. Building Control check that the structure meets Building Regulations. Official guidance is on the Planning Portal.
When You Need a Structural Engineer to Check Supports
Wall removals and large openings
Any time you remove a load bearing wall or make a large opening you need an engineer. The support type, base stiffness and bearing all affect beam design.
Home extensions and wide spans
For rear and side extensions, beams often carry roof and floor loads over wide spans. A correct model of fixed vs pinned supports can make the difference between a light UB and an over-stressed beam.
Loft conversions and steel frames
Loft conversions use several beams working together. Some act as simple supports, others as part of a frame. An engineer checks which members can be assumed as pinned and which need more stiffness.
Step-by-Step Process for Getting the Supports Designed
- Send your drawings or sketch showing spans and openings.
- Provide photos of existing walls, floors and foundations where possible.
- Engineer reviews the load path and decides which supports can be treated as pinned and which need extra stiffness.
- Calculations are prepared for beams, columns, padstones and foundations.
- Drawings are issued with clear details for supports, plates and fixings.
- Submit to Building Control and give the pack to your builder to follow on site.
FAQ – Fixed vs Pinned Supports for Homeowners
Q: What is a pinned support in simple terms?
A pinned support holds the beam up but lets the end rotate. It behaves like a strong hinge.
Q: Does a fixed support always give a smaller beam?
Not always, but a correctly fixed end often reduces the bending in the span, which can allow a lighter section. The detail must truly act as fixed, not just be drawn that way.
Q: Does Building Control care whether supports are fixed or pinned?
Yes. They review the calculations and details. If the support detail does not match the design model, they can ask for revisions.
Q: Can a builder decide on fixed vs pinned supports on site?
No. The engineer chooses the support model and designs the connection. The builder follows the drawings.
Conclusion
Fixed and pinned supports are not just technical labels. They decide how beams bend, how frames behave and how much load your walls and foundations carry. Getting the support model right at design stage avoids sagging, cracking and arguments with Building Control later.
If you are planning wall removal, an extension or a loft conversion and want the supports checked properly, you can use this page to lead people straight into your RSJ, extension or loft enquiry funnels.