How the two differ in kind
Both of these do one job: collect a point load out of the beam and deliver it to something solid underneath. What separates them is how each one buys the capacity to do it, and the answer is not strength. A column of any ordinary storey height fails by buckling — bowing sideways and losing the plot long before the material itself is crushed — and resistance to buckling is governed by stiffness, not by how strong the stuff is. Structural timber's modulus of elasticity is a small fraction of steel's, so timber cannot win that contest with quality of material. It wins it with quantity: a post fat enough that its slenderness never becomes the governing problem in the first place. Steel takes the opposite route, putting a modest amount of very stiff material as far from the centre of the section as it can get it, which is the entire idea behind a hollow section. Fill that tube with concrete and you have added cheap compressive material exactly where the steel was doing least work, while the concrete braces the thin wall from inside and stops it folding locally. The outcome is the thing people notice first: for one storey and an ordinary beam reaction, the steel column occupies a fraction of the plan area the timber post needs. In an open-plan room, that plan area is the whole reason anybody is reading this.
Before the footprint decides anything, though, there is a question underneath it that changes the answer completely, and most people do not know it is there. Buckling capacity falls with the SQUARE of the unbraced length — the distance between points where something stops the column moving sideways. Halve that distance and the critical load goes up roughly fourfold; leave the column standing free from floor to beam and you are asking it to solve the hardest version of its own problem. This is why a post buried in a stud wall and a post standing alone in the middle of a room are not the same member wearing different clothes. The wall restrains the post continuously, its effective length collapses, and timber's stiffness disadvantage largely stops mattering. The free-standing basement or porch column has no such help, and that is precisely the condition where timber's answer becomes a post of a size people start objecting to. Shape enters here too. A rectangular timber post has a strong axis and a weak one, and it will buckle about the weak one regardless of which way you were hoping it would lean, so the small dimension governs and the large one is partly wasted. A round tube has the same stiffness in every direction — there is no weak axis to find — which is a quietly efficient thing to be when the load has no fixed opinion about which way to push.
So where does the answer actually land? Genuinely split, for a great many ordinary supports, and it is worth saying that plainly rather than crowning a winner. What is not split is which questions settle it. First: does the footprint cost you anything where this thing actually stands? In a corner, in a wall line, at the end of a basement partition, the timber post's bulk costs nothing at all and the case for steel largely evaporates with it. Standing in the walking route between a kitchen island and a table, every centimetre is felt daily for as long as you own the house. Second: will the base ever be wet? A timber post's end grain sitting on a slab that sweats, or on a porch deck that drains toward it, is drinking, and it is drinking at the most heavily loaded section of the member. Third: is the geometry finished? A timber post is cut to length in a minute by whoever is standing there; a steel column with plates welded on is a fabricated part with a lead time and a length frozen at the shop. And whichever you choose, keep this in proportion: the column is the easy half of the problem. What sits under it — a footing sized for a concentrated load, not the slab that happens to be there — and what it lands on at the top decide more about whether this works than the choice between timber and steel ever will.
The factors that actually differ
| Solid timber post | Steel / concrete-filled steel column | |
|---|---|---|
| How it gets its capacity, and what that costs in floor space | Bulk. Low stiffness is compensated with cross-section, so the post is sized until slenderness stops governing — which makes it a substantial object in the room, and one you can see and lean on. | Geometry. A thin, very stiff wall held far from the centre, with concrete filling the middle to carry compression cheaply and brace the wall against local folding. Far less plan area for the same job. |
| Weak axis | A rectangular post has one, and it will buckle about it. The small dimension governs the whole design and the large dimension is only partly earning its keep. | A round section has none — equal stiffness in every direction, which suits a free-standing column that cannot know which way it will be pushed. A square tube has none either: equal about both principal axes means equal about every axis through the centre, so there is no weak direction to find in that one either. |
| What braces it at mid-height | Inside a stud wall it is restrained continuously and the effective length nearly disappears, which is where timber posts are at their most competitive and least noticed. | Also improved by restraint, but rarely needs it — the usual reason to reach for steel is precisely that nothing is available to brace against over a full storey. |
| The base, and water | End grain on a damp slab or a wet deck wicks, and it wicks at the most heavily loaded section. Needs a preservative rating that matches the exposure and a standoff base that keeps the cut end up out of standing water. | Corrodes rather than rots, and a coating plus a base plate bedded clear of the wet handles it. The hidden risk runs the other way: an unfilled tube that admits water at the top and cannot drain at the bottom rusts from the inside where nobody inspects. |
| Behaviour in fire | Chars at a rate that can be relied on, and the char shields the sound timber behind it — so a solid post can be sized with a sacrificial thickness and left exposed, which is why exposed timber posts are ordinary. | A slim section heats fast and loses strength with it, so an applied protection is the usual answer where a rating applies. Concrete fill genuinely helps: the core keeps carrying while the shell softens, which is one of the strongest arguments for filling rather than leaving hollow. |
| Getting the length right, and changing it | Cut on site, to the millimetre, by whoever is holding the saw. A floor build-up that changes after the post arrives is absorbed without a phone call. | Fabricated to a length with the plates already welded on, so the dimension is frozen at the shop. Tolerance is taken up at the base with shims or levelling nuts and then grouted solid — a real system, but one that absorbs millimetres, not a revised floor level. |
| Finishing around it | It is timber, so casings, trim, panelling and a shelf bracket all fix straight to it with what is already in the van. Left bare it reads as a deliberate part of the room. | Nothing fixes to it without drilling, welding or a bolted collar. And boxing it out in studwork to make it look like a post hands back the floor space that was the reason for choosing it — worth deciding before, not after. |
| How the load leaves the bottom | Strong along the grain, weak across it — so the post is rarely the limit. Where it lands on a beam, sill or plate, the load is crossing the grain of THAT member, and crushing there is one of the commonest things overlooked. | The reaction leaves through a welded plate, and the plate's size is a design decision rather than a consequence of the member — so the bearing area is chosen deliberately, and is often larger than the post it replaced rather than smaller. What that buys is control over the pressure on what is below; it does not remove the need for a footing sized for a point load, which the timber post standing on the same slab needed just as much. |
| How it arrives and gets stood up | Off the shelf at the yard, carried in by hand down an ordinary staircase, stood up and trimmed the same day. No drawing, no lead time, no second trade. | A made part with a shop drawing ahead of it. Filled sections are usually erected empty and filled through the top, which adds a concrete operation on site and a cure before the composite capacity is really there. |
Which one, and when
Choose solid timber post when…
- The post stands in a wall, in a corner, or anywhere its bulk costs you no usable floor — the strongest argument for steel simply does not apply here.
- The length is not final, or the floor build-up may still change: a post that is cut to fit absorbs on site what a fabricated column would have to be re-ordered for.
- It will be seen, trimmed out, or fixed to — timber takes screws anywhere along it, and a post left exposed can be sized to char rather than dressed in applied fire protection.
- The job is dry, interior and modest, and you would rather the crew already standing in the room finished it today than wait on a fabricator for one small part.
Choose steel / concrete-filled steel column when…
- The footprint is genuinely in the way — a walking route, an island clearance, a sightline across an open-plan room where every centimetre gets noticed for years.
- The column stands free over a full storey with nothing to brace it, and the load is large for the height: this is exactly where timber's answer becomes a post nobody wants in the room.
- The base will be wet, or will be wet eventually — a porch, a garage, a slab that sweats — and you would rather manage a coating than manage end grain in water.
- The geometry is settled and permanent, so freezing the length at a fabricator costs nothing, and slenderness is doing the deciding rather than convenience.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Can these two calculators tell me what size column I need?
- No — and the reason is worth understanding, because these two tools sit at OPPOSITE ends of the same failure curve. The Euler calculator returns the elastic critical buckling load: the load at which a perfectly straight, perfectly elastic column of that stiffness and that unbraced length goes unstable. It contains no material strength at all, so for a stocky post it returns an enormous number the timber would never reach, because it would crush first. The composite tool returns Pno, the cross-sectional squash capacity — steel yield across its area plus confined concrete across its own — and it contains no length at all, which is why it says in its own confidence note that the required slenderness reduction has not been applied. One answers 'when does it go unstable, ignoring strength'. The other answers 'when does it crush, ignoring height'. A real column fails somewhere BELOW whichever of those two is lower, on a curve that blends them, and neither figure has had a safety factor or a load factor anywhere near it. Use both to understand what drives a column and to compare the shape of two options. Do not use either to specify one: the load coming down out of that beam is the hard part, and a support that holds up floors above is an engineer's calculation that building control will want on paper.
- Which one is cheaper?
- They are cheap in different shapes, which is why no figure belongs here. A timber post is a material purchase and almost nothing else — bought at the yard from stock, converted with the saw already on site, stood up by the people already in the room, and finished with fixings anyone has. Its cost tracks its size and very little else. A steel column is a fabricated part, and fabrication cost is largely FIXED per column: the drawing, the setup, the two welded plates and the coating cost much the same whether the column is short or tall, so a single one carries the whole overhead and a run of them shares it. Add a lead time you cannot compress, a base detail with shims and non-shrink grout, and — if it is being filled — a second concrete operation and a cure. Then there is the cost that appears on neither quote: if the column stands where floor space is genuinely scarce, the slim one has already paid for itself in a way no invoice records, and if it stands in a corner where nobody will ever touch it, it has bought you nothing at all. Price both against where the thing actually stands.
- What does the concrete inside the steel tube actually do?
- Two favours, in both directions, which is what makes it composite rather than just heavy. The concrete carries compression, which is the one thing concrete is genuinely good at, and it does so in the middle of the section where the steel was contributing least to stiffness anyway. In return the tube acts as permanent formwork and as a hoop that confines the concrete, and confined concrete carries more than the same concrete standing in open air. Meanwhile the concrete braces the tube wall from the inside, so a thin wall that might otherwise buckle locally — dimpling inward under load long before the column as a whole gave up — has nowhere to go. AISC recognises the confinement effect explicitly and treats shape as part of it, crediting round sections more than rectangular or square ones because a circular hoop grips its core evenly all the way round while a flat wall bows outward at mid-face and grips hardest at the corners. Fire is the third benefit and often the unadvertised one: as the outer steel heats and softens, the core is still cool and still carrying, which buys a filled column time an empty one does not have.
- Can I just swap an existing timber post for a slimmer steel column?
- Sometimes, and the column is the easy part of it. Three things need answering first. What is under it: the old post's load has to keep going somewhere, and it is the same load either way — a base plate spreads it over whatever area the plate is sized for, which changes the pressure on the surface but does nothing about what is under that surface. A pad or footing sized for a point load is a requirement rather than a detail — a floor slab is not a footing and was never asked to be one, and if the timber post was standing on one, that was already a problem before the swap. What happens at the top: the beam has to bear properly on the new cap, and packing the difference with whatever is handy is how a good column ends up in a bad load path. And how the load is held while the swap happens: the post is carrying something right now, so it needs propping to one side of the work before it comes out, and that temporary support is its own design, not an afterthought. If the answer to 'what does this post hold up' is anything other than certain, that is the question to resolve before the tools come out.
- Does a timber post need to be treated, and what should it stand on?
- It depends entirely on what the base will meet, and the base is where posts die. Dry, interior and clear of concrete, an untreated post is ordinary. Sitting on a slab, in a garage, on a porch deck, or anywhere it can be splashed or stand in a puddle, it needs a preservative rating that matches that exposure — and ratings are not interchangeable, with ground contact being a different specification from above-ground use. Whatever the rating, keep the cut end up out of the wet: a standoff base that lifts the timber clear, lets air at the end grain and gives water somewhere to leave does more for a post's life than any amount of treatment applied to a member left sitting in water. Two details that get missed. A field cut exposes untreated timber in the middle of a treated piece, so cut ends want an end-grain preservative. And treated timber is corrosive to ordinary fasteners, so the connectors and bolts at the base have to be a compatible grade or the fixing fails before the post does.
- What about an adjustable screw-jack steel column?
- They are common in basements and they solve a real problem, but they are frequently misunderstood. The threaded adjustment exists so the column can be set and levelled precisely during installation, and to take up settlement once under supervision — it is not a jack for cranking a sagging floor back up over the years, and treating it as one is how a beam gets pushed harder than anything above it was designed for. Their rated capacity also depends on how far they are extended, because extension is unbraced length and unbraced length is squared in every buckling calculation there is, so a column run near the top of its range is not the column its label describes. Some jurisdictions restrict adjustable columns in permanent structural use, or require the adjustment to be fixed once set. If the support is permanent and the geometry is known, a fixed column of the right length is the simpler and stronger answer; the adjustable one earns its keep where the final dimension genuinely is not knowable in advance.