Concrete versus wood—which one gives you more strength per dollar?
As material prices fluctuate, it’s important to think about
what gives you the most bang for your buck.
Let’s look at a simple beam first, take a 2 by 12 spanning 10 feet. With this we can support about
1200 lbs before failure, and this would cost about $33 usd. Assuming grade No.2 or better
and exterior use pressure treated for a better comparison to the weatherability of concrete.
What if this 2 by 12 beam was concrete instead? Well, pure concrete achieves less than 10% the
strength of reinforced concrete, so we would only be able to take about 130 lbs that way--so
let’s add a rebar. Now we can take 2300 lbs before failure—more than 10x the load with just one No. 3 bar added!
And, this would only cost about $31 usd (assuming 4000 psi @28 days and a #3 rebar).
Of course, it’s unusual to have a 2 by 12 concrete beam, but the logic hear extends
to multiple widths. For example, the comparison is the same for a built-up, three-ply 2 by 12
and a similar size concrete beam. (Note that self-weight, material resistance factors,
and size-effect factors have been considered here, and that prices used are based on USA
northwest pricing for material only. These assumptions are consistent throughout the video).
Let’s catalogue these strength per cost values and compare them to a
few more scenarios to see which material is best.
Let’s compare a 2x4 wood framed deck to a 4” (10cm) slab. Both cases will
assume sufficient compaction of the substrate. For the wood deck, we’ll assume it’s supported
on posts and add up costs for the joists, nails, hangers, and some plywood on top.
This comes out to about $350 usd, and would be able to support about 4000 lbs distributed.
For the slab, we’ll assume it’s placed directly on grade with a soil bearing
stress of 25 kPa (3600psi), which is on the low side of normal for compacted ground.
Since the concrete is placed directly on grade, and concrete is much stronger than soil, the
strength of the slab on grade will be equal to the soil strength below. This means that the concrete
deck would be able to take a distributed load of nearly 50,000lbs. And the cost for this comes in
a little over $600 usd. Note that reinforcing is not needed for strength in this case, but a few
bars should be included anyways to help control shrinkage and cracking over weather cycles.
In this case, the concrete wins again and by a much larger margin. And, this is without
saying anything about long-term durability of these materials being exposed to the elements.
However, this is not strictly a comparison of wood versus concrete strength per price, because
in this case the soil strength is governing the concrete strength. The comparison does highlight
the versatility of concrete though, because it’s easily formable, generally inert, and has high
compressive strength, which allows the slab to have strength equal to the soil in this case.
Let’s try a more direct comparison too, by checking a suspended deck.
For a 4 inch (10 cm) concrete suspended slab.
We'll need some formwork
which will be similar to the wood deck we just priced, and we’ll need structural reinforcing bars too.
Adding these prices comes to about $1000 usd. And in this case the slab
can support about 16,000 lbs. In terms of strength per dollar,
this is a little closer to the wood deck—but concrete is still winning.
How about comparing a wood column to a concrete column?
For a 6x6 wood post 10ft long, this would run $58 usd and be able to carry about 20,000 lbs.
For a concrete column with the same dimensions,
the cost would be about $200, but the strength would be around 110,000 lbs.
Adding this to our strength per cost table, we can see that both materials are more cost effecting
in taking compression load than they were in bending as beams, but concrete is still winning.
So, to summarize and list a few other relevant factors.
Concrete comes out on top in the scenarios analyzed here—but there
are a few important criteria to note. The prices here are for material only,
and the labour costs for wood and concrete systems will vary. Labour cost is a
significant factor for pricing construction work, and can be difficult to estimate.
However, we can do a sensitivity analysis on our numbers to see how labour price may affect
the results. Even if we say the labour for wood frame is zero, and the labour for concrete is
3x the material cost, concrete still comes out on top in some of the cases we analyzed simply
due to having much greater strength than wood. Another important factor is the grade of wood.
We can buy 5 dollar stud grade interior rated 2 by 4s, but they will only allow stresses of 5MPa.
Spending 10 dollars for a No.2 structural grade exterior rated would allow 10 MPa stress.
Twice the cost, but also twice the strength—so this would not
change our results one way or another since we account equally for both strength and price.
Scalability is also an important aspect of strength and cost analyses. For example,
our 2x12 concrete beam can be scaled in width and depth to get an exponential increase in strength
with a fairly linear increase in price. However, the same beam in wood can only be increased in
width by adding plies. If additional depth is required, a different material such as glulam or
other engineered lumber would be required, which would result in an exponential increase in cost.
A final point in favor of wood construction is general workability. Wood can be much easier
to work with since it’s fairly soft and light compared to concrete, and comes in standard sizes
ready to fasten and frame. In comparison, sure you can get more strength per cost out of concrete,
but it’s also more labour intensive to place, and much more difficult to move
or modify afterwards (if not impossible). Analyzing structural materials to take your
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