Regarding joists: today's 45x220 vs. 70x170 from 1920?

[Martin_B]

For floor joists, 45x220 planks are most often used today, and it is the width (height in this case) of 220 that makes the floor stable to walk on. In the past, for example, 70x170 was used instead, which admittedly has a slightly higher volume (more wood in the same area) but only has the width (height in this case) of 170, which means that when used in floor joists, it flexes a little more when you walk on that joist.

But then I have an interesting question here.

How does today's 45x220 compare to 70x170 from 1920, considering that the wood was much better back then. When I feel the planks from the 1920s, it quickly becomes apparent how incredibly hard and heavy the wood is compared to today's wood in today's 45x220s. The difference is alarmingly large.
Shouldn't the old 70x170s have just as good load-bearing capacity, if not better, than today's (relatively "soft") 45x220s?

 

[prototypen]

Calculated by the height/width ratio, the old beam should have been 75 mm for the same load-bearing capacity, but it's marginal. However, old wood was probably three inches seven, so it's a bit thicker than what you mention.

Wood quality......... There is probably no direct truth, with more thinning, the forest grows faster and this summer it has apparently rained in Småland so the forest has grown extra for 300 billion or something like that.

Knots were present in the wood before, and people even skimped back then. It seems like they didn't have the same understanding of the laws of mechanics back then, so they couldn't really utilize the wood properly.

Today, you can buy sorted wood with better quality.

Protte

 

[Martin_B]

Calculated on the height/width ratio, the old beam should have been 75 mm for the same load-bearing capacity, but that's marginal. But old timber was probably three inches seven, so it's a bit thicker than what you mention

Yes, in reality, they are slightly wider than 70 as you say. More like 75 if not 80 sometimes. But how do you calculate the comparison for "same load-bearing capacity"? You don't mean the volume now, right? Because there are wood I-beams (or whatever they are called) that are used quite frequently in the USA, for example. And then it's typically a 2"x2" or something like that at the top edge and one at the bottom edge with a plywood or OSB board between them (recessed into the 2"x2"). Standing on edge, it becomes very strong and provides high stability for the floor. However, they are weak laterally, but that matters less when they're used for joists. Regardless, the point I'm trying to make is that these "wood I-beams" have much less volume than both 45x220 and 70x170 but certainly provide stable joists if not more stable since they are "higher." So it can't be the volume used to compare "load-bearing capacity"? (No one said it either, I'm just wondering)

Nowadays, you can buy sorted timber with better quality.

Is it really true that you can buy better timber today?? It's one thing that they are better sorted, sawn, planed, etc. today. But the wood itself isn't as good, is it?

If I compare two planks (for example) of the same dimension where one is today's K24 and the other is from the 10s, 20s, the difference is enormous. The old timber almost feels like rock-hard oak in comparison. It also weighs more. Looking at the growth rings, the old timber is much denser.

So can today's timber really be as strong and durable? It sounds strange if it is. Carpenters and people who work with wood usually point out the significant difference between today's timber and timber from the past - to the older timber's advantage, of course.

 

[prototypen]

Doubling the height quadruples the bearing capacity. Square the ratio of the height difference

(220/170)^2x45=75.36.

Wood gets harder over time, so today's wood will be muuuuch better than future wood.

Protte

 

[AnneFi]

It was not easier to find better timber in the past.
The neighbor of my parents told about when they built their house themselves in the 1920s. (my parents' house was built in '29 and the neighbor's a few years earlier)
She told how her husband went everywhere to find wood for the mullioned windows that he crafted himself.
It had taken a very long time before he had gathered enough wood to make all the windows.

 

[Martin_B]

Doubling the height quadruples the load capacity. Square the ratio of the height difference

(220/170)^2x45=75.36.

Wood hardens over time so today's wood will be muuuuch better than future wood.

Protte

Oh, I see thanks for the formula! Just one more question. Doesn't the thickness have something to do with load capacity? I mean if it's 45 or 70-80mm? Because a 45x170 can't have the same load capacity as a 70x170, right?

 

[Gabbe1]

The formula for the moment of inertia I=b*h*h*h /12
The formula for section modulus W=b*h*h/6

 

[prototypen]

No, but I have based it on the height and width of the new rule and then added the height of the old rule, and the answer is the thickness the old one should have for the same load-bearing capacity.

Protte

 

[Martin_B]

I understand Thank you for the explanation and the formulas!