I need help calculating the load for a deck that will stand on footings. I don't really think it's necessary, but I find it interesting and would like to understand how to do the calculations.
I've used the Svenskt Trä calculator (https://www.byggbeskrivningar.se/dimensionering/altanbjalklag-ute-pa-plintar/) to calculate a deck, 5.4x4.0m on 12 footings. See attached image.
The calculator provides the loads:
Dead load 0.3kN/m²
Live load 3.5kN/m²
I guess that snow load is not included in this, so in my case, it adds another 3.0kN/m² (snow zone 3).
If we then calculate that 1kN roughly corresponds to 100kg, does that mean the construction will "weigh" approximately 0.3 * 5.4 * 4.0 = 6.48kN or 648kg? With live load and snow zone, it becomes (0.3 + 3.5 + 3.0) * 5.4 * 4.0 = 146.88 kN or 14688kg?
Since the deck will have more or less even load over the entire area, that should mean each footing will have a maximum load of 146.88 / 12 = 12.24kN?
I have more thoughts, but I think I'll start with the above to see how right or wrong I have understood it so far. Thanks in advance!
I've used the Svenskt Trä calculator (https://www.byggbeskrivningar.se/dimensionering/altanbjalklag-ute-pa-plintar/) to calculate a deck, 5.4x4.0m on 12 footings. See attached image.
The calculator provides the loads:
Dead load 0.3kN/m²
Live load 3.5kN/m²
I guess that snow load is not included in this, so in my case, it adds another 3.0kN/m² (snow zone 3).
If we then calculate that 1kN roughly corresponds to 100kg, does that mean the construction will "weigh" approximately 0.3 * 5.4 * 4.0 = 6.48kN or 648kg? With live load and snow zone, it becomes (0.3 + 3.5 + 3.0) * 5.4 * 4.0 = 146.88 kN or 14688kg?
Since the deck will have more or less even load over the entire area, that should mean each footing will have a maximum load of 146.88 / 12 = 12.24kN?
I have more thoughts, but I think I'll start with the above to see how right or wrong I have understood it so far. Thanks in advance!
Realized now also that maybe it's not necessary to account for both full useful load and full snow load at the same time...
I was actually looking at pre-cast plinths now, and for example, Benders plinths have a maximum load of 4kN or 8kN if you place them on a concrete slab.
I was actually looking at pre-cast plinths now, and for example, Benders plinths have a maximum load of 4kN or 8kN if you place them on a concrete slab.
As I wrote yesterday, it doesn't seem reasonable to count both the maximum live load (3.5kN/m²) and the maximum snow load (3.0kN/m²), because you wouldn't have 20 people on the deck with all the other considerations for live load while also having 3m of snow over the entire deck...
But even if you take a middle ground, say 5kN/m², that's still too much for the footings...
5kN/m² * 5.4m * 4.0m = 108kN
108kN / 12 = 9kN, which is more than what Benders footings can handle in any case, especially when they're placed with concrete slabs underneath.
So I don't know if I've calculated wrong, or if it's simply that I can't put into perspective how much 5kN/m² actually is. I don't know, maybe it's even unreasonable to reach 3.5kN/m² (as the Svenskt Trä calculator uses) on a "normal" deck.
But even if you take a middle ground, say 5kN/m², that's still too much for the footings...
5kN/m² * 5.4m * 4.0m = 108kN
108kN / 12 = 9kN, which is more than what Benders footings can handle in any case, especially when they're placed with concrete slabs underneath.
So I don't know if I've calculated wrong, or if it's simply that I can't put into perspective how much 5kN/m² actually is. I don't know, maybe it's even unreasonable to reach 3.5kN/m² (as the Svenskt Trä calculator uses) on a "normal" deck.
I continue with my thoughts even though I haven't received any answers yet... 😅
Under the footings, I plan to insulate against frost with XPS250, which (for a short time) can handle 250kN/m². The footing I'm looking at is 185x185mm, i.e., 0.034m². Am I correct in thinking that if the insulation can handle 250kN on 1m², it can handle 250 * 0.034 = 8.5kN on 0.034m²?
However, I will have a concrete slab of 400x400x50mm between the insulation and the footing, so that the footing can handle 8kN instead of 4kN. Calculating in the same way, it's 0.4 * 0.4 * 250 = 40kN.
Here, at least, it seems like there should be no issues. I'm more worried about my calculations in earlier posts, about the imposed load + snow load (even if only partially considered) exceeding 8kN per footing...
Under the footings, I plan to insulate against frost with XPS250, which (for a short time) can handle 250kN/m². The footing I'm looking at is 185x185mm, i.e., 0.034m². Am I correct in thinking that if the insulation can handle 250kN on 1m², it can handle 250 * 0.034 = 8.5kN on 0.034m²?
However, I will have a concrete slab of 400x400x50mm between the insulation and the footing, so that the footing can handle 8kN instead of 4kN. Calculating in the same way, it's 0.4 * 0.4 * 250 = 40kN.
Here, at least, it seems like there should be no issues. I'm more worried about my calculations in earlier posts, about the imposed load + snow load (even if only partially considered) exceeding 8kN per footing...
What kind of ground is it? You seem to assume that the ground is immobile and the pillars are the weak point...
350kg/sqm is high!! And a rough estimate. To put it into perspective, that's 4 people weighing 87kg standing on 1 sqm. In total, you would need 86 people on your patio... It's hardly likely to happen more than once during the patio's lifetime. And even if it's over what it was dimensioned for, it probably won't crash but rather settle a bit. Then there's a bigger risk that there are defects in the wood (natural or created when the patio was built) that give way first. The dimensioning tool is good. But you also have to apply it to reality. The organization behind it is pushing to sell more of the industry's products...
What other stuff are you going to have? Table, chairs, grill, etc. weigh nothing in comparison. And if you're going to have a hot tub or pool on it, you should construct accordingly with supports directly under the pool, not a rough estimate where you theoretically calculate that the load spreads 5m further away.
350kg/sqm is high!! And a rough estimate. To put it into perspective, that's 4 people weighing 87kg standing on 1 sqm. In total, you would need 86 people on your patio... It's hardly likely to happen more than once during the patio's lifetime. And even if it's over what it was dimensioned for, it probably won't crash but rather settle a bit. Then there's a bigger risk that there are defects in the wood (natural or created when the patio was built) that give way first. The dimensioning tool is good. But you also have to apply it to reality. The organization behind it is pushing to sell more of the industry's products...
What other stuff are you going to have? Table, chairs, grill, etc. weigh nothing in comparison. And if you're going to have a hot tub or pool on it, you should construct accordingly with supports directly under the pool, not a rough estimate where you theoretically calculate that the load spreads 5m further away.
Hehe, nah it's more about it being easier to calculate since there are datasheets/specifications for those things. I assume it's harder to calculate ground bearing capacity as it depends on so many parameters. By the way, it is silt/clay.
Sounds anyway like what I wrote at the end of post #3, that I probably needed to realize how much 3.5kN/m2 or 350kg per sqm really is. We won't have any hot tubs or similar, it's more about possibly building a canopy that will partly be supported by the posts. But that is still much less load than what a hot tub, with a relatively small area and high weight, would exert.
Sounds anyway like what I wrote at the end of post #3, that I probably needed to realize how much 3.5kN/m2 or 350kg per sqm really is. We won't have any hot tubs or similar, it's more about possibly building a canopy that will partly be supported by the posts. But that is still much less load than what a hot tub, with a relatively small area and high weight, would exert.