One of the flooring assemblies I looked at for our renovation was 23 psf dead load. It ended up adding 22,000lbs for just the first floor. So much engineered wood...
23 psf dead load is the **distributed** dead load per every sqft, for the entire floor area. If your 22,000 pound is the total weight covering a 1,000 sqft area, for example, then each sqft's dead load is just 22 psf, which is just right.
22,000 pound is the total weight. It does not mean every sqft of the floor area is 22,000 pound heavy, or else that floor will definitely collapse.
Take a 6”x24” board of plywood. That’s 1 square foot.
Fully support it with concrete underneath it and around it. Then place weight on it. It’ll take about 600k pounds to destroy it. That’s its theoretical compressive strength.
Then take a similar board, nail/screw it to two 6” long boards to represent floor joists with a 24” span. Place a 40lb child on it and tell them to jump. That’s a 40lb live load in a building situation.
The floor loading is VERY different that the compressive strength of a material. The floor loading is dependent on the design of the support structure for the floor. The compressive strength of a material is the direct pressure that can be placed on an area of the material before it starts to fail. If you place a 4x8x1/2" piece of plywood on a large concrete foundation that was completely solid under the wood- you could load the plywood to the 4,500psi before it starts coming apart. If you support that same plywood only at the ends with an 8 foot open span, you could not put even 20 psf on the surface before it sagged and failed.
Thanks for the explanation! So it's basically the theoretical material strength vs. the practical load-bearing capability when used for flooring. I'm guessing the actual psf that plywood (or any other material) could support for flooring will be dependent on the flooring structure of the building it's built in, and would differ from building to building, if I understand correctly then?
It's not just theoretical versus practical. Compressive strength is actual practical compressive strength, if you have an application where that particular material property is the limiting factor. But it's not the relevant parameter for the application of flooring supported by joists.
It's more the fact compressive strength would be an immovable surface the size of the sheet under it and an ~700,000lb load covering a square foot of the top versus a 40lb dumbell setting in the middle of a sheet with nothing under the weight.
one of the best books Ive read, as a non-engineer, is [https://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835](https://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835)
it really helped me develop an intuition about building materials and how they 'work' in a structure. and I designed a built a woodshed and it didn't collapse with 3 feet of snow on it, so I'm 1 for 1.
It’s just that compressive strength is not the reference value relevant to your application. You want to look at bending strength of plywood when spanning support points. The compressive strength only matters for areas directly over a joist. But between joists the plywood will bend down under load. (Think of placing heavy weights on a weak and/or long book shelf).
It may be more intuitive to try to calculate what is the deflection (downward) of the plywood under different loads than try to examine the load that fully brings it to failure.
For a spanning application the material is actually experiencing compression in the upper layers, and tension in the lower layers simultaneously. This much more complex and torturous to the material..and then adding the dynamics of variable forces. If you were to use a round piece of plywood as a pressure pot lid with clamps it would experience similar shear-like stresses.
More that you're looking at the bulk strength of a material (plywood) versus the required strength of a structure that happens to be made of that material (floor).
You could make a car out of steel, or aluminum, or carbon fiber, etc. But the material strength of any of those alone doesn't tell you whether it will pass crash testing.
Failure is compression is different from failure in bending. Bending gives both compressive and tensile load in the cross section. However, if tensile and compressive strength are equal the only difference in bending is the moment (force*distance), which has leverage effect and therefore gives more load with less effort (load). Hence it appears thst the material has failed with lesser load, but the cross section fails at same load internally as in compressive or tensile failure.
That 6000 PSI number is related to the allowable stress WITHIN the sheet - the innate strength of the fibers and glue. The 40-70 PSF number is how much WEIGHT a sheet of plywood can hold up when it’s supported every 12”, 16”, etc. The WEIGHT it can hold in a building application is derived from how much internal stress the sheet can handle.
Given a loading scenario (say 40PSF applied to the plywood), you can calculate the stress inside the sheet. That stress has to be less than 4500PSI (or whatever the actual material strength is, for this situation).
If you check carefully, I believe it is a **maximum** live load of 40 psf allowable, and **NOT** your minimum 40 psf, as if infinite load is possible.
And with a margin of safety of 2, the ultimate live load possible is 80 psf.
That 6,000 psi is the maximum load tolerable before deformation takes place, and before it subsequently breaks. So anything less than 6,000 pound (or 3,000 pound, just to be safe) within a square inch is reasonable and makes sense.
This refers to the compressive strength. But if you refer to the beam, which involves tension, then the analysis will be different, such as checking the plywood's 2nd moment of inertia and the deflection at the middle of the span.
The actual load on the material is much different than the load being applied. For instance, a 2x4 spanning 10 feet with a 150lb person standing on the middle will cause loads in the wood of almost 1500lbs per square inch. put 600lbs in the middle it will certainly break. You also have to consider how much the structural component will bend when a given load is applied.
This is correct for converting in^(2) to ft^(2), but I believe the equation for pounds per in^(2) to pounds per ft^(2) should be:
6000lb/1in^(2) = xlb/1ft^(2)
And simplifying:
6000lb/1in^(2) = xlb/144in^(2)
Then solving for x, you get 6000\*144 = 864,000lb/144in^(2) or 864,000 pounds per square foot.
As I've learned from other replies, the issue here is that this is the compressive strength of the material, which isn't really applicable for the most part when talking about flooring, as the material is not being compressed over most of it's area, but rather supporting weight above a relatively open area.
Edit: fixed superscript
one uses psi for stress, while the other uses psi for pressure.
One is a material property, while the other is how much force a construction has to carry.
If you would load a short piece of plywood (lets say 1 inch long) that is 1 square inch in size. you can load it with that amount of force before it breaks.
the beam is bending, which increases by how far it is from the wall (this multiplies) and that the 4500/6000 number is only compressive strength, bending
will create a far larger stress in the top and bottom edges than the middle, so the stress is not evenly distributed inside the beam.
"I've found multiple results listing the psi to be in the range of 4,500 to 6,000 psi." But the concrete I've ordered was only 3000 psi. Is plywood stronger than concrete in compressive strength?
I wouldn't be terribly surprised if that were the case to be honest. Given the relative softness and flexibility of plywood compared to concrete, I could definitely see it lasting longer under compression tests than concrete. Here are some of the links I looked through showing compressive strengths of plywood and other wood:
[https://www.matweb.com/search/datasheet\_print.aspx?matguid=bd6620450973496ea2578c283e9fb807](https://www.matweb.com/search/datasheet_print.aspx?matguid=bd6620450973496ea2578c283e9fb807)
[https://workshopcompanion.com/KnowHow/Design/Nature\_of\_Wood/3\_Wood\_Strength/3\_Wood\_Strength.htm](https://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/3_Wood_Strength/3_Wood_Strength.htm)
I wouldn't be too sure of that analysis. Mineral based construction materials like concrete, rocks, and The Earth tend to be just strong as hell in compression. Not so much in shear or bending, but they preform great in compression.
Compressive strength is very different from what plywood can support spanning joists.
3/4" plywood works great as shims to level a crawler crane on timber mats can't walk a crawler crane through your living room
> can't walk a crawler crane through your living room Hold my slide rule.....
One of the flooring assemblies I looked at for our renovation was 23 psf dead load. It ended up adding 22,000lbs for just the first floor. So much engineered wood...
23 psf dead load is the **distributed** dead load per every sqft, for the entire floor area. If your 22,000 pound is the total weight covering a 1,000 sqft area, for example, then each sqft's dead load is just 22 psf, which is just right. 22,000 pound is the total weight. It does not mean every sqft of the floor area is 22,000 pound heavy, or else that floor will definitely collapse.
Yes I'm fully aware. What gave you the impression I understood otherwise? Most flooring assemblies have 10 dl/sqft or less.
Nuh uh, the gubmint and their building codes can’t tell me what to do
Take a 6”x24” board of plywood. That’s 1 square foot. Fully support it with concrete underneath it and around it. Then place weight on it. It’ll take about 600k pounds to destroy it. That’s its theoretical compressive strength. Then take a similar board, nail/screw it to two 6” long boards to represent floor joists with a 24” span. Place a 40lb child on it and tell them to jump. That’s a 40lb live load in a building situation.
I doubt you would have to tell them to jump. Source: am father
> I doubt you would have to tell them to jump. You could even tell them to stand still.
Jumping, spinning, falling down... you can test most live loading scenarios with a toddler.
Or my drunk neighbor .
I said I was sorry bill, don't have to put me on blast with your internet friends too.
The floor loading is VERY different that the compressive strength of a material. The floor loading is dependent on the design of the support structure for the floor. The compressive strength of a material is the direct pressure that can be placed on an area of the material before it starts to fail. If you place a 4x8x1/2" piece of plywood on a large concrete foundation that was completely solid under the wood- you could load the plywood to the 4,500psi before it starts coming apart. If you support that same plywood only at the ends with an 8 foot open span, you could not put even 20 psf on the surface before it sagged and failed.
Thanks for the explanation! So it's basically the theoretical material strength vs. the practical load-bearing capability when used for flooring. I'm guessing the actual psf that plywood (or any other material) could support for flooring will be dependent on the flooring structure of the building it's built in, and would differ from building to building, if I understand correctly then?
Flooring is not being compressed everywhere. Across a lot of area it is bending, and this is much harder for plywood to support
It's not just theoretical versus practical. Compressive strength is actual practical compressive strength, if you have an application where that particular material property is the limiting factor. But it's not the relevant parameter for the application of flooring supported by joists.
It's more the fact compressive strength would be an immovable surface the size of the sheet under it and an ~700,000lb load covering a square foot of the top versus a 40lb dumbell setting in the middle of a sheet with nothing under the weight.
one of the best books Ive read, as a non-engineer, is [https://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835](https://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835) it really helped me develop an intuition about building materials and how they 'work' in a structure. and I designed a built a woodshed and it didn't collapse with 3 feet of snow on it, so I'm 1 for 1.
It’s just that compressive strength is not the reference value relevant to your application. You want to look at bending strength of plywood when spanning support points. The compressive strength only matters for areas directly over a joist. But between joists the plywood will bend down under load. (Think of placing heavy weights on a weak and/or long book shelf). It may be more intuitive to try to calculate what is the deflection (downward) of the plywood under different loads than try to examine the load that fully brings it to failure.
For a spanning application the material is actually experiencing compression in the upper layers, and tension in the lower layers simultaneously. This much more complex and torturous to the material..and then adding the dynamics of variable forces. If you were to use a round piece of plywood as a pressure pot lid with clamps it would experience similar shear-like stresses.
More that you're looking at the bulk strength of a material (plywood) versus the required strength of a structure that happens to be made of that material (floor). You could make a car out of steel, or aluminum, or carbon fiber, etc. But the material strength of any of those alone doesn't tell you whether it will pass crash testing.
Failure is compression is different from failure in bending. Bending gives both compressive and tensile load in the cross section. However, if tensile and compressive strength are equal the only difference in bending is the moment (force*distance), which has leverage effect and therefore gives more load with less effort (load). Hence it appears thst the material has failed with lesser load, but the cross section fails at same load internally as in compressive or tensile failure.
Only a tiny fraction of installed plywood is in compression. Most of the stress on plywood is shear.
Shear and bending.
That 6000 PSI number is related to the allowable stress WITHIN the sheet - the innate strength of the fibers and glue. The 40-70 PSF number is how much WEIGHT a sheet of plywood can hold up when it’s supported every 12”, 16”, etc. The WEIGHT it can hold in a building application is derived from how much internal stress the sheet can handle. Given a loading scenario (say 40PSF applied to the plywood), you can calculate the stress inside the sheet. That stress has to be less than 4500PSI (or whatever the actual material strength is, for this situation).
Correct
If you check carefully, I believe it is a **maximum** live load of 40 psf allowable, and **NOT** your minimum 40 psf, as if infinite load is possible. And with a margin of safety of 2, the ultimate live load possible is 80 psf. That 6,000 psi is the maximum load tolerable before deformation takes place, and before it subsequently breaks. So anything less than 6,000 pound (or 3,000 pound, just to be safe) within a square inch is reasonable and makes sense. This refers to the compressive strength. But if you refer to the beam, which involves tension, then the analysis will be different, such as checking the plywood's 2nd moment of inertia and the deflection at the middle of the span.
The actual load on the material is much different than the load being applied. For instance, a 2x4 spanning 10 feet with a 150lb person standing on the middle will cause loads in the wood of almost 1500lbs per square inch. put 600lbs in the middle it will certainly break. You also have to consider how much the structural component will bend when a given load is applied.
Math is wrong I believe. 1’sq = 144”sq 6000psi/144=41.667psf Edit:format
This is correct for converting in^(2) to ft^(2), but I believe the equation for pounds per in^(2) to pounds per ft^(2) should be: 6000lb/1in^(2) = xlb/1ft^(2) And simplifying: 6000lb/1in^(2) = xlb/144in^(2) Then solving for x, you get 6000\*144 = 864,000lb/144in^(2) or 864,000 pounds per square foot. As I've learned from other replies, the issue here is that this is the compressive strength of the material, which isn't really applicable for the most part when talking about flooring, as the material is not being compressed over most of it's area, but rather supporting weight above a relatively open area. Edit: fixed superscript
one uses psi for stress, while the other uses psi for pressure. One is a material property, while the other is how much force a construction has to carry. If you would load a short piece of plywood (lets say 1 inch long) that is 1 square inch in size. you can load it with that amount of force before it breaks. the beam is bending, which increases by how far it is from the wall (this multiplies) and that the 4500/6000 number is only compressive strength, bending will create a far larger stress in the top and bottom edges than the middle, so the stress is not evenly distributed inside the beam.
"I've found multiple results listing the psi to be in the range of 4,500 to 6,000 psi." But the concrete I've ordered was only 3000 psi. Is plywood stronger than concrete in compressive strength?
I wouldn't be terribly surprised if that were the case to be honest. Given the relative softness and flexibility of plywood compared to concrete, I could definitely see it lasting longer under compression tests than concrete. Here are some of the links I looked through showing compressive strengths of plywood and other wood: [https://www.matweb.com/search/datasheet\_print.aspx?matguid=bd6620450973496ea2578c283e9fb807](https://www.matweb.com/search/datasheet_print.aspx?matguid=bd6620450973496ea2578c283e9fb807) [https://workshopcompanion.com/KnowHow/Design/Nature\_of\_Wood/3\_Wood\_Strength/3\_Wood\_Strength.htm](https://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/3_Wood_Strength/3_Wood_Strength.htm)
I wouldn't be too sure of that analysis. Mineral based construction materials like concrete, rocks, and The Earth tend to be just strong as hell in compression. Not so much in shear or bending, but they preform great in compression.