7 ms·

I’m not sure what you’re talking about with “the average concrete foundation is 200 lb/square foot” but I assume you’re talking about allowable bearing pressure

by abduhl 4y ago

I’m not sure what you’re talking about with “the average concrete foundation is 200 lb/square foot” but I assume you’re talking about allowable bearing pressures. First there’s two things to note: (1) a 2400 sf house is living space, its foundation’s bearing area has little relation to this number, especially if it’s not slab on grade and (2) the actual loads going into the foundation are not calculated in a way even remotely close to what your post implies. Regarding allowable bearing pressure, the allowable bearing pressure specified in the building code for the most likely soil condition can be found in CBC 1806 and it is 3000 lb/square foot for sandy gravel (assuming good fill was placed for the original house and the solar addition sits on similar structural fill). Your Seattle garage would be supportable by a 3 ft by 3 ft foundation (call it 4x4 after foundation weight is added). A 480kip house could be supported by a 12 ft by 12 ft slab. Even taking the lowest presumptive value (1500 psf for clays, which is incredibly unlikely to be the bearing strata for a modern or even recent home) would only double these areas (8x8 and 24x24). 64000 lbs of reinforced concrete is almost 450 cubic feet (17 yards!). This is 2 entire concrete trucks’ worth of concrete, which is absurd for a residential foundation. Assuming a 2 ft thick slab this is 225 square feet of bearing area. This is enough to support your ENTIRE hypothetical home, not just to support a solar array. More likely the slab is under 18 inches thick, or maybe even 12. So it’s entirely believable that a 4 story building with 2400 sf of living space (or 3600 or 4800 sf) exists and is supportable by 17 yards of concrete. The architect is right, you are wrong. There is simply no reason to have this much concrete for a residential solar addition. For your information and to head off argument: I am a licensed civil engineer in California. The rules and procedures for little shit like a solar array foundation can be followed by almost anyone who can do simple arithmetic by using the CBC, you do not need a license to do it correctly. You need the licensed engineer’s stamp so that there is someone (insurance) to sue if things go wrong.

robomartin 4y ago
> I am a licensed civil engineer in California. The rules and procedures for little shit like a solar array foundation can be followed by almost anyone who can do simple arithmetic by using the CBC, you do not need a license to do it correctly. You need the licensed engineer’s stamp so that there is someone (insurance) to sue if things go wrong. Thanks for posting this (and the rest of your analysis). I'll repeat what my wife says all the time to her patients: "Your Google search is not equivalent to my medical degree". This kind of thing comes-up all the time online. People think they understand something and don't seem to bother to make sure they actually do. I am not a Civil Engineer. I am an EE, with enough of a physics background and multidisciplinary experience to work my way through a project like this without any issues. I have done a lot of construction work. I did all of the design work because (a) I enjoy this and (b) I could save some money this way. I knew full well that a licensed Civil Engineer would have to come into the fold eventually. The more work I did in order to ensure all was structurally sound, to code, etc., the less design iterations we would require while paying a Civil Engineer. As it turned out, I got most of it right. We had to make some adjustments based on the wind loading the county wanted (my house would be shredded at those speed, but my solar structure will stay put). Here are a few renderings from Solidworks: https://i.imgur.com/xhbpxBe.png https://i.imgur.com/xhbpxBe.png https://i.imgur.com/gO91PaO.png https://i.imgur.com/gO91PaO.png https://i.imgur.com/JUao20F.png https://i.imgur.com/JUao20F.png The civil engineering firm was able to use the ACAD drawings I produced from Solidworks, that saved me a bunch of money. As a point of interest, I installed fans under each section to help cool the panels and improve output. I don't have data on how well this works yet (it's been four years, I should have it!). It was quite an experience to start with a pile of lumber on the driveway to personally cutting and milling every single component and then assembling the thing. The best part, my kids learned a ton in the process.
- abduhl 4y ago
  No problem. People think that structural engineering is some kind of mystic art because the licensing and code regimes exist; however, the truth is that it's normally very straight forward if you have a basic grasp of Newtonian physics and a simple project (for some complex projects it is a mystic art - skyscrapers, tunnels, bridges, etc. are all situations where you have to invoke arcane spells like FEM, empirical approaches, and archaic portions of the code). This is especially true for shallow foundation design where the concrete behaves well (because the loading is so simple) and the code specified soil parameters are excessively conservative. Rather than looking at the code and going through the straight-forward analysis contained in the tiny little section that's applicable (and appropriately titled) most people will see that the code is hundreds of pages long with hundreds of sections and give up before realizing that the building code contains provisions for almost every scenario (building with concrete? there's a section. building with steel? there's a section. building with timber? there's a section. wall? section. beam? section. foundation? section. special circumstances like earthquake or wind or frost? section). Engaging with the code like you did assures that you are not taken for a ride by an unethical contractor or engineer, and it's something I would recommend every owner does before they embark on a project, but honestly a lot of the simple stuff (like a light foundation) is stuff that people used to just do themselves without designing anything and it's not any more safe because you have to get it stamped now. Your initial gut reaction for this simple stuff is usually within an order of magnitude, and often it will be more than is required because people don’t have a good sense of how strong modern building materials are. As an aside and based on your other post, a slab might not have been the most economical solution for your particular problem. You probably could have just installed some small diameter piles under each solar panel post. I'm surprised your engineer didn't recommend it as this would have much more easily addressed the wind loading issue. Value engineering like this is the real benefit of engaging a knowledgeable/experienced engineer. Cool Solidworks model. Easy 3D modeling has very much changed the game for small scale projects by allowing people to build a project on their computer and figure out all the little details that you'd normally screw up without an engineer. Having a working 3D model means that your design is probably at a higher level than most. It's cool to take something from an idea to a concept to physical reality.

by abduhl 4y ago

robomartin 4y ago

> I am a licensed civil engineer in California. The rules and procedures for little shit like a solar array foundation can be followed by almost anyone who can do simple arithmetic by using the CBC, you do not need a license to do it correctly. You need the licensed engineer’s stamp so that there is someone (insurance) to sue if things go wrong. Thanks for posting this (and the rest of your analysis). I'll repeat what my wife says all the time to her patients: "Your Google search is not equivalent to my medical degree". This kind of thing comes-up all the time online. People think they understand something and don't seem to bother to make sure they actually do. I am not a Civil Engineer. I am an EE, with enough of a physics background and multidisciplinary experience to work my way through a project like this without any issues. I have done a lot of construction work. I did all of the design work because (a) I enjoy this and (b) I could save some money this way. I knew full well that a licensed Civil Engineer would have to come into the fold eventually. The more work I did in order to ensure all was structurally sound, to code, etc., the less design iterations we would require while paying a Civil Engineer. As it turned out, I got most of it right. We had to make some adjustments based on the wind loading the county wanted (my house would be shredded at those speed, but my solar structure will stay put). Here are a few renderings from Solidworks: https://i.imgur.com/xhbpxBe.png https://i.imgur.com/xhbpxBe.png https://i.imgur.com/gO91PaO.png https://i.imgur.com/gO91PaO.png https://i.imgur.com/JUao20F.png https://i.imgur.com/JUao20F.png The civil engineering firm was able to use the ACAD drawings I produced from Solidworks, that saved me a bunch of money. As a point of interest, I installed fans under each section to help cool the panels and improve output. I don't have data on how well this works yet (it's been four years, I should have it!). It was quite an experience to start with a pile of lumber on the driveway to personally cutting and milling every single component and then assembling the thing. The best part, my kids learned a ton in the process.

abduhl 4y ago
No problem. People think that structural engineering is some kind of mystic art because the licensing and code regimes exist; however, the truth is that it's normally very straight forward if you have a basic grasp of Newtonian physics and a simple project (for some complex projects it is a mystic art - skyscrapers, tunnels, bridges, etc. are all situations where you have to invoke arcane spells like FEM, empirical approaches, and archaic portions of the code). This is especially true for shallow foundation design where the concrete behaves well (because the loading is so simple) and the code specified soil parameters are excessively conservative. Rather than looking at the code and going through the straight-forward analysis contained in the tiny little section that's applicable (and appropriately titled) most people will see that the code is hundreds of pages long with hundreds of sections and give up before realizing that the building code contains provisions for almost every scenario (building with concrete? there's a section. building with steel? there's a section. building with timber? there's a section. wall? section. beam? section. foundation? section. special circumstances like earthquake or wind or frost? section). Engaging with the code like you did assures that you are not taken for a ride by an unethical contractor or engineer, and it's something I would recommend every owner does before they embark on a project, but honestly a lot of the simple stuff (like a light foundation) is stuff that people used to just do themselves without designing anything and it's not any more safe because you have to get it stamped now. Your initial gut reaction for this simple stuff is usually within an order of magnitude, and often it will be more than is required because people don’t have a good sense of how strong modern building materials are. As an aside and based on your other post, a slab might not have been the most economical solution for your particular problem. You probably could have just installed some small diameter piles under each solar panel post. I'm surprised your engineer didn't recommend it as this would have much more easily addressed the wind loading issue. Value engineering like this is the real benefit of engaging a knowledgeable/experienced engineer. Cool Solidworks model. Easy 3D modeling has very much changed the game for small scale projects by allowing people to build a project on their computer and figure out all the little details that you'd normally screw up without an engineer. Having a working 3D model means that your design is probably at a higher level than most. It's cool to take something from an idea to a concept to physical reality.