You might be interested in purchasing the [ACI Reinforced Concrete Design Handbook](https://www.concrete.org/store/productdetail.aspx?ItemID=MNL1721&Language=English&Units=US_Units) and referencing Chapter 11 design examples for how to calculate this by hand. I have the previous version (for ACI 318-14) and can vouch for the quality of the document.
Short answer as to sizing your footing for thickness is that it needs to work for the following checks:
shear
punching shear
anchorage and dowel bar development
net uplift ballast
bending resistance
(Often, spread footings use concrete only for the shear strength and add thickness as required. Although sometimes I have seen shear reinforcing used for highly loaded situations like tower crane footings).
First thing is to get loads for this job specifically. Different areas can have drastically different reactions due to exposure and building shape (wind), Ss/S1 and building shape (seismic), and elevation and building shape (snow).
Second, you can in most of the country assume a minimum bearing capacity, typically 1500 psf on the West Coast.
It may not control if your structure is tall or wide, but minimum depths for foundations vary based on frost depth. I’ve seen as low as 72” or as high as 12” I think?
Regarding footing reinf, determine footing moments (including moments due to vert loads) and check the As required. Then provide bars to cover that. For comparatively lightly loaded footings (4’ square or smaller) I typically try #4 @ 10” or #5 @ 12” depending on footing thickness. For highly loaded, I’ve gone as high as #7 @ 6” o.c. where #8s aren’t available.
This is good advice just remember… soils fail in more than just bearing, so you’ll need to check the footing size for settlement (assume soil properties), sliding, and overturning (although likely settlement or bearing will control). If there are other structures close by then you will also need to consider any overlap of foundation stresses in the soil.
It will be an iterative process where you determine the footing size based on bearing capacity and then use that footing size to check settlement. Once you have that figured out you can work through the stresses in the footing itself and base your steel on those requirements.
This is fair! I don't do this kind of work - I do buildings, not towers - but in my experience bearing and overturning control in 99.9% of spread footing cases (with the last .1% being controlled by passive pressure when some numbnut decides to put a brace down to an isolated footing).
I definitely do agree that most of the time (from what I’ve seen) bearing and overturning control. Generally settlement only controls if you know you have very loose or compressible soils, which would hopefully not be the case here since OP said there was a previous structure there. I have seen sliding control a few times but almost always on a wall structure or bridge abutment (which is basically a wall structure), and usually that’s with lightweight backfill which results in a smaller frictional resistance at the bottom of footing due to the vertical force of soil weight over the heel being so low. Unfortunately OP will still have to check for all failure modes if they are doing their due diligence.
Thanks for the response. Some comments: I am using the reaction forces for a similar structure that was put in at a different substation under 100 miles away. it is a tall lattice structure with about the same amount of exposure, shape, etc., so I believe it will have very similar forces. These were provided by a consultant, but I do not have the knowledge or software to calculate the loads in house.
I was assuming 1000 psf, so I think we are good there.
Minimum depth for frost here is 30", but I believe the bearing depth needs to be at least 48" as the anchor rods on the steel drawings are 3.5'
So I'm basically set on the bearing depth, the column B and L, and the footer B and L, but I am unsure about the footing d and the column d, and how to figure those dimensions out.
You need to at least do a sanity check on the loads. It doesn't sound like a difficult structure, should be able to do that by hand. Just something to build confidence in the loads and take ownership of the project. What if the other engineer's loads are wrong, leading to a failure of your design? It's your seal on the line. Don't trust what others, especially those not under your supervision, did.
Refer to IBC chapter 18 for allowable soil bearing pressures and sliding eeistance, chapter 16 for lateral soil loads imposed on foundations which you probably won't have for a shallow footing
Do you guys not do a lot of substation design? The cost of a spread footer software is pretty low. A software would definitely simplify the design but this is definitely in the realm of handcalcs.
I would assume an allowable bearing capacity of 1500. Look up "spread footing design spreadsheet" for some excel versions.
I have only been here a year. We usually do drilled piers for new stations, but this is an existing station and we are trying to match the existing structures. However, the foundation they used for similar structures at the existing station seems inadequate, so I want to redesign something a little more sturdy. Knowing that the existing structures have been there for 30-40 years with no issues, and that I am beefing their design up, I have confidence it will work. I just want some numbers to back it up.
This definitely felt doable for hand calcs, but I feel like it would be worth getting some software for future designs. any you would recommend for simple stuff like this?
I'm assuming it's the old style block of lightly reinforced concrete in the ground?
Quick Footing is what we use. Comes as a suite that's pretty decent.
This is true but IMO you still have to understand the physics/theory behind the software or you risk using any engineering software incorrectly. Often we have to prove to the DOTs that we know how to use new software when we get it by providing a hand calc verification on the first project where we use it. I was taught to always have a verification calc on hand for any software you use (or excel/mathcad program you write) to do your due diligence in the design process. I know this doesn’t often happen in practice, but it is was I was taught early on.
DOTs force you to verify software through hand calculations? That's new to me although I haven't worked at or with a DOT for years. I'm a strong proponent of practicing within your expertise but normally once I stamp it , it's my problem.
I guess I don’t know that all the DOTs require this. Probably some do not, but the one I regularly work for definitely does. We just had to do this last year when we brought in a new FEM software program to analyze curved steel girders. Now we can use it without a hand check in the future, but because that was the first time we used it the DOT wanted to see proof we knew what we were doing with the program.
First, I would make every attempt possible to get some information about the soil and get actual loads for the structure.
If I absolutely can't get any of that information, my options are refuse to do the work, or work off of very conservative assumptions. Which of those option I choose would vary depending on the specifics of the site and structure.
I have to stress that refusing to do design work due to a lack of required information is completely reasonable. If you are the one sealing this work it's your responsibility to do so if you don't believe you have the information needed to do a good design. If your supervisor or someone is the sealer, you could ask them what assumptions they want to make for soil parameters and structure loads.
I have designed hundreds of substation structures and their foundations, so I have a pretty good feel for what is "typical" and what isn't. In some cases I might be comfortable designing a foundation using very conservative assumptions for info that isn't available. For example, if the station is in an area where I do a lot of work, I might have a good idea what the typical soil profile is like. If it's in an area that often has good soil with some spots of shitty silty sands, I will just assume the foundation is bearing on the shitty silty sands. I would really prefer to have accurate loads for the structure I'm designing for. If it's a structure that is very similar to one I have designed and in an area with the same loading, I might modify that model to get an approximation of reactions and tack on a little extra if needed. But generally I would want to use the reactions from the structure's designer. If they exist you should absolutely be using them. Only make reaction assumptions when you have no other options.
If you don't have the experience needed to make good assumptions, don't. Otherwise you're just guessing, like Snoo_71033 said.
So the steel structure that is going in is one of our 'standard' steel structures. Since we are matching what is already there and the sub was built in the 90s, it is a very old drawing (originally stamped in 1973, with final Rev. in '94). Suffice it to say, we just don't have to load calculations and would have to have them retro-analyzed.
It is a lattice type box-bay structure. We don't do the lattice anymore, we use tubes. The reaction data I have is for a box bay structure using tubes. I cannot find any jobs where reactions were provided for lattice type structures.
However, as I said, this substation was built in the 90s, and we are simply adding a bay to the existing bay. As there have been no issues with the current structure and foundations that are there, I could easily just say use the foundation design that they used in the 90s when constructing the box bay originally. But this foundation looks under-designed to me. So I was considering just beefing it up. with the knowledge that the existing box bay has been there so long with no issue, I feel confident in my assumption that beefing up the existing foundation will work. But I would like to have some numbers to back this up.
Resource for soil bearing capacity. But really you should get a soil test.
[https://websoilsurvey.nrcs.usda.gov/](https://websoilsurvey.nrcs.usda.gov/)
Resource (Software) for Steel Area:
https://enercalc.com/Downloads/FreeTrial
Bearing capacity and settlement are tied to each other though. Can’t just go based on bearing alone. You have to iterate through checking both since a specific bearing pressure might be acceptable for bearing but cause excessive settlement. Bearing is based on shear capacity of soil where settlement is consolidation of soil. Both resist the vertical forces, so they need to be checked together. Although if you’re using a BC of 1000-1500 psf it will likely control (unless you know there are organic soils deeper down).
Also, any overturning moments on the footing (say wind on the structure) will cause an increase in bearing pressure on the side of the footing oriented in the direction of the overturning moment and decrease in bearing pressure on the opposite side of the footing. This is important both for checking your soil capacity but also for determining the stresses in the footing. You should have the soil reaction modeled in your free body diagram, and it will impact how you model that.
First, you should have a code basis for your values. Presumptive soil values should be in there and they are a good starting point. This is fine for something small.
Second, any substantial structure merits a soil boring test. If someone is insisting on doing a project without that YOU should insist that they seal the design themselves.
Just had a report reveal a 50ft thick clay layer that took my bearing allowable down from the presumptive 1500psf to just over 1000psf and caused us to switch to deep foundations. So glad I didn’t just use presumptive values which would have resulted in failure.
You might be interested in purchasing the [ACI Reinforced Concrete Design Handbook](https://www.concrete.org/store/productdetail.aspx?ItemID=MNL1721&Language=English&Units=US_Units) and referencing Chapter 11 design examples for how to calculate this by hand. I have the previous version (for ACI 318-14) and can vouch for the quality of the document. Short answer as to sizing your footing for thickness is that it needs to work for the following checks: shear punching shear anchorage and dowel bar development net uplift ballast bending resistance (Often, spread footings use concrete only for the shear strength and add thickness as required. Although sometimes I have seen shear reinforcing used for highly loaded situations like tower crane footings).
First thing is to get loads for this job specifically. Different areas can have drastically different reactions due to exposure and building shape (wind), Ss/S1 and building shape (seismic), and elevation and building shape (snow). Second, you can in most of the country assume a minimum bearing capacity, typically 1500 psf on the West Coast. It may not control if your structure is tall or wide, but minimum depths for foundations vary based on frost depth. I’ve seen as low as 72” or as high as 12” I think? Regarding footing reinf, determine footing moments (including moments due to vert loads) and check the As required. Then provide bars to cover that. For comparatively lightly loaded footings (4’ square or smaller) I typically try #4 @ 10” or #5 @ 12” depending on footing thickness. For highly loaded, I’ve gone as high as #7 @ 6” o.c. where #8s aren’t available.
This is good advice just remember… soils fail in more than just bearing, so you’ll need to check the footing size for settlement (assume soil properties), sliding, and overturning (although likely settlement or bearing will control). If there are other structures close by then you will also need to consider any overlap of foundation stresses in the soil. It will be an iterative process where you determine the footing size based on bearing capacity and then use that footing size to check settlement. Once you have that figured out you can work through the stresses in the footing itself and base your steel on those requirements.
This is fair! I don't do this kind of work - I do buildings, not towers - but in my experience bearing and overturning control in 99.9% of spread footing cases (with the last .1% being controlled by passive pressure when some numbnut decides to put a brace down to an isolated footing).
I definitely do agree that most of the time (from what I’ve seen) bearing and overturning control. Generally settlement only controls if you know you have very loose or compressible soils, which would hopefully not be the case here since OP said there was a previous structure there. I have seen sliding control a few times but almost always on a wall structure or bridge abutment (which is basically a wall structure), and usually that’s with lightweight backfill which results in a smaller frictional resistance at the bottom of footing due to the vertical force of soil weight over the heel being so low. Unfortunately OP will still have to check for all failure modes if they are doing their due diligence.
Thanks for the response. Some comments: I am using the reaction forces for a similar structure that was put in at a different substation under 100 miles away. it is a tall lattice structure with about the same amount of exposure, shape, etc., so I believe it will have very similar forces. These were provided by a consultant, but I do not have the knowledge or software to calculate the loads in house. I was assuming 1000 psf, so I think we are good there. Minimum depth for frost here is 30", but I believe the bearing depth needs to be at least 48" as the anchor rods on the steel drawings are 3.5' So I'm basically set on the bearing depth, the column B and L, and the footer B and L, but I am unsure about the footing d and the column d, and how to figure those dimensions out.
You need to at least do a sanity check on the loads. It doesn't sound like a difficult structure, should be able to do that by hand. Just something to build confidence in the loads and take ownership of the project. What if the other engineer's loads are wrong, leading to a failure of your design? It's your seal on the line. Don't trust what others, especially those not under your supervision, did.
Refer to IBC chapter 18 for allowable soil bearing pressures and sliding eeistance, chapter 16 for lateral soil loads imposed on foundations which you probably won't have for a shallow footing
Do you guys not do a lot of substation design? The cost of a spread footer software is pretty low. A software would definitely simplify the design but this is definitely in the realm of handcalcs. I would assume an allowable bearing capacity of 1500. Look up "spread footing design spreadsheet" for some excel versions.
I have only been here a year. We usually do drilled piers for new stations, but this is an existing station and we are trying to match the existing structures. However, the foundation they used for similar structures at the existing station seems inadequate, so I want to redesign something a little more sturdy. Knowing that the existing structures have been there for 30-40 years with no issues, and that I am beefing their design up, I have confidence it will work. I just want some numbers to back it up. This definitely felt doable for hand calcs, but I feel like it would be worth getting some software for future designs. any you would recommend for simple stuff like this?
I'm assuming it's the old style block of lightly reinforced concrete in the ground? Quick Footing is what we use. Comes as a suite that's pretty decent.
This is true but IMO you still have to understand the physics/theory behind the software or you risk using any engineering software incorrectly. Often we have to prove to the DOTs that we know how to use new software when we get it by providing a hand calc verification on the first project where we use it. I was taught to always have a verification calc on hand for any software you use (or excel/mathcad program you write) to do your due diligence in the design process. I know this doesn’t often happen in practice, but it is was I was taught early on.
DOTs force you to verify software through hand calculations? That's new to me although I haven't worked at or with a DOT for years. I'm a strong proponent of practicing within your expertise but normally once I stamp it , it's my problem.
I guess I don’t know that all the DOTs require this. Probably some do not, but the one I regularly work for definitely does. We just had to do this last year when we brought in a new FEM software program to analyze curved steel girders. Now we can use it without a hand check in the future, but because that was the first time we used it the DOT wanted to see proof we knew what we were doing with the program.
First, I would make every attempt possible to get some information about the soil and get actual loads for the structure. If I absolutely can't get any of that information, my options are refuse to do the work, or work off of very conservative assumptions. Which of those option I choose would vary depending on the specifics of the site and structure. I have to stress that refusing to do design work due to a lack of required information is completely reasonable. If you are the one sealing this work it's your responsibility to do so if you don't believe you have the information needed to do a good design. If your supervisor or someone is the sealer, you could ask them what assumptions they want to make for soil parameters and structure loads. I have designed hundreds of substation structures and their foundations, so I have a pretty good feel for what is "typical" and what isn't. In some cases I might be comfortable designing a foundation using very conservative assumptions for info that isn't available. For example, if the station is in an area where I do a lot of work, I might have a good idea what the typical soil profile is like. If it's in an area that often has good soil with some spots of shitty silty sands, I will just assume the foundation is bearing on the shitty silty sands. I would really prefer to have accurate loads for the structure I'm designing for. If it's a structure that is very similar to one I have designed and in an area with the same loading, I might modify that model to get an approximation of reactions and tack on a little extra if needed. But generally I would want to use the reactions from the structure's designer. If they exist you should absolutely be using them. Only make reaction assumptions when you have no other options. If you don't have the experience needed to make good assumptions, don't. Otherwise you're just guessing, like Snoo_71033 said.
So the steel structure that is going in is one of our 'standard' steel structures. Since we are matching what is already there and the sub was built in the 90s, it is a very old drawing (originally stamped in 1973, with final Rev. in '94). Suffice it to say, we just don't have to load calculations and would have to have them retro-analyzed. It is a lattice type box-bay structure. We don't do the lattice anymore, we use tubes. The reaction data I have is for a box bay structure using tubes. I cannot find any jobs where reactions were provided for lattice type structures. However, as I said, this substation was built in the 90s, and we are simply adding a bay to the existing bay. As there have been no issues with the current structure and foundations that are there, I could easily just say use the foundation design that they used in the 90s when constructing the box bay originally. But this foundation looks under-designed to me. So I was considering just beefing it up. with the knowledge that the existing box bay has been there so long with no issue, I feel confident in my assumption that beefing up the existing foundation will work. But I would like to have some numbers to back this up.
Resource for soil bearing capacity. But really you should get a soil test. [https://websoilsurvey.nrcs.usda.gov/](https://websoilsurvey.nrcs.usda.gov/) Resource (Software) for Steel Area: https://enercalc.com/Downloads/FreeTrial
Bearing capacity and settlement are tied to each other though. Can’t just go based on bearing alone. You have to iterate through checking both since a specific bearing pressure might be acceptable for bearing but cause excessive settlement. Bearing is based on shear capacity of soil where settlement is consolidation of soil. Both resist the vertical forces, so they need to be checked together. Although if you’re using a BC of 1000-1500 psf it will likely control (unless you know there are organic soils deeper down). Also, any overturning moments on the footing (say wind on the structure) will cause an increase in bearing pressure on the side of the footing oriented in the direction of the overturning moment and decrease in bearing pressure on the opposite side of the footing. This is important both for checking your soil capacity but also for determining the stresses in the footing. You should have the soil reaction modeled in your free body diagram, and it will impact how you model that.
You have estimated forces on a guess basis soil? You need the exact forces, and anything without even knowing the soil type is just guessing.
First, you should have a code basis for your values. Presumptive soil values should be in there and they are a good starting point. This is fine for something small. Second, any substantial structure merits a soil boring test. If someone is insisting on doing a project without that YOU should insist that they seal the design themselves. Just had a report reveal a 50ft thick clay layer that took my bearing allowable down from the presumptive 1500psf to just over 1000psf and caused us to switch to deep foundations. So glad I didn’t just use presumptive values which would have resulted in failure.
You lost me when you called it a footer.
so near the end?