How to size an isolated footing

hello my name is rich nolan from nolan engineering in upstate new york and today i'll show you how to size a footing a concrete footing this would be to support a post that might be underneath a beam in your basement if you're a homeowner or builder you should be able to do the simple math that's involved here so there's about four things to consider for footing the depth the thickness any rebar that goes inside the footing and then the footprint or the size of the of the footing we'll address each one of these the depth other than for frost protection is really not that important here in new york we need to have the bottom of the footing at least four feet deep and that's prevent the ground from freezing underneath it and lifting it if you're not in new york um you might not have to go four feet deep if you're inside of a basement you don't have to dig down another four feet your basement's probably already four feet deep if you're inside of a heated building you would also not need to necessarily go down four feet other than frost protection and perhaps to get through some bad material if you have some organic material at the surface you need to get rid of there's no benefit for going any deeper than you have to it doesn't help the footing uh carry any more load or not so ideally if you're in your basement you can put the footing right on the dirt floor or even right on top of an existing concrete slab where you can cut that concrete slab alone so four foot deep in new york for four frost protection the thickness of the footing is important so the post does not punch through the foot if you have a footing concrete footing that's sitting on the ground and you have a point load from a post city on there you would not want that footing to break through or punch through the minimum thickness is six inches per code not only do you not want the footing to punch through but you would not want your slab to bend your footing to bend like this and break the calculation of a footing thickness is more involved that would be something an engineer would have to do so i'm going to give you a general safe rule for most residential projects if you do a 12 inch thick footing you will not have any risk of punching through or breaking through the next thing to consider would be the rebar the reinforcement inside the footing again the reinforcement helps with the slab not punch or the footing not punching through or bending and breaking so i'm going to give you another general rule for that typically most footings are 24 inches by 24 inches and they will put two number four rebar which is half inch thick each way so if we're looking down on a footing let's say it's 24 inches by 24 inches they will lay two rebar this way and two rebar this way another important rule is that rebar the two this way to that way should be three inches minimum from the bottom of the footing and the purpose of that is so there's any cracks in this footing that water doesn't get through and get to the rebar and rust it out so i'm going to give you another general rule for the rebar footings up to 24 inches to 24 inches should have two number four each way anything bigger than 24 by 24 inches just put number four rebar 12 inches on center each way so let's say you have a footing that turns out to be 36 inches by 36 inches then you want three this way three that way again uh at least three inches from the bottom if you have a smaller footing than 24 inches on center maybe a 16 inch by 16 inch then you would still stick with the two rebar each way next we'll talk about the most important design criteria of a footing and that would be the footprint the footprint is by far the most important aspect of the footing it's the footprint the size of the footing that's spreading that point load from your post out over the soil you can imagine if you took a pencil would be very easy to insert it into say jello versus if you took something that was much larger you perhaps would have a much more difficult time sticking it through the jello or whatever material it is so it's that footprint that's very important the next important thing you need to determine is your soil bearing capacity the soil bearing capacity is what will also determine the size of your footprint there's a code chart in the building codes that have soil bearing capacity for different sized soils you could also have the soil tested by a geotech engineer that could tell you that i'm going to give you some general rules the lowest soil berry capacity in this chart is 1500 pounds per square foot uh that's for clay stuff like that and then there's uh 2500 pounds per square foot is what most engineers would design for most soils can easily take 2500 pounds per square foot are gravel bedrock that could take 10 000 pounds per square foot if you're looking for general number to use i wouldn't use less than 1500 pounds unless you have terrible terrible soil something that's very sloppy and wet and if you think you have that you should contact an engineer for most soils i recommend using 2500 psf unless you have clay then maybe you could drop it down to 1500 psf that's pounds per square foot so in other words if you have a footing that's one foot by one foot the area of that footing the footprint is one square foot so that could support 2500 pounds per square foot and now we'll do in a design example let's say we have a ranch house that's 30 feet long and 24 feet wide and this is the basement and there's a beam down the center with two posts that support the first floor let's say the floor joists run that way so over here is your basement as your first floor say it has trusses so there's no roof load coming down so this beam is supporting one floor and it has two posts and we need to size that footing right there the first thing we have to do is determine uh how much each one of these posts and footings uh how much load it sees so we need to determine the tributary area of the trib area um this beam here is supporting the floor joists that run over it uh so it supports um halfway back in this direction and the floor joist halfway back in that direction the foundation walls take the other half of the floor joist we'll just pick this post here it is uh supporting the beam halfway that way and halfway that way and they'll say the posts are equally spaced at 10 feet so that would make this 10 feet here and this would make this 12 feet this way which is also half the span on either direction so this area here is what we call our trib area and it's how much area that one post and footing support so the tributary area would be 12 feet times 10 feet or 120 square feet next for the loads typically floor loads will use a dead load of 10 pounds per square foot you could go a little higher 12 i wouldn't go past 15 unless you have concrete over your floors or something live load if it's not a bedroom we used 40 pounds per square foot so our total floor load would be 50 pounds per square foot so the load on that post would be our 120 square foot times what am i doing here 50 pounds per square foot and that should come out to 6 000 pounds so one post and one footing has to support 6 000 pounds now if we use our soil bearing capacity of 2500 psf that'll tell us that the size of the footing would be 6 000 pounds divided by 2500 psf and that should be if i find my right notes here we'll need 2.4 square feet to a footing area footprint area to support that load now if you want to be safer you could use a lower number 1500 pounds and you get a higher required square footage now to get the length of this each side of the footing i need to take the square root of that the square root of that gives me 1.55 foot per side if i want to get that in inches i'll multiply by 12 it says my footing has to be 18.6 inches by 18.6 inches now if you happen to be using a round footing you would um have to use the area of a circle to come up with the diameter but most people use square footings now 18.66 is an odd number if we're doing drawings for this house i'm going to round up to 20. so we follow all the previous rules so i'm going to call this footing out as a 20 inch by 20 inch by 12 inch thick footing i'm gonna say with two number four rebar that's half inch each each way and that's how we would show it on the drawing 20 inch by 20 inch square footy 12 inches thick with two number four half inch rebar each waist set three inches off the bottom and that footing would support the six thousand pounds that you would need actually a little bit more over six thousand pounds that you would need so that's basically um how it's done okay so in conclusion the depth is only important to achieve frost protection or to get to acceptable soils the thickness code requires six inches minimum but if you don't want to do engineering calculations that are fairly detailed to be honest with you if it's for a house if your footing is less than say three foot by three foot you're safe to go with a 12 inch footing the thickness is important to resist shear shearing through the footing punching through it or bending the footing and breaking it and bending rebar general rule again rebar resists shear failure and bending failure of the footing general rule for footing is 24 inches square and smaller two number four rebar each way set three inches from the bottom anything bigger than 24 inches start spacing your rebar in each direction at 12 inches on center again set three inches from the bottom uh the next thing uh you need to determine is your soil bearing it would not go less than 1 500 pounds per square foot unless you have terrible soils i would recommend using 2500 pounds per square foot that's what most engineers use maybe 2 000 or the tables in the icc the building code or call your local building department or you can have a soils test done by a geotech engineer i'll tell you exactly what to use the footprint of the footing is the most important so determine your footing size i wrote these two equations down for you once you determine the load on your footing that's important to know what that load is divided by the soil bearing pressure reliable soil bearing that you pick take the square root and multiply it by 12 that will give you your footing size in inches if it um you're using a diameter if you're designing a footing for a deck that's very similar it'd be 4 divided by pi times the the load on the pier divided by the soil bearing the square root of that times 12 and that would give you the diameter of the footing before we exit here we do have a product if you don't want to let's say you need a new footing in your basement because you have a sagging beam or some other problem or you're taking on a load-bearing wall you don't want to cut your slab mix concrete put rebar in we do have um these plates that you can use instead and i'll show you those in a second so if you have an existing home that has a concrete floor in your basement or even a dirt floor instead of pouring a concrete footing you could use our product called the insta footing plate we have two sizes we have a 12 by 12 and a 16 inch by 16 inch square plate they're both a half inch thick steel they can be laid right over the existing concrete and because if you lay them over the existing concrete because the load acts through the concrete at a 45 degree angle the size of this uh the load supporting capability of this is much higher than if you just place it on the dirt for example you can place this one right on the ground this is one foot square so this would be good for 2500 pounds but if you were to place it on a three and a half inch thick slab it actually good for over 9000 pounds our 16 by 16 inch plates are good for twelve thousand five hundred pounds so how they work is you just put these down on your concrete slab you can use two of these holes to anchor it to your slab you can make it with a half inch bolt or tap on screws you can put a wood post in here or four by four pressure treated a six by six four by six whatever size wood post you want and adjust these uh to where it hits the post nail them to the post and tighten these down if you want to use a steel lolly column just disregard these you put the steel leak this bolts right over the bottom of the base plate on the steel lolli column um these are a lot quicker and easier than uh pouring in a concrete slab the only thing is you have to be aware there will be a half inch raised section around here so if it's a finished basement it might not work for you but if it's not a finished basement and you have an area where you need more post this is an excellent solution

Channel: Nolan Engineering, PLLC

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Id: ZEXuVQ0-7Gw

Length: 15min 51sec (951 seconds)

Published: Sun Jan 17 2021