Footing required for 22 ton point load
I will soon be constructing a two-story garage that will have 6 point loads from two beams. Two of those points will carry 44,135 pounds each, including live and dead loads. The other four points will carry approx. 5 – 6 tons each. The building site consists of clay at footer depth. The foundation will be 5 rows of 8″ concrete block, and at each point load the block will be grouted full with concrete and rebar tied to the footer.
I realize that this is pretty general specs, but I would like to get an idea of what I might need for footers. And I know that you are going to say get an engineer to look at it. I just want to make sure I have enough, and if anything, I will make it overkill.
I would guess that a 2′ x 2′ x 1′ deep footer would be enough at each of these points. But then, that is only a guess. Any input?
Thanks,
Bryan
“Objects in mirror appear closer than they are.”
Klakamp Construction, Findlay, Ohio
Edited 7/7/2005 3:43 pm ET by Bryan Klakamp
Replies
You aren't going to know the answer without some soil tests. Too many variables otherwise.
OUCH!!!
over 2 tons per sq ft on CLAY!!! You are in for serious settling.
My references say about 1000#/sq ft for clay, that would be a 6 ft square footing or more.
I just found an 1867 arch. book, I'll see what they said back then and add an edit later. .
Edit, all the 1867 "Achitectural Encyclopedia" had to say on the subject was to NOT build on clay, as the campnile and cathedral at Pisa so attest.
Edited 7/7/2005 4:54 pm ET by JUNKHOUND
You're right about what we'll say about engineers. You'll pay less for the engineer than you will for all that extra concrete you'd use "just in case".
Depends on the makeup of the soil, as has already been stated. "Clay" is a pretty broad term, but generally it doesn't have the greatest bearing capacity.
It really is as simple as finding the allowable capacity per SF (via a soils engineer) and dividing the load by the allowable KSF. I seriously doubt you'll get it with only a 2x2 footing, since that would be 10KSF- that's the type of loading you can expect when you're bearing on rock- not clay.
As JUnkhound said, something along the lines of a 5x5 or 6x6 is far more likely, and it'll end up being deeper than 1' as well- probably 2'- 2 1/2' easily.
Please check with a structural and soils engineer though, before someone gets killed.
Bob
Another consideration with clay is that some clays are expansive. They expand when they get wet, and shrink as they dry. The adobe we have here in LA is like that. It can break concrete footings. If your clay is expansive, it might be better to go deep, where the moisture content is less variable. You need a soils engineer for this one.
-- J.S.
I'm guessing that an engineer (do consult one) will tell you to use at least 12" block in that area. Also remember that some of the point load will be distributed downward at roughly 45 degree angles through the adjacent block helping to distribute the weight to a larger area. Expect the engineer to specify a fair number of re-bar in the footing.
Woody
Two 22+ ton point loads? What kind of garage is this, anyway?
Re: "Two 22+ ton point loads? What kind of garage is this, anyway?"A man has to have a place to park his matched set of his-n-her M-1 tanks. At 60 tons each. Vanity plates marked 'Bambi' and 'Thumper'. Wouldn't do to leave those sweet powder-pink and blue paint jobs in the rain.Whole garage has to be on posts. It would be terrible loss if the tracks were to get wet when the creek rises. Wouldn't do at all. These are sweet, and heavy, babies that deserve every protection.
Thank you for your interesting responses.
Here is a little more information. This is a two-story garage, gambrel style roof. The second story and roof will be supported by two 3-ply 18" LVL's. The center support which is offset in the beams 40 foot length by two feet is the one that will have the 22 ton load. I-joists will span 20' between the two beams, for a 20' x 40' room. The center support is to have a minimum bearing length of 7 5/8". The design for the beams, floor system, and trusses has been done by the truss designer. This load is for a 10 psf dead load and a 40 psf live load. The ends of the beam will have the 5 and 6 ton loads. Basically, the entire second floor and most of the roof will rest on these beams.
Hope this helps.
Thanks,
Bryan"Objects in mirror appear closer than they are."
Klakamp Construction, Findlay, Ohio
you can do it another way. put the bldg on a mat fdn, instead of footing. pour your whole slab about 2 foot thick with heavy rebar and the structure would just float.
Bryan,
This is the picture I get from your description.
I know its wrong, but that's what you've been saying, that your house has no walls around the first floor, just two columns holding up everything.
View Image
Maybe now you can see why we are all a little cornfuzed.
Samt
I'll try to be a little more specific.
The building is 32' x 40'. At 6' in form the 40' walls on each side will be a 40' long beam. This means that there will be two beams 40' long, 20' feet apart. These beams will support the I-joists for the 20' wide second floor room. There will be triangular trusses 6' wide by 8' high, approx 19/12 pitch. These trusses will form the roof and walls for the second floor. On top of these trusses will sit 20'7" long 4/12 pitch trusses to form the upper part of the roof for the second floor. This will make it a gambrel-style roof. The posts in question will sit 18' from one end, 22' from the other end. At one end of each beam will be a post that will support 5 tons, and at the other end will be a post that will support 6 tons. Again, these loads are the combined dead and live loads.
I hope that adds a little more clarity to the picture. This means that you will have to redraw your drawing.
Again, thank you for your concerns. I am in the process of having footings engineered to properly support these loads.
Thanks,
Bryan"Objects in mirror appear closer than they are."
Klakamp Construction, Findlay, Ohio
Ah, Findlay, Ohio. Out near the great black swamp. After your engineer gets back to you, tell us what soil bearings he used, and what his pad designs were.
Here is a good text on the land on which you are building. I used to travel over it weekly, shuttling between northeastern Indiana, Perrysburg, and Bowling Green. You don't want to lose attention while driving those county roads around there. Those drainage ditches are deep ones, and would wreck your car, and you, if you wandered off pavement.
"It is hard to believe that there once lay a terrible swamp beginning in the vicinity of South Boundary Street and running as far as Findlay, Ohio, and east and west from the city of Sandusky nearly to Fort Wayne, Ind.--40 miles wide and 120 miles long. It was the Great Black Swamp, an oozing mass of water, mud, snakes, wolves, wildcats, biting flies, and clouds of gnats and mosquitoes. It was nearly big enough to cover the entire state of Connecticut.
Water, often up to the belly of a horse, stood on the surface until it evaporated in the hot summer months. When it rained, or thawed in the winter, it was water and muck. Much of the swamp was covered with an almost impenetrable forest of giant oak, sycamore, hickory, walnut, ash, elm, maple and cottonwood trees, except in a few prairie areas where limestone just under the surface would not support timber growth.
Not even native Indians went into the swamp except to hunt, and unless you could follow a blazed trail, it was easy to become hopelessly lost since you could only see but a few yards ahead.
The swamp was made 20,000 years ago when the last glacier retreated.
The enormous weight of the mile-high ice pack pressed down and scooped out the earth beneath it to create a depression about 10 feet lower where Perrysburg sits on the river bluff. Thereafter, until it was drained, water stood in the wetland and clay in the ground prevented it from soaking in. When water was standing, even large fish from the Maumee River and other streams could all over areas now covered by corn and soybean fields. One man in Perrysburg told of ice skating to what is now Weston, nearly 17 miles southwest of here.
There was no end to the variety of sicknesses and maladies spawned the mosquito-infested swamp. There was cholera, typhoid and milk sickness, but chief among them were malarial fevers generally known as ague for which people kept quinine powder on the table, along with salt and pepper, to sprinkle on their food.
The fevers caused people to have chills, or the shakes, and according to a doctor of the time it took them from three to five years to get over it. The shakes occurred from about the first of July until the first frost. They took hold of people and literally shook them up. The doctor wrote that so violent were the chills and shaking that when they came on, the very bed and floor would rattle.
The Black Swamp was Ohio's last frontier, and beginning in the 1840s, it took several generations of determined farmers to drain it and make ours the rich, flat farmland it is today. What started it all was pretty much the idea of the medical profession which believed that it was bad swamp air that caused the fevers.
They were ignorant of the fact that it was blood-sucking mosquitoes, but at least they were on the right track. Along with this, when canals and railroads came through here they created markets for the vast timber resources, most of it in the swamp. And still another good reason for beginning the tremendous job of draining was the realization that it could be done. People learned from trying to build roads that they could dig ditches and the water would flow toward the nearest stream of river.
Until then, early farmers tilled just the highest ground, with some effort to build shallow, open ditches around a plot or field, or one leading to the nearest creek if available. As more settlers came, farmers would sometime cooperate in extending their adjoining ditches.
Finally, in 1850, the Ohio legislature passed the first law and this resulted in people throughout Northwest Ohio cooperating in wide-area drainage, with ditches deep enough to drain the swamp water into Lake Erie via the Maumee and Portage Rivers.
Individual farmers continued to dry out their fields by plowing trenches across them, by using wooden troughs laid underground, and eventually with tiles and pipe introduced by European farmers.
It took back-breaking labor and one of the greatest underground drainage systems in the history of the world to create the productive farmland we now drive by and take for granted just outside of Perrysburg."
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Edited 7/9/2005 11:34 pm ET by Ima Wannabe
Great story IW.
now we are going to have to "restore the wetlands" & convert back to the malaria & other maladies.
I think I understand now,
There are 4 corner posts, 2 at 5 tons, 2 at 6 tons, and 2 semi-center posts at 22 tons each, for a total of 66 tons.
And no bearing walls on the first floor.
Anyway, from the sound of Ima's post your soil may have as little as 500 lbs/sqft capacity.
You really need to get a soils engineer to look at your situation. I'm thinking piers may be needed.
Samt
In places here on the coast, where sometimes structures are built on filled estuaries and former swamps, we drive piling to point of refusal and THEN pour the footings.
Well, with that last description, I'm thinking that your engineer is likely to want a drilled pier rather than a poured footing. (That's simpler than having two or three widths & depths of footing to carry the varying loads.)
At least that's been my experience with the montomorillite-laden (highest expansion factor) black prarie clay around Dallas.
But, that's back to "you need a local engineer who has seen your, actual site in person" answer. Might be that you could actually use driven piers, too--or some other foundation.Occupational hazard of my occupation not being around (sorry Bubba)
Sounds like you've done your homework. Honestly, I think your reactiona may be a bit conservative. No way are both your roof and floor going to be fully loaded at the same time over the entire structure. Also - beam design programs take the wort unbalanced load case and apply them to each reaction. If you hand calc the reactions you may get lower numbers.I think BOCA allows for a live load reduction of some sort if the tributary area of a beam is over 400 square feet. But I don't recall exactly, and I haven't seen that used much. But to get any sort of reduction, you'd probably have to hire an engineer and get your building department to O.K. it. But even at that, I don't think the 2X2 pads you mentioned are going to be adequate. A soil test and a soils engineer are probably your best bet. Sure beats trying to fix these later. I hope you'll let us know how this all turns out.
Q: Why coudn't Hellen Keller drive?
Because she's a woman
How have you arrived at your point loads?
Allowable bearings for soils run from a low of about 2,000 psf up to 8,000 psf. Let's say your conditions are in the middle, at 4000 psf, which is what clayey sands, sand-clay mixtures, clayey gravels, and gravel-sand-clay mixtures will do.
Your 44,000 point load will need to be distributed over 11 square feet, and if using a square pad, you minimum size would be 3'-4" square. Your pad thickness and rebar sizing and spacing will be dependent on the size of the pier that brings the load to the pad.
What is your engineer doing for you? Or are you really going to build this thing after just soliciting our opinions here on Breaktime?
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Or are you really going to build this thing after just soliciting our opinions here on Breaktime?
Why not, what better advice coould he get<G>
On a serious note, load your truck up at the gravel pit and get the scale weight when leaving. Park your truck over 3 holes dug down to where your foundation will sit. Put a square foot 1/2" thick steel plate in each and hydraulic jacks with 4x4s up to the truck. Jack up the truck. See how far the truck and each plate has sunk the next day. You can look in the corp of enginneers "soil handbook" and do a rough calculation for your soil properties. Use the handbook for footing design. Heck, even designed a small earthen dam out of the hadbook once, didn't even know breaktime existed then (probably didnt) to ask advice.
<<< "... Or are you really going to build this thing after just soliciting our opinions here on Breaktime?...">>>
Brian- I just ignore people who say things like that- If you don't answer them, they go away and try to dictate to other people-
In North Carolina, the presumptive bearing capacity for clay in the residential building code is 2000 lbs/sq ft. We do not have expansive clays( or at least I haven't run across any - never underestimate Mother Nature's surprises), for which the capacity would be even less. Without soil tests to back me up, I would never use more than 2000 lbs/ sq ft even for my own stuff and I've been known to pinch pennies till Lincoln screams for mercy. I also have been a practicing civil engineer dealing with Carolina clays for more than 30 years and have developed a pretty good "feel" for bearing capacity in our clay. Nevertheless, wouldn't go with my "feel" with out valid bearing capacity tests.
You build it, no soil data, the building settles - risk versus reward doesn't work for me.
Good luck.
It depends on the soil structure. I calc that to work out to just under 290# PSI. I don't believe that cals will handle that.
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Here's one more vote for an engineer, and unfortunately I'm not licensed in Ohio. Gut feeling based on soils bearing capacity guidelines say that your footings for those kind of point loads should be at least 5' square and probably 2' thick or more. Get soil tests done and spend the money for a basic footing design by a licensed engineer.
Here are the specs that an engineer designed for me:
I have designed a spread in the footing to accommodate the post load. There should be a 16"x16" pilaster beneath the post supported at the center of the spread footing. It should be filled with concrete with reinforcement (1/2" rebar in each cavity should suffice unless there are substantial lateral loads). The footing needs to be spread to 5' 6" in width beneath the post. It can be 8" thick, as is the rest of the footing. The reinforcement parallel to the wall should be either 9 #3 bars spaced at 7.5" or 5 #4 bars spaced at 15". Perpendicular to the wall, the reinforcement should be one of the following:
16 #3 bars spaced at 4"
9 #4 bars spaced at 7.5"
6 #5 bars spaced at 12"
or
5 #6 bars spaced at 15"
This design has been based on an allowable soil bearing pressure of 1500 psf, which is rather conservative but codes sometimes call for soils investigation by qualified professionals if a higher value is utilized.
One more thing to consider:
What type of post would you use to support this beam? The truss design calls for a minimum bearing of 7 5/8". Using a figure that I found in a post by Boss Hog, I used 565 psi for syp lumber. This would give me a total of 22,617 pounds that a 5.25" x 7.625" syp post could handle. Obviously not enough to carry the load.
Any suggestions?
Thanks,
Bryan
"Objects in mirror appear closer than they are."
Klakamp Construction, Findlay, Ohio
Edited 7/11/2005 12:34 pm ET by Bryan Klakamp
What type of post would you use to support this beam?
Given your bearing requirements--I'd be sorely tempted to a 4x6 tube steel column with a full-thickeness welded support plate tope & bottom.
A 5" diameter 56K steel column might work, but then you'd need some design for the support flanges for the bearing plate, too.Occupational hazard of my occupation not being around (sorry Bubba)
Thanks for your suggestion.
I had a local company quote me for a 4" x 6" steel column with a 1/2" top and bottom plate. They suggested going with 3/4" plates, and also specified the column with 1/4" thickness walls.
Total cost a little less than $600 for two columns.
Bryan"Objects in mirror appear closer than they are."
Klakamp Construction, Findlay, Ohio
Total cost a little less than $600 for two columns.
Not a bad price.
Any BI grief about the base/footing for the columns?Occupational hazard of my occupation not being around (sorry Bubba)
No inspections in my county, unless it's commercial. Plus, this is in the country, not city. Only thing the township is concerned with is meeting the setback requirements.
Bryan"Objects in mirror appear closer than they are."
Klakamp Construction, Findlay, Ohio