I am working on a little project where they would like to put a couple of vaults on a floor that I have no written information on.
The inspection dept said they did not have anything on file, the original builder said the pad was filled with red clay, packed with a loaded dump truck so hard that a backhoe/loader could not scrape some high spots out.
It was a nominal pour of 3 1/2″ with some spots that were a little thinner because of the hard pack. No steel or mesh but fiber reinforced product. These babies weigh around 5500# with average load and have a footprint just under 10 sqft. I am waiting for data sheets from Diebold but not here yet.
I guess worst case is floor cracks and have to move them out and cut floor out and repour pads.
I have not been able to find any online data, any suggestions? Asked my inspector and he said he was not an engineer. It appears that the bottom is flat and will spread fairly evenly. A little over 550/sqft but it can’t fall far (:-).
Thanks in advance.
Bob
Replies
Its going to crack. Fiber is not a load factor product, only rebar is. and at 3 1/2 inches, that not enough concrete for a patio. Will it crack, yes.
.
Wrong mix notwithstanding, it doesn't sound like a lot of weight to me.
~550/s.f. is less than some cars - lots of 3-1/2" slabs here (but there's usually at least remesh in a garage)
I'd be inclined to place them and see what happens.
Unless there's a good alternative location you might as well give it a shot.
The customer needs to understand the risk though...
Put a hard rubber (think tire tread) mat under them and go for it. A large car weighs 4000# on 4 hard rubber foot prints of less than 1/2 sqft each.
The mat (tire) eliminates the micropoint loading of slightly out of flat concrete.
SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI 84310.51
The real problem is how are you going to get them into position without cracking the full lenght of the pathway.
Any constant load on a curing slab will cause major cracking as the slab shrinks.
Gabe
This slab is about 13 yrs old by the way. Thanks for input all.
Bob
If the slab is that old, I would strongly suggest sounding the slab for hollow spots before locating your safes.
A dump truck is not a compactor and if the work done by the original contractor is sub standard, it's a roll of the dice regarding the slabs ability to resists failures.
Gabe
I think your greatest risk will be while you are placing the vaults; I've seen floors crack / spall and otherwise be damaged by the weight on the cart wheels, when the equipment is being positioned. The rubber mat is a good idea; I'd carry it a point further, and construct some sort of "pallet" to place between the vaults and the floor. I have found a surprising amount of moisture passes through slabs, and setting things directly on the cement is seldom wise. I agree that the slab is too thin for it to bear the load. No matter how well packed that ground is, it will change with the seasons. I don't suppose you can pour another slab on top, with the appropriate steel mesh and connections? Another name for that slab is "foundation." The foundation is the one place you don't want to cut corners.
An update for all and thanks again for all the input even the tire pressure excursions.
Gave all available data to a Lic arch in the next county. Suggested putting layer of rubber or commercial grade rubber backed (dense) carpet on floor then a sheet of 1" plywood at least the footprint. I intend to probably almost double the size. The vaults are 40.5" x 33" Will probably go to 4' x 4' which will take it from 9.2 to 16 sqft. Doubt if ply will spread load out much more than that.
Checked floor with straight edge and it is not a pool table. The vault company uses a pallet jack type trolley and plywood end to end leapfrogging to get in place. They are empty shells (3900#) when installed and then the inards are installed. By the way drainage is great and this is packed above grade over gray clay also well drained.
I think I have done my due diligence, I am not installing or guaranteeing anything. I notified the owner's son and will be sending him copy of archy letter with seal.
Thanks again, Bob
If you do it and it works, you're good.
If it doesn't work OR if you rebuild the slab first, it's the same cost.
SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI 84310.51
your tag line and your post do not agree with each other.
There's right and there's right.
The client is saying I don't want to spend money I don't have to. It's a binary solution set, either it works as is or it gets replaced.
In order for the contractor to know whether or not this slab will work as is, he can do cores, ultrasound, xrays, reverse engineering, etc. That would tell him if the slab should work or may not work Or he could try it and see if it works. That will tell (short term) him it should work or it definately will not work. That's twice as good as the expensive tests, right there. Long term, it could prove to work just fine.
IMO, the right way to test this slab is to put the vaults in place for a year, charge the HO for that and . . .
MAKE DARN SURE the HO knows that "This is a test," and more money may be spent later.
Putting the vaults in place is the surest and cheapest, albeit, potentially slowest, way to test the slab.
One has to recognise that all the tests in the world can only indicate in this marginal case. That the only way to know the slab will work before loading it, is to replace it. The only way to know the slab won't work as is, is to try it.
On the other hand, if the HO is easily scared, think of the profit. . .SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Thanks, a little more background. The floor is covered with commercial vinyl tile, fairly even, will check it closer tomorrow. Building is one that was built for furniture store showroom with loading dock (one vault location) that was closed in. Being leased to a very large Credit Union for local branch in temporary lease (three years) situation.
I usually work for the owners in small time maintenance and repair for their three connected buildings two of which are much older and in poor condition. Kind of fell into doing the conversion for the CU. They have their own contractors for store front aluminum and glass partitions, fixtures, carpet, data/comm lines, security.
I am going to tell the manager tomorrow what my concerns are and then draft a letter to owner (actually her real estate attorney son) and Credit union with my crude evaluation and recommend that they test and correct, or take their chances and be prepared to repair. Best solution is to get something that weighs less for this leased location.
This place is rock throwing distance from the Police station so I don't think anyone is going to load up a vault that weighs half that amount and put it in their pickup.
I am not the one that did the slab, selected vaults or locations, or indicated suitability for the use. My inspector is supposed to drop by this week and will discuss it further.
Bob
Structural engineer here...it is nice to have useful information for once. Your slab will crack. The subgrade, slab thickness and lack of welded wire mesh or rebar all point to this. Even a reinforced pad on top of the existing slab could cause problems. I recommend saw-cutting the existing slab around the area where the vaults are to go. Then, either remove this portion and place a new, thicker pad in its place, or place a new pad on top. Your goal is to let the vault loaded area move independent to the rest of the room. A typical 4" slab on a good subgrade and reinforced with mesh is designed for around 400 psf...you don't have that. By the way, those who are speaking of the weight of automobiles...a parking garage in CT is required to be designed for 40 psf, or a 3000 lb concentrated load on 20 sqaure inches. Also, no matter how large the vehicle is, it is always exerting a load on the slab approximately equivelant to the tire pressure.
it is always exerting a load on the slab approximately equivelant to the tire pressure.
That's a ridiculable statement. It would mean that a caddilac and a volkswagen would be the same load as long as they both had 32psi in their tires and that a 10speed would excert a greater load as it would have 70 psi.
Or that we could increase ferrys, carrying capacity by just lowering all vehivles tire pressure to the minimum required to keep the rims off the deck.
I understand that you meant that as long as the rims aren't sitting on the slab, the load /sqinch can not exceed aproximately the relative air pressure in the tires. But using air pressure in a tire to the compute the load a vehicle puts on a slab is stupid
SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Edited 1/22/2007 3:05 pm by SamT
He's saying that its PSI load is approximately equal to tire pressure. The difference between vehicles is in the footprint.Of course, even poor concrete can stand something on the order of 1000 PSI in a concentrated load. At issue is the ability of a slab to spread weight without failing. This places areas of the concrete in tension (its worst mode for strength), as the slab flexes like a drumhead.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
He's saying that its PSI load is approximately equal to tire pressure. The difference between vehicles is in the footprint.
LOL I know what he said. I disagree with him. My GF's young teenage daughter also disagrees with him. After thinking about it for a minute.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Sorry Sam, PSI is exactly that. Pounds per square inch. This is the principal behind every hydraulic device in existence. How else can a few pound pressure on a bottle jack lift a car? Surely you don't think you need to "push down" 200 lbs to lift a 2000 lb car? Now, this is something that is easy enough to experiment with; but, the short version is that the area of the tires' footprints times the air pressure in the tires is exactly the weight of the car. Or bike. The whole point of a concrete floor is to spread the load over a larger area .... that is, to keep everything from sinking into the mud. To do this, the concrete needs both thickness, and something to help hold it together (steel). I think we've pretty well established that the existing slab is inadequate for the proposed load.
see above.
was gonna delete. . . but I dint wantcha madd at me for an "Unread Msg"
Edited 1/22/2007 9:51 pm by SamT
Found this somewhere on the web ...
The "30 pounds" that many people speak of is really a contraction of the whole description, which is "30 pounds per square inch", and is a measure of pressure. To obtain the force that this pressure can resist, you have to multiply the pressure times the area of contact of the tire, which would be "pounds per square inch" times "square inch", resulting in "pounds", which is force.
Let's estimate the contact area of a tire on the ground. Let's say the tire tread is 5 inches wide. Now, when I look at the tires on my cars it looks like the contact area of the tread is about 6 inches long (the "length" of the tread that is on the ground). That's 5 inches times 6 inches which is 30 square inches. Multiply that times 30 pounds per square inch and you get 900 pounds. Since my car has 4 tires, and 4 times 900 is 3600, the tires on my car can hold up 3600 pounds of car. Now, my car doesn't actually weigh 3600 pounds, so I have overestimated the contact area.
What actually happens is that the contact area adjusts so that the force is just equal to the exact weight of the vehicle. What happens if you load the car with lots of extra weight is that the tires get squashed down more and therefore there is more contact area on the ground.
John Link, MadSci Physicist
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Also, no matter how large the vehicle is, it is always exerting a load on the slab approximately equivelant to the tire pressure.
Are you saying that you agree with that statement?
SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Edited 1/22/2007 9:52 pm by SamT
... always exerting a PRESSURE on the slab ...There's a need to be a little more precise in terminology.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
You don't say!
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
So if I took the size of the footprint and the psi of the tires I could calculate the weight of the car.
In theory, discounting the stiffness of the rubber itself.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
the footprint of a 195/65R15 is about 170 mm2
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Depends on the weight of the vehicle. And 170 mm2 sounds pretty tiny.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Yeah Dan it does sound odd. I found that on a tire web site, and it was giving an example. Thinking through it, the contact area is a function of the air psi and the vehicle weight.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Could that be per hundred pounds or some such?
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
The tire provides a good deal of strength. So it is very unlikely that the tire pressure is an accurate measure of the pressure of the tire against the ground.0 air pressure is certainly going to provide the wrong answer.
The standard car tire is quite flexible, relative to the weight of the vehicle. This is primarily because stiffness implies heat buildup and lost fuel mileage, something to be avoided. As a result the contact pressure of the standard tire will be quite close to the tire's air pressure (if you average the contact pressure a bit to account for the tread pattern)."Run flat" tires and heavy-treaded truck/implement tires, of course, are stiffer.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
and of course as the temp rise of the air so will pressure so temp would be a factor too..
Yes, but the air-pressure/contact-pressure relationship still holds.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
This here explanation is a pretty good one. It's actually correct, but it is missing a few qualifeirs.
That basically means that it is sometimes true.
For instance, when driving on sand, letting pressure out of the tires will result in a larger and softer contact patch, which means a lot less load/"2, this Lets you "Float" on top instead of bogging down. On Ice, or concrete, it would tend to prevent cracking, provided the material can handle the total load of all contact patches.
Increasing the load a tire is carrying has very little effect on tire pressure because the only compression acting on the air inside is through the contact patch. Drastically increasing the load will make very little difference in tire air pressure.
That's why my tires "slumped" about 2" when I put a full ton of gravel in the bed. Oops. Glad it was only a 1/4 mile run. Tried a 1" "slump", that came to 1500# of gravel on my rear tires. That's ok. I also pumped the tires up to max pressure after that.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
In regards to your link:One should not take what appears on the internet as truth.
noSamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Sam,
Is it possible to compute the weight of the vehicle using the tire pressure and an exact measurment of the contact area of the tires?
I can see that it is possible but also doubt it. (easily confused here) Wondering for instance what happens when I jack the car up with two floor jacks that each have 4 steel wheels 1" wide with only 1/32" of contact ea. wheel.
I come up with .25 sq. in. of total bearing surface and that means that my garage slab is capable of holding a 4000 lb. truck on .25 sq. in..
Or 576 (144 / .25) x 4000lbs = 2,304,000 lbs. / sq./ ft.
Now even if I have "flat on one side " steel wheels that each have 1 sq. /in of surface and I have 8 wheels I still get 4000lbs. / 8 sq/in. and that is 500lbs. sq./in.
Which happens to be 72,000 lbs./ sq./ ft.
Whats the problem with the safe on the slab again.. ?
I did like the idea of precutting control joints to prevent cracks from spreading.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Whats the problem with the safe on the slab again.. ?the problem is not the safe but the concrete itself. think of the slab as a beam. when weight is applied the slab will bend, this cause cracking. to counter act the bending you need, thickness, psi and rebar. If you add more rebar the thickness and psi can be less. if you have high psi then rebar and thickness can be less. Also thickness apply to rebar to. a certain amount of coverage is needed for the steel itself. you need a 1/2 inch at the bottom, rebar needs to be in the bottom 1/3 for weight. upper 1/3 for tension. so 1 1/2 inch for concrete, 1/2 inch #4 bar that leave 1 1/2 inch of corvage. not enough. you need three more iches, so you looking at a 5 inch slab with #4. I think it needs #5 bars. Now we talking psi. 4000 will do it. 4000 poured dry is better.#4 5 inch thick 4500 psi, now we need compaction of subsoil, control joints and stress cracking fiber or mesh..
Can we start a separate folder for the engineers among us?
Treat every person you meet like you will know them the rest of your life - you just might!
brownie,
I am not unaware of the working characteristics of concrete nor of what it takes to make it work. I am not an Engineer , but not a novice in the field either.
I am also not unaware that if the subgrade is capable of handling the loads placed on it without deflection or deformation that the concrete only has to transfer the loads placed on it to the subgrade. I could park my truck on a sheet of window glass without shattering the glass if the glass were uniformly supported across it's entire bearing surface by a substrata that was capable of withstanding the loads without deflection or deformation. Is there something wrong with my math when I use the floor jacks as an example? These are used all the time on unreinforced slabs without failure of the slabs.
Prevent the subgrade from deflecting or deforming and the concrete itself has little tension load in it's lower third.
Dry , hard packed clay can support enormous amounts of weight. The OP stated that the substrata had been rolled with a loaded dump truck , a crude form of "proof rolling" that is done all the time and that a blade had a very difficult time grading after that . Seems to me that the substrata might well be able to support the load.
The assumption here has been that the sub strata is incapable of handling the load, therefore putting stress on the concrete that it cannot handle without reinforcment. Not an unreasonable assumption, but an assumption never the less.
If the OP had stated that the substrata was solid granite would any of this be an issue in terms of the placing of the safe on the slab?
I am not disagreeing with what you have posted at all, or questioning your knowledge or that of the other posters here.
Just pointing out that despite of all our math and calculations there are real life examples that defy our logic.
> I could park my truck on a sheet of window glass without shattering the glass if the glass were uniformly supported across it's entire bearing surface by a substrata that was capable of withstanding the loads without deflection or deformation.I was told by my grandfather that when they wanted to service the overhead streetcar wires, they used to lay down a sheet of glass and drive the service vehicle onto it.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Some very good points here. With concrete slab on grade, the grade preparation is everything. The slab is only redistributing point loads to the dirt. An unreinforced slab can do this at approximately a 60 angle, so a pair of floor jack wheels distribut their 6000 pounds to approximately 275 square inches of dirt (4" slab and wheels 6" apart). That is a soil pressure of just over 3000 pounds per square foot which is a decent subgrade. Increase the load too much, and your little pyramid of concrete will sink into the dirt and begin to crack etc. A reinforced slab can distribute the load through different mechanisms.
A dumptruck could actually do a fantastic job compacting a subgrade if the wheels are actually run over every spot. Larege pneumatic tire rollers doing a highway subgrade actually use the PSI principles previously discussed to apply a greater compactive effort than could be done with a steel drum roller. Of course, mechanical devices like a vibratory plate compactor or a jumping jack also can achieve high levels of compaction.
Regarding the though that tension is in the lower third, this is simply not so for a grade supported structure. The interaction between a rigid surface and the semi-flexible soil is a complex affair. Suffice to say that the top of the slab will also go into tension in many locations. This is a continuous beam example, not a simple beam supported on two points. It's like running an I-joist from wall to wall for a 30' house with a beam in the center; the top of the joist is in tension over the center beam.
Again, great concrete cannot make up for a poorly prepared base.
Regards
Thank You tuolumne.
As I said , I am not an engineer, but I can understand the point about the slab going into tension. After thinking about the tire pressure issue I can see it's validity when one factors all the parts into the equation.
I was not attempting to cast serious doubt on what had been posted about the function and composition of concrete slabs, rather point out the problems inherent in not taking the whole system into account when looking at a situation.
I appreciate you taking the time to reply to my post, I had begun to wonder if any one would.
Thanks Again.
(isn't there a river and meadow bearing your knick name as well?)
I hope you will contribute more often. Your comments generated the best discussion I have seen around here in a while.
Being leased to a very large Credit Union for local branch in temporary lease (three years) situation.
Cutting out a portion of the slab and providing a proper foundatuion is part of the cost of doing business and preparing the property for the branch. It gets written off over the term of the lease. Tell your project manager at the CU that either he tells you in writing how to do it, or else he leaves it to you and you're going to do it right.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Saw the concrete to match to the perimeter of the vault. If the concrete fails, no one will ever see it.
Edited 1/22/2007 10:42 am by GHR
One important consideration is the damage that cracking may do. If cracks travel 20 feet it could result in additional repair expense. Sawing some control joints around the vaults would probably be wise.
Another issue is if the vaults actually sink an inch or so -- will that render the doors inoperable? How much sinking or tilting can be tolerated and still operate the doors, keep whatever's inside in order, etc?
And you need to know if the vaults require uniform support or will be OK with point support. If the slab fails the wrong way the support under the valuts will not be totally uniform.
I wonder if it might not be cheaper to install pilings than to tear up and replace the slab?