Calculating Rebar diameter for Concrete Landing 5

[azonicbruce]

First of all, let me say, WOW! I can't believe the wealth of knowledge that can be found on this site. First time posting and hope to be able to contribute.

I'm supposed to be a design engineer, but manufacturing engineering is my background, so I'm VERY new to working with structural design.

Here's the problem. We are going to be putting in a new concrete landing on our site. Approximate size is 190' x 100' by 8" thick.

I'm wondering if there's a good beginning reference for calculating what size rebar we will need for reinforcement. Typical items being placed on this pad weigh about 400 tons. The contact patch they have with the concrete is small (about 36 sq. in. each footing, 4 footings).

I think were going to become more and more involved with foundation/concrete design, so what a good BEGINNER's reference?

Thanks

 

[AlohaBob]

You need more information.

Check reference Ringo, Anderson -
Designing floor slab on grade.

That's quite a leap in scope of responsibility.

 

[cap4000]

Another highly recommended article for you is: Concrete
Floors Slabs on Grade Subjected to Heavy Loads. The ARMY manual# TM 5-809-12 Chapter 15. You might be able to search this with google and then print it. Make sure you have a thick solid soil/gravel sub-base. Are you sure an 8 inch slab is enough?? Good Luck.

 

[ERV]

A couple of excellent refernce books:

http://www.pubs.asce.org/BOOKdisplay.cgi?9991625

http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471395765.html

The second book comes with a handy software program called "CONCAD", very useful for reinforced concrete design.

 

[Focht3]

I'm wondering if there's a good beginning reference for calculating what size rebar we will need for reinforcement. Typical items being placed on this pad weigh about 400 tons. The contact patch they have with the concrete is small (about 36 sq. in. each footing, 4 footings).

Hmmm,

36 in[sup]2[/sup] is 0.25 ft[sup]2[/sup] - so the contact pressure, p, beneath each footing will be:

p = 400*2,000/4/0.25 = 800,000 psf = 800 ksf ~ 5.6 ksi!

While you need to design the concrete for the (very large) imposed loads, be sure to look at the soil-related issues too!



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[AlohaBob]

Dido Wow,

That's 800 kip point loads. I have no doubt you are beyond slab on grade. YOu probably need a pile foundation and an engineered structural slab - pile cap.

 

[azonicbruce]

Thanks for the replies so far. Those books/links look like good places to start.

Truth be told: In my mind management should have paid to have a structural expert be in charge of this project, but because I am an "engineer" it's assumed that I know everything engineering. Like I said, since we'll probably be involved in determining these types of things in the future, I feel I'd better do the best I can to learn it now so I can make more accurate decisions when the next project comes along.

Many of the specifications were chosen from suggestions from others within the company and concrete companies who we will be contracting this work out to. For example, 8" was chosen because our current pad is 6" thick, and of course, is beginning to crack. Other suggestions were to use 3/4" rebar spaced 12" apart, and 10guage wire mesh throughout. Also, since it was determined that we might possibly build a structure over this pad, we added footings. Again, arbitrary numbers of 36" deep and 18" thick were thrown out.

The company that finally got the contract has come back to us saying we probably don't need the wire mesh, and that we only need 1/2" rebar. We feel that it's possible that they are simply trying to cut costs rather than give us sound advice, which is why I decided to come here and find out where I could go so I could start learning this stuff and find out for myself.

Any other suggestions would be appreciated. In the meantime, I have a good chunk of info to research and learn. Thanks

 

[Focht3]

Where is your site? (city, state only)

Is this a cargo dock? (I have been thinking about what could carry such large loads...) The loads are way too high to be hauled by trucks...

If these are being placed behind a bulkhead, watch out. These loads could easily overload the bulkhead, anchorage, walers, etc. Pavement failure is a trivial problem by comparison. If the site is along the waterfront, insist your employer hire a qualified firm to perform the design. DO NOT proceed on your own!

You're dealing with unusually high point loads for pavements; [blue]AlohaBob[/blue] may be right - you may need a structural system. Then again, management may be willing to accept some problems in return for a reduced cost. But management needs to understand the risks inherent in their decisions -

First, the contact pressures are a problem by themselves. Since the contact pressure is greater than the likely compressive strength of the pavement concrete I'll guess some of the damage has to do with surficial failure of the concrete. This can be reduced by resting the 9 inch square legs on "thick" steel plates to distribute the load and reduce the contact pressure. If the containers are always placed at specific locations (which seems doubtful), you could embed larger panels of steel plate into the surface of the concrete. You could also put more reinforcement in these specific areas to support the plates.

You could always go to a double matted slab; but this won't be much cheaper than [blue]AlohaBob[/blue]'s approach.

And you still haven't addressed the overall stability of the load/pavement/soil system -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[AlohaBob]

Those rotten buggas. The serviceability for what you are doing would likely far weigh cost in the process of making good decisions. Most likely less costly in every respect. Glad to be of help. Give me a ring I'll be right over.

 

[azonicbruce]

OK, here's some more info. Focht3...we're in El Paso, TX. While we are close to the Rio Grande, I wouldn't exactly call it waterfront. We're well over 1000 feet from the river. Also, the river has been completely bone dry for quite some time. Don't know how that affects it though.

I also just learned that flexural strength we requested from the concrete contractors was 3000psi concrete. On page 3-3 of TM 5-809-1 it only goes up to 700psi concrete. So I wasn't able to get a quick comparison for stationary live loads, but regardless, here's another problem.

While the original intention was using the pad for the assembly of parts (we build car shredders), it's very possible that they will want to drive our portable crane over this thing. I'll need to investigate but I'd think the crane is pretty dang heavy.

Regarding Point Loads: The original number I gave was very approximate. Depending on what we're assembling at the time, the contact area between the pad and part ranges anywhere from 300sq. feet to a couple square inches. Yes, inches.

I will try and post some pictures and weights of what I'm talking about. Will have to check with my supervisor first.

 

[azonicbruce]

OK, here are some pictures that may shed some light.

http://www2.et.byu.edu/~brb6/EngTip/Site_01.JPG

In this picture you'll see our crane, and the relative size of the parts we work with.

http://www2.et.byu.edu/~brb6/EngTip/Site_02.JPG

Here are the relative sizes of parts to a person

http://www2.et.byu.edu/~brb6/EngTip/Site_03.JPG

Part on the far right weighs about 60,000 lbs

http://www2.et.byu.edu/~brb6/EngTip/Site_04.JPG

This part weighs about 15,000 lbs. As you can see the points of contact are very small.

http://www2.et.byu.edu/~brb6/EngTip/Site_05.JPG

This part is about 90,000 lbs. Same thing...small contact points

When all parts are completely assembled, they add up to 300-400 tons. The new concrete pad we are intending to build is supposed to accomodate 6 workstations, so anywhere from 3-6 of these workstations will have units being assembled.

 

[azonicbruce]

Oops. Corrections to the above weights.

http://www2.et.byu.edu/~brb6/EngTip/Site_03.JPG

140,000 lbs not 60,000

http://www2.et.byu.edu/~brb6/EngTip/Site_04.JPG

34,000 lbs not 15,000

http://www2.et.byu.edu/~brb6/EngTip/Site_05.JPG

98,000 lbs not 90,000

 

[aggman]

My $.02,
You are way over your head here. First off, the charts that only go up to 700 psi are the TENSION stresses for the concrete, which is a fraction of the COMPRESSIVE strength of the concrete (3000 psi). Second, by my quick calcs, based on your numbers, the three parts add up to 272,000# = 136 tons, which is way shy of your 300-400 tons. Designing for a patch load of 136 tons is very much different than 400 tons. I think you need to explain to your bosses the difference between mechanical engineers and structural engineers. Explain to them that you can not make any sort of an educated decision about this and that you trying to decide on the concrete design is no better than them pulling it out of there you no what. I would explain to your management that the best thing to do would be for them to let you take a class in concrete design, if they really want you to know how to do this. In the meantime hire a structural engineer to design the slab, and maybe if you get lucky he will go through their calculations in an effort to give you some insight. If they refuse to do this, then I would start looking for a new job right away, because management that takes that kind of attitude is only a lawsuit or two away from being out of business.

 

[CTW]

Can you post pics of the components that make up the remaining 164+ tons or a fully assembled shredder? I'm assuming a majority of the remaining weight comes from motors, drives, gearboxes, etc. When the shredder is fully assembled, what is the contact area per support (or is this the 36 sq. in. you mentioned in your first post)?

Also, the 700 psi you saw in TM 5-809-12 is the flexural strength of the concrete. Which should correspond to a 28 day concrete strenth of 6000 psi. Although that is a good reference, it's not applicable to this situation as there are too many other variables to consider. Something else to keep in mind is that the pad may be loaded prior to the concrete achieving its full strength, therefore a reduced compressive strength should be considered in the design. However, for this project, a high early strength concrete may be beneficial.

 

[Focht3]

Ah, El Paso! I'm in San Antonio - my great-uncle taught a Sul Ross (Alpine, TX). My family vacations tend to go through your city - Carlsbad, Grand Canyon, Philmont Scout Ranch (near Cimarron, NM), Telluride, CO. I've been to/through El Paso a lot.

Eight hundred kip car crunchers? How do you deliver them? That's at least 10 pieces - and semis - to get to a road-legal 80 kip truck load limit. Are most of them heading to Mexico?

The odds are that your subgrade is gravel or coarse sand - perhaps rock (although I don't think so from the pictures.) While these give good support, it can be uneven.

What will happen if your design doesn't last?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[azonicbruce]

aggman,
Just to clarify, the parts I listed above are not the only parts that get put on the pad (at least that's what I've been told). There are many more parts, so they do indeed add up to about 300-400 tons.

I agree, though, I am probably in over my head. I'm trying my best with what I have to work with, though. Being fresh out of college isn't helping either.

 

[azonicbruce]

Focht3,
I've only been working here for about 2 months (also my first real job), and haven't quite been caught up to speed on how things work around here. From what I've seen and heard, the shredders do get delivered in pieces. In fact, we just shipped that blue part in the picture today. It got put on a truck and had an Oversized load truck following.

What will happen it if doesn't last? Hmm, well they'll probably say "let's just blame it on the new guy", and I will have learned a valuable lesson in life, is my guess.

 

[jheidt2543]

Once the parts are all assembled, do you do an operation test of the shreader? If so, you are not only designing for the static gravity loads but also the dynamic loads. What about the final destination, are you responsible for the shreader's foundation at that end too?

 

[Wood2coal]

8" does not sound thick enough I would at leats push it up to 10". Check ACI 318-02 chapter 11 for shear strength, and cross check it with AASHTO. This is actually more of a geotech question than a structural one. As long as your soil conditions and base material is good thickness is more important than rebar, if your building on some pretty lose soil than you might have to use a double mesh. I would also consult with a local contractor in the area, they know what works for that area, and sometimes their insight can be more valuable than the calcs.
Just make sure your design meets the standards of ACI, AASHTO, PCA, and also your County. Then no one can blame you if something goes wrong.

 

[sacem1]

Hello azonicbruce:

really it sounds you are lost out there, all the advice given before is really good and you should take really a good attention to it specially about getting a structural guy to do the design but if you find yourself in the need of a quick decision even though you do not know where to turn lets do this:
Assuming there is no river or water course near, that the ground is levell for at least 100' around your slab and that the soil is reasonably stable you should first compact your soil and if possible remove about 8" thick and replace it with a good quality fill (macadam or similar) well compacted and watered after that you'll make your slab at least 12" thick with the highest grade of concrete you can get your concrete company to come out with and add 1/2" rebars @ 10" between centers in both directions forming a mat at 2" separation from the compacted soil and another 1/2" rebar grid at the same 10" separation this one 2" from the top, that will leave 8" of concrete between both mats, some form of 1/4" hangers from the upper to the lower mat and spacers to gurantee the rebars stay in place with the impact of the concrete being filled, have several vibrators for the day the concrete is poured in and divide your slab in squares no bigger than 20 x 20' with 1" separation between adjoining individual slabs and 1" rebars at 12" centers at the middle of the thickness of the slab crossing from one slab to the other with a 48" lenght each one , about 24" into each slab, after curing the concrete with permanent wetting for at least 2 weeks let it attain full resistance for another 2 weeks before loading anything on it to be sure you do not overtstress the slab before it attains full strenght.
If the soil is exceptionally good you cut cut the slab thickness to 10" but I would stay on the safe side.
I´m sure that if a structural engineer makes the calcs and gets a soil resistance study you could save more than what he'll cost but if as I said you are stuck with giving a solution, I do not think you'll get fired for using this as a solution.
Good luck
SACEM1