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Calculating Rebar diameter for Concrete Landing
5

Calculating Rebar diameter for Concrete Landing

Calculating Rebar diameter for Concrete Landing

(OP)
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

RE: Calculating Rebar diameter for Concrete Landing

You need more information.

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

That's quite a leap in scope of responsibility.

RE: Calculating Rebar diameter for Concrete Landing

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.

RE: Calculating Rebar diameter for Concrete Landing

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 in2 is 0.25 ft2 - 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!



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RE: Calculating Rebar diameter for Concrete Landing

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.

RE: Calculating Rebar diameter for Concrete Landing

(OP)
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

RE: Calculating Rebar diameter for Concrete Landing

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; AlohaBob 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 AlohaBob's approach.

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



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RE: Calculating Rebar diameter for Concrete Landing

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.

RE: Calculating Rebar diameter for Concrete Landing

(OP)
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.

RE: Calculating Rebar diameter for Concrete Landing

(OP)
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.

RE: Calculating Rebar diameter for Concrete Landing

2
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.

RE: Calculating Rebar diameter for Concrete Landing

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.  

RE: Calculating Rebar diameter for Concrete Landing

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?



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RE: Calculating Rebar diameter for Concrete Landing

(OP)
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.:)

RE: Calculating Rebar diameter for Concrete Landing

(OP)
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.:)

RE: Calculating Rebar diameter for Concrete Landing

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?

RE: Calculating Rebar diameter for Concrete Landing

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.  


RE: Calculating Rebar diameter for Concrete Landing

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

RE: Calculating Rebar diameter for Concrete Landing

2
There is a reason that engineerning is taught and practiced in different disciplines - and this is a great example why!

I'm certain the civil, structural and geotechnical engineers in this thread and forum would baulk if their employer asked them to design a conveying system or select  bearings, a housing and the lubricating oil!  In fact, in my country it is against our professional body's Code of Ethics to practice outside an area of competency.

The slab design will be a relatively simple task for a suitably experienced structural engineer and recommend that you engage one in this case.

RE: Calculating Rebar diameter for Concrete Landing

Your doing the right thing by taking the initiative to learn more about the proper way to design and construct this type of slab.  It sounds like you don't have much say in the final design, but I wouldn't worry about it.  Just use this opportunity to learn.  Take notes during the construction phases about concrete strength, thickness, rebar size and spacing, location of rebar in slab, curing method, jointing, subbase/soil conditions, and anything else that may seem useful.  Be sure to take plenty of pictures.  
The good thing about this project is that the slab is not going to fall down.

RE: Calculating Rebar diameter for Concrete Landing

To dbuzz:

I'm sure that no engineer is going to answer or recomend something that they do not know or are in doubt of, however the truth is that in many situations an engineer has to develop other fields in order to be able to solve problems that arise in their work, thats why I agree completly with CTW in that azonicbruce should study and learn from this experience.
In my own experience, 31 years of profeeesional practice up to now, I have found that having a basic plan, layout or design helps a lot in refining and recalculating the final design.
I expected feed back for my recomendation either up or down from my values which are given by experience and studies as really I am an Industrial engineer wich also have studied civil engineering and done an specialization in machine design in Switzerland, right now I design machinery for heavy mining equipment and manage my own small manufacturing firm in wich we export a lot of finished components to the States, Chile, Panama Canal Authority, etc. and in the years of experience acumulated I had to solve several heavy foundation designs for the Ball and Rod Mills that we also manufacture and have to install for our customers.
Hope I did not trespass any ones turf, if I did sorry.
SACEM1

RE: Calculating Rebar diameter for Concrete Landing

(OP)
Well, I thought I'd all let you know that it doesn't matter now. The contractors already came and are starting construction. I guess the least I could do is keep you updated as to what ends up happening with this situation. I'll be taking a lot of pictures during the construction to see how it is getting built and what types of things I can consider next time around.

Thanks again for all the help. It's nice to know that there are people that are willing to help. In fact, I know that I'm going to have a few Statics questions in the near future, so you'll probably see more posts from me. :)

RE: Calculating Rebar diameter for Concrete Landing

Did you decide the pavement thickness and design, or did someone else?  (My money's on the successful bidder -)



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RE: Calculating Rebar diameter for Concrete Landing


Azonicbruce

My curiosity is with Focht3.  I have been following the posts and was wondering who made the final decision.  When I first read your post I was sure you were going to get "beat up" for practicing outside your area of competance.  For the most part everyone was helpful and your honesty contributes to that.  For thoes that like to play the "discipline" fiddle dont let it be a discourage.   Accumulate good reference material, put in the time and effort to research your designs(against this material) and try to find an experienced engineer to bounce ideas(and designs) off of.  If you have received a bs in engineering you should have the math and physics (the tools) to get a fundemental understanding of a variety of topics provided you do the proper research with good engineering reference material. It is of upmost importance to know when your in over your head and need outside assistance.  Further formal education in areas as mentioned in a prior post is a great tool to broaden you areas of knowledge.

Good Luck,

DDS1

RE: Calculating Rebar diameter for Concrete Landing

(OP)
Focht3,

Here's the deal. I was asked to be in charge of determining what we would need to complete this project. As far as the actual design, I simply based my models on what I was told to make them. At first was told to make it 10" thick, but then the "maintenance" guy said we probably wouldn't need THAT much, so they told me to change it to 8". He also said we should use 3/4" rebar spaced 12" apart in a grid pattern. Then we should use 10guage wire mesh. I guess he had been in charge of other concrete projects, but whether or not he knew what he was doing I don't know. Then, the VP of engineering told me that we probably will want footings. So I added those in. He told me they should be about 36" deep and 18" inch wide.

Since the landing was going to be placed on a slight slope, I had to determine how much dirt would be needed to make it level. Armed with nothing but a laser level and a metal ruler, I was able to model up what the fill would look like.

So really my main responsibility was to determine how much concrete, rebar, and dirt fill it would take to complete the project. With my model I was able to determine how many cubic yards or dirt/concrete, how many feet of rebar, etc. So if the contractor comes up short on materials, I'll be willing to take responsibility for that. It wasn't until the contractor said we didn't need this or that that we started to question whether or design was correct or not.

So I started to investigate, started asking questions, and here I am getting feedback from professionals who really know. I've definitely learned a good lesson working in the "real world".

I guess what's frustrating is that because I'm just out of college, I'm really not "competent" in anything; I simply don't have the experience. Yet it seems that in the environment I work in, there is nobody who really knows what they're doing either, so there's nobody to guide me and let me know what I'm doing right, and what I'm doing wrong.

Like you said DDS1, I have a basic understanding of these concepts because of my education, and I have some knowledge/tools to give me place to start, but not much to help me see if I'm going about understanding/solving the problem correctly (except for places like here). What really stinks is that I got good grades in school, but can't seem to remember those things when I really need to! I took statics, dynamics, and material properties almost 3 years ago. Should I be remembering everything I learned? Or is it normal to not remember everything you learned in college?

Sorry for my long posts. Again I appreciate everyone’s help.

RE: Calculating Rebar diameter for Concrete Landing

Azonicbruce,

No, you shouldn't remember everthing you learned in college.  Therefore the comment on good reference material.  You hopefully have your college textbooks but this is only a start.  One of my most valued possessions is the library of reference material I have accumulated through out my career.  I had a couple mentors who "guided" me through the early years.  I to was "insecure" once about what I knew or thought I was supposed to know or remember.  Experience, Experience, Experience.  But you've got the right attitude and you've got eng-tips and I can tell by many of the posts I have read on these forums that there are alot of people here who have also played the role of mentors in their daily professions to many up and coming engineers.

With that said, do the research if your name is associated with the design.  You mentioned the "maintenance man".  I have dealt with many maintenance men and contractors.  As mentioned do you homework because the maintenance man and contractors who were so eager to guide you into failure will be the first ones to step back and say "Im not the engineer".  

Your contractor that has suggested things you did or didnt need, does he have an engineer on staff.  He may have and that engineer may have provided this information and may be someone you can speak with. When someone suggest something, question them as to what they are baseing their suggestions on.  It will provide you insight as to if they have any idea what there talking about because youve done you homework or if not now have some to do. Keep you chin up and your ego down.

Good luck,

DDS1



RE: Calculating Rebar diameter for Concrete Landing

azonicbruce,

It really is not important or is it possible to remeber everything that we learn throughout life, what is important is that we learn from our mistakes. That is something that can only be learned with experience. As an engineer of more than 30+ years, I call it a gut feeling and start from there. With no experience, a person does not have a starting point, so reliance on the gut feeling of others has to be the start. The difference between good engineering practices and bad engineering pratices is research. I think that you are off to a good start, honesty about what you don't know is a rare trait, never be afraid to ask questions about what you do not understand. No answer is far better than the wrong answer in engineering.

RE: Calculating Rebar diameter for Concrete Landing

I hate engineers that try to bluff their way through problems they clearly don't understand!  Guys like that aren't very bright - trying to "buffalo" the real experts!  It used to make me mad - now it makes me laugh!



Your honesty is refreshing.  Stick with it!

I am much less concerned about pavement failure than I would be about a structure's failure - particularly since it's in your company's marshalling yard, and the design is in-house.  Finally, I have a hard time seeing anyone hurt as a result of an inadequate design.  Waste of money?  Yes.  But it isn't a fatal mistake; and it might work for some time.

Hang in there!



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