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Heavy Machinery loads on Slab on Grade design
4

Heavy Machinery loads on Slab on Grade design

Heavy Machinery loads on Slab on Grade design

(OP)
Does anyone have any insight or tips on a slab on grade design for heavy mining trucks?  For example, a mining truck manufacturer needs a parking lot designed for their finished heavy machinery and one of their trucks has a single wheel load of 62k but is spread in 13.2 ft^2 contact patch (large tires).  That results in about 4.7 ksf pressure.  I have other ones as well as a tandem trailer with 150k load to analyze. That company typically uses 12" thick slab with #5 bars in each direction and on 6" of agg subbase, but I'd like to determine if this section will work.

I have the ACI Slabs on Grade manual but it doesn't address that kind of contact area size. Those are designed for forklift or standard truck traffic it seems.  I also have the Army Manual TM 5-809-12 and it does a little better, but they base the loadings on Design Index, with 10 being the highest (120k track laying vehicles). Nothing really seems to fit the bill with the Army Manual, but a DI of 10, Subgrade modulus of 150psi, flexural strength of 530psi results in a slab thickness of about 8.5". I don't exactly have a track vehicle though.

Slabs on Grade have always seemed like black magic to me, as there are tons of charts in the ACI/Army publications but no real equations to show how those numbers were determined. When you are outside the chart limits, no direction is provided. Thoughts?

RE: Heavy Machinery loads on Slab on Grade design

Could you check with your local DOT pavement engineers?

 

RE: Heavy Machinery loads on Slab on Grade design

"Slabs on Grade have always seemed like black magic to me" - I am in the same boat.
What I found helpful though is making a quick FE model of SOG. Just don't forget to apply loads at the corners and edges of the slab. As expected, results would be very sensitive with respect to the soil modulus K. If you dont have K value you can find estimated ranges for various soil types in the literature (like Bowles, etc).

For certain types of soil you may need to increase the subbase and/or slab thickness, or to provide top and bottom rebar. Cost analysis will tell what is the most feasible option.

RE: Heavy Machinery loads on Slab on Grade design

(OP)
Thanks.  Local DOT won't touch it since its private work unfortunately.  They don't deal with this kind of loads either.  I finally found at the end of the ACI book that "unique situations outside the charts of this book will require a FEA to design the slab".  Exactly as sasa2k said. I also found within that book that there are no equations that the charts were derived from, but rather a FEM slab program was used to develop the charts.

 

RE: Heavy Machinery loads on Slab on Grade design

(OP)
This slab is huge (area-wise) and will have joints at 15' spacings but there will be dowels at the joints and the slab will have temp/shrinkage bars.  How does everyone determine the extents of the FEA model for the slab size?  Should I model it as a 15' square and apply loads at the edges and at the center, or should I consider a larger area since there are load transfer devices at the joints?  No expansion joints will be used except around the perimeter of the slab, where it meets with existing.

RE: Heavy Machinery loads on Slab on Grade design

It is my understanding that RISAFoundation can handle patch loads on SOG's. My next thought is runway design. There must be a procedure used by the aircraft industry.

RE: Heavy Machinery loads on Slab on Grade design

(OP)
I only have StaadPro, if that makes a difference :(

RE: Heavy Machinery loads on Slab on Grade design

jbuening

FEA can be done in STAAD you just have to read the manual and..... you may be out of your element so you may need to seek guidance with someone who is at least familiar with designing systems using FEA.

I have messed around with FEA in STAAD before but have never used the results because of the item discussed above.

Splitrings

If I remember correctly from my studies (a long time ago), runway design was more about the sub-base than it was about the topping.  I seem to remember them being ridiculously thick (including the sub base).

 

RE: Heavy Machinery loads on Slab on Grade design

Regarding slab design of heavy loads, I suggest you either follow the limiting stress approach (typical of ACI 360, chapter 7) and ignore reinforcing (reinforcing is for crack control only, not for strength),
OR
you design using active reinforcing - essentially a conventional reinforced concrete design using ACI 318 considering factored shears and moments, almost like an elevated slab, except with soil springs (where you take the upper and lower bound to capture expected behavior).
I always prefer the latter, but I think the slab gurus generally prefer the former (they argue more akin to pavement design, but really more like voodoo).

RE: Heavy Machinery loads on Slab on Grade design

I've got another document titled "Rigid Pavements for Air Fields" but I'm having trouble uploading it.  It's Army Technical Manual TM 5-825-3.  I'll keep trying to upload the file.  

RE: Heavy Machinery loads on Slab on Grade design

(OP)
Thanks everyone for the tips thus far.  I actually don't have the ACI 360, apparently the one I have is PCA - Concrete Floors on Ground.  Guess I'll invest in the ACI version!

CTW, I already have that one, which is the Army/Air Force manual.  Unless you know more than I about the manual, it divides the loading into Design Indexes 1-10. DI 10 is essentially a 120k track-type vehicle, but mentions nothing about track size. I've seen track-type equipment with 15' long and 2' wide tracks, so that 120k is distributed into a larger area. I guess the part I'm struggling with is relating all these charts to the type of vehicles I'm working with.  One vehicle has a wheel contact patch of 43.6"x43.6" and load of 70k, which is at the high end of area contact patch.  Another is a truck tandem axle trailer with wheel load of 27.5k but a standard 20"x20" contact patch.  The Army manual divides it up into either forklift or track-laying vehicles.  I'll look into the other one for Air Fields to see what it says.

My gut feel says the 12" slab on 6" agg subbase is sufficient, but need to back that feeling up with calcs.  I will get the ACI 360 and go with the limiting stress approach.

RE: Heavy Machinery loads on Slab on Grade design

Sorry, I didn't mean to upload TM 5-809-12.  I must have accidentally grabbed that one.  The other document that is linked in my first post has some good information.  Check out Rigid Pavements for Air Fields before you purchase ACI 360.

RE: Heavy Machinery loads on Slab on Grade design

I have always taken a different approach to this type of design.  For a slab like you are looking at I would design it using Roark's Formulas using Table 26 Case 13.  This is a continuous plate supported continuously on an elastic foundation of soil modulus "k".  This is essentially what all of the mentioned documents are doing in one method or another.  I look at my tire loads and patch areas then convert that to an equivalent circle for the circular patch loading.  You can use the ACI document to get your allowable concrete stresses but you are more or less limiting yourself to an infinite fatigue life for the bending stress.  I have designed many slabs for your type of application using this method and they have all been successful.  I also use the reinforced method for the joint spacing so that the joints can be spaced out more.  The customer's spec on the slab and rebar would be on par with what I would typically have with any type of subgrade strength at all.

RE: Heavy Machinery loads on Slab on Grade design

(OP)
Hmmm, thats a book that I don't have (Roark's Formulas).  Aggman, is that something that can be regurgitated here or would I have to buy the book?

RE: Heavy Machinery loads on Slab on Grade design

You need to buy it in my mind.  It's a vital piece in any good structural engineers library.  I use mine weekly.  It's essentially the winkler foundation theory though.

RE: Heavy Machinery loads on Slab on Grade design

There is a new edition of Roark's coming out in September. Chk here:

http://www.amazon.com/Roarks-Formulas-Stress-Strain-8th/dp/0071742476/ref=sr_1_1?s=books&ie=UTF8&qid=1312323878&sr=1-1

Combination of Army Technical Manual for preliminary sizing and Roark's Formulas to verify FE results should be sufficient.

About FE - you can use STAAD, however I found it very cumbersome. You can model only a typical area including joints and apply the appropriate boundary conditions to simulate the rest of the structure (or just model a complete SOG - computers are fast enough noways for this type of analysis). At joint location you need to release the moments. Support all the nodes on springs (constant equal to k x tributary area). Most of the modern FE software will do this automatically. Apply pressure loads (area equals to contact patch area) at various locations including center, edges, and corners.
You may find that when the load is at the corner or edge of the slab top rebar is required.

BTW - I remember there was an example in the STAAD manual that describes modeling SOG.
Cheers
 

RE: Heavy Machinery loads on Slab on Grade design

(OP)
Wow, good stuff everyone!  I think I got it down to a reasonable analysis, but keep the information coming if you have it...as it will benefit me in the future as well as many others.

RE: Heavy Machinery loads on Slab on Grade design

(OP)
Bringing this one back up.  The question has came up about expansion joints.  Keep in mind this is a very large (600'x400' or so) outdoor parking lot for large mining trucks, subjected to midwest temperature swings.  The slab is 12" thick with temp/shrinkage and contraction joints at 15' centers each direction.  This parking lot will butt up against an existing parking lot on all four sides.  There is absolutely nil that I can find in any ACI/PCI/Army manual about expansion joints, their needs, and what sizes are needed. They each have a short paragraph on them, stating they aren't typically needed for indoor slabs.

Should I provide an expansion material like PJF between the existing and proposed slab?  If so, I should  also use load transfer dowels with the end into the existing debonded correct? What formula do you typically use to determine the thickness of the PJF, if provided? Bridges, which I'm more accustomed to, have a definite expansion and we typically use 0.0000065/°F with 80° swings. Using the 600' length I'd have 3.75" total movement, meaning 1.875" on each end.  This doesn't account for expansion of the adjacent slab though.

Any guidance you can provide would be appreciated!   

RE: Heavy Machinery loads on Slab on Grade design

"... a study of the concrete pavements constructed from 1922 to 1931, inclusive, was made. The data collected were analyzed and the committee submitted its findings and recommendations to the Chief Highway Engineer on February 1,1932. The investigation revealed that during the period mentioned, 4,633 miles of pavement had been constructed without expansion joints, that 3,940 joints had been cut in these pavements, and that up to the end of 1931, 3,400 blowups had occurred on the 4,633 miles. The study showed that the rate at which blowups occurred increased with the age of the pavement; for example, pavements built in 1922 averaged two blowups per mile in 1931. During the same period (1922-1931, inclusive), 3,417 miles of pavement were constructed with 4-in. open joints, of which only 269 miles, all in one district, had been constructed prior to 1928. In the pavements built prior to 1928, the ends of the slabs adjacent to the joints were not strengthened by edge thickening, a procedure which was followed in the pavements built during and after that year. The data assembled by the committee showed that the 4-in. joints closed at the rate of approximately one inch per year, and that blowups could be expected about the fifth year after construction, although scattered blowups might occur earlier. This conclusion was based not only on measurements of the width of the joints in pavements of various ages, but also on data as to the occurrence of blowups and the necessity of widening existing joints and cutting new joints to keep blowups to a minimum. That blowups may occur early in the life of a pavement, in spite of 4-in. joints, was demonstrated by the fact that five blowups occurred in one district during the year the pavement was built. The committee agreed that expansion space of some kind should be provided, but did not feel that the data available justified definite recommendations as to particular types. While the 4-in. joints were not considered satisfactory, it was recommended, in the absence of amore suitable type, that their use be continued during 1932. In the meantime, it was thought advisable to construct a number of experimental sections in which joints with smaller openings spaced at shorter intervals would be provided. It was suggested that both 1-in. joints spaced 200 ft. apart and 2-in. joints spaced 400 ft. apart be used, and also that 4-in. joints, preferably of the mechanical type, be provided every 1,000 ft. It was felt that the strengthening of the slab ends adjacent to joints should be accomplished by some means other than the edge thickening then in use."

UNIVERSITY  0F ILLINOIS BULLETIN
Vol. 45 December 3, 1947 No. 23
ENGINEERING EXPERIMENT STATION BULLETIN SERIES No. 365
EXPERIENCE IN ILLINOIS WITH JOINTS IN CONCRETE PAVEMENTS

Surely more current info in

ACI 224.3R-95
Reapproved 2001
Joints in Concrete Construction
Reported by ACI Committee 224
Chapter 6

"6.3—Expansion or isolation joints
Expansion or isolation joints are constructed with a clean break throughout the depth of the slab to permit movement (Fig. 6.1d). Expansion joints are no longer used in mainline pavements,* except that expansion joints with dowels for load transfer are used at bridges. Isolation joints are used at fixed structures like manholes and drainage inlets, and at T or other nonsymmetric intersections. The clear distance across the joint is often maintained at about 3/4 in. (20 mm), although openings of 1/2 in. (12.5 mm) and 1 in. (25 mm) are also used. Since the joint has no aggregate interlock, it is necessary to provide some type of load transfer. Thickened edges [Fig. 6.1(h)] have been used at expansion joints to reduce or eliminate the need for dowel bars. When thickened edges are specified, consider cost, constructibility, and the restraint it may provide to slab contraction.
The structural adequacy of an expansion joint is determined to a large extent by its load transfer device. If adequate load transfer is provided, deflection of the slabs is minimized, and pumping action is reduced. It is necessary to maintain the joints, periodically, and in some cases to replace the filler material in the joint. Common types of fillers include fibrous and bituminous materials and cork. It is essential to seal the joints periodically to prevent infiltration of surface water. Resealing of joints is best accomplished during a cool period, when the joint has opened, thus permitting placement of a sealant. Expansion joints also may gradually close up in pavements that have unsealed contraction joints that can fill with incompressible material. This is a very undesirable condition that should be  avoided by proper design, construction, and maintenance. Nearly all states have discontinued the use of expansion joints, except at fixed structures, because they appear to be unnecessary and are difficult to construct in a slip-form paving train. However, one state successfully uses expansion joints in lieu of contraction joints. A few states use contraction joints with every third or fourth joint being an expansion joint; this system causes the adjacent joints to open and fail.
For airport pavements, isolation joints should be placed between new and old concrete slabs and between different pavement features, such as ramp-to-taxiway and taxiway-torunway.

*At one time, blowups were a major consideration for joints in highway pavements. These typically occurred when incompressible materials entered unsealed joints, often in the winter when joint widths were greatest. In summer, the pavement expanded in response to daily and seasonal temperature changes. For a joint containing incompressible material, compressive stresses developed that lead to failure in some cases. Properly designed pavements with sealed and maintained joints are not susceptible to
blow-ups. True expansion joints in pavements are needed only in very unusual conditions of construction or with unusual materials. See PCA (1992b)."

So it seems that if you decide to place expansion joints they may need to be mechanical, sealed and properly maintained for proper operation, indluding the proper specifications for maintenance.

RE: Heavy Machinery loads on Slab on Grade design

What did they use last time??

RE: Heavy Machinery loads on Slab on Grade design

You've got a couple of advantages = You "know" your (current) heaviest loads, and you're on dirt, so you can control the compaction.  Doesn't sound like you're needing fill or partially excavated, partially filled un-compacted dirt.


Make you're FEW program NOT the small 15x15 (foot ?) single slba:  Take you're current heaviest truck and size your load for at one, if not two, increases in capacity.   (If these were pickup trucks, and you know you have Ford F150's, size the load for an F-250 pulling a trailer, as an example.  Don't go overboard, but you've got to assume the loads are never going to decrease over the life of the slab.

Next, figure out the true footprint and tire loads.  Some of these earth movers/rock haulers have six tires, some 4.  All will have heaviest loads in back under the back axle.  So calculate based on two trucks parked next to each other.   NOT one truck with the one evenly divided between all six tires, and only one tire on a 15x15 foot square.   

Edge loads:  Assume you have "parking stripes" prohibiting trucks from parking next to each right on the edge.   (Otherwise, you're sure to have some time in the future when four drivers all pull right out to the edge of the lot in a nice row, with their trucks tire-to-tire-to-tire.   )   

If they're not parking near the edge, then the drive-on forces will be greatest right when those loaded double-tire rear axles pull on-to the edge of the concrete as the truck moves from dirt side to concrete pad.  Maybe make the outside 18 to 36 inches deeper with more rebar around the edges?    

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