Crane pad design....CE's please read...i could use your help

I have been asked to design an aggregate crane pad for a senior design class here at Penn State. Our professor assigned this as a preliminary assignment to see where the class stands in their knowledge. The crane will be a Link Belt HTC-8675. I have calculated the weight to be 2038.3psf (assuming 4 outriggers using 48"x48" pads under the outriggers). I still have yet to make it to the library to check up on the ASTM standards. If you guys, especially any Civil Engineers in here, could give me some guidance in designing this particular pad. Again this is a rough estimate assuming the ground is stable, water table is deep, etc... He really didn't give us any specifications on soil properties for the site so I don't have those to give you.

again any help would be more then appreciated.

-Bryan

I have calculated the weight to be 2038.3psf (assuming 4 outriggers using 48"x48" pads under the outriggers).

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I'm no CE, but I have to ask for a clarification--I've seen cranes with two of their outriggers in the air, because the load was outside of the envelope, and the machine was balancing on the other two outriggers. Does your weight calculation account for the load, and the fact that when you reach out with it, the weight on each of the outriggers changes accordingly?

digger242j said:

I'm no CE, but I have to ask for a clarification--I've seen cranes with two of their outriggers in the air, because the load was outside of the envelope, and the machine was balancing on the other two outriggers. Does your weight calculation account for the load, and the fact that when you reach out with it, the weight on each of the outriggers changes accordingly?

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Digger,

If you've seen a crane up on two outriggers then:

A: That crane was severely overloaded, or the outriggers where not placed properly.

B:According to what I learned when I started on cranes that use outriggers when you set up you are supposed to lift the wheels completely off the ground so that you don't set up pivot points that do cause outriggers to come off the ground.

the load that i gave accounts for a 9cu bucket of concrete too...the crane should be on 4 outriggers...granted when the load goes out, yes there will be more weight on the one side outriggers but as i mentioned this is a very fundamental analysis so i am going to assume that all 4 outriggers are undergoing the same load

I'm no engineer, but here are a few things to take into consideration:

With no idea of existing soil condition, I would reccomend over excavating and re compacting at least 3' of soil to 95% relative compaction, and follow with a layer of aggregate base, also compacted to 95%.
You can do engineering calcs for soil stability based on the "R" value of the soil, and of the base rock.
Standard specification books will have values for these.
This will allow you to specify the thickness of base needed to give you a high enough bearing load to support your crane.

Be sure to accomadate Diggers question regarding your calculated total load.
Total crane weight, plus crane load, may be exerted on 2 of the 4 outriggers at any one time.
Remember the difference for static load (weight of crane) and live load (load, wind, etc)
Use the total load to calculate your bearing pressure, and remember the safety factor. There are standards for this, and I am not sure, but I think at least 1.5 to 1

For size, do you need to move, or just sit in one place?

Remember access to and from the street, or wherever the crane must travel to and from.

This is an area that sometimes gets neglected, and leads to accidents.

Others may have more detailed info , this is just from the top of my head.

when I put in crane pads at The turnpike at KOP any soil that was in question we dug down 4 foot and put in 24inch minus or they call it "B" rock. Capped with 2B stone. we set 400t I beam off of those pads.

thanks JDOFMEMI...i am using a FS of 2, we are not given any type of site plan...the assignment paper literally says "using this crane, design an aggregate pad to support its weight". The FS of 2 should more then accommodate the added stress when the boom extends. The crane will be sitting in place and not required to move.

basically what this is going off is a case study we are looking at right here in State College PA...it was an apartment building built 3 or 4 years ago where the crane pad failed and we are just told to design a better one. State College is in the middle of a mountain range and to get the foundation they had they actually had to blast rock, so an addition 3' excavation with 95% probably isn't required

what i was thinking was 2 feet of #4's followed by 10" of a 3A modified on top...obviously this is a preliminary plan and i need to crunch the numbers, or even see if this would work but any more help is greatly appreciated

ask a local geotechnical engineer if you are in school what kind of plate load test result they get locally. This is done when designing footings of buildings. I do this test 2-3 times a year and the last time I did this test I jacked a D8 completely off the ground using s 24*24 plate and it deflected 1 inch in recompacted red clay. one trick is to use metric it is simpler it use and cross reference.

Be carefull here.

If u check the crane manuals u will in some/all of the charts find the maximum weight of the stabilizers on a given weight.
Some cranes got a spezial chart in the back of the book where the max "stabilizerweight" is calculated.

A hint here, when u swing your crane over one of the outriggers u will have nearly all the weight on that outrigger.
U can make a calculation, as in US, u got 85% safety before the buzzer starts.

Then u will have the total weight on one pad, and the three others supporting your "security" load.
If then the operator uses his override switch, the load can be increased futher.

Digger,

If you've seen a crane up on two outriggers then:

A: That crane was severely overloaded, or the outriggers where not placed properly.

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Gee. Do you think so? :cool2

B:According to what I learned when I started on cranes that use outriggers when you set up you are supposed to lift the wheels completely off the ground so that you don't set up pivot points that do cause outriggers to come off the ground.

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I'm sure you're correct.

The crane in question (at least in this instance) was an old 15 ton Pettibone. (Picture below--that not the actual crane. I just googled up a picture of one, but it's identical.) It had a different load chart for over the front of the crane than it did for 360 degrees around. That's because it set up longer on its outriggers than it was wide, and I'm sure the engine in the rear added some counterweight as well. Fortunately, it was very forgiving in that respect.
They'd try to reach out as far as they could with it, and more than once I saw the rear stabilizers starting to lift up, and a few times they were actually all the way off the ground, but I never saw them tip it over, (at least not forward.) I had the misfortune of running it sometimes too, and more than once I was out there, exploring the edge of the envelope myself.

Once, I had to set it back on its feet, after they tipped it over sideways. (The tip of the boom hitting/smashing the air compressor they were lifting kept it from going all the way over, but the wheels on the one side were six feet off the ground.)

Once, it was set up on dirt, near the top of a bank. Reaching out, with a bucketful of concrete, the bank gave way under the left front stabilizer, and the concrete bucket fortunately ended up resting/balancing, on top of the wall that was being poured. A nice, real-world demonstation of the concept that the weight on any given outrigger changes with the position of the load.

I was on the recieving end of that one, standing on a scaffold next to the wall. That's why I took an interest in this thread. I just wanted to be sure the future engineers, and everybody else, keep that kind of stuff in mind.

You'll find a chart similar to this one in the manual.

This one is from our Grove RT760E.(I had my manual CD laying around)

Horisontaly u find padpressure, verticaly u find load and one line is for each radius.
Chart is in metric.

Sorry for the bad quality its a photograf from my pcscreen.

example:
50t load, 3m radius, gives 36t padpressure.

With this chart u then know the weight on the pad, now its easy to make a calculation on the size of the pad and get ground pressure.
If i, as here, got a 1m2 pad i get 36t/m2 ground pressure.
Soil and compaction tells u how much groundpressure u can put pr square meter/feet.

thanks guys so far....i have all my weight calculations done, and i searched all the relevant ASTM standards on aggregates today but couldn't find anything that suited my fancy..i have been getting so swamped with work this week, and to make it worse i am traveling up to Syracuse tomorrow at 5am to watch a little football so this assignment has been getting put on the back burner...any more help on how i should properly calculate the bearing capacity of aggregates would be helpful, like i said i plan on using 2' or #4's followed by a few inches of modified to make it easy to walk/drive on

Hi dirt digger,
It would be a fundamental mistake to assume equal loading on all the pads. You have to calculate the maximum weight on an individual pad. I assume this will occur when the crane is at full capacity and slewed at roughly 45 degrees in plan to put the boom directly above one outrigger. If it was me, I would assume all the crane + load to be acting on one pad for design purposes. I am a degree qualified mechancial engineer so my soil mechanics knowledge is limited and has not been exam tested. Therefore i cannot tell you what plan area you need for the rigger. It would seem a reasonible assumption though that the ground is firm and has good load carrying capacity since the crane will need good ground to access the site. The ground under the pads is likely to be or made to be of similiar bearing capacity to the crane access road. With this in mind you would probably need bigger pads for an all terrain crane than an ordinary crane of same capacity. As a starting point for assumptions on soil bearing capacity you could easily calculate the soil pressures developed under the tracks of a crawler crane. Every manfacture will publish the track size, crane weight and maximum lift. The soil pressure underneath the outriggers for a crane should be roughly similiar.

Some good comments for you here. As has been stated, pressure is not equal on all 4 pads. With no load and crane boomed up, pressure may be sorta equal, but no exactly. With no load, the biggest weight is the crane counterweight. With the crane over the rear, and no swinging, take a load and boom out. When you get to the maximum lift of the load, you will be at 90% of tipping. At that point your 2 rear outrigger will be light, but not quite raised. Close to 100% of the weight (crane and load) will now be on the 2 rear outriggers. If you were to boom up a llittle, and swing left or right, the outrigger you are swinging over will get an even larger load then it had. 2000 to 5000 psf is not uncommon for crane pads. Rather then recompacting soil, larger pads are usually usued. 4x12 or 6x12 are common timbers used for extending pad sizes. Draglines and other big cranes use 12x12 mats. Simple statics will help you to determine pad loads. Crane and load weights and center of gravity are all that are required. Sum your loads about the outrigger closest to the load and you are good to go.

I googled the crane listed it has 5 jacks....

maybe tenstar geo grid fabric first ... we use it alot under temporary roads for cement & dump trucks when the condition are less than ideal. then a 4" rock on top....about 1' to 2' thick.... the tech name escape me at the moment....



I worked one place that used 8 x 16 railroad ties with a 3/4 steel plate on top for a 80 ton Grove...

Normal pads for small cranes (35t and under) are usually 3' x 3'.... 2 layers of 2x6, 1 layer 4x4.... easy to move....

the 4' wire spool ends make good pads.. couple end nailed together... heavy but easy to move.

I seen several cranes turned over because the operator was to lazy to pull the pads off the fenders... do a google search.... you will probably find pictures of cranes .... turned over with pads laying on the fenders.... with a couple of outrigger jack sunk in the asphalt or whatever....

boys i have long since graduated college....thanks though!! haha

Got this from Link-Belt website, picked these parameters just because this is the configuration that I normally use. 8675 model not listed. This close to the max permitted capacity (85%) before tipping. Need to have access to this part of the website (Link-Belt Preferred)


Link-Belt Constructon Equipment Co., Lexington, Kentucky - HTC-8670 (F2)
Model HTC-8670 (F2) Telescopic Boom with 12,000# CTWT
Mode B Boom on Fully Ext. O/R
7500 lbs load @ 100 ft radius 115 ft boom + 36.5 FT fly, w / 2 DEG fly offset
45.9o boom angle Main pontoon: 452 in2; Bumper pontoon: 201 in2
99,905 lbs gross vehicle weight (GVW) Date: 11/14/2009 - v 1.0
Rear O/R Max Load: 97,400 lbs; Front O/R Max Load: 97,400 lbs; Bumper O/R Max Load: 50,000 lbs;
Slew Angle For Highest O/R Reaction Click & Drag the Boom or Input Slew Angle

O/R Reaction Over 360o (Based on Input & Crane Config)
Right Front Right Rear Left Rear Left Front Front Bumper
77,507 lbs 65,849 lbs 69,235 lbs 72,566 lbs 53,285 lbs

Cranes have to be level to pick ..less than 1 degree of plane..

Long boom on a heavy load is worse than a short boom on a heavy load with the crane of level ..the severity of the capacity reduction is directly related to how much the crane is off level ..

do the math ..200 ft of boom and the crane off level 3degrees..how far off center is the load ..
and when reaching the load is actually further away as you boom down on the down side ..on the upside of the plane ..you'll have to boom down further to put the load out
so to have the outriggers go up in the air ..leveling the crane is important..

For that Link belt crane ..over the years depending on who's doing the engineering ..Ive had drawing's with the weight of the machine divided into 4 with the entire load added to one jack.. or the entire machine divided by 2 and the load applied to one jack..

do your calc's first with the machine level ,maximum counter weight ,maximum main boom angle on the shortest boom length ... the superstructure swung with the counter weight over one outrigger..

With the newer higher capacity European gear coming online allot ..the worst outrigger loading isn't with a load on ..
Its actually shortest boom ,all the CW ..boomed up to its uppermost point and swung to one outrigger..
Case in point ...AC500-2...
boom weighs about 96000 lb (memory here)
CWT 396,000 lb

with just those weights in mind and the boom at 83 degree main boom angle with the super structure and what it weighs swung over to one outrigger the load is well...you may as well have it all on that beam ..

And searching your going to find regardless of boom amount or configuration ..unless you exceed the load moment and cause the crane to rotate shifting the machine onto one rigger ..you not going to get a more extreme outrigger loading ..

And as far as the crane pad goes ...with a 4'x4' square of 6"x6' hardwood ..I'll only ask that you give me 24" of 3" crush run,(not clear).. with another 12"of 3/4 crush run (not clear).. beat it with a bucket ..make it level ..and I'll stick my crane there ...That's essentially what i would want if it was setting up on what would be typically referred to as a bog or a quagmire ..really bad ground .... There's also the chance I'll want to go deeper if the excavator by itself is sinking more than 6-8 inches driving in ..in that case ...get me a road ..and go deep so that the crane will essentially be set up on 4 pier's of stone and rock..