LIFTING WORLD FORUM ARCHIVE (READ-ONLY)

TO ENTER THE NEW LIFTING WORLD FORUMS, CLICK HERE...
FORUM ARCHIVE HOME | SITE SAFETY | GROUND BEARING PRESSURE AND MOBILE CRANE RIGGER MATS.

Author Topic
ryendry
03/05/2007
There is currently an upswing in the size of rigger pads been calculated from the given bearing capacities of made up ground (from the Main Contractor) and the Point Loadings of the Riggers.

Frankly I do not accept that all of a sudden a typical 35te Truck Mount needs rigger pads at 4square meters (i.e. total area 2m x 2m) to carry out standard steel erecting duties on compacted made up ground. Some one somewhere is making a massive error in the calculation of the figures and it is costing steel erectors a fortune in time and money.

So lets look at the standard methods.

To calculate the Pin Loads

CPCS method.
0.65 x (Total Crane Mass + Total Load Mass)

Typically for Steel Erecting this gives a value of around 26Te (=26000 Kg).

This however requires converting to KN (Kilo Newtons) and taking the standard equation Force (N) = Mass (KG)x Acceleration (10m/s2 (this is acceleration due to gravity which at sea level on earth is 9.776m/s2, 10 is close enough for our purposes)).

From this our 26000 KG is a Force of 260000 N or 260 KN.

Now it has been posted elsewhere on this forum that a rule of thumb for GBP is 10-15Te / m2, which converts to a high end value of 150KN/m2.

The blindingly obvious size of rigger pad is therefore:

THE LOAD / CAPACITY

or

260 / 150 = 1.73 sqaure metres.

BUT

If you do this calculation you don’t actually get an area at all. You get a ratio.

To explain, the equation has units of KN / m2 as the denominator (top) and divisor (bottom) which effectively cancel each other out. The devisor has to be a value in KN to leave us with an area. This value of 150KN / m2 is not the figure to be working with. It is not even the correct type of measurement.

In order to get a value for the Area of the Mat the Pin Loadings in KN/m2 have to be divided by a load in KN!

Is this the error that is being made?

To further compound the issue, bearing ratings are often given in the CBR scale, which has no direct conversion (in the same way that the Beaufort Wind Scale cannot be converted to an speed in mph).

As a final note. Many will not be aware of the massive loadings that Cherry Pickers impose. A genie S65 (not a massive machine) according to genie's own literature imposes in the region of 570KN/m2 in certain configurations. Twice that of the typical crane. If the ratings that the MC's are given why are we not having pickers falling over left right and centre?

Please feel free to pick holes in this argument. i would rather know that I'm utterly wrong than the current abysmal and confrontational state of affairs that exist now.
ryendry
03/05/2007
Ok so one problem I have ironed out for myself.

What is required is a Jack Loading in KN and a Ground Capacity in KN / m2.

When the latter is divided into the former we have an area for the Mat.

From our example then we have a Jack Load of 260KN and therefore a required rigger pad area of 1.73m2

Still doesn't explain why all them pickers are staying up in the air and not sinking into the ground!

Something is wrong here and Crane Operators, Hirers and Users are being made to pay.
webmaster
03/05/2007
The one certain thing with the both types of machines that you mention is, they are designed and tested on CONCRETE. Neither come with a recommendation by the manufacturers to use only on concrete, but they're still expected to perform as though they are!
The machines automatically lose up to 25 percent or more of their lifting capacity in reality on some terrain. The size of rigger pads were an after-thought.
ryendry
03/05/2007
Excellent and valid response Mr Webmaster. Doesnt help in my constant rows (erm differences of opinion) with Main Contractors though.

What would be useful would be a simple and above all else difinitive way of measuring the ground bearing capacity. Could even be as "rough" as a BS for any given make up of ground. At the moment I'm willing to bet a whole shiny pound (and thats a lot to risk for a yorkshireman) that the majority of MC's are thinking of a number and halfing it.
billybiglift
03/05/2007
I would take that shiny dubloon off you, if i were smart enough. In some sites, ground bearing pressure we need to work to is 5 tonne per metre squared, which means huge mats, for small cranes. This, in reality is counter productive, as most hiring companies supply flat plates, which bow at either end when crane is rigging, leaving mat with less surface area in contact with ground than the standard 1.5 x 1.5 trackway we use. I have found that if customer wants really reduced ground loadings, get in touch with Demag/Liebherr and get a disc of ground loadings, and they are a lot less reduced than the crane mass/load mass equation.
ryendry
04/05/2007
Thanks for the response Billybiglift.

With respect to the "large" plates bowing, I completely agree with what you say although I have a sneaking suspicion that some materials engineer will say that the bowing of the plate is irrelevant as all of the underside surface is still in contact with the ground (in most cases) as the ground depresses into a bowl under it.

Indeed, the main reason that I searched out this forum and posted, is a site on which one of my collegues is working where they have given a ground rating of 60KN/m2, and left him carting 2x2m Steel Road Plates around, For 6 cranes. I told him about the Picker loadings and suggested he shuts down work for fear of the pickers falling over on such apparently soggy ground. This tactic can backfire however as more than once, I have been told to get Tracked Pickers in.

You are also correct in saying that Liebherr and Demag will supply the Exact loadings for any given configuration, which are typically 65-70% of those calculated. I have not yet managed to get a disc off anyone. The tech departments are also becoming loath to do this. Obviously more and more are using the service to get the lowest possible Jack Loads.

Edited by - ryendry on 04/05/2007 08:20:48
billybiglift
04/05/2007
send me an email and ill send you a disk, up to 200 tonne g/loadings.
billybiglift
04/05/2007
www.freewebs.co.uk/billybiglift
ryendry
04/05/2007
Nice site Billy.
Sent an email to you.
ryendry
04/05/2007
well i would have done if there was an actual email address there
paulnw
04/05/2007
Dont shoot me down here but, i think over the past 5 months cpcs have changed the rule on how ground preasure loadings are calculated.

Its no longer 0.65 x (crane weight+weight lifing)
but is now 1 x (crane weight+weight lifting)

paul
too high
04/05/2007
Say again Paul....
ryendry
04/05/2007
yes i agree with toohigh.. Im really not wanting to shoot you down Paul but offering a smorgersboard of three targets....

But to the point you make, IF CPCS have really altered the calculation to 1x(Mass of Crane + Mass of Load) that is really going to compound the problem.

That example in the first post of this thread. We now have a maximum of 40Te going through one rigger and a big fat zero through the other 3 (4 if there is a front jack). I assume that the three are floating just over the mat. Now technically this is feasible. This would be the case as the crane was at its tipping point. The Chances of this happening on just One Jack and not Two nust be infintisamly small.

I can imagine that they have made this change to ensure that the ground doent give way as a crane reached its tip over point. This does seem a little like over-kill though.

Anyway back to our example. We now need not 1.77m2 (1.33x1.33) mats but 2.66m2 (1.66x1.66)mats. Thanks a bunch.
paulnw
04/05/2007
the reason why the change is clear.

CPCS have done there calculations on the worst case senario. And although to everyone working out there on cranes the fact of getting a crane to balance soley on one rigger is something for the record books, they have had to base the calculations on just that because, this is the worst case senario.

i dont make the rules and if i did by god would i change many of them however as i don't..................
skittles
06/05/2007
hi gents can i ask a question, not being well up in the maths department and considering most cranes travel with round pads, how do you calculate metre squared of a circle
paulnw
06/05/2007
where a round mat is say 0.9m diameter, devide this by 2 to give you the radius.

then R2 = 0.45 x 0.45 = 0.21

therefore area of mat is (pi) 3.14x0.21 =

0.66 sq m

be careful when you write the calculation down on paper as (sq m) and (m sq) are two totally different answers.

hope that explains it.
SADE
06/05/2007
If we take your original 26 Te and divide it by 0.66 m sq ( 900 mm diameter mat) Your resulting ground bearing pressure will be 39.39 Ter per m sq. No site foreman will ever accept this as he has probably been on a crane appreciation course!!!!and as such is totally terrified of taking any responsibility.
billybiglift
07/05/2007
The metre is hypothetical, it isnt there. so the 26 tonne remains 26 tonne into matted area. Site agents start getting worried when the mat size isnt a metre, and you start having to multiply the point loading to achieve the ground bearing pressure.IMHO
ryendry
07/05/2007
You are absolutely correct SADE you would indeed get a value of around 39Te / m2.

In fact for most STeel Erecting applications with 25-40 Te Cranes the typical value will be around 250-350 KN jack Load, which is then put through a Pad of typically 0.7m sq.

This in turn gives a Bearing of 350-500 KN / m sq.

Notice how this is a comparable value to that of the Average Cherry Picker.

To get back to the point of the thread.

Firstly I wanted to confirm that we were working these values for Bearing Pressure out correctly. After a fundamental mistake of my own in the first post, corrected in the second, I believe that yes we are.

There is still room for debate as to how the Jack Loading itself is calculated. This idea that we should use the Maximum Possible figure (i.e. when the crane is balanced on one rigger) is an absolute nonsence. The Constant of 0.65 gives a value that is typically 130% of the actual value (calculated by the manufacturer) so going up a further 35% cannot be acceptable. Im all for working safely, but the risks from lugging around 4-8 sq m plates far out weigh the chances of a crane ending up balaced like this.

To go further, this is I believe one of th fundametal problems with Safety Initiatives at the moment, through out the industry. The further drive for safer practises is concentrating on one scenario (whats the worst possible scenario, on matter how unlikely for e.g.) and ignoring the implications of carrying out the necessary works to negate that scenario (lugging the huge mats around that are required e.g.)

2. On top of all this, If the Ground Capacity that we are being given, 50-100KN is correct, how are the Cherry Pickers managing to drive around without falling over!

I believe that as a sector, we need to be putting pressure on the MCG to sort this out. Their figures for Ground Capacity do NOT add up in light of the evidence, and by implemeting what they belive to be correct (because thats what they have been told) they are putting us all at a different but still very real risk!
billybiglift
07/05/2007
If the site was a large site, with a lot of work on it, surely it would be worth the while for site manager to get relevant ground condition information, and supply mats of equivelent size. Telehandler could be utilised for delivery/positioning. 4 lots of 3m x 2m x 25mm road plate would be adequate for most jobs, if it was a larger crane job, the crane would/should have their own ribbed mats.
ryendry
07/05/2007
Totally agree with you BillyBigLift. And indeed on Larger sites with 3-6 Cranes all erecting steel, a telehandler is a pre-resquisit anyway. (I know thats spelt wrong). Even still this is IMHO a totally unecessary cost in both time and money that does absolutely nothing to improve safety, and infact brings in whole new risks to the job that would not be there if using the standard [man-liftable] pads that cranes of this type are supplied with.

The point of this post is that I do not accept that the figures that we are given by the MC are any where close to being accurate, and as and when I am proven correct (pre-assuming that I will one day) what can be done about it?

With regards to "Big Cranes" and "One off Lifts" that kind of work is outside the scope of this thread. In this type of work, the Lift Plan would cover every possible individual lift in entirety. A level of detail that is utterly impossible in the General Steel Erecting LP that is the acceptable norm for Steel Work.
TO ENTER THE NEW LIFTING WORLD FORUMS, CLICK HERE...