Mobile Crane Outrigger Pad Loads 4

[DaveVikingPE]

Subject as above...

I've had some success in locating manufacturer's published maximum outrigger pad (or float) loads. Mostly, Grove seems to not mind sharing this information.

I'm looking for a formula for computing outrigger loads. I've found one in and Austrialian mobile crane standard, but nothing from ASME or SAE.

 

[JStephen]

You can get some simple approximations. A detailed answer would probably require additional information from the manufacturer.

For example, in many cases, crane capacity is limited by tipping, with some (fairly small) factor of safety. Assuming the crane actually tips, you'll have the entire weight (crane + load) on one or two outriggers, depending on rotation angle.

If that kind of quick check shows that you're okay, then you're done. Otherwise, I think you'd need information like center-of-gravity of the rotating part of the crane, with boom at different positions, and that information is not readily available from load charts.

 

[DaveVikingPE]

"...and that information is not readily available from load charts"

Don't I know it... Typical vexing problem: I have a pier and the owner is getting "a 100-150 ton mobile crane." OK, which one. "Don't know yet..." In my quest to be precise, I've gone overboard in looking for the hard way out. Even though I have lots of information on different cranes - and it would be a lot of fun to calculate outrigger loads - I should be spending more time on the larger problem...

The Australian Mobile Crane Code (2006) has this formula:

11.2.6 Calculating pressure applied by outriggers...
If a crane is designed in accordance with AS 1418.5: Cranes, hoists and winches – Mobile cranes, the crane will overturn within the stability part of the load chart when the maximum safe working load (SWL) is multiplied by a factor of 1.33. In reality, a crane will not approach this condition, provided the operator does not overload the crane. However, a reasonable approximation for maximum ground pressure applied by the outriggers is detailed below.

Pressure (tonnes per m2) applied by outrigger feet:
Pout = 0.6 x (total crane mass + lifted load)/ (individual outrigger area)
Pout = 0.6 x (CM + L)/area
================================

The formula give results that are higher than what's in Grove's literature, but it's a start.

 

[JStephen]

If the crane and load to be lifted are indefinite values for the future, I'd say make a very conservative assumption and see what it does to the design. If it's no problem, then you're okay. If it has a major impact on the design (and thus cost), run it back by the owner. See if you can specify a maximum outrigger load to be used and leave it at that. After all, your design has to be adequate for cranes that haven't even been built yet.

They have some monster motor cranes in different places, up to 800 tons or more, so it's not feasible to just design for the biggest crane that could ever possibly be set there.

You might also inquire as to load weight, load placement, etc. They may anticipate a large crane due to the reach, but not be planning to pick up loads that are that heavy, in which case the load+crane total would be a lot lower.

 

[MGMtulsa]

To Dave Viking PE -- Grove and Terex have been great at supplying pontoon loads. Linkbelt has an excellent internet program for specific models. Signup and you are off and running.

JStephen is on the right track (no pun) to determine what a maximum load can be. That is, at tipping, you have a determinate load consisting of crane weight plus hook load.

The problem becomes more difficult for a lesser load when it is not tipping, and is distributed on 4 pontoons. The load is indeterminate. (Pontoons at the ends of outriggers.)

I have analyzed a number of bridges with crane loads. Generally, decks are not adequate to carry a pontoon load. For a 70 ton crane, I've seen pontoon loads in the 60 to 70 kip range. This compares to an HS20 wheel load at 16 kips.

Sometimes the maximum pontoon load is not on the lifting side. Sometimes it is on the counterweight side, and might even occur during erection. Linkbelt's program includes wind loads, additive to the heaviest loaded pontoon.

Years ago, I developed my own pontoon loads. But these are subject to some approximations, if not outright errors. Now, I go immediately to the manufacturer for pontoon loads.

When looking at thin bridge decks, the pontoons generally need to be over the girders. Small cranes will allow more margin for approximations and errors. Larger cranes, less margin.

Small and large are relative to the size of the bridge, size of the lifted load, span, spread, etc.

Michael Mills
Tulsa, Ok.

 

[DaveVikingPE]

Thanks, folks. A star for each of you...

I'm going to see what Terex and Linkbelt have in another round of web surfing. What bugs me about these kinds of projects (other than I really like piers and I don't get to do enough of them) is that I cannot trust the mobile crane operator to put the outriggers where they're supposed to be. Solution: assume they'll be put anywhere.

 

[archprecast]

In many cases, the crane rental company has software to do that analysis. I got crane+load specific answers from the dispatcher. However, in many cases cribbing or something else is used between the pavement/surface and the outrigger pad in order to spread the load. Cranes often crack slabs.

http://www.caryconcrete.com

 

[DaveVikingPE]

Thanks, archprecast!

A star for you too... Good insight. Cracking the slab is my #1 concern. I will include a discussion of crane rentals and what the owner should require from the crane supplier, etc.

 

[oldrunner]

There are also instances where the crane will overload the pier. Then we will place timber mats to spread the load. These are 12" thick by 4' to 6' to up to 20 feet long.

 

[pba]

If you can find it - Get hold of CIRIA Special Publication 131 'Crane Stability on Site - an introductory guide' Published in 1996 and revised 2003. This is a UK publication which gives information on typical crane types and their methods of use.

Below is an extract which has some relevance to this problem:

"A survey of major crane manufacturers quoted figures concluded that the following values are maximum theoretical loads on each support for typical mobile cranes:

30 tonne capacity truck mounted telescopic - 33 tonnes
50 tonne capacity all-terrain telescopic - 40 tonnes
80 tonne capacity all terrain telescopic - 61 tonnes
120 tonne capacity all terrain telescopic - 80 tonnes
160 tonne capacity all terrain telescopic - 95 tonnes

I trust that is of some help.