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Active Member
JamesRJatho
Posts: 6
Registered: ‎11-28-2012

Looking for info on whether ASM is the right tool.

131 Views, 6 Replies
04-01-2013 09:18 AM

I've been using AutoCAD for quite some time but never dove into any kind of simulation software. I wanted to see if anyone has an opinion on this kind of work being realistic in Autodesk simulation.

 

I work for a major crane company doing lift planning, meaning all the calculations and preparations necessary to ensure that our cranes can perform the necessary lifts within all parameters such as setup space, avoiding obstructions, etc.

 

One issue that is becoming more prevalent is ground bearing pressure. It's always been common practice to place wooden mats under the outrigger pads or crawler tracks of a crane to spread out the forces being put into the ground. There are quite a few lazy ways of calculating ground bearing pressure beneath the mats, but all of these ignore the physical properties of the mats themselves. Our customers are asking for more and more specific calculations of ground bearing pressures to ensure we don't exceed any limits that might cause a failure.

 

Here's the type of scenario I'm looking at:

 

The crane in question is mounted on two "crawlers" that essentially just look like the tracks on a tank. The effective area pushing into the mats below is 29.9' by 4.3'. The mats below are 12" thick by 4' wide by 20' long. Jobsites will usually have a restriction on the amount of allowable pressure, usually around 2500lbs per square foot. Crane simulation software will calculate the amount of pressure underneath the crawler tracks. Just as an example, I recently worked on a plan where the front end of the crawler was putting out 57psi of pressure into the mats, and the rear end was at 16psi.

 

Is it realistic to use Autodesk Simulation to find information such as how much pressure is being exerted by the mats into the ground below, as well as other info such as the stresses in the mats and any deformation?

 

Please let me know if any clarification is needed, though I tried not to be vague in my description.

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Valued Mentor
AstroJohnPE
Posts: 498
Registered: ‎08-30-2012

Re: Looking for info on whether ASM is the right tool.

04-01-2013 02:43 PM in reply to: JamesRJatho

Hi James,

 

The analysis that you describe should be easy, provided that you have the material properties for the mat and the ground. Another consideration is whether the mat behaves like an isotropic material (steel), orthotropic material (wood), or something more exotic (hyperelastic, rubber). The more complicated the "stress-strain curve", the more unlikely it is that you will be able to find the material properties, and the more complex the analysis becomes.

 

One note about the results. The software will not provide the contact "pressure". The results will be the force at each node. From this, you can estimate a contact pressure by dividing the force by the area of the element.

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Active Member
JamesRJatho
Posts: 6
Registered: ‎11-28-2012

Re: Looking for info on whether ASM is the right tool.

04-02-2013 05:41 AM in reply to: AstroJohnPE

Well that's great to know. Most mats are made of oak, and usually the only property we know about the ground is what the allowable ground bearing pressure is, measured in lbs per sqft.

 

What is the most reasonable way to represent ground in ASM? Do I just need to make a massive box or flat plane that represents the soil?

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Valued Mentor
AstroJohnPE
Posts: 498
Registered: ‎08-30-2012

Re: Looking for info on whether ASM is the right tool.

04-02-2013 06:33 AM in reply to: JamesRJatho

If all points of the mat remain in contact with the ground, I would suggest just using some type of constraint on the bottom of the mat. For example, "General Constraints" would simulate a rigid soil, or "3D Springs" could be used to represent a soil with some known or assumed stiffness. This would result in the fastest run time.

 

If some part of the mat lifts off of the ground because of the displacement, then some type of gap element will be required. You could draw a big "box" of ground like you suggested, or use the Copy command to generate the gap elements "by hand" from the bottom of the mat. The stiffness of the gap elements would then mimic the one way support that the ground provides in real life.

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Active Member
JamesRJatho
Posts: 6
Registered: ‎11-28-2012

Re: Looking for info on whether ASM is the right tool.

04-02-2013 06:39 AM in reply to: AstroJohnPE

In the time I've been doing this, I've never seen a mat actually deflect enough to be noticed visually. The real problem we run into is that when we move into a refinery/chemical plant/whatever we're usually given the allowable ground bearing pressure that corresponds to a permaneant structure. I've been told by PEs in the same field that the reality is it should be up to 3 or 4 times as much for a mobile crane on crawlers, but plant safety people won't accept it.

 

I guess my next question is how should I simulate the actual tracks pushing into the mats? My first thought was draw a row of the 12" by 4' by 20' mats in AutoCAD, put a general constraint on the bottom, then have an area of pressure pushing down on them in ASM that is the same area as the crawler tracks (29.9' by 4.3') but I can't seem to figure out how to use the pressure tool for a specific area that isn't the entire surface of the top of the mats.

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Valued Mentor
AstroJohnPE
Posts: 498
Registered: ‎08-30-2012

Re: Looking for info on whether ASM is the right tool.

04-02-2013 06:05 PM in reply to: JamesRJatho

Pressure and other types of surface-based loads are applied to entire surfaces in the model. (Use "View > Appearance > Color By > Surface" to visually see each region.) To apply a surface load to a region that is smaller than an entire face, you normally split the face in the CAD software. Search the Help for "surface splitting" for other ideas.

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Active Member
JamesRJatho
Posts: 6
Registered: ‎11-28-2012

Re: Looking for info on whether ASM is the right tool.

04-03-2013 11:39 AM in reply to: AstroJohnPE

Well I figured out the surface issue. Basically just made a surface for each side of the mat but divided the top into three distinct surfaces so that I can apply pressure only to the one in the center. I then combined all of them into a single surface, opened with Simulation, and it worked perfectly.

 

I'm sure I'll have a whole other series of questions shortly.

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