## Approximating the forces on a beam for recovery towing operation

## Approximating the forces on a beam for recovery towing operation

(OP)

We have a large 200 tonne catamaran-style dredge that we are planning to tow with a couple of D11 dozers up a hill. We are designing a platform that it will sit on and be lashed down to, and this is what the dozers will be attached to using towing strops. This platform will have a large welded beam across the front of it, however, I need to establish the how the loads are being applied to it so we can size the beam in our beam calculators.

In the attached sketch, we have assumed that the dozers may stop and start, so the entire force from one may be applied to a single side at any one time. We have also assumed that the structure of the dredge cannot/will not take any of the load through it. We have assumed this because we don't know if the structure is strong enough to handle these loads. So, essentially we have two pontoons acting independently. I figured we could assume a point load where the centre of mass is in these pontoons, which is essentially the reaction forces. However, this could also be a distributed mass as the skid plate will be connected along two lengths of the beam, I'm not 100% sure. If point loading is a worst case scenario and makes the calcs simpler, we are happy to assume that. The link below is an Excel file showing how I calculated the reaction forces from the moments.

Beam Calcs

Now, technically the beam will move when the required force is applied. This is problematic for beam calculators as it's not constrained. I have done a force balance calculation to find the reaction forces from the location of the pontoon CoGs and modelled these as forces. There is some residual force/moment at the end of the beam, which was required to be added into the model as a fixed joint to constrain everything, however this is negligible.

So me question is, is this the correct way to set up this beam analysis? Are my assumptions reasonable? Something just feels off about it and it doesn't fill me with confidence. Any input would be appreciated.

In the attached sketch, we have assumed that the dozers may stop and start, so the entire force from one may be applied to a single side at any one time. We have also assumed that the structure of the dredge cannot/will not take any of the load through it. We have assumed this because we don't know if the structure is strong enough to handle these loads. So, essentially we have two pontoons acting independently. I figured we could assume a point load where the centre of mass is in these pontoons, which is essentially the reaction forces. However, this could also be a distributed mass as the skid plate will be connected along two lengths of the beam, I'm not 100% sure. If point loading is a worst case scenario and makes the calcs simpler, we are happy to assume that. The link below is an Excel file showing how I calculated the reaction forces from the moments.

Beam Calcs

Now, technically the beam will move when the required force is applied. This is problematic for beam calculators as it's not constrained. I have done a force balance calculation to find the reaction forces from the location of the pontoon CoGs and modelled these as forces. There is some residual force/moment at the end of the beam, which was required to be added into the model as a fixed joint to constrain everything, however this is negligible.

So me question is, is this the correct way to set up this beam analysis? Are my assumptions reasonable? Something just feels off about it and it doesn't fill me with confidence. Any input would be appreciated.

## RE: Approximating the forces on a beam for recovery towing operation

As shown, this looks like a great way to try and rip the beam off the skid plates, or get the entire thing cocked sideways and introduce shear load on the dredge platform where it connects to the pontoons.

## RE: Approximating the forces on a beam for recovery towing operation

It needs two dozers to pull it out, one isn't enough. We can't use wire rope, site standards, strops are all we can use. The rigging plan for this is already done and not our area, our job is to build the platform.

## RE: Approximating the forces on a beam for recovery towing operation

## RE: Approximating the forces on a beam for recovery towing operation

The skid is dragging on the ground. All load calcs have been done for that. Please ignore how the load is being applied and rigged up, it's already all decided and signed off by a team of site engineers, so we have no say on that. Our area now is the skid with the given loads.

## RE: Approximating the forces on a beam for recovery towing operation

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Approximating the forces on a beam for recovery towing operation

The safety factors all come from mine standards. The load is the max drawbar pull for a single D11T in first gear pulling at a specific angle with a specific grade of road and includes dynamic loading while accelerating. Safety factors will come in at the end. However, I'm conscious about getting sidetracked on the rigging and towing aspect here as our focus is currently on how to model the beam with the current loads that we have been given.

The bracing is to keep the two skids apart while turning.

Cheers

## RE: Approximating the forces on a beam for recovery towing operation

The bracing matters as well. After all, it's going to prevent the skids from coming together and bending the beam further.

I don't see reason to do a beam bending calc without more info. There's so much missing from the picture. As far as I can tell, they want to drag it on an angle as shown.

Since the beam is unconstrained, you need to change the angle of the beam until the friction forces at each pontoon COG are equal distance on either side of the pull point. Then you can solve for the actual moments and forces within the beam by resolving the angled pull and friction forces into x and y components for the calculators.

## RE: Approximating the forces on a beam for recovery towing operation

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Approximating the forces on a beam for recovery towing operation

As another aside, since this is a mine, how strong of belting do you have? Consider putting the dredge on (ST 10000 or something very strong) belting with appropriate pulling (I could elaborate on that) and drag it.

## RE: Approximating the forces on a beam for recovery towing operation

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Approximating the forces on a beam for recovery towing operation

I am trying to follow your calculations and I assume the 1.3m and the 7.5m are the horizontal distance between the two skid centrelines?

Also what the incline of the slope you are dragging this thing up and the coefficient of friction?

What is the distance between the reaction forces and the applied pulling force shown on your free body diagram?

I don’t believe your approach is correct but there is a lot of information not given.

I would be more concerned about the size and strength of the weld that you are using to attach the beam to the skids.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

## RE: Approximating the forces on a beam for recovery towing operation

Is the view of beam from the side correct? Maybe a side view would help clear this up.

"as the skid plate will be connected along two lengths of the beam," Eh??

SO are the pontoons connected via a flexible central anchoring point or welded to the beam or what?

I suspect the answer is going to be don't pull with only one dozer.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Approximating the forces on a beam for recovery towing operation

Your beam size (and whether/not you even need the beam) will depend on the stiffness of the pontoon structure and the unevenness of the ground. Typically you'd start the design by sizing the skids for an acceptable ground pressure while operating on max slope, and design the tow points last. Point loads on the skids are your biggest enemy, both the skids and entire structure need to be flexible enough to prevent damage. When you finally get to designing the tow points you need to keep in mind that they will see both bending and torsional effects simultaneously.