impact force question

[par060]

I am designing a saftey cage for a bulldozer that is used in deforestation. I need to find the force that is created when a tree falls over and strikes the cage. The trees are 30m tall and have a diameter of 1m....the density is 12 KN/m^3. I know these are bigger and denser than the trees in yuor back yard but this is in a jungle and this is the design criteria.

I have tried useing mechanics...but the forces I am coming up with are astronomical so I don't think I'm useing mechanics correctly.

I am grateful for any help

 

[IRstuff]

You're describing a 64,000 lb object falling on a vehicle whose roof is not that substantial, so I'd believe that the cab would get crushed in most case.

There have been already some discussions about this type of analysis, with about the same title, so you might try search, e.g.:
thread404-74498
thread507-86811
thread507-69901

TTFN

 

[GregLocock]

Those threads are mostly to do with elastic collisions. I strongly suspect that you will only be able to absorb that sort of impact by designing a plastic (ie deformable) structure.

If this so then you can use an energy balance to determine the final deflection, as most crash structures use a constant force construction, crudely this just gives you masstree*g*.5*height=F*d

Alternatively you could use a spring and damper to hold the roof up, in which case you could use the impact solution for a single degree of freedom system.

You need to do a bit of work to establish the striking velocity and the worst bit of the trunk to be hit with (the top is moving faster, but the middle has more effective mass). This has got something to do with baseball bats.


Cheers

Greg Locock