Falling Brick Problem 6

[SteveGregory]

Could someone tell me how to calculate the force of a brick falling 30 feet and what kind of damage I could expect on 2 layers of plywood? The plywood would be used to protect some translucent fiberglass roof panels.

 

[kslee1000]

I think this a phenomenon of transferring potential energy to kinetic energy, so can be solved mathematically by equating the equations to find the free fall velocity before impact:

Potential Energy = Wy, in whih W=weight, y=distance
Kinetic Energy = mV^2/2, in which m=mass, V=free fall velocity before impact.

From here time "t", acceleration "a", and impact force = ma can be find.

 

[Bribyk]

It not that easy, you need a deceleration time to find the impact force, m*a. To calculate "a" you need an impact coefficient which is empirically tested or just a guess. This is far easier, cheaper and more accurate to just test. Also, more fun.

The most likely scenario is a stack of bricks gets bumped and few loose bricks fall together. You might want to strap two together and test that as well, as stated above.

 

[desertfox]

kslee1000

If the brick is in free fall then the acceleration is constant ie:- due to gravity 9.81m/s^2

desertfox

 

[IFRs]

What is unknown is the decelleration after the brick strikes the wood. You could guess at a deflection of the plywood and from there back-calculate the time to go from fast to stop, and then the acceleration. However, I'd just go and drop the brick. If lacking a tall enough structure, you could throw it upwards to the desired height and let it fall onto the wood, if you can toss it straight up (don't forget to get out of the way!).

 

[kslee1000]

desertfox:

Thanks for pointing out the obvious mistake in my previous response.

For anybody interested in calculating "impact force" from a free falling object, please visit the site linked below.

http://www.livephysics.com/tools/cl...ated-to-impact-force-from-falling-object.html

 

[kslee1000]

Another link for more mathematically orientated persons.

http://www.jw-stumpel.nl/bounce.html

 

[RWF7437]

Hard to see how this link can possibly be helpful:

http://www.livephysics.com/tools/cl...ated-to-impact-force-from-falling-object.html

It calculates the average impact force, NOT the maximum impact force.
It uses the force of an object dropped on "dirt" but doesn't say whether that dirt is rock, mud, silt, peat or what.
It doesn't account for the area of contact at impact.

Aren't these factors important ?

 

[kslee1000]

The linked site provides simple graphical explanation of free fall - impact phenomenon accompanied with calculator to demostrate simple calculation method. However, it failed to address the derivation of the most important factor -"delta d", distance travelled after initial contact of the falling and recipient materials, which, again, can be solved mathematically by knowing the elastic properties of these materials.

The second link provides better, in depth, explanation. Please check it out, and let me know your thinkings.

 

[hokie66]

Too much messing around. I would just put the plywood down, hope it works if bricks fall, and if not, replace the translucent fiberglass sheets. They only last a few years in high UV conditions, anyway.

 

[GregLocock]

Hang on, the OP is trying to protect his GF panels while building above them, not applying a much tougher test of making them brick proof in general.

I'm with rb1957, a wheelbarrow full of bricks and a slab of beer and a video camera is all that is required.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kslee1000]

There is no need for any kind of testing. Go walk on street where has on-going construction to see how the side walk is protected for falling objects (I think most city has ordinace/requirement for such protection). You may do your own test by sneak into one of such site and throw a brick to find the result, get someone ready to bail you out though.

For persons curious in finding way to figure out scientifically the force/impact from free falling, for many other cases that are too difficultcostly to carryout the tests, the web has many interesting sites to explore. With some engineering judgement and assumptions, reasonable solutions could be find.

 

[beton1]

Put a net above the plywood.

 

[concretemasonry]

Just make sure you are comparing apples with apples.

Your weight of the individual brick is obviously in error at 4#. Most Speed brick used in the U.S. are 12#+. The weight you are using is very similar to a normal common cored brick weight and not the size of the unit you described. A Speedbrick is normally an over-sized hollow concrete masonry unit with the visual appearance and general properties of a clay brick, but is used in larger scale applications. There is a weight difference by a factor of 3!!!

Any calculations and assumptions are obviously in error and a waste of time if the assumed weight is not correct.

Dick

 

[Bribyk]

Ala Mythbusters... you could put some shipping/packing G/shock recorders on your dropped bricks to see what the max deceleration is (they're pretty cheap). Then you'd have some data as well that may be useful for future projects (or in court).

 

[drawoh]

SteveGregory,

I assume you know how to work out the velocity of a brick that has dropped thirty feet. This gives you the kinetic energy to be absorbed by your plywood.

Can you make an assumption about the distance travelled while the brick decelerates safely to a halt? You know initial velocity. You should be able to calculate acceleration. From this, you know the force, and the design criteria for your plywood shield.

Are you certain the brick will not be followed by more bricks, and then the dropper of the bricks?

JHG

 

[SteveGregory]

Thanks everyone for your reponses. I know how to calculate velocity and kinetic energy. It's the impact, deceleration time and the unknowns ("does the brick land on an edge or corner") that add to the complexity.

This industrial plant has a lot of good windows to do a drop test from and plenty of plywood available, if I can get my clients to play along with me. And I'll be wearing my safety glasses and hard hat just like they do on Mythbusters.

 

[jheidt2543]

DON'T DROP THE BRICK! Case closed!

 

[EddyC]

I would solve it this way:

1. Potential Energy=Kinetic Energy=Spring Energy

2. Spring Energy=

a) Bending Stress
b) Shear Stress
c) Through Thickness Compression Stress
d) Membrane Stress

3. Bending Stress example:

a) Delta=PL^3/48EI (Deflection of simple beam w/conc load)
b) k (Spring Rate)=P/Delta=48EI/L^3
c) Spring Energy=0.5*k*x^2
d) x (Design Deflection)=(Spring Energy/(0.5*k))^0.5
e) P(Design Load)=48EI*x/L^3
f) M (Design Moment)=PL/4
g) fb (Design Bending Stress)=M/S

4. Do similar for Shear, Compression, Membrane stresses.

 

[racookpe1978]

Drop one brick. Look.
Step sideways
Drop two bricks. Look.
Step sideways.

Drop three bricks.

...

 

[racookpe1978]

Memo to self: Put the test piece of plywood (3/8? 1/2 thick?) ABOVE the target piece of plastic - won't do any good to have the plywood (or its supporting frame!) directly ON the plastic pieces as the plywood bounces and recoils and shakes around.

Your time is worth money, do'nt waste it by playing with mathematical approximations of a problem.