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Falling Brick Problem
6

Falling Brick Problem

Falling Brick Problem

(OP)
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.

RE: Falling Brick Problem

F=ma and then use the force as an impact load, analyzing the plywood in sections as a skinny beam.

RE: Falling Brick Problem

"force" doesn't really apply ... the brick will hit at some velocity (not hard to calc ... potential energy = kinetic energy), so it'll some impulse (=mv) which the pylwood will absorb over some (small) time period.  i'd expect the plywood would absorb the energy of the brick by bending.

part of the problem is does the brick hit end on or side on (less severe, as the energy of the brick is distributed over a larger area of plywood)

RE: Falling Brick Problem

(OP)
It turns out the brick is a "speedbrick" that is 8"x12"x3.2" and it weighs 4 pounds. The worst case would be landing on the 8"x3.2" side.

RE: Falling Brick Problem

No, the worst case is that is lands on the short edge.

The time period of the force action is very short (0.01 sec?, maybe less).  So assuming or calculating a velocity deals with instanteous acceleration, to determine F=ma.

RE: Falling Brick Problem

I strongly recommend you try to get a brick cannon built by your company for testing. In the name of safety, of course!

-- MechEng2005

RE: Falling Brick Problem

Actually...the worst case is landing on a corner!

RE: Falling Brick Problem

Landing on the corner is worst case, (point load).

RE: Falling Brick Problem

arh, you don't need a "brick cannon" ... just a 30' crane, or toss the brick off a 4 storey building ... i'd put up some netting (cricket nets ?) 'cause the damned thing could bounce somewhat !

RE: Falling Brick Problem

Send the brick to Texas Tech.

They have an air cannon they use to shoot 12' long 2x4s @ 125 mph at walls to determine the wall sections acceptable to prevent penetration. - It has got to be a lot of fun doing it!


Dick

RE: Falling Brick Problem

Why go to the expense of calculating when you could actually do it as a previous response said.

RE: Falling Brick Problem

(OP)
I think the test is a superb idea and probably a whole lot more fun. My brain was beginning to hurt just thinking about trying to calculate it.

RE: Falling Brick Problem


I was involved in a canopy project where plexiglass panels in a steel frame were installed to protect against falling debris at building entrances. It was a two layer system with panels spaced about 6" to 8" apart. The first plexiglass panel (thinner) was anticipated to break, reducing much of the energy of the falling concrete and the second panel (thicker) would catch the debris. This was an attempt to reduce bouncing of the debris off the canopy by catching it between the layers.

We tested the assembly by ... you guessed it ... having the contractor drop chunks of concrete off the roof onto a series of test panels ... that was the fun part.

RE: Falling Brick Problem

I also had the opportunity of watching the Texas Tech 2x4 cannon recently.  Even the professors got a kick out of watching the test even though they have seen it over and over again.

And to think, most of my research was done electronically....

RE: Falling Brick Problem

Forget about "F=ma". Falling bricks ain't statics.

Testing is probably the best idea. Remember the chunks of foam that NASA calculations showed couldn't possibly punch thru the shuttle?

A couple of simple tests showed how wrong the theory was...

RE: Falling Brick Problem

I think the worst case is if something else fall  :).  But really, Ill be really supprised if the brick can go through since it will break to pieces on impact.  Haven't you watched Myth Buster?  Just test it smile

Never, but never question engineer's judgement

RE: Falling Brick Problem

The guys is just trying to protect some light panels with plywood, and you guys suggest he send his brick specimen off to Texas Tech for research testing??!?!?  I'm pretty sure he probably didn't write that into his fee proposal. How unrealistic some engineers get sometimes!

I'm pretty sure he could throw the brick faster than gravity would drop it. Throwing it by hand is probably much cheaper than funding University research.

Wood is very impact resistant.  Look at the Load Duration factor, Cd, for impact.  It's twice that of live loads.

We're assuming one brick will drop, not fifty. Call-out 2 or 3 layers of ply/osb and call it a day.

 

RE: Falling Brick Problem

1st off vandede, most of us are suggesting testing in situ (simply dropping a brick off the building) rather than using the "brick gun", which obviously is going to be very expensive (and not as much fun).

2nd, d'ya reckon you can throw a big honk'n brick at something like 14 fps and prove (probably with a radar gun) that you achieved this target ... simpler (and cheaper) to drop a brick, no ?

RE: Falling Brick Problem

The art of engineering!

RE: Falling Brick Problem

Where did the "magic" criteria of just one brick come from?

If one comes down there could be many more or the head joint mortar between 2 bricks could cause two to come down together.

Just get up above and throw a couple of brick bonded together. Because you are giving it some initial velocity the impact velocity will be slightly more than just gravity would produce.

A crude test, but does give better results than the calculation with a number of assumptions.

I also would you look into the weight of the Speedbrick. - It sounds a little low since most are in the range of 10# to 12# if they are made to ASTM specs.

RE: Falling Brick Problem

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.  

RE: Falling Brick Problem

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.

RE: Falling Brick Problem

kslee1000

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

desertfox

RE: Falling Brick Problem

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!).

RE: Falling Brick Problem

Hard to see how this link can possibly be helpful:

http://www.livephysics.com/tools/classical-mechanics/solve-problem-related-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 ?

RE: Falling Brick Problem

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.

RE: Falling Brick Problem

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.

RE: Falling Brick Problem

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Falling Brick Problem

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.

RE: Falling Brick Problem

Put a net above the plywood.

RE: Falling Brick Problem

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
 

RE: Falling Brick Problem

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).  

RE: Falling Brick Problem

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

RE: Falling Brick Problem

(OP)
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.

RE: Falling Brick Problem

DON'T DROP THE BRICK!  Case closed!

RE: Falling Brick Problem

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.

RE: Falling Brick Problem

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

Drop three bricks.  

...


 

RE: Falling Brick Problem

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.

RE: Falling Brick Problem

Wow, this thread has been going on for over a month.

Why don't you call-out some really good brick ties so they don't fall off the building in the first place?

RE: Falling Brick Problem

Steve,

This is first year dynamics, any good text will cover it.

RE: Falling Brick Problem

Simply absorb the energy of a falling brick by 2 or 3 layers of 1/4 plywood separated by , say, 1".The plywood should be "loosely" supported at a few points. The impact will be distributed in the sheets.
A large number of bricks simultaneously could be a problem.
This is the principle behind micrometeroid shields used on some spacecraft (aluminum or plastic sheets separated)

RE: Falling Brick Problem

It is amazing to see the mind of engineers make a simple problem so complex. Again and again I read problems and solutions on this website that disregard a big part of what we all do: economics. I realize that alot of the previous posts are tongue in cheek and I admit are very amusing. What I haven't noticed is the otherside of the story, i.e. the guys in the field who called the engineer to say "We might have some bricks falling below, what should we put down?". The cost of two a 3/4" rated panels..heck make it 3 layers = approx. $90. The cost of loosing the glass that is being protected = much more than $90. The cost of my entire crew waiting 1 day for the engineer to tell me that 3/4" panels are more than ok = more than $90....I think everyone gets the point.  

RE: Falling Brick Problem

Quote (racookpe1978):


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

Drop three bricks.  

...

racookpe1978

    This is the scenario I was refering to in my post.  At some point, someone is going to implement jheidt2543's suggestion to cause the bricks to stop dropping. Whatever has been put in place to stop falling bricks is going to have to stop you as well.  

               JHG

RE: Falling Brick Problem

somebody please put an end to this thread.

RE: Falling Brick Problem

(OP)
Thanks everyone. Go home now and enjoy your weekend!

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