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
RE: Falling Brick Problem
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
RE: Falling Brick Problem
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
-- MechEng2005
RE: Falling Brick Problem
RE: Falling Brick Problem
RE: Falling Brick Problem
RE: Falling Brick Problem
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
RE: Falling Brick Problem
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
And to think, most of my research was done electronically....
RE: Falling Brick Problem
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
Never, but never question engineer's judgement
RE: Falling Brick Problem
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
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
RE: Falling Brick Problem
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
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
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
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
RE: Falling Brick Problem
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:
RE: Falling Brick Problem
http://www.jw-stumpel.nl/bounce.html
RE: Falling Brick Problem
http:
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 second link provides better, in depth, explanation. Please check it out, and let me know your thinkings.
RE: Falling Brick Problem
RE: Falling Brick Problem
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
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
RE: Falling Brick Problem
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
RE: Falling Brick Problem
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?
RE: Falling Brick Problem
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
RE: Falling Brick Problem
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
Step sideways
Drop two bricks. Look.
Step sideways.
Drop three bricks.
...
RE: Falling Brick Problem
Your time is worth money, do'nt waste it by playing with mathematical approximations of a problem.
RE: Falling Brick Problem
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
LOL...
3/8" plywood would stop the brick. There are no numbers that should be applied to this situation.
MDJ
www.windspeedbyzip.com
www.groundsnowbyzip.com
RE: Falling Brick Problem
This is first year dynamics, any good text will cover it.
RE: Falling Brick Problem
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
RE: Falling Brick Problem
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.
RE: Falling Brick Problem
RE: Falling Brick Problem