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.

 

[SLTA]

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

 

[rb1957]

"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)

 

[SteveGregory]

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.

 

[Ron]

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.

 

[MechEng2005]

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

-- MechEng2005

 

[Ron]

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

 

[civilperson]

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

 

[rb1957]

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 !

 

[concretemasonry]

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

 

[jike]

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

 

[SteveGregory]

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.

 

[SkiisAndBikes]

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.

 

[enginerding]

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....

 

[Tomfh]

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...

 

[COEngineeer]

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

Never, but never question engineer's judgement

 

[vandede427]

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.

 

[rb1957]

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 ?

 

[beton1]

The art of engineering!

 

[concretemasonry]

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.