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stopping force/ cmu wall ftgs

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Stopping force on a log:
 
I think Scott pretty much had it. You need to know how much time it takes to stop the log (deacceleration). I believe this would be independent of the stiffness or type of stop. The term for this force is IMPULSE.
 
Basic equation:
 
m1*v1+F*dt = m1*v2
 
m= W/g,  g=32.3ft/s^2, W=lb
v1= velocity before
v2=velocity after
 
Since velocity after the impulse is zero, just solve for F. Assume dt= 1 second:
 
F= (1/s)*(6200lb/ 32.2ft/s^2)*(200ft/60s) = 642lb
 
 
Smaller the dt, larger the impulse. Same theory with traffic control barriers, extend the dt to lower the force.
 
 
Wall ftgs under CMU wall:
 
By most codes even interior walls need to be designed for a Plat=5psf. This can go up if you have a partially enclosed/ open building. This creates a moment, which means you have to reinforce the wall. By ACI 530 at least 1/4 of vertical wall reinforcement must be fully developed, which means you need a footing of some type. This reason is in addition to all the other great reasons others have mentioned. I have always put CMU walls of any tpe on some type of reinforced ftg, any additional expense is worth the cost and contractors are used to this. Metal stud non-brg may or may not require anything but a slab, I usually throw a small TE if I can. Make sure the top of your wall is braced off properly to the roof structure, you do not want a cantilever.
 
 
HTH,
 
Andrew Kester, EI
Longwood, FL