Shoring Practice Session
for Southwest Portland NET's
Sunday, April 18, 2004
starting at 1 pm


x marks the Hillsdale NET staging area
behind the bleachers at Wilson High School


.
What does it weigh?  -- Let's estimate:
Jersey barrier
12' 5-1/2" long
X-sectional area = 2.86 ft2(measured with CAD software)
Volume = 35.6 ft3   Sp gr = 2.4 - 2.9 (Army TM 5-742)
This calculates to a weight of 5320 lb - 6430 lb.

Slip it under . . . 
. . . carefully!

Log-cabin cribbing and log-cabin fulcrum.
Log-cabin cribbing
fulcrum failure!
Heavy Jersey barrier -- flat victim
.
A Lesson or Two
using  some video frame grabs:
Anatomy of a failure
The near crowbar is now applying its force at the point of the small red arrow.  The failure will start when the crowbar transfers its force to the 4X4 below.		 The concrete Jersey barrier is here starting to be lowered.  Four crowbars on this end are manned by four NET people.  (The four crowbars must apply a lifting force of about 3000 lbs, 750 pounds per bar.  The mechanical advantage we see in the photo is about 7.2, so the downward force--by hand--at the end of an individual bar is about 100 pounds.  Note that an individual would need to apply about 400 pounds were he alone.)

The concrete has just been raised, the cribbing removed, and now the concrete is starting to move down.  Part of the problem is the overhang (large red arrow) of the top 4X4 beyond the lower 4X4.  The crowbar will soon press down on the lower 4X4 starting the collapse of the fulcrum.  Note that, at first, the forces on the various log-cabin pieces are predominantly vertical, but as the crowbar is tilted upward, horizontal forces appear throughout the system.  These horizontal forces require some minimal frictional forces if the fulcrum structure is not to fail.
The crowbar touches the 4X4 putting a torque on it that begins to raise its left end.  Horizontal forces are now distributed throughout the "log cabin."
There is a potential problem with the above log-cabin cribbing.

When a crowbar is supported at the center of a piece of lumber which is supported at its ends, it's relatively easy to exceed the strength of the lumber.  This is a classical problem in elementary structural engineering:

Stress in the beam varies with position.  It's zero at the neutral axis and increases with distance up or down from the neutral axis  The greatest (tensile) stress occurs at  x, and there it is proportional to the ratio, b/a.  Unless b/a is kept rather small a wooden beam such as this will fail.  Think of that ratio as a lever arm that concentrates the force of the crowbar onto x.
The horizontal force on the 4X4 at the tip of the white arrow has overcome the friction holding it in place moving it the length of the white arrow.

The bar becomes steeper.  The vertical component of the force on the bar remains the same, so the horizontal component gets greater--becoming infinite when the bar is vertical.  Conclusion: The weight of the Jersey Barrier will push the fulcrum away unless we restrict the height we raise the end of the bar. Lowering the weight off the cribbing must be done in small steps.

During the recent cribbing practices (Rodeo and this practice) this error was frequently made resulting in momentary loss of control.  This could be hazardous.
The 4X4 has moved even further (white arrow).  The force on the fulcrum by the crowbar is shown resolved into its horizontal (h) and vertical (v) components.  Because the vertical component will remain essentially the same, the horizontal component gets very large as the crowbar approaches the vertical.  Because friction is proportional to the vertical force, friction cannot be relied upon to resist the motion of the white arrow:  the structure will fail at some point well before the crowbar becomes vertical.

A little understanding of elementary physics is very helpful when we design and use fulcrums.

 (See the sidebars to the left.)
We successfully raised, and lowered, a three-ton object with comparative ease!
We used techniques we learned in NET training.