We DO NOT "pre-weaken" all load-bearing elements prior to implosion...that's what the explosives are for! This would create an EXTREMELY unsafe working environment and, further, be totally against OSHA regulations. We may, however, selectively remove and/or modify portions of certain load-bearing walls and remove selected stairwell portions near the base to assist the structure in becoming more flexible during its decent, further allowing us to precisely control the rate and direction of fall.
Chicken wire would most CERTAINLY be a lot cheaper; however, it's not what we use ;)
When wrapping a reinforced concrete column with protective covering materials, we use a combination of heavy duty chain link fence (usually 11 gauge) and a non-woven geotextile fabric (we use a brand called Nicolon, made by a company called Mirafi...it's heavy duty, high tensile strength, VERY puncture-resistant, "felt-like" material used underneath road base to help prevent erosion...usually 10 oz.). We refer to this as "at-source, soft cover/protection. Essentially, these materials work together much like a "catcher's mitt." Dependant on the explosives load in those columns and the proximity of adjacent properties/utilities and other improvements to remain, we may use a double wrap of fence only, double wrap of fabric only, a combination of the two, or whatever the situation calls for. In a structural steel building, plywood boxes are custom constructed for EACH AND EVERY charge placement location. The boxes are carefully placed over the linear shaped charges. Often, we also add panels of used conveyor belting inside the box to further help "knock down" the flying copper sheathing that comes off the charge when it detonates (for every action there is an equal and opposite reaction). In addition, we may also opt to wrap these boxes with the fence and fabric mentioned above.
Another common practice in protective covering operations is what we call "curtains." If you look at some pictures of structures about to be imploded, you'll note that certain floors have, literally, a "curtain" of black fabric around them. That's the geotextile fabric I mentioned above. Often, there is also a curtain of chain link fence hung inside the fabric curtain as well. This goes even further to prevent flying debris from leaving the structure's periphery.
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