huangyhg

demolition of 5-story reinforced concrete building
i have a question about logistics and risk management with regards to demolition of a heavy industrial building in an urban setting.
we are about to demolish a 5-story reinforced concrete structure in an urban setting. it was once a factory building, and approx 12 bays on each floor were designed for heavy live loads (10" thick flat slabs with 30" dia. concrete columns and shear caps.
after removing everyting but the concrete structure, the demo contractor proposes to attack the conctete slabs with a hydraulic excavator with a universal processor (concrete cracking jaws) and another with a concrete pulverizer, and yet another with a demolition grapple.
the contractor's work plan proposes to work from the bottom up (since access to the top of the building will be logistically difficult. the plan states:
"the second floor slab will be removed in sections of 200 to 300 square feet between columns followed by the removal of the same quantity of slab directly above the section just removed. once the section of roof slab has been demolished, slab removal between the next columns will begin. once roof slab has been removed, exposed column will be demolished, starting from top down."
while i have studied engineering, i am a construction program manager--not a practicing structural engineer. i am acting as the owner's rep, and my experience with demolition work is limited. this project appears to be one of the more challenging jobs that this demo contractor has undertaken.
won't removing the lower slabs destabilize the columns? it sounds spooky to me.
i am concerned and would appreciate any helpful advice from anyone experienced in such matters. we are in the hazmat abatement and interior demolition stage. the demolition specs don't require a formal submittal of an engineered demolition plan.
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demolishing the slabs is ok, but the problems will start once the main beams between the columns are removed. there will be a loss of strength in the frame and re-alignment of the load path, is this predicted? in most buildings the outside wall and frame of the building is the stiffest part, so it's normal just to go in and chomp away at everything. with a designed industrial building, it may not be the same, it is designed as a internal structural system with little external wall action.
i would remove the slabs, walls and leave the beams, on all levels, then from the top, design the selected beams for plastic structural failure, to become pinned at the joints and midspan. this will still tie the columns together.
i would get a structural engineer to look at it. most cities require some sort of demolition plan - and this to ensure that adjacent properties are not put at risk.
why are you involved with the demolition? the contractor is the 'expert' in construction and the manner of undertaking the work should be determined by him. the more you get involved, the more liability you assume. as a caution, with the original useage, the concrete could be well reinforced and detailed with liberal redundancy. is the building designed for seismic, too?
his approach appears to be reasonable; he might have to cable some of the columns to provide lateral support assuming that these elements have been removed. what sort of insurance does the contractor have and what is his experience.
you mentoned that it's in an urban setting. how much 'space' does he have around the perimeter?
thanks to all for your responses. i agree with dik & jae in the sense that i don't want to take on liability, but as the construction program manager, the client still looks to my firm to help steer them away from trouble. i certainly don't want anyone to get hurt. i'd prefer to have a demo engineer involved, and we might go that route, but we will likely face a change order (maybe it's a price we have to pay), since the bid docs don't require it.
we are surrounded by roads, parking lots and the backyards of some old, historical homes that are ripe for renovation, but currently unoccupied. there is a courthouse on the other side of a 4-lane road.
rtmote--thanks for the structural advice. sounds reasonable. i will look at the framing plans to see if i can discern some main beams in the flat slabs. as long as they retain a section of slab along the grid lines, sounds like it might work.
the contractor has typical insurance (commercial general liability and commecial umbrella liability ($2m per ocurrence and $4m aggregate.)

your client doesn't want to get involved, either... you might want to undertake a pre-demolition damage report for the surrounding buildings... removing a mass may cause some 'springing up' of the adjacent soil. cutting away the slabs results in a redistribution of stiffness...
dik
the pre-demo property survey sounds like a great idea. i'll take lots of photos.
are there beams in this slab? the mention of column capitals seems to indicate flat slabs and therefore rtmotes comments about beams would not be applicable.
if this is indeed a flat slab then i would advise only a top down approach, who knows what will happen with the vibration of machinery e.t.c.
i fail to understand how acces to the upper levels would be easier once the bottom floors are removed?
the use of a demolition engineer should not be seen as optional, the price will be negligible compared to the project, also if something does happen then you are the most knowledgible party involved in the decision making and could therefore be held liable.
merry christmas
csd
as mentioned above i definately would get a consultant involved if the is any doubt based on the contractors experience. i've seen two reinforced concrete building demolition jobs go wrong in a flash.
here is one of several companies that do consulting work on such jobs.
sounds like a job for those crews that come in with shaped charges, detonator cord, etc. is it called implosive demolition? i've seen pictures of buildings that fall in on themselves with minimal disturbance to nearby structures.

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