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Re: heritage building

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Andrew,
 
        The term heritage would seem to be a rather loosely defined term that should seem to imply a building of some architectural significance or was associated with some historically significant use and occupancy.  Buildings that are protected by law would be deemed "Designated Historical Resources".  The "designation" could be imposed by any of three levels of government: municipal or city; provincial; or federal.  It can also be designated by more than one level.  Which ever level of government designated a building could also undesignate it.
 
        The cracks in the roof slab appear over the beams (perpendicular to the span of the slab) and are visible from the TOP of the slab, exactly where you would expect them to be.
 
        There is one bay that has cracks visible on the underside of the slab.  These cracks are parallel to the slab span; they are quite wide, at least 1/8" wide; and would appear to be shrinkage cracks.  There is one short crack at midspan connecting two of these cracks.  Someone has reinforced this bay using S8x18.4 spanning the 15' to 16' direction (parallel to the concrete slab span) spaced four to five feet apart in a pattern similar to
                    HH HHH HHH
These steel beams are connected with rivets and/or square headed bolts indicating that this may have been done during, or very shortly after, the original construction.  I will be completing the pattern and adding one or two extra beams to maintain a 5' minimum spacing with support at midspan of the concrete slab in this area.  This will also serve as the laydown area for any building materials lifted onto the roof.  HVAC units will be lifted directly into place (they will not be wheeled around on the roof slabs) and will be supported directly on the beams via separate steel frames.
 
        We could reinforce the roof.  Two methods that have been considered in addition to supporting from underneath as per the one reinforced bay are: clean and sandblast the slabs, add 2" or 2.5" of new concrete, negative (top) steel and temperature reinforcing (This may seem radical to southerners but this is done as repairs to bridge decks subjected to salt damage in Canada quite regularly.); and put sleepers over the beams and add a new roof using cold formed steel shapes (wood is not acceptable because of combustibility).  Either method would probably cost well in excess of $50,000.00.  I will have approval to spend this amount of money if I deem it necessary; but, based on conversations I have had with other engineers, with replies to my posting, and with private replies to my posting I presently do not believe it is necessary.
 
        Thank you for writing.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Tuesday, April 25, 2006 2:08 PM
Subject: heritage building

Daryl,
Do Canadians use the term "heritage" for a building of historical purpose (in the US "historical), that may have government protection from demolition or renovations?
 
Where are the cracks in the roof slab? Are there any in the center of the slab, parallel to the beams, where we may expect a tensile crack to form in a simple span? Did the cracks at the supports propogate to the top of the slab, if the slab was visible from the roof?
 
How bad would it be to remove the concrete slab and replace with a steel joist and deck system? Or take measures to support it from underneath?
 
Sounds interesting, it is sometimes a tough call to make on existing structures that are performing and convincing the masses that spending the money is necessary.
 
I just had a job where I was checking an existing floor slab for a new piece of medical equipment, and it was super close to the allowable... I was talking to my associate , we'd prefer it be way under (easiest), or way over where we have to add framing or strengthen it (additional service fee rqd)....
 
 
Andrew Kester, PE
Structural Engineering Consultant
Lake Mary, FL