Would stainless steel rebar be a solution to this problem?
Gary
Jordan Truesdell, PE wrote:
> Daryl,
>
> I had the same reaction as you - steel strongback temporarily,
> replacement with p/c after knowing you had a 5-7 day window for
> replacement. 5-7 days is short, so it may depend how flexible the
> schedule is, and getting everybody in sync.
>
> It sounds like the roof design needs a lot of thought. I presume the
> atmosphere inside is essentially heated sulfuric acid. I'm not
> qualified to speak to the specifics of the material interactions, but
> the long term corrosive atmosphere suggests lots of cover, "special"
> concrete, and either coated steel or corrosion resistant reinforcement
> material. Is strand even an option, or should might you consider a
> thicker cap with non-prestressed steel? Sounds like an interesting
> project.
> Jordan
>
>
> Daryl Richardson wrote:
>> Fellow engineers,
>>
>> I have a project involving a sulphur pit with a roof which is
>> on the verge of collapse. I would like to have some help in the form
>> of brainstorming to find suitable ways and means of carrying out an
>> emergency repair. Any ideas, no matter how "wild and wooly" would be
>> welcome.
>>
>> The particulars of the subject facility are the following.
>>
>> A sulphur pit is a reinforced concrete structure resembling a
>> swimming pool with some type of roof structure which may be steel,
>> aluminum, or concrete in nature. The purpose is to serve as a
>> holding tank for liquid sulphur (melting temperature about 243
>> degrees F.) during the process of removing sulphur (an undesirable
>> impurity but saleable by product) from natural gas. These pits seem
>> to suffer from severe cracking due to the high temperature gradient
>> through the walls and from extreme corrosion of the concrete due to
>> the nature of the service (sulphur vapor + water vapor + oxygen in
>> the space above the liquid level). The life of a sulphur pit is
>> commonly less than 10 years between major restorations.
>> Petrochemical processing facilities like to operate for four to five
>> years between major shut down periods for scheduled maintenance.
>>
>> The subject sulphur pit is rectangular, 33 feet by 60 feet,
>> of unknown and unimportant depth (probable guess about 12 or 16 feet)
>> with steam pipes (for keeping the sulphur molten) a foot or so above
>> the floor. The wall thickness is probably about 12 inches. The top
>> is abut 30 inches above grade.
>>
>> The roof structure is precast concrete panels 12 inches thick
>> by 6 feet wide spanning the 33' direction. There are four voids
>> about 14 inches wide by 7 inches deep filled with some type of
>> particle insulation. The main reinforcing is post tensioned grouted
>> duct with about 1.5 inch clearance to the bottom; there seem to be 8
>> strands. The cement type is Type CSA 10 (ASTM Type 1), 35 MPa (about
>> 5,000 psi) mix design, with 4.5%air. The panels are seated on a
>> Styrofoam strip and calked with a Tremco material. The roof has been
>> in service for 15 years!! All of this information comes from the
>> drawings; the "as built" situation may be somewhat different. The
>> drawings do NOT bear the stamp of a professional engineer!
>>
>> The apparent condition of the roof is as follows.
>> 1.) There is a visible "sag" in the panels (perhaps as much as 2
>> inches). I have not yet checked the curvature due to the thermal
>> gradient through the roof slab.
>> 2.) I was informed that there was originally a camber of 3 inches
>> (which I find doubtful but I guess it's possible).
>> 3.) One panel (the third from one end) has a noticeable sag of at
>> least 1 inch more than the panels adjacent to it. I suspect that at
>> least some of the strands in this panel have failed; but how many??
>> This panel also shows signs of a horizontal shear failure at one end;
>> there is a horizontal split about mid height across the full width of
>> the panel and the top portion cantilevers out over the bottom portion
>> about half an inch.
>> 4.) I am recently informed that fires in sulphur pits are a very
>> frequent occurrence. Apparently these fires do not cause ay
>> significant problem from an operating perspective; the operators can
>> put them out very quickly once they are discovered.
>> 5.) It is a standard procedure in ALL facilities (not just this
>> one) that no one is ever permitted to walk on the roof of an
>> operating sulphur pit.
>> 6.) This plant appear to be able to take this pit out of service for
>> not more that 5 to 7 days without shutting the facility down. No one
>> will tell me the cost of such a facility shut down but my guess is
>> about $200,000 to $300,000 per day. The next scheduled major
>> maintenance shut down is in about two years.
>>
>> Possible repair procedures already considered.
>>
>> 1.) As a short term repair while the plant is "hot" (kept in service)
>> span beams (say W16x36 at 6 feet spacing spanning in the 33'
>> direction) a foot or so above the existing roof, build a working
>> platform on top of these, suspend the existing precast roof from
>> these beams using a mechanism similar to a drywall anchor that could
>> be dropped through a hole cored through the precast voids. Once the
>> mechanism was activated it would resemble an inverted T supporting
>> all or most of the ribs in the precast. Such support could be
>> achieved at the third points (or even the quarter points) of ALL of
>> the precast panels forming the roof. I formulated this scheme before
>> I was aware that a shut down of 5 to 7 days was possible; I now
>> prefer the next alternative, never the less, the owner wants to give
>> this scheme a more formal consideration. The risk of failure with
>> this scheme seems to me to be fairly small (but not zero) with a good
>> and careful contractor; however, the consequences of failure are,
>> admittedly, very high and could involve serious injury or worse.
>> 2.) Design and install a new precast roof. It would seem possible to
>> have panels precast in advance and installed within the 5 to 7 day
>> window. The risk of failure would seem to be no more than the risks
>> normally associated with construction and the consequences of failure
>> would seem to be only financial, related to not meeting schedule.
>> One decided advantage would be the opportunity of inspecting the
>> upper portion of the pit walls and to better plan the work to be done
>> in the next shut down in two years.
>>
>> As I said above, any other ideas or thoughts, no matter how
>> "wild and wooly" would be gratefully received and would be treated
>> respectfully.
>>
>> Thank you for anything you might submit.
>>
>> Regards,
>>
>> H. Daryl Richardson
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