Post-Tensioned building demolition

[ampersand]

I am beginning to prepare a structural demolition plan and shoring plan for a multi-story post-tensioned structure. I have assessed the equipment the contractor will be operating on the decks, and I have designed the shoring required under the working surface before demolition of a floor may begin. But, when it comes to beginning to release strand tension, I need some help. I have seen contractors smash up PT bridge with hydraulic hammers, allowing the strands to detension as the hammers hit them. But on this project, which is near other buildings, I need to ensure that no wedges or strands eject, at all. I am trying to figure a strong, but not to expensive, means of doing this- some kind of reusable temporary encasement around the strand live ends... Any thoughts here?

 

[dhengr]

Ampersand:
Which ends are the live ends (potential live ends) when you cut the strands? Consider hanging some blasting mats made out of old truck tire carcasses over the ends of ten strands at a time, then move the mats to the next slab section. Inside the blasting mats affix a piece of heavy steel plate to the mat, hanging with the mat and btwn. the mat and the slab edge (strand end hardware). Maybe this steel plate has some 6x6 angles at its top, so that they hook on the slab and locate the steel plate and mat. Then just lay the top half of the mat down on the slab, at the slab edge. I’ve never done this, but it should work with a little more thought and planning.

 

[Canuck65]

Ampersand,

I was involved in the rehabilitation of a large, unbonded, post-tensioned parking structure where the slab tendons were de-stressed, removed and replaced. I personally witnessed the de-stressing of upwards of a thousand tendons. Keep in mind, every situation/structure is different. In the parking structure, the longitudinal tendons were in bundles of 2 or 3 spaced at intervals of approximately 3' o/c. We marked their locations on the top surface over beam locations so the contractor knew where to cut and they were close to the surface. When it came time to de-stress, the contractor used a quick cut saw to cut into the concrete and through the tendon. We tried to de-stress near midspan (friction of tendon in sheath helps minimize force at stressing ends upon release). The contractor used a combination of plywood and sandbags at each anchorage, however, in our case we never had a tendon emerge from the original concrete bulkhead ... it was more of a precaution. I do like the idea of blasting mats though, especially at the early stages. They can be moved by a bobcat type machine and can cover several anchorages at a time. Start with multiple layers for a test and go from there.

 

[ampersand]

Thanks, dhengr and Canuck65.

I don't actually know which ends are dead/live- I will apply some protection detail to both ends.

Also, if I may add another question- do you think my 40-billable-hour budget, to produce the shoring plan/calculations, and demolition plan/sequence, is reasonable? I would prefer to ask for more budget sooner than later, if I will likely need it.

 

[hokie66]

I assume you are talking about unbonded PT. If it were bonded, your problem would be much less, and just another reason to use bonded PT.

 

[StructuralAddict]

I believe the article in the following link will be helpful for you:

http://www.brandenburg.com/CompanyLiteraturePDF/Post-Tension Concrete Structure Demolition.pdf

 

[dik]

I think your 40 hour budget is out by a factor of 2 depending on what you are providing. If multi storey, are you providing a re-shoring set of drawings? Can you just do some notes and leave it to a 'qualified' contractor?

Dik

 

[racookpe1978]

A forty hour weekly budget is reasonable ... unless you are spending an 80 hour week but only getting paid for 40.