Post tensioned slab capacity 1

[jml1]

I am investigating a 30 year old cast in place concrete mall parking structure in PA with grade level and roof top parking. The roof slab is 5 inches thick, reinforced with half inch diameter 270 ksi unbonded tendons at 3 feet on center with a parabolic draped profile. The typical slab span is 18 feet with some 23 foot spans over roadways below. The spans are continuous over PT beams spanning 63 feet with a 12" by 36" overall size.
Several spans appear to be sagging as much as 1 to 2 inches as measured with a string-line from beam to beam. (It is apparent to me that severe snow loads and related snow removal operations have taken their toll over the years. Soffit cracking is not widespread nor severe at this time.) Is their a way to analytically determine what I believe is a reduced live load carrying capacity, at the present time, based on center of span negative camber measurements or will some kind of physical load testing be required with strain gauges etc.?

 

[ishvaaag]

Better than a real check, no analytical procedure can be so reassuring of the actual capacity.

Anyway a check as if new is a good start. Then proceed on what, apart of excessive loading, can have brought the structure to its present state. It could be that compounded creep and shrinkage effects were not properly estimated and this adds to the ability of the slabs to sag. Loss of posttensioning steel by corrossion might also cause additional deflection.

Of course, check the deflection at the initial condition; I would say that most times that excessive deflections appear too little steel was placed at construction time; and in fact adding steel is one of the remedial measures one is permitted to take sometimes at design time to correct the issue. I have seen (deflected) catenary-like beams of 6 inch sag at old dintels of garage size openings, and about the same in strip fundations 80 cm deep, 20 ft spans. Reinforced and prestressed concrete can sag, I always remember the 4 to 5 ft recoverable deflection on prestressed mats for electric lines in a photo of the Leonhardt's text; also of course the like strips of paper hanging façade panels of parking structures after earthquake events.

As soon you feel the structure is salvageable for the wanted use, propose the load test; do it in accord with some standard procedure, preferably never going above the service level load, and keeping it no less than 24 hours at upper level of loading. Register the live deflection for the test echelons' and see if the deflections are manageable and still respond to an elastic pattern.

Then devise if necessary remedial measures in accord with the intent of the owner and safety requirements.

 

[BAretired]

I would be concerned that corrosion has occurred in the tendons. If that is the case, the situation will worsen over time and should be examined by someone experienced in inspecting post-tensioned unbonded systems. There is no analytical way to predict the extent of corrosion.

Also, snow removal has to be handled very carefully to avoid unbalanced loading, i.e. large piles of snow in some spans with no load in others.

BA

 

[JAE]

And unbonded tendons can fail quickly.

 

[dik]

Was the 23' span slab thickened? to 7-1/2" or something of that ilk?

The proportions and stand are reminiscent of slabs I've been involved with back in the 70's. The main beam span is a little large. Most parkades I've done have approx 60' span.

Has the deflection become greater (concern)? has the slab received a topping (added weight)? has there been any maintenance (concern)? even 40 years ago, I used to photocopy a servicing manual for parkade owners.

Dik

Dik

 

[Brad805]

Parabolic profile in a 5" slab? Those were some dedicated contractors. I would be a little suspicious about how close the strand layout was to the drawing.

The 1" deflection wouldn't be overly shocking, but the 2" is pushing the limit for sure. Long term creep and deflection is not an uncommon problem considering the slenderness of that cross section.

I think the critical thing is to determine if any corrosion has occured. Something like ground penetrating radar might be in order assuming this facility lifespan is to be extended for another 20 or 30yrs. Unbonded tendons were not uncommon at all in the 70's and 80's.

Brad

 

[jml1]

Brad, I was able to observe two exposed top tendons crossing a support beam in a deteriorated/delaminated area with only about 3/4 inch cover, which is also what was detailed for the low point of the same strands at midspan of the slab in another area of the parking deck. How would GPR help in determining tendon corrosion? Wouldn't tendons need to be exposed at their end anchorages to perform a "lift-off" test to determine the residual prestress in a strand by applying a hydraulic jack to the remaining tail at the anchor end and stressing until there is movement in the wedges or the tendon? (Do you think it is possible to expect 20 to 30 additional years. I thought most structures of this type in this environment were only expected to have a useful life of about 40 years tops.) Thanks to everyone for taking the time and effort to comment. It is much appreciated.

 

[Brad805]

Yes, GPR does not make a lot of sense for corrosion. I had that in the back of my mind from another project I was thinking about. My error. The structural method you propose would be one option. You could also consider some electrical resistance methods. You will need a corrosion expert to give you some better advice. Either way you need to expose some bar ends once you confirm their is cause to do so.

As to the lifespan, it really all depends on your clients goals and how hard a life it has had until now. I know of retrofit projects undertaken to repair structures that were not seriously compromised. There are a few retrofit options these days when a client is willing to evaluate the options. If all the strands are approaching their end, then I would agree that the lifespan of this facility is limited.

Have you checked the original design or calculated the theoretical long term deflections? I think that would be helpful to know that there was not some fundamental design problem and understand if your observations are reasonable or if you do have a serious problem.

Brad

 

[BAretired]

I thought most structures of this type in this environment were only expected to have a useful life of about 40 years tops.

I would expect much more than 40 years of useful life for a concrete structure, but if the useful life is to be limited, let it be limited by obsolescence, not structural collapse.

BA

 

[rscassar]

Does anyone find lift off testing to be detrimental to the future performance of the tendons?