PT slabs one way vs 2 way action with shallow band beams

[aaronPTeng]

All,

I have designed several PT structures over the years, lately I have started to see a great deal of cracking occurring (not on my structures) between the columns in the top of the slabs 90 degrees to the beams. My take on this is the slabs are spanning 2 way rather than the 1 way the designer has wanted. One particular case had a 7.5m beam direction grid and 8.2m slab span, with beams 2400(w) x 220(d) and slabs 150mm thick. Typically these structures I see with these issues have a 250(d) beam.

In my designs I typically narrow the beam and go a deeper section to try and get the band depth being 2 x slab thickness. In a run of 2D software this gives good results, in FEA it will still try and span the 2 directions, setting the slabs to 1 way just produces unrealistic results.

Is the band depth 2 x slab thickness adequate? with stiff sections in FEA you can produce results which limit the secondary moments in the slab direction and ensure a concrete section with P/A can handle this,

My frustration is when I have a design and place a slightly thicker beam and the builder or client will complain, even with a review of cracking of previous structures, the response is always, all concrete cracks? Does it matter?

I want to know other peoples thoughts on this situation?

 

[hokie66]

I think 220 is too shallow for a 7500 span, but doubt that is the reason for the cracking perpendicular to the bands. That is restraint shrinkage cracking. The band shrinks slower than the slab, thus the slab cracks. This is typical in band beam construction, PT or conventionally reinforced.

 

[aaronPTeng]

Hokie

these types of shrinkage cracks I have seen before however I do not believe these are shrinkage as these types of cracks would be more common if the beam was larger.

Was looking for anyone else's experience with regards to 1 way slab and beam arrangements acting a "two way" first as the beam elements are not stiff enough,

regards,

 

[hokie66]

I agree that the slab as you described it would work in both directions. But the cracks in the top, in midspan, still sound like shrinkage to me.

 

[aaronPTeng]

ah hokie, I re read, I was not clear enough, they were not mid span, they were from column to column in the slab direction

 

[hokie66]

In that case, they are flexural cracks. It just indicates to me that the beams are too shallow. I would expect these floors to be quite bouncy.

 

[dik]

Is the total thickness of your slabband 370 or 220. If the former, then it is too flexible and the 150 slab is likely cracking because of rotation of the slabband at the ends. I've encountered similar effects with drop panels rotating within the drop and putting the entire slab section in tension... cracks wide enough to allow a business card to pass through...

Dik

 

[dik]

'former' should be replaced by 'latter'...
Sorry...

Dik

 

[aaronPTeng]

hokie, dik,

Correct there too skinny, bouncy and to flexible. The slab band is 220mm over all. Looking at it from a reporting aspect,
When these cracks occur the slab can support itself in a one way manor between the bands, for strength etc,

Besides some nice ugly cracks and a trampoline for a floor, with respect to durability as it is a PT slab there is no reinforcement in the location of the cracks (1 duct passes through this location) Does it serve a durability problem? Clearly it is not good design practice and I do not design in this manner however it is an issue I am seeing lately.

Maybe it is the case of "the computer said it was ok in rapt" no offence to rapt as i use this program also along with FEA and it is great.

 

[rapt]

AaronPTeng,

Agree that it is a two way slab with a thickening in the column strip in one direction. Unfortunately a lot of designers either do not understand or believe the ACI/PTI logic on this and design a two-way slab as one way anyway! This happens all through Asia and the Middle East especially, to the point where the slab tendon parallel to the bands is actually included in the design using the depth of the band (Adapt design group does this all the time and has convinced many others to do it).

If it is going to be done as one way, a single tendon in the slab parallel to the beams is useless. You are least need top and bottom reinforcement where the tension stresses are going to be. The cracks will be caused by a combination of bending stress and shrinkage restraint (the thicker band area shrinks significantly less than the slab area inducing tension into the slab and compression into the band area. You will probably eventually find some cracks in the slab at mid-span in the bottom perpendicular to the bands also.

I think that both the slab and the band are too thin. I did a quick RAPT run on the slab and do not believe that RAPT would say it will work either but have had to guess at loads and PT quantities etc. Deflections were still failing with an enormous amount of PT in the slabs. If you can provide me with the full details or the RAPT runs I would be interested to look at them.

 

[hokie66]

I am confident that RAPT did not say that design was OK. Durability, serviceability, perhaps strength...there are lots of problems with that structure. The band beams are not deep enough.

 

[rapt]

AaronPTeng,

One I forgot to mention: Many designers still do not realise that they have to check combined deflection in 2 directions. For a flat slab, column strip in one direction + middle strip in the other, for a beam and slab, beam deflection + slab deflection.

These deflections should be compared with the diagonal span length!

 

[aaronPTeng]

rapt,

Thanks for your comments, These particular designers run with P/A of around 2-2.8 MPa in bands like this (note end spans are 5.4m). Hokie, i am aware that the bands are not deep enough and have noted that from the start, I defiantly do not design in this manner.

I was more interested to see if other engineers have seen this similar type of design as in certain areas of Australia I see it happening more frequently. When I based my design of beams in this case it is more getting the slab to work than ensuring I have a stiff enough beam section for this slab to work in one way action. I was interested in others experience,

rapt do you also see this problem in Australia?

Regards

 

[Ingenuity]

I wonder is this situation, or similar, are becoming more common with the prevailing consulting engineers in Australia stating "PT design, detailing and responsibility by the PT contractor"?

 

[aaronPTeng]

Ingenuity,

I am a consulting engineer but I design the PT also, I have worked for PT contractors where i have been stuck in the above situations, when you explain to the design engineer the above,that the L/250 slab run they send you does not take into consideration the beam deflection and has p/a too high with beams not stiff enough,

Its always easier if the consulting engineer designs the pt in my opinion, it leads to more efficiency rather then simplistic vertical elements to suite the consulting engineer at impact to the slab for the "pt contractor" to work out.

 

[rapt]

aaronPTeng,

No, because RAPT would not do it as far as I can know. No software developer can control what users do with their software. But to get this design out of RAPT you would have to try very hard to not following logical design approaches and to force it to give stupid results.

I have seen similarly bad designs in SE Asia , India and Middle East. They have tended to come from FEM slab packages being used by designers who do not understand what they are doing, especially with regards to deflections. Many concrete designers have never calculated real deflections before, simply relied on L/D ratios. This is allowed by design codes for RC members but can give very inconsistent results, from conservative to very un-conservative. It is not allowed for PT design, so a designer who starts using software to do the design and it gives him a deflection, he accepts it, without understanding what he is getting. With some software, it will be short term uncracked deflection, which for concrete is completely meaningless. For RC members, proper long term deflection is about 6 times short term uncracked. For PT members it is somewhere between 0 and infinity!

Then they start doing PT members and use a multiplier approach (all codes say you cannot, with good reason as the numbers are completely meaningless) but unfortunately some software still uses them. And the results are meaningless. Software provided by at least 3 people (3 different programs) very high in PTI folklore still has this problem! If your permanent load deflection under short term conditions is zero, then multiplying it by a factor to predict long term effects will result in zero. Because shrinkage and creep deflection is not directly related to load deflection, this number is meaningless. The guess of 2 for RC members works for some situations for RC members, even though it can vary from 1.5 to about 5 or 6, depending on your design. You cannot guess for PT members, and PT members often do crack, even if some software does not realise it.

The other problem I find is that people blindly apply the ACI/PTI flat slab logic to slabs, ignoring the differentiation between column and middle strip areas. It is patently wrong for flat plates with no depth changes and UDL loadings. It is completely wrong for flat slabs with drop panels and also band beam and slab structures and transfer structures and results in severely under-designed buildings. Add to this some weird and wonderful tendon layouts based on the average moment logic in this design method, where apparently, according to some, the tendon positions in the slab in the 2 directions is irrelevant, and you end up with some seriously bad designs, some of which do more than deflect and crack too much unfortunately!

But can you blame software for not picking up the inadequacies of its users! To some extent, maybe, but the responsibility has to be on the designer to know what he is doing and to question when told to use illogical design practices or when a computer program is not giving sensible answers. They might be sensible if the designer understood what the software was doing with the numbers given to it!

 

[aaronPTeng]

Rapt,

thanks for the response, maybe it is back to the engineers relying of software to much, However that is not a debate i would like to start as i am sure it has been covered on here enough and that is one where many people differ opinion.


"according to some, the tendon positions in the slab in the 2 directions is irrelevant" also that scares me!
A great deal of posts i read on here shows a very mixed view of PT in the US, maybe due to sick buildings?

I thought a great deal of people in the middle east would be basing designs on either Eurocode or British codes? Or do you find these places in india/middle east etc are using software which is based on the ACI/PTI logic?

 

[hokie66]

aaronPTeng,
Careful with the terminology. The term "sick buildings" normally refers to buildings in which people get sick, due to mold or other contaminants. I don't think deficiencies in PT design and construction, as bad as they are, are directly related to human illness.

 

[rapt]

The BS8110 section on Flat Slabs is a single sentence referring designers to use Technical Literature. This has tended to be PTI information as it is the most readily available plus TR43. TR43 edition 1 was based on ACI/PTI logic as in those days most PT slabs in the UK were unbonded PT and ACI is the only code that really covers it (BS8110 logic in nearly all for bonded PT). Edition 2 covered the column/middle strip approach but still allowed the ACI approach as it had been in the earlier edition and people were used to it, and because it is easier to use, designers still use it.

In Asia, many still base their design on the ACI/PTI logic for flat slabs as it is easier and is now accepted practice as that was how the software they were using did it and seminars put on by PTI people pushed that approach and the buildings did not fall down! Unfortunately they have extended the ACI logic to slabs with drop panels (as the PTI manual did in the past and may still do) and band beam and slab structures as is done (at least until a couple of years ago) by Adapts own design group in USA (and they do a lot of contract design for Middle East consultants and PT companies).

The problem of very thin deflecting slabs is not confined to PT slabs. The trend is getting much worse for RC slabs also and for much the same reason. Computer software that gives short term uncracked deflections as the actual deflection. Most design software based around FEM programs has had this problem, at lest in the past. Some are trying to offer better solutions as time progresses, some ignore the problem, but unfortunately the programs have been used incorrectly for many years so there are a lot of problem buildings out there. But as long as the program lets the user know that the deflection being reported is short term uncracked, it is the responsibility of the designer to understand the implications and allow for it in their design. For instance, if you read the UK Concrete Centre on applicability of FEM deflection results to reinforced concrete member design, they recommend multiplying the results by between 4 and 6, depending whether you want to compare to code L/D ratios or real world deflections!

 

[aaronPTeng]

Rapt,

Thanks you are very knowledgeable and help full

regards,