Slab Bands 3

[slickdeals]

I know this item has been discussed before in this forum, but I wanted to ask a few more questions (specifically with how Ram Concept analyzes it).

I have a grid of 11m x 8m with some really heavy loads. I am using a 300mm thick slab with a 500mm x 2700mm slab band (in the 11m direction).

According to PTI and Bijan Aalami's recommendations, as long as the thickness of the band is <= 2t and width >= 3*overall thickness of band, then the behavior generally remains two way.

My question pertains to the design strip in the 11m direction. Ram Concept has the option of choosing either full width or Code T-beam for these strips.

For one way shear checks, the program is using only the width of the 2.7m wide band. Is this appropriate or should the one way shear check be based on a 8m wide strip?

For flexure checks, I think (conservatively) the band beam should be analyzed as a T-Beam and not a two-way slab.

Any other thoughts/suggestions are welcome.

 

[csd72]

slick,

Where i come form these are called band beams and the practice is a little different. I have always analysed these as one way T-beams supporting one way slabs.

Not saying you are wrong just making it clear where I am coming from.

Now the beam is much stiffer than the adjacent slab so any shear will be distributed in accordance with stiffness and the beam will take at least 90% - might as well say 100% as your program assumes.

You need to also check punching shear for a band this wide.

Personally I would do similar for flexure but if you insist on doing it as a two way then I would use the T-beam as the column strip and be slabs as the half middle strips. In the other direction design as you usually would for a flat plate with the beams acting as drop panels.

 

[hokie66]

I just design these as one way in each direction, with the width of the band used for both shear and flexure. Sure, you get some two way action, but that is just a bonus.

 

[slickdeals]

Do you normally have stirrups in the band beams? I am using the Indian Code (IS 456 and IS 1343). Sadly, they are very outdated. Yet, the local engineers want to stick by it.

What is the average precompression that you provide in these band beams?

If punching shear does not work based on the band width, do you make it thicker or resort to punching shear reinforcing? If you resort to punching shear reinforcing, then do you treat it similar to a drop panel (ie zone having 2 thicknesses)

 

[hokie66]

Band beams commonly do not require stirrups, except for nominal supports for the top bars, but if your loads are heavy, stirrups may be needed. Just take it case by case. Bands can normally (in most codes, I believe) be considered as one way slabs, so the minimum shear reinforcement is less than for beams.

I don't know that there is an average precompression. Band beams are used both post-tensioned and without tensioning, so there is a wide range.

Punching shear is not controlled by the band width, but rather by the depth. I hardly ever consider using punching shear reinforcement. Just make the band deeper, or drop the section near the column.

 

[csd72]

It has been shown by testing that the shear ties can be less than fully effective if the legs are spaced too widely apart.

 

[rapt]

Slickdeals,

Are you asking about analysis or design. You have said analysis, but the questions would appear to be design ones!

Regarding Analysis, most specialist PT designers I know outside USA would not analyse a floor with beams in Finite Element software. Basically because you are assuming a one way system while the software is analysing the complete floor as it theoretically acts as a plate with some stiff elements, but you as a designer do not want to reinforce it that way. The other comment I keep getting from designers on this is that the torsional stiffness effects they get from such software in these cases is compeletely unreasonable and they seem to get some unexplainable results!
Make sure you turn on the switch in Ram Concept to include Mxy momnets in design (it is off by default so they care calculated in the analysis but ignored in design). This is especially important in irregular grids and with section changes!

Regarding Design.
Yes, there will be two way action, there always is in design incorporating beam and slab and band beam and slab. Elastically, that is how the moments will distribute, relative to the stiffnesses of the different areas of the floor. However, as a designer you are reinforcing it as a one way slab and beam/band system.
This is where definitions get blurred.
PTI/Alami/ACI318 allow you to design a two way (flat) slab assuming a moment applied on a full panel width with no thought to column/middle strips, as long as a consistent load path is supplied by the reinforcement/tendons eg banded/distributed tendons. This works for ultimate strength, as long as the loads are relatively low stresses, the loads are uniform and the concrete depth does not vary (no band beams, drop panels etc). Unfortunately, they also allow this logic for service design (crack control, and deflection). In these design areas it does not work, the same as yield line design only satisfies untilate conditions, not service limit states (this ACI flat slab method is actually yield line by subterfuge)!
Unfortunately, PTI and Aalami and some other USA "experts" use this ACI average moment logic for slabs with drop panels and band beams. There is it completely illogical.
Yes, there is some moment is the slab area parallel to the band beams. But no, you cannot therefore assume that the full width of the slab can be included as part of the beam and that any reinforceemnt/tendons in the slab area can be included in the Tshapes section capacity using a depth equal to the depth of the band beam. Similarly with Drop Panels, this assumption cannot be made in support areas (I know of many companies in Asia/India currently doing this but they are wrong!).
So after a long explanation above, the beam should be designed as a T section with limited flange width (no, it is not conservative, the reverse is the case, the USA method is unconservative and results in gross underdesign). Any reinforceemnt/tendons in the slab parallel to the bands should not be included in the bands. If you weant to go to the trouble, any moment in the slab (someone above suggested this might be 10% but it will depend ont he design) could be resisted y reinforcement in the slab based on the depth of the slab only, but then you must allow for the concentration of moment in the other direction near the support (like a column strip).
The bean beam must be designed for shear (any slab should also), but wide flat beams really do not nned to be limited to the normal beam shear minimum shear rules, the slab rules would apply (AS3600 and ACI cover this, BS8110 deos not and the Indoan code has followed BS code on this). I would normally have shear ties for the first couple of metres from the column face even if they are not needed then only if needed by calculation and also to support transverse reinfrocement and tendons.

Punching shear needs to be checked as the band is so wide that punching can occur.

 

[haynewp]

Can you point to references where bijan/ACI/USAexperts are using an entire slab width to resist bending?

 

[rapt]

Haynewp,

Do you mean for a flat slab without any drop panels etc (completely flat soffit) or slabs with drop panels and band beams?

 

[haynewp]

Flat slabs since I think you have mentioned it before, but bands too since this is what the thread is about.

 

[rapt]

Haynewp,

For Flat slabs, it is in ACI code, PTI manual, all Adapt manuals and any lecture material by Aalami and Adapt 2D and 3D software does it.

For drop panels , there used to be an example in the PTI manual that did this, I assume it is still there. Also, Adapt software does it this way for 2D and is the default method for their 3D FEM software.

For Band beams, Adapt design group were recommending and using this design logic, as well it was in lectures by Aalami and in their manuals. There has also been at least one discussion on it on the Adapt and PTI discussion forums several years ago. There was also a discussion with Ken Bondy on it on the PTI forum several years ago where he was suggesting that even for beams (not band beams) with heavy longitudinal brick loads on edge beams, that the full slab width and all tendons in it be used as the flange of the T beam.

 

[haynewp]

I will have to check my PTI and Adapt literature when I get to the office, I did not recall this was the recommendation for bending. I remember Ken talking about that on the old PTI forum but do not recall if he recommended the entire width in that case, this was about 9 years ago. I think there is an option in Adapt to do this but it was not mandatory.

It would seem there would have been problems if making such an assumption during their careers if that was the case. I don't see how assuming a 60 ft width of slab resists bending when the slab is only 45 ft span (for example) shouldn't have been an issue for them over all those buildings.

 

[rapt]

Haynewp,

Yes, the PTI discussion with Ken was 9-10 years ago and he was definitely using the panel width (half panel for an edge beam). I can be certain about it as I was the person on the other end of the "discussion".

In Adapt PT for drop panels it is mandatory, as they offer no column/middle strip solution. For beams, Bijan answered to several queries on their forum that it should be done that way. And Adapt PT used to do it by default as I understand Adapt Floor still does (or at least its own design office does when doing designs for others).

I know for certain that it is being done widely in Asia with drop panel slabs. If you do the calculations, in general they are under strength by about 17% on a normal office slab. This is not enough to cause "problems". Just provides the client with an under strength end product!

 

[haynewp]

I remember the discussion with Ken.

Bijan's paper on beams does not say that:

http://www.adaptsoft.com/resources/ADAPT_T901_Effective-Width-PT-beamsr.pdf

 

[rapt]

Heynewp,

No, it doesn't inh this doucment, but it does not nominate how to calculate the width either.

But hs is definitely in print in other documents/discussions saying to use the full width (no I do not have any copies as they were rubbish so I binned them)!

 

[csd72]

I cannot find anything in rapts comments that I disagree with.

Very succinct and informative as always. Worth a star for the shear amount of time typing it if nothing else.

 

[haynewp]

It looks like he used 16xthickness of the slab in the calc.

Back to to the flat slabs, I think PTI/ACI/Bijan would have tested using the entire slab width for bending sometime along the way and it would have shown this understrength. I am not saying I disagree with you(rapt) so don't have a hernia. I do wish someone from those groups would say something in their defense if that is their philosophy. Sorry to the OP for getting sidetracked.

 

[slickdeals]

Regarding shear reinforcing in the band beam system:

Provided the band beam is not thick enough to predicate a one way behavior (i.e. sticking to the PTI limits), then can the one way shear capacity be calculated based on the tributary width rather than the width of the band beam?

Is there any good literature on band beam + slab systems?

 

[haynewp]

I don't know what the references state but logically the band will still be stiffer than the slab meters away from it and should be treated like a beam for shear and not use a trib width to resist shear.

 

[rapt]

On shear I agree with Haynewp, carried by the beam. That is how you are forcing the whole thing to work by reinforcing it as one way. So that is how you have to design for shear.

RE Flat slabs (No sign of a hernia yet, just get frustrated with bad engineering sometimes! And there is some very bad engineering involved in this logic), there was a test done in Austin Texas in the 1960's that all of this is based on. But the tests were never completed as I understand it due to a failure of testing equipment (If Ingenuity is still around on this site, he has researched this and could clarify it for us). They basically showed that the collapse load of the slab will be ok, and that is expected based on yield line theory. The problems are
- serviceability is not ok as service cracking is dependent on the elastic stress pattern. Collapse is ok because everything redistributes to the load paths you have provided, so as long as you provide a logical load path with sufficient strength it will not collapse. But to get to this load path from the elastic one you require cracking, deflection and redistribution. You need to have your reinforcement where the elastic stresses are going to predict the cracks, not where your selected load path has it to control these cracks. And you get extra deflection.
For example, imagine the simpliest version of this, a 2 span beam. You can make it stand up by reinforcing +ve moment only to carry wl2/8 in each span, with no top reinforcement. But you get a really big crack at the support and your deflection is about 2.5 times higher. You might think it is only a small crack! If the span is 10m and the depth 500mm and you allow a deflection of L/250, the crack width is about 16mm (If I remember correctly from a calculation I did several years ago). To restrain this crack, you need the amount of reinforcement you would have originally needed to carry the continuous moment, because that is the moment causing the crack. So you end up having to reinforce the top for wl2/8. So you have wasted bottom reinforcement!
- it does not work with band beams and drop panels. The stress you can generate in any reinforceemnt is dependant on the section depth that it experiences, it cannot be dependant on the depth 5m away.
- where you have copncentrated loads, you have to provide a logical load path for those loads. You cannot average their effects over with areas without getting thosde loads to those areas first.