Slab Bands
Slab Bands
(OP)
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.
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.






RE: Slab Bands
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.
RE: Slab Bands
RE: Slab Bands
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)
RE: Slab Bands
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.
RE: Slab Bands
RE: Slab Bands
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.
RE: Slab Bands
RE: Slab Bands
Do you mean for a flat slab without any drop panels etc (completely flat soffit) or slabs with drop panels and band beams?
RE: Slab Bands
RE: Slab Bands
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.
RE: Slab Bands
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.
RE: Slab Bands
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!
RE: Slab Bands
Bijan's paper on beams does not say that:
http:
RE: Slab Bands
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)!
RE: Slab Bands
Very succinct and informative as always. Worth a star for the shear amount of time typing it if nothing else.
RE: Slab Bands
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.
RE: Slab Bands
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?
RE: Slab Bands
RE: Slab Bands
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.
RE: Slab Bands
I apologize for asking so many questions, but I do really want to try to get a better understanding of how things are done in countries outside the US. I am currently working in India and would love your input on any issues (code/construction related) that you have encountered here.
In the PT flat plates that you design, do you provide reinforcing similar to that of a mild reinforced slab i.e., reinforcing in the column and middle strips in both directions?
How do you provide your cables? I am sure you don't provide the banded/distributed pattern? Do you provide concentrated bands in the column strips and spread out tendons in the middle strips?
If its not too much trouble, can I ask you to post a typical flat plate floor designed in order for me to see how the tendons/reinforcing get laid out?
I presume you carry on the same logic for flat slabs (with drop panels).
I know your answer for band beam systems, you essentially design them as a continuous beam supporting a one way post-tensioned slab.
Thanks for all the help.
RE: Slab Bands
Answering out of order,
2 With bonded PT, I would always use a column/middle strip pattern in both directions. About 70% in the column strip and 30% in the middle strip. I would only use banded/distributed if the column arrangement required it (lined up in 1 direction only).
Unbonded PT (I would never use it) has to be banded as weaving of tendons is very difficult. THis results in a different response to Q1, which is why I reversed the order.
1 C/M strip tendon layout
- over the column - always provide top reinforcement in the column strips, concentrated over the column
- bottom - only if the stresses are high enough to require it for crack control or if there is significant restraint to shortening (multiple cores etc) - AS3600 limit fior this would be .25 root fc. me countalways on a bottom mat of reinforcement.
Banded/distributed layout
- over the column - always provide top reinforcement in the column strips, concentrated over the column (at least ACI minimum but bonded PT area can be subtracted)
- bottom - always provide a bottom mat independant of stress level (ignore ACI .2 root fc limit)
In all of these cases I would ignore the ACI L/6 termination points for the topm and bottom reinforcement. Bottom reinforceemnt should extend to the support.
3 it is too much trouble. I do not design any more so have nothing available. Look at the moment distribution across the slab. That is how the tendons should be layed out, with tendons near to column for shear transfer to the column and then the spacing increasing towards the middle of the bay. Look at the C/M strip layouts in TR43 second edition. It is acceptable to have the column strip tendons equally spaced in that strip width and the middle strip tendons equally spaced in the middle strip with, but I would normally close up the spacing in the column strip at the column to ensure there is a tendon either side of the column close to the column for transfer. Once you do this, it is normally to then increasing spacing towards the centre.
4 Flat slabs with drop panels are exactly the same as flat plates except that the column strip tendons drape to the edge of the drop panels at internal columns (not at edge columns where they still drape to the centre of the column). The middle strip tendons still drape to the centre of the columns. The tendon distributioon in this case will not be 70/30 as the column strip tendons are much more effective than the middle strip ones as they have the extra depth of the drop panel in the negative moment zone. M/BD2 logic as well as load balancing logic will both show this.
RE: Slab Bands
End of 1 C/M should have been
"some countries always insist on a nominal bottom mat of bottom reinforcement, irrespective of stress conditions."
RE: Slab Bands
In Australia, it seems like metal deck forms (Bondek) are used in lieu of traditional formwork. I am looking at an option of being able to use them on a project in India. Columns are on a 11m x 8m grid.
I came across a presentation in PTIA (http:
There is a line that suggests that these bondek systems can be used as a two-way system also.
Does anyone have any additional information on this?
RE: Slab Bands
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Slab Bands
Yes, using it in conjunction with a band beam with bondek spanning the short way.
RE: Slab Bands
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Slab Bands
RE: Slab Bands
11x8m grids, are the 11m spans band beams? Then yes, design as one way but put enough flexural reinforcement over the support in the secondary direction because the structure will naturally want to behave that way.
I heard a saying once when studying strut-tie that the structure will want to behave the way it want's to behave or something similar.
I can post photos of a cracked slab similar to what you described with no reo in the secondary direction, but it's the same photo which got me into troule the first time.
Anyways good to be back.
T-beam for flexure checks.
Also check the beam shear (one way shear) but take the width of the band, it will be ample.
RE: Slab Bands
For example, columns are represented by their true geometries in the BEM, thus no fake singularities around columns in contour results of the slab. Same goes to beams, the actual beam width is a factor. The BEM requires you to place your elements on the boundaries too, no internal meshing, saving time on the effort put into choosing a suitable mesh as done by users of the FEM software.
A software that uses the BEM is the PLPAK. It is a new developed software that supports the analysis of shear deformable plates. Post-tensioning has been added to it around 3 years ago, and the results have been compared to those of the FEM software since then. What I love about the PLPAK is the accuracy it has at solving punching shear, much better than any software I have come across.
There are much more advantages to using the PLPAK in post-tensioning than other software like ADAPT and Ram....check out all the details on its website: http://be4e.com
RE: Slab Bands
You get some 2way action in all beam/slab systems. There is always some stiffness in the slab in the transverse direction. The amount of 2way action you get will depend on the relative stiffneess of the slab to the beam in that direction. Thius can be allowed for in any design as long as the detailing of the system is consistent with this in providing the proper 2way load path. In doing this, the main slab design must allow for a concentration bear the support to allow for the "column strip" created in the other direction as the slab is not considered to be carrying load in both directions, so the transverse load has to be carried to the support by this "column strip".
Once metal decking is added, the transverse slab stiffness will reduce markedly, depending on decking type used and slab thickness. You could easily lose 1/3 of the slab thickness resulting in a stiffness reduction of the cube of this, so lots! So the amount of 2way action would be minimal, and you have complicated the detailing a lot.
I do not think it isd worth the effort!
AndyBWilson,
Your posts would appear to be bordering on the inappropriate! I hope you can justify your comments regarding punching shear solutions. We software developers are not supposed to be "marketing" in out answers!
RE: Slab Bands
Apologies for the mistakes in the last. Fixed below (it is early Monday morning!!)
You get some 2way action in all beam/slab systems. There is always some stiffness in the slab in the transverse direction. The amount of 2way action you get will depend on the relative stiffneess of the slab to the beam in that direction. This can be allowed for in any design as long as the detailing of the system is consistent with this in providing the proper 2way load path.
In doing this, the main slab design must allow for a concentration near the support to allow for the "column strip" created in the other direction as the slab is not considered to be carrying load in both directions, so the transverse load has to be carried to the support by this "column strip".
Once metal decking is added, the transverse slab stiffness will reduce markedly, depending on decking type used and slab thickness. You could easily lose 1/3 of the slab thickness resulting in a stiffness reduction of the cube of this, so lots! So the amount of 2way action would be minimal, and you have complicated the detailing a lot.
I do not think it is worth the effort! Normally it is ignored in beam/slab systems because we want the simplicity of obe way systems to construct. It is even more the case with slabs with metal decking where the benifit is so much less.
AndyBWilson,
Your posts would appear to be bordering on the inappropriate! I hope you can justify your comments regarding punching shear solutions. We software developers are not supposed to be "marketing" in our answers!
RE: Slab Bands
Thank you for the heads up, I really appreciate the support from you since I am new here.
I was just pointing out that the boundary element method can solve the near exact punching shear around any columns in slabs. That is possible because the whole geometry of the column is taken, not represented as a node like in finite element software.
I just think you could have a go at using the PLPAK once and giving me your opinion on it. I believe it is more economical to use it since no singularities appear in the slab region when presenting moment contours and so on.
Most people here are involving the finite element method in their discussion. You could try to consider the boundary element method too since it competes with the previous mentioned method.
If you still see this as inappropriate, please explain. Thank you.
http://be4e.com/ A more realistic approach to structural modeling!
RE: Slab Bands
I will check in more for why boundary element method developers think that punching shear is more accurate using their software and reply as soon as I can.
http://be4e.com/ A more realistic approach to structural modeling!
RE: Slab Bands
I can't see it myself, as far as most codes go the shear design is based on empirical methods, so all you have is the M* and V* applicable to the design. Punching shear in my opinion should always be done by hand. Sorry Rapt that goes for your program too.
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Slab Bands
The codes use empirical methods because the FEM is deficient as you know. I check with the developers and found this:
The shear calculation is ignored in the basic derivation in the finite element analysis, then it is calculated backward using simple equilibrium equations for every element.
However, this is not the case in boundary element formulation, where the shear variables are solved in the boundary and internal solutions directly.
The main difference is that the finite element works on the derivative of the elasticity equations (when u deffrentiate (x2+2x+3) u get (2x+2) so u r loosing terms) , but in boundary element we work on the direct integral form, so all the variables are kept.
RE: Slab Bands
I had the same initial reaction as rapt about some of your posts, as they seemed to be promoting the be4e software. We are naturally suspicious of salesmen. Perhaps you can clarify if you are an employee of that company so we can know how to take your posts. We have quite a few software developers on the site, and their technical input is valued, but full disclosure is necessary.
RE: Slab Bands
Thank you for the feedback, I am not a developer, just a user (4 years until now) and I prefer using the PLPAK because of some things it facilitates, and I have done several practical examples such as foundations and post-tensioned slabs with it. I was asked to specify the reason why punching shear would be better, and I asked the customer support on their website yesterday and replied with the answer I got from them today.
RE: Slab Bands
Agreed. RAPT provides a calculation, but the engineer needs to look at it and possibly expand on it for each case, but that SHOULD be the same for all calculations done by all software!
AndyBWilson
I am very doubtful about your punching shear answer. Punching shear is basically based on the column reaction and moment. Are you/they saying the FEM results for those are that wrong? Sounds like propaganda to me (and I am the last person on this site to defend FEM as most would know). Punching design is normally the application of an empirical code formula to a shear head affected by a moment and a reaction as RE said.
RE: Slab Bands
I still don't see it, you may get better moment distribution, but this may help for punching shear, but again it is an numerical formula that is the basis of the calculation. I know that Mohammed Nazief, et al has published a paper suggesting the moment parameters are better defined than the ACI code, but not the complete punching shear calc as such, personally I thought his paper was very poor on the subject as he didn't include any punching shear test results only did a paper comparison. In his paper, I think he he was relying on a numerical analysis for shear component.
I am not saying that BEM doesn't have it good parts, as you are reducing a integration but don't be fooled into thinking that all the problems with FEA are removed by BEM. The post processor part of the equation is generally where all the errors are hidden. The M*,V* and N* are generally not all that different for all analysis methods (ignoring a few doggy ones form the ACI) if used by an experienced person that has experience with the method.
Rapt,
Agreed, I like to do a separate hand calc check of all answers (a few rules of thumb), but punching shear is the only calc I always do completely by hand.
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: Slab Bands
RE: Slab Bands
RE: Slab Bands
Yes, but the bond properties of deck vary significantly. Some decks have very poor bond and cannot be used in this way. Others are designed to provide relatively good bond with special embossments etc and can be used this way. You need to test the deck. And you need to allow for the developemnt length which is dependent on the bond properties of the deck and much longer than for normal reinforcing. So you cannot just assume normal reinforcing properties.
Eurocodes have a code on this, EN1994-1-1, chapter 6 I think.
RAPT software includes this design procedure. It can be used for any deck type as long as you have test data for that deck type to define the bond properties.
Unless you have very high live loads, decking is normally of no benifit with PT as you already have sufficient +ve moment capacity from the PT. Decking actually hurts as it increases your deflections due to shrinkage restraint effects.
RE: Slab Bands