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Re: Slab bands

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Gail,

I tried to respond to thew original post on this with the following but it never posted so I will repeat it here,

John,

There is no problem with using slab bands as long as all design considerations are met. The FE analysis will give great moment pattern results. The main thing is that the designer must make sure he/she uses them properly.

One Way reinforced System
If the whole slab/band system is detailed as a one way system, as is normally the case, with equally spaced reinforcement in the slab spanning to the band which then carries everything to the columns, then this is different to what has actually been analyzed which is a 2-way system. There will be significant areas of slab with reasonably high moments which have not been reinforced for these moments. These will be
1 in the bottom of the slab and require a large amount of reinforcement parallel to the bands.
2 at the support between and parallel to the bands
3 over the supports perpendicular to the bands. (as a minimum for this reinforcement an extra band of reinforcement should be added over the top of the column in the slab direction to carry the moment created by
the "cantilever" effect of the band from the face of the column as the band is too wide to transfer the load direct to the column as a beam would. This should show up in the FE moments but has probably been "averaged" by the designer over the whole width when it should be concentrated in a width D either side of the column).

Normally only nominal support reinforcement will have been supplied for these 3 areas and in some areas none will be supplied (top parallel to and between the bands). As this is not adequate to carry the moments, these moments redistribute to the "one way system" or load path supplied. In good PT designs this redistribution usually only occurs at loads above the service loads as the stresses at service are limited due to the uplift and axial compression action of the PT.

In RC designs, this cracking and redistribution can occur at service loads so is much more of a worry. This redistribution can lead to uncontrolled cracking and significant increases in deflections above those expected for the system.

Added to this deflection will be the effects of cracking in the slab which most FE programs will not allow for. The basic deflections provided will be grossly inaccurate unless normal cracking has been allowed for in their calculation and will be added to by the redistribution deflection.

As long as the width of the band is greater than 3 times the depth, the shear design of bands is normally done as for beams but ignoring the minimum shear reinforcement requirements until the applied shear is greater than the shear capacity, rather than the beam requirement of .5 shear capacity. You would have to check to see if the FE design program is actually doing a beam/flexure shear design at all, many do not.

Two Way reinforced System
If, however, the reinforcement has been distributed as per the FE moment pattern and both punching shear and beam (flexural shear) have been checked everywhere and the effects of cracking and long term effects have been allowed for in the deflection calculations then there should be no problem.

The whole moral to the story is that you should analyse the system the same way as you are designing it. If the ultimate load path is going to be one way, then the analysis and design need to be "one-way" for the ultimate design. They should not be "averaged" from a "two-way" analysis. Unfortunately, you will need another analysis at service as the crack control and deflection calculations need to be done on the elastic moment pattern which is not one-way, rather than on the ultimate load path analysis.
If you analysis it by FE then you should design it for the results of the FE is the locations where they occur, not "average" them and move then to suit different load paths.

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 99003351504)
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