The design approach, which involves creating slab strips and then designing based on
tabulated code coefficients, is covered in the training manual. Discussion there also extends to the option of using an FE strip. In this section we will look at some comparisons between the two approaches and also look a little closer at the FE modelling options and how these might affect design based on an FE analysis.
For this sort of regular slab it would be considered quite reasonable to use the traditional strip method outlined in the codes. The importance of setting the slab type correctly is noted again, refer back to the training manual if you have any doubts on this.
When you design the strip X1 on this basis the design moments are given in the output shown below.
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Now we can change the strip to an FE Span Strip by updating the slab strip properties – first check the FE Strip option under the General tab then choose the appropriate type of FE strip on the FE tab as shown below.
Now remesh the model and then go into the FE Postprocessor to review the analysis results.
In the view above the moment diagram is determined along the actual cut line, in the table below you can see the peak moments captured from the wider area of slab as indicated on the plan view in the background.
We can compare the hogging and sagging moments in the table above with those reported using the coefficient method earlier. Where we now have a hogging moment of 7.5 kNm, it was 4.6 kNm. Where we now have a sagging moment of 4.8 kNm, it was 4.5. Before
continuing it is worth emphasising that these peak moments indicated above are very close to or exceed the moments determined by the empirical code approach.
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More interestingly, the peak hogging does not occur at the first internal support, at that support the FE analysis gives a moment of 4.4 kNm,whereas a traditional empirical approach gives 5.6 kNm. Traditional idealisations will assume that beams are rigid lines of support, but this support is deflecting. The deflection diagram for the strip cut line can also be viewed as shown below.
The relative beam deflections are obvious and in a stiffness analysis this will clearly have an impact on the design shears and moments. In effect we have built-in modelling of support flexibility in an FE analysis.
Effects of Adjusting the Beam and Slab Stiffnesses
The empirical code method takes no specific account of support flexibility, we can attempt to emulate this by adjusting the relative stiffnesses of the beams and the slabs.
Increase the beam stiffness multiplier to 1.5 (arguably to account for the flanges), and decrease the slab stiffness to 0.01. We would not suggest that such a large adjustment be used in practice.
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In the view below we can see deflection contours and the moments determined on the strip.
These moments are actually more in line with the moments determined by the empirical approach. The hogging moments are more regular but the peak is still a good bit higher than determined by the empirical approach, 7.0 kNm as opposed to 5.6 kNm.
The sagging moments are similar but a little lower than determined by the empirical approach, 3.2 kNm as opposed to 3.4 kNm being the most extreme variation. (Ignoring the angled slab).
The empirical approach is deemed to include for load patterning and moment redistribution.
BS8110 also advises that where the loads are not patterned, there should be a 20%
redistribution of support moments with a resulting increase in sagging moments. In the current model that would mean reducing the 7.0 kNm hogging to 5.7 kNm and increasing the 4.4 kNm sagging (end span) to about 5.0 kNm, and the 3.2 kNm sagging (internal span) to about 4.4 kNm. This gives very good agreement on the hogging moments, but the sagging moments are higher than determined by the empirical approach.
Note that for reasons made much clearer in the chapter on Flat Slab Models, the simple principles of redistribution as traditionally applied to 2D frames do not make sense within the more complex analytical geometry of a 3D FE model.
Returning to the previous model where the beam and slab stiffnesses might be regarded as more reasonable, we did not have the extremely high hogging moments to start with. To some degree by actually accounting for the beam stiffness we have shifted the BMD reducing the hogging moments and increasing the sagging moments. Would it be safe to reduce the hogging moments further? Should the sagging moments be amplified?
It is important that we do not regard the empirical approach as the correct answer, it is one possible method that comes with documented limitations. When we analyse a complete 3D subframe model we are dealing with a good analytical model, and it probably does not make sense to start making lots of adjustments to try and make it’s results fit with those derived from a table of coefficients.
Before accessing the postprocessor you do have the option to apply moment adjustment factors as shown below.
We are unaware of any authoritative texts providing analysis and design guidance relating to FE modelling and analysis. In considering the requirements to adjust the moments we are probably considering serviceability rather than safety issues. This is all discussed in greater depth in the chapter on Flat Slab Models. In that chapter we suggest that you should be very cautious about applying negative adjustment factors of less that 1, it is suggested that you might apply an adjustment to the positive (sagging) moments of up to around 1.2. Once again, we emphasise that this is not the same thing as 20% moment redistribution.
If we return to the original beam and slab stiffness settings you can then review the results after introducing a positive (sagging) moment multiplier as shown above.
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In the view below the hogging moments are unchanged, but the sagging moments are factored up by 1.2.
Orion provides the tools to make such adjustments, but in the absence of any authoritative guidance on the subject we cannot make any definitive suggestions. In this particular example the unaltered moments seem to provide a reasonable set of design forces, when the option of amplifying the sagging moment is used an even safer set is generated. If in doubt we can only suggest that you consider making this sort of adjustment.
Effects of Wood and Armer Moment adjustments on a Regular Slab
Refer back to the section Plots Including Wood and Armer adjustments in this chapter for an introduction to this. Consider the same strip (X1) swap to view the Design Moments, this diagram shows the hogging and sagging moments after applying Wood and Armer
Adjustments. Note that in some places both a hogging and a sagging moment is generated.
In this case the 1.2 factor is still being applied to positive (sagging) moments. Comparing with the previous diagram all the forces increase slightly, but the moment at the right hand end where the support beam is angled to the line of the strip (and to the line of the reinforcement) is more significantly increased.
The adjusted design moments are never (by definition) less than the unadjusted moments.
Designing based on adjusted moments is optional. Comparing the above with the moments calculated based on tabulate code coefficients, you might feel even more justified in not applying an amplification factor of as much as 1.2 to the sagging moments.
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Reinforcement Design
As you exit from the FE postprocessor a dialog is displayed.
The transfer option needs to be checked in order to pass FE results back to Orion for strip design. The interpolation option is of little significance in beam and slab work. It is more important in Flat Slab Models and it is therefore discussed in the chapter Flat Slab Models.
You can choose whether or not to use the design moments including Wood and Armer adjustments.
Back in the main Orion graphical editor, you can select one or several strips. You can then right click and select options to run a check design on any existing bars, or delete the bars and then update the strip to design new bars.
You could for example design strips based on the empirical code method, and then update the slabs to FE Strips and check the steel provided on the basis of FE results.
This may in fact be a very attractive option in many cases. In this example (since minimum steel requirements dominate) the original steel works for the revised moments from FE.
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