fea for regular two way slabs

[calculor]

Does anybody have any information or case studies comparing fea to equivalent frame analysis of two way slabs. Are there any significant savings to warrant this type of analysis?

 

[dik]

I'm not sure about FEA methods, but about 40 years ago, Per Christoffersen did some plastic model studies and the results were similar, except the distribution of the reinforcing in the top mats could be improved. In addition, generally the middle strip top reinforcing could be redistributed to the top mats, simplifying the placement of reinforcing.

Dik

 

[calculor]

I'm using hillerborg's method on most of our slabs; which results in some savings but the usefulness is mostly with unusal type slabs. However we are beginning to see more clients indicating other firms are offering fea as well as significant savings in reinforcment as well as slab thickness. In fact I have recently saw a 7 inch slab used with 21 foot column spacing. I have a very limited background with fem but I cannot see any savings that would warrant the fees to complete an analysis of this magitude for a regular type slab.

 

[AllanBommer]

For very regular structures, the two analysis types will result in very similar design forces/moments and hence there will be little difference in the design.

For irregular structures, FEA will "find" a more efficient load path, as the effective strip width guesses in EFM will tend to be conservative (or if the engineer is trying to be aggressive, perhaps unconservative). Here, FEA will lead to a more efficient design (and one that is still safe).

Most post-tensioning suppliers in North America use FEA, presumable because it results in more efficient PT designs.

-Allan Bommer, PE
Bentley Systems, Inc.

 

[AllanBommer]

I cannot see any savings that would warrant the fees to complete an analysis of this magitude for a regular type slab.

The FEA design tools have improved significantly in the last ten years.

For regular structures equivalent frame design can still be faster than FEA-based design (especially if you can get away with one strip in each direction), but once the structure starts to become less regular FEA-based design starts to shine.

There are multiple vendors (including my employer) who sell FEA-based slab design tools that are practical for use for all your office slab design.

-Allan Bommer, PE
Bentley Systems, Inc.

 

[rapt]

Allan,

As you know I would agree entirely with your first comment in your first reply.

Regarding more efficient load paths generated by FEM analysis, yes this is also correct. The important thing that is often missed by designers however is that the slab must be reinforced in a manner consistent with the load path generated by the FEM analysis. Reinforcement must be supplied in the slab to match the pattern of the bending momets, not based on averages etc.
If the reinforcement pattern does not match the FEM bending moments, redistribution will occur which could reduce the accuracy of the original analysis and load path. Obviously, minor variations to make a slab constructable are ok but the more irregular the slab, the more important it is to follow the FEM load path and this is dependent on the user knowing what he is doing, the software cannot do it for you.

Calculor

RE Thin slabs by FEM,
Designers must be aware of the calculations that are being done by the FEM software and the relative accuracy of the results. REM analysis does not mean that concrete design results reported are accurate. The "design" is a separate addon after the analysis.

For example, if you do a flat slab design in most (not all) FEM programs, the deflections reported will normally be based on uncracked sections and will not allow for long term effects. Some programs may apply a long term factor and some may attempt to fudge cracking effects and a couple actually try to allow for cracking and long term effects.
If the program is reporting short term uncracked deflections, the real deflection could be 6 to 10 times as large as this value.

The designer must be aware of what the software (not only FEM based) is doing and what its results really mean and how they should be modified to be used in design decisions.

I know of many cases where the designer didi not reralize this resulting in slabs with excessive deflections.

 

[dik]

Alan: for PT work, a properly run FEM can be very accurate because most FEM's work with elastic materials and post-tensioning causes a slab to behave in a very elastic manner. In addition, concrete for post tensioning tends to have a higher strength and higher strength concretes tend to behave more elastically.

Dik

 

[rapt]

dik,

A large number of the PT slabs designed and built around the world are designed to be cracked at service loading, at least at the critical sections. So they do not behave in a very elastic manner!

 

[patswfc]

There are no big savings to be had when using FEM for regular, simple slab design.

Its merely a tool to calculate design forces. The basic design principles dont change just because you use FEM.

It does seem to be used more often and can be very useful and provide some savings in material when used to design irregular shaped slab or slab with openings. The savings are due to the FEM being able to calculate the design forces more accurately than what we can achieve by hand, due to conservative design assumptions that we have to make when analysing irregular slabs.

As mentioned already, be very wary of deflections calculated using FEM.

 

[calculor]

Okay, for irregular slabs how does fem compare with plastic analysis. Based on a number of papers I have read it still seems fem is not the way go, other than doing a nonlinear fem.

 

[dik]

sorry rapt...
relative to regular reinforced concrete PT slabs behave more elastically... didn't mean to imply that they are perfectly elastic... Measurements of office building loads indicate that actual live loading over more than 200 sq ft is approximately 20% the design loading. Realistic flexural tensile strength for concrete slabs can be estimated to be 8*SQRT(f'c). At realistic service loads, the slab is likely/mostly uncracked.

Calculor...
If the slab is reinforced based on an elastic solution with no redundancy... then the plastic analysis would fail at the same load as the elastic analysis... When I mentioned a plastic model, the floor plate was actually constructed from an acryllic material, loaded and the curvatures measured.

Dik

 

[rapt]

dik,

No problem.

1 the designer is commissioned to design a slab for a specific loading. Also, as soon as they start putting light weight compactus at mid span as most government offices do (in Australia anyway) then you are getting real design loads similar to the required ones and permanent live loads higher than the code assumptions.

2 In deflection design, the concrete is assumed to have a tensile strength equivalent to this (in our software anyway). But under sustained load this tensile strength drops off fairly quickly.

Calculor
There are some high end FEM programs around that purport to to be able to allow for non linear concrete properties, cracking, shrinkage, creep and reinforcement. Using one of these, you may be able to do a plastic design. But you will not get one on a consulting engineers budget.
I know of someone who wrote a simple beam program to do this, takes hours to run for a 2 span beam with simple loads.