Design Strip - Critique

[slickdeals]

Folks,
See attached word document that shows the design strips for a irregular slab. I would appreciate your feedback on the adequacy of the design strips to capture/design for the forces on the slab.

 

[ishvaaag]

Use FEM with some reliable program and forget trying to put something where it doesn't fit. Many empires and businesses fall through outreaching too much, and the use of inadequate tools is the structural equivalent of such lack of insight.

 

[slickdeals]

I am not sure where you are going with that statement. The slab is being modeled in Ram Concept. Owing to the irregular geometry of the support layout, I wanted a review of the design strips that I have modeled.

 

[asixth]

Looks like you already have the ram model set up. Run the analysis without defining strips. Plot the moment contours and deflection contours and then define your strips and possibly run some bands where required. Be careful with ram because it will take Mxy out of the design.

 

[rowingengineer]

I once wrote:

"Design Moment distribution (not redistribution)
Points of high amounts of reinforcement with show up on contour plots often, these should be distributed as column and middle strips. This is due to the micro cracking reliving the slab at the support locals to the surrounding areas. There is a temptation to provided reinforcement to resist the peak moments, this should be avoided. This means design strips and sections must be defined for the serviceability and strength checks. The advantage of FEA is that design strips can be defined after the slab has been modelled. Design strips can be defined by code definitions or points of zero shear, engineering judgment should ensure that the deign strip is designed for load acting on it. The points of zero shears is especially useful for complex geometries, defining the strips based on FEA result should result in more economical reinforcement for complex support situations refer Concrete Society report TR43 (2).

Most computer programs will report moment and reinforcement in contours, the reinforcement and moments should be distributed across the column and middle strips as appropriate keeping in mine all detailing requirements of the code. For FEM it is recommended that bending moment be taken at the centre of the column. This is because of the uncertainties in the modelling with regards the column. A useful rule of thumb for verifying the results is that top reinforcement in the column strip will be in the order of twice the area of the bottom reinforcement (i.e. not the same as, or 4 times as much as, the bottom reinforcement)."




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

 

[ishvaaag]

The statement above is along the lines of axisth; you can proceed from Mx, My and Mxy or principal stresses for longitudinal reinforcements; shear stresses may be a more trickier if the program is not doing something about with the correct contours, but you will have still the shear passed along floors and the local shear stresses with which to proceed. Certainly I wouldn't go with bandings of such irrational aspect as given in your pdf, whichever the program producing it.

 

[JoshPlumSE]

There are a number of different ways to establish your design strips for irregular supports:

1) ACI strip method... approximately. Obviously, the strip method doesn't work perfectly for irregular supports. But, it appears that this is approximately what you have done. This support layout is particularly tricky, do I can easily see your desire to seek other opinions.

2) Zero Shear Transfer Method: If you plot the shear force contours for the slabs (if Ram allows this), then you should see the contour lines which correspond to zero shear force in the slab. If you look at these zero shear locations as being equivalent to the "mid-points" of rectangular support layout, then you can use the ACI method to establish column and middle strips.

For rectangular slabs, this should provide results very similar to the ACI definition for a perfectly rectangular layout, but is more theoreticallly derived. Thus it can be extended out into more complex support arrangements. I believe the Adapt guys discuss this in a PTI publication called "Design Fundamentals of Post-Tensioned Concrete Floors".

3) Zero Moment Method: I've used a variation on the zero shear transfer method where I look at the change in sign of the moment contours to see approximately where to switch from column strip to middle strip.

Usually, I use the two contours (moment and shear) together to establish the strips. But, in the end it ends up being "engineering judgement" based on the FEM output. Though I should point out that I usually end up being a bit extra conservative and shortening the width of the design strip a bit around the maximum moment locations.

 

[dik]

I think that the strip method, with a little judicious care, can give as 'correct' a solution as a FEM approach. In the y-direction, would consider the slab fixed at the ends of the shear walls and increase the tributary width to reflect this. In addition, at the ends of the walls I would concentrate some of the negative reinforcing at the wall due to the 'knife edge' type of support. In addition, I would put concentrated positive reinforcing in the same area, but to resist/minimise cracking from the high concentration of shear. An FEM model will likely bear the above out. The trickier parts are the torsion in the intersecting beam and the non-colinear walls at the end... but, they can be accommodated in a similar fashion.

Other than that, it looks like a flat slab or sorts... Reinforcing may not be optimal, but, there's a bit of a price to pay for an irregular shaped floor slab.

FEM will give you a bit of an understanding in how the slab will behave and/or deflect, but, it will not necessarily give you a more precise solution... it will show you where you have high degrees of fixity that should be accommodated. With the 'knife edge' conditions, this fixity will drop off a bit as the slab cracks.

My 2c, Dik

 

[slickdeals]

With the way things get done in India, the slab gets designed by a PT supplier. However, I had my own model created to understand how this slab would want to behave and for me to validate their analysis and results.

It is a flat slab, but it has supports around the perimeter because of a large slab depression for balconies/viewing decks.

I will post the zero shear plots, but I have to read through the internal workings to see how these plots work because of the 45 degree angle of this slab relative to X & Y axis.