the width of the strips 1

[mostsdemand]

hi all engineers i urgently need to know about the momment contour method that i shall use it to determine the width of the strips if i want to determine the width of the strips of longitudenal bars in yy direction i shall determine the width of the perpindicular contour of the momment in x direction and viceversa as i made in shear contour method i will be so thakful if to reply with an attachement as much as possible

 

[JoshPlumSE]

We're talking about RISAFloor ES, correct? Much of the ACI requirements based on support lines and such will be handled automatically. Though, certainly there are times where you might want to override the program calculations based on these contours.

Since so much of RISAFloor ES is automated, the help file doesn't give as much guidance in how this could be done manually. But, the RISAFoundation help file (which currently requires users to set their strip widths) has a good write-up on the subject:
Setting the Width of Design Strips
One of the most important design considerations is how wide to set the Design Strip. If the width is set too large, then the program will average out the moments and shears over too wide of a region. This would result in unconservative design moments and shears. Similarly, if the strip is set too small, then the effect of stress risers in the FEM analysis will be over estimated and the design will be over conservative.

The setting of the design strip widths is truly a matter of engineering judgment. RISA Technologies, LLC makes no endorsement on what methods would be most appropriate.

ACI Definition of Strips (Section 13.2 of ACI 318-11)
This section of the ACI code is really intended for elevated slabs. But, the concepts can be extended into mat foundations as well. The requirement for "column strips" is that the width on each side should be set to 25% of the span length or width whichever is smaller. Then the "middle strip" is defined to span between the edges of the column strips.

This method requires engineering judgment for column grids that are not perfectly aligned and rectangular. In addition, when the column strip becomes very small then the middle strip may become very wide so that the entire slab is included in either a column strip or a middle strip.

The ACI strip method listed above is based on essentially 1/2 of the mid-span tributary lines. The hand calculation methods would have you design for the full tributary moments over this smaller width which should be conservative. Computer methods (like RISAFoundation) will design for the average moment over the assumed design width which should result in a more efficient design.

Zero Shear Transfer Method
The Zero Shear Transfer method used the shear force contours perpendicular to the span of the slab to set the design width. This should provide a result very similar to using the mid-span tributary lines, but is a bit more theoretically derived for non-rectangular column layouts. This method is described in greater detail in the PTI publication Design Fundamentals of Post-Tensioned Concrete Floors. Ideally, this method should give design strips of similar width to the ACI strip method. However, it is more rationally derived and should work better for cases where uneven column spacing makes the strip method difficult to apply.

Zero Moment Method
In a similar fashion to the zero shear transfer method, the Zero Moment method uses the moment contours to identify where the moment changes sign. This can be used to set the design strip width approximately equal to the distance between zero moments.

Shear Perimeter Method
Another basis would be to set the design width equal to the pedestal width plus a distance 'd' or 'd/2' on each side. This will end up being a more conservative assumption for flexure than the other methods listed. As such, it would be more appropriate for situations where shear or punching shear failures are a primary concern. Examples would also include cases where the pedestal is very large such as for a vertical vessel or grain silo. This is similar, though not identical, to a method given in the NEHRP document GCR 12-917-22 (Seismic Design of Reinforced Concrete Mat Foundations).

Hybrid Method / Engineering Judgment
A variation on these methods would be to start off setting the column strip using the ACI strip method. Then, if necessary, the width could be modified based on considering the other methods. This is especially true for situations where the column grid is not aligned or rectangular.

In addition, when the middle strip widths get too large, they could be set to values closer to the column strip width. The middle strip would normally be centered on the area with the highest mid-span moments. This would neglect lower moment regions between the column and middle strips. Hence the strips would designed for a higher moment per unit width. This reinforcement could then be extended into the lower moment regions between strips. Or the user could set up another design strip for these lower moment regions.

 

[JoshPlumSE]

I gather from your post that you're trying to use the "Zero shear transfer method" as described in my previous post. Or, some method very similar to it. The best reference I can give you for that method is that PTI publication. I don't think we ever wrote up a blog topic with good graphic images explaining the concept in detail. Therefore, I would suggest obtaining a copy of that document.

If you're trying to test yourself and your ability to apply it in a complex situation, then I would suggest creating a couple of very simple models with rectangular layouts. And, verify that the contours you're looking at would give you essentially the same design strip widths as the ACI method. Then start making the model a bit less rectangular or evenly laid out.

 

[mostsdemand]

i have made this for a rectangular layout and made sure about the methode but i stil have aproblem to use this method as my country most of enginees uses another method wich is too conservative so ican't use this metod without using any other method giving the same result such as momment method to confirm and i try to search the pti publication on goagle.

 

[JoshPlumSE]

See the link below to a site where you can purchase that PTI publication:
Design Fundamentals of Post-Tensioned Concrete Floors

To me, design strip widths are inherently a matter of engineering judgment. There's guidance you can give of course. But, two reasonable engineers can arrive at different conclusions.

I can't give you much advice other than to obtain a copy of that document and point to that as an authoritative reference on the subject as justification for why that method may be followed for setting strip widths.