Structural theory behind Design Strips in two-way slabs (as per CSI-SAFE)

[canadiancastor]

I've been using CSI SAFE and other software that allows the integration of design moments along a strip of a fixed width for my two-way slab design for over 10 years. Someone has asked me today what theory / reference this practice is based on, and to my surprise I was not able to find anything in CSI references or anywhere else on the internet (apparently NOT the same as the strip design method by Hillerborg!). Does anyone know of a reference that states that it's safe to average out moments over a strip for design found by FEA plate analysis for design? Or at least explain why I'm finding it impossible to find such a reference?

 

[rapt]

It is not practical to design for a varying moment across the width as your reinforcement spacing would be continually varying. Normal allowed logic is to consider 2 strips, column (support) strip and middle strip with the total moment in each strip averaged over the strip. Most codes then have rules requiring a minimum amount of reinforcement over the width of the column in the top and bottom, so this will normally control the reinforcement distribution.

If you have concentrated load effects it is probably better to use thinner strips in those areas so that the concentrated effect is not spread too far.

PT design in USA has come up with the idea of using full panel width. But it also requires that the PT tendons be placed in a "load balanced" layout. So equally spacing in both directions does not work as it is not a load balance configuration. The best way is still column/middle strips in both directions with about a 70/30 distribution. But in USA with unbonded PT, they normally do 100/0 in one direction with 50/50 in the other. Both are load balance configurations, but the 100/0-50/50 option requires significant redistribution to make it work. Normally extra reinforcement is supplied over the supports to assist with this as well as crack control. It should also be supplied in the bottom of the slab.

 

[KootK]

Indeed it is not the same as Hillerborg. The method of which you speak is based on equilibrium and lower bound-ish design principles.

The best reference that I know of from a theoretical perspective is shown below. There are many texts that will tell you how do it, but this is one of the few that goes into detail about why it makes sense to.

 

[JStephen]

Not exactly your application, but this idea is implied in 13.2.6.4 of ACI 318-14.