Strip Layouts for Mat Footing Designs

[KootK]

I'm working on a partial mat footing. It's partial in the sense that it supports a couple of elevator shafts and a hand full of columns but not the entire building footprint. It's a pretty irregular layout and I'm struggling to set out the design strips in a logical fashion. Visualizing my mat as an upside down suspended slab isn't helping as much as I'd hoped.

Does anyone know where one might find guidance on strip layout for combined/mat footings? ACI 15.4.4.2 deals with reinforcement distribution in rectangular footings but it's not apparent how to apply it in my case. I seem to remember seeing somewhere that the ratio of the maximum moment in a strip to the average moment in a strip should be limited to some value (1.5?). Anybody know of a code clause along those lines?

Thanks,

KootK

 

[slickdeals]

Sketch would help.

 

[KootK]

I'm looking for general guidance on good practice rather advice for my specific situation Slick.

 

[JoshPlumSE]

Are you talking about using hand-calc methods of design strips, or are you trying to use design strips with an FEM solution as a way to bridge the gap between pure analysis results and reinforcment design?

The reason why I ask is because I did some research a few years ago, but I focused mostly on using design strips in conjuction with FEM results.

There is a PTI publication called "Design Fundamentals of Post-Tensioned Concrete Floors" that I found to be a good resource on the topic...even though I was focused on conventional not PT slabs. In particular they looked at couple of different methods to establish design strips. The first based on a tributary width concept which is easy to grasp. The 2nd was focued on a "zero shear transfer" method where you look at the FEM shear contours and see where the shear goes to zero and use those locations to help determine the edge of the your strip. If I recall correctly, they validated this FEM contour method vs the trib width method to show how they form similar strips when used on easily understood layouts. But, I believe it works betters for the more unusual (i.e. non-orthogonal) strip layouts.

 

[KootK]

I'm using FEM this time but I imagine similar principles would apply for hand calculations. I have that book at home. I'll check it out tonight. Because I'm looking at footings, the zero shear concept isn't so straight forward. Often, that method would lead me to have one or two very wide strips, with the bending moments varying rather significantly within those strips.

 

[BAretired]

The Hillerborg Strip method is worth a look.

BA

 

[asixth]

The way that I set out strips on any concrete design are the lines of zero shear. Because it is unlikely that you have a line of zero shear for a continuously supported mat as opposed to a slab spanning onto columns, I would run strip at regular intervals, probably somewhere between 2-4 times the mat depth.

 

[KootK]

Thanks for help guys!

 

[steellion]

If you aren't worried about being a little conservative, use your FEM to determine the region of maximum reinforcement and just apply that bar spacing across the entire footing. For a small area, you probably won't be adding that much steel, and the contractor will appreciate the simplicity.