## strip method for two-way slabs

## strip method for two-way slabs

(OP)

The strip method (by Hillerborg and Wood & Armer) seems like the most convenient way to analyze a two way slab with irregular dimensions or boundary conditions (aside from FEM modeling).

I have a few questions about the method for those who use it in their practice:

Does ACI 318 allow strip method for analysis? The two other methods -- DDM and equivalent frame seem too restrictive for irregular slabs.

I find the strip method convenient, but it requires that one makes seemingly arbitrary decisions about apportioning loading and moment between supports (e.g. fixed on one side, strong band on the other). How do you justify dividing the total moment between negative and positive (usually a ratio of 1.5 to 2.5 seems to be used) moments?

Thanks

I have a few questions about the method for those who use it in their practice:

Does ACI 318 allow strip method for analysis? The two other methods -- DDM and equivalent frame seem too restrictive for irregular slabs.

I find the strip method convenient, but it requires that one makes seemingly arbitrary decisions about apportioning loading and moment between supports (e.g. fixed on one side, strong band on the other). How do you justify dividing the total moment between negative and positive (usually a ratio of 1.5 to 2.5 seems to be used) moments?

Thanks

## RE: strip method for two-way slabs

Mike McCann

MMC Engineering

## RE: strip method for two-way slabs

No, I mean reinforced concrete.

## RE: strip method for two-way slabs

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## RE: strip method for two-way slabs

## RE: strip method for two-way slabs

## RE: strip method for two-way slabs

Every time I am designing a slab which isn't regular, I always perform some form of finite element analysis just to make sure I have made a valid assumption of how the structure is going to behave. Sometimes it can be very difficult to make correct assumptions on the stiffness of load paths, particularly when dealing with a bi-dimensional element such as a slab.

## RE: strip method for two-way slabs

Do you only use the moments from your FEM slab models or do you also accept shear and torsion values? From my experience, people tend to derive the shears for a plate from tabulated coefficients or from some other hand method, rather than using, say, SAP2000 values. And yes, Roark's is another source for plate stresses.

Thanks

## RE: strip method for two-way slabs

## RE: strip method for two-way slabs

I have similar concerns however:

1) How to safely apportion the span / support moments. This is straight forward for slabs with fixed / simple boundary conditions (Moody's charts help here). However, for the much more common case of partial fixity or back-span slabs, I'm not at all clear on how to assign the moments.

2) To my knowledge, Hillerborg doesn't do anything to address rebar cut-off locations. I've been developing moment diagrams based on the "assumed" support moments and calculating my bar lengths that way.

It's a shame that the method hasn't been developed further as it seems as though it could be quite powerful. I suspect that the strip method has been largely ignored by codes and designers for two reasons:

1) It doesn't address deflections in any direct way and;

2) The method isn't easily adapted to computer applications.