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Re: Two way slab designed as one way

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Alex,

You can design this 2-way slab as a 1-way slab.  Even if you leave out all
reinforcing in the 16 ft direction, which is not advisable, the slab will
support the design load and then some.  If it is detailed properly, it
should theoretically reach the collapse load for a 1-way flexural member as
the cracks develop in the short direction and cause the slab to behave as a
series of independent strips spanning in the short direction.  As long as
you provide a continuous load path where there are no weak links, the
structure will eventually carry the load the way you envisioned (designed)
it.  

The problem is, you will have undesirable cracks at service load conditions
because a 1-way design for this slab is 'forcing' it to behave in an
unnatural way.  A more efficient approach (least amount of reinforcing
steel) is to use the two way design.  But, I have to believe that in the
days before computers, many 2-way slabs of similar proportions were
designed as one-way members.  Although there may have been many problems
related to servicability with these 1-way designs, I don't think there's a
general concern about the adeaquacy of the strength.

You can read a technical discussion on this topic by Park and Gamble,
REINFORCED CONCRETE SLABS where they disucss the differences between lower
bound and upper bound solutions and a theoretical dicussion on achieving
the most efficient design.  The most efficient design would be to provide
reinforcing to match the elastic plate solution which is obviously not
practical for construction.  Park and Gamble also discuss the lower bound,
equilibrium based 'Strip Method' in Chapter 6 which the 1-way design
approach for your example can be classified as being.

Walt Sawruk
EQE International, Inc.
Shillington, PA



At 07:31 AM 10/10/1998 +0800, you wrote:
>
>I would like your opinion on designing two way slabs as one way.
>
>Ex.   12 ft x   16 ft  slab designed as one way  on the 12' direction.
>
>My opinion is that it  is possible but it would be more economical to design
>it as two way slab.
>
>
>Alex C. Nacionales
>A.C. Nacionales Construction
>
>