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# Re: Two way slab designed as one way

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Two way slab designed as one way
• From: "Geoffrey G. Melchor" <melbec(--nospam--at)saipan.com>
• Date: Fri, 09 Oct 1998 21:53:53 -0700

```Alex,

If the ratio of length to width of one slab panel is greater than 2,
most of the load is carried in the short direction to the supporting
beams.  Slab in some cases carried by columns.

One way slab analyse with a unit strip of slab cut out to the supporting
beams with "h" as the depth.  This strip is analyse by the method for
rectangular beam.  Since all the laods must be transmitted to supporting
beams, reinf. should be placed per. to supporting beams.  And bars in
the other direction to control shrinkage and temp. cracking.  And this
analysis assumes Poisson's ratio of 0 and this will also expand
laterally in the opposite direction which is small effect and can be
disregarded because of adjacent slab.  One way slab are normally
designed with tensil ratio below 0.75pb, and this is for reason of
economy.

In two way slab, the larger share of the load is carried in the short
direction (wa/wb = lb^4/la^4)(inversely proportional to the fourth power
of the ratio of spans, although actual behavior is more complex).

Fractions of total W transmitted in two directions: (for a 12x16)
Wa = 0.76
Wb = 0.24
see the difference above and compare Wb with your Wa in one way analysis

Geoffrey Melchor

Alex C. Nacionales 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
>

```