one-way slab 3

[mikhelson]

Two questions about 1-way slabs.

1) When are 1-way slabs preferable to 2-way slabs? I can only think of the ease of design and perhaps the geometrical requirements of a particular project, i.e. column locations.

2) When calculating live load reduction based on floor (or flat roof) area, do you consider the total floor area or a single span of the one way slab? For example, think of a continuous 1-way slab spanning beams 20' o.c. in the short direction. For the purpose of live load reduction, do you consider a single span of slab (let's say, 20' x 40') or a total floor area consisting of several such spans (e.g. 100' x 80')?

 

[271828]

1. It depends on what system you're using. In some cases, you'll want beams and girders on a grid. Usually, the slab will naturally span between the beams because the beam spacing is much closer than the girder spacing, so it's more of a one-way situation. If you have a flat slab or flat plate, then you obviously have a two-way slab. Of course, there are exceptions and some systems that don't neatly fit into one or the other category.

2. I don't have my ASCE 7 here, but I seem to remember that one can't reduce LL for the design of a one-way slab. Somebody correct me if I'm wrong.

 

[JedClampett]

There are specific instances where it's worth the trouble to use a two way slab. It's a pain to use and there's a certain amount of black magic inherent to the method. You can use a lot less concrete (with higher reinforcement ratios), meaning less shoring, less dead load and more headroom.

 

[jike]

Slab aspect ratio will determine whether it is a one way or two way slab. I typically use 1.5:1 or 2: 1 or greater as a 2 way slab.

 

[SteelPE]

ASCE 7-05

Limitations on One-Way Slabs:

The tributary area, AT, for one-way slabs shall not exceed an area defined by the slab span times a width normal to the span on 1.5 times the slab span.

 

[mikhelson]

SteelPE: thanks a lot.

 

[SteelPE]

Little correction... sorry.

ASCE 7-05

Limitations on One-Way Slabs:

The tributary area, AT, for one-way slabs shall not exceed an area defined by the slab span times a width normal to the span of 1.5 times the slab span

 

[jike]

Oops, I meant 1.5 to 1 or greater for one way slabs.