two way VS one way concrete slab reinforcement 1

[johndeng]

Hi it might sound silly, correct any of my points if I am wrong.
I think two way slab is using about 75% of the loading of one way concrete slab reinforcement calculation. So for both directions, total reinforcement should be at least 1.5 times of the one way slab reinforcement, am I right? If so why do we still use two slab, is it better using only temperature reinforcement on the other direction?

Thanks

 

[driftLimiter]

I don't know how you come to this conclusion.

The flexural demand on the concrete section will depend on the proportions and geometry of the system especially span length in each direction.

If the span length was the same in both directions, the section should experience less moment demand in either direction than it would in a one way span arrangement.

This is my intuitive sense here, you have the same load all attributed to one section in a one-way slab, and in a two-way each direction takes some proportion of that total load.

 

[rapt]

it depends on your definition of a 2 way slab.

Are the supports continuous? or is it a flat slab supported by isolated columns.

For a one way slab, supports are continuous in 1 direction, so the slab only spans 1 way. The continuous support spans the other way, so if that is a beam, it is reinforced for 100% also, so 200% total of the 2 directions.

 

[dik]

If a two way slab with sufficient edge beams, the slab reinforcing can be greatly reduced when compared to a two way slab without edge beams or a one way slab.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[human909]

Hi it might sound silly, correct any of my points if I am wrong.

Intuitively it does sound a little silly. However as somebody who isn't an 'expect' in concrete slab design like possibly other posters above it is a question I've often asked myself.

Normally the crunch point comes in how you are calculating your bending moments and deflections. If you are suitably calculating them then you'll find both are significantly less than the one way span situation and you end up with significantly lower steel reinforcing requirements in each direction along with a thinner slab.

If you don't suitably account for the dual span reduced moments and deflection then you can readily end up with more steel.

 

[andrews2]

What others have said is correct. You reinforcing is dependent on a lot of other factors, and you are neglecting the presence of beams and girders that are necessary in a 1-way slab system.

However I think your question is asking why 2-way slabs are used when they are typically both thicker and more heavily reinforced than their 1-way counterparts, and the answer is labor cost. 2-way slabs use more concrete and steel, but these are cheap (In the US at least) when compared to labor, and the most expensive labor in concrete construction is in formwork. If a contractor can cut down on formwork by not having to form beams every 10-15 feet and just use a single flat form under the entire floor, there can be significant savings even if they are paying more for steel and concrete. Not the mention the decrease in construction time that comes with that.

For concrete construction, I always try to use the rule that more concrete is worth it if it reduces form work. I believe this is the reason many in the construction world prefer 2-way systems whenever they are practical.

 

[Tomfh]

I think a lot of the confusion stems from the fact that we use the term “two way slab” to refer to two very different configurations; The first being slabs which are supported on beams and walls on all sides, which have far more support than one way slabs, and secondly, those slabs supported on columns only, which have far less support than a one way slab.

 

[hokie66]

Another advantage of flat plates and flat slabs is increased headroom for provision of services, resulting in less floor to floor height, which reduces facade cost.

 

[rapt]

In most areas I have practiced,

Flat slab/plate is normally column supported, so the slab is also supplying the support strips to carry the loads to the columns.

Two way slab has continuous supports,either walls or beams.