One Way Slab

[WiQi]

Hi,

for one way slab ABCD I have longer dimensions AB and CD, main reinforcement is in shorter direction spanned on the longer direction and temperature/shrinkage reinforcement in longer direction spanned on shorter direction.

how to find the negative reinforcement length above temp/shrkg reinforcement?

 

[Once20036]

Where's the negative moment coming from?
Do you have a one way slab continuous over several supports? Are you considering the ends to be fixed?

I`m a little confused on your description of the bars - It almost sounds like you have 3 layers: short main reinforcing on top of long temp bars on top of more short bars. Is that the case?

 

[WiQi]

@once20036, let me rephrase my problem statement at bottom reinforcement is in both directions.in shorter direction main reinforcement is provided and in perpendicular direction to main reinforcement temp reinf is provided. at top there is negative reinforcement layer above the bottom main reinforcement. in practice there is negative reinforcement above temp/shrinkage reinf so my question is why this reinforcement is provided as main reinforcement has been provided in other direction. there are only two layers one at bottom and other at top. consider any case of one way slab either fixed at two ends and two ends free or all four ends fixed or all four ends continuous.

 

[KootK]

In theory, there is no need for top steel on the edge parallel to your main span. However, any edge loading is going to result in some negative moment there so it's generally good practice to provide some nominal top steel. The Canadian concrete code requires a certain area of steel in these locations but is silent with regard to the length of these bars. I've seen various people do various things:

1) Match the 1/2 length of the main span top steel.
2) Provide bars that are five times the depth of the slab.
3) Bars long enough to meet corner top bar requirements.
4) C15M1200 @ 400 and call it a day.

Another good reason for these bars comes about if there are facade attachments near by.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

If the short side is free to deflect, there is no need for top bars perpendicular to that edge. But if there is a support present, bars should be provided, particularly at the corners, where twisting will occur. If there is continuity over that support, top bars are required to control cracking. In short, it depends on the geometry, loading, and acceptability or otherwise of cracks.