Bottom Reinforcement in Slab

[tmgczb]

According to Chinese Standard, bottom reinforcement in slab shall be anchored into supports. with an anchorage length not less than 5d, and at least extending to beam centre.
I want to know if in ACI there is some requirement for it.

 

[skewl]

9.7.3.8 in ACI 318 discusses the termination of reinforcement at supports and may be what you are looking for.

Is your support at a simply-supported end (positive moment region) or at an interior support (negative moment region)?

 

[ProgrammingPE]

One-Way Slabs

At simple supports, at least one-third of the maximum positive moment reinforcement shall extend along the slab bottom into the support, except for precast slabs where such reinforcement shall extend at least to the center of the bearing length.

At other supports, at least one-fourth of the maximum positive moment reinforcement shall extend along the slab bottom into the support at least 6 in.

Two-Way Slabs

Positive moment reinforcement shall extend to the edge of slab and have embedment, straight or hooked, at least 6 in. into spandrel beams, columns, or walls

At least two of the column strip bottom bars or wires in each direction shall pass within the region bounded by the longitudinal reinforcement of the column and shall be anchored at exterior supports.

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[tmgczb]

negative moment supports

 

[tmgczb]

Thank you very much.
It seems that 8.7.4.2.2 is applicable to slabs without beams.
For the case shown in my picture, I think 8.7.4.1.2 is applicable, but it doesn't distinguish positive or negative reinforcement.

 

[centondollar]

The anchorage of reinforcement is not dependent on a standard or the sign of the bending moment. You always need to anchor reinforcement beyond the inflection points for bending moment, because forces in reinforcement must be transferred to the concrete through bond (complete bond is assumed in ordinary design) in such a way that a "slipping" of the reinforcement bars does not happen.

Have a look at Eurocode 2, EN1992-1-1, chapter 6.3 and chapter 8. There, anchorage distance demanded by a certain force in longitudinal reinforcement is described as a superposition of the effect of shear force and bending moment ( F_s = M/z + shear) at a chosen point in a beam or slab; there is even a formula which directly presents the required anchorage length as a function of the internal lever arm and angle of cracking. I am sure that ACI 318 or some American textbook have similar provisions that are based on the same principles mentioned above.

 

[tmgczb]

Thank you very much.

There indeed are some functions in ACI 318 for development length of reinforcement.
But here I want to know a detail, about if the bottom reinforcement has to be continuous or could be placed span-separately.

 

[jayrod12]

I guess the question would be why? I don't see why you couldn't do it. Provided that you have the appropriate anchorage beyond the support for each slab bay.

 

[tmgczb]

Is there any clear requirement for splice of temperature-shrinkage reinforcement?

 

[jayrod12]

You having two threads on the same topic is confusing. You asked about bottom steel. Now temperature steel, whereas you have another thread asking about temperature steel. How do you expect to keep anything clear?

 

[tmgczb]

You are right. Because I am working at site now, so I am always eager to learn some conceprts.