Top Re-bars in RC BEams

[Sanira]

Hi, is there any requirement in ACI-318 for minimum bars to be used in the top in singly-reinforced beam? I know we use longitudinal rebars at top to hold stirrups. However, I couldn't find any section in ACI for the size of bars to be used. Can we use any size bar or is there any minimum requirement? Thanks.

 

[KootK]

- I know of no codified requirement for top bars in a simple span beam.

- I've been adhering to old fashioned rules of thumb that sometimes show up in the literature that say that, no matter what, top steel should be provided at the ends of beams to provide a negative moment capacity of at least 1/4 to 1/3 the mid-span positive moment capacity.

- I know of no requirement governing the size of top bars other than that I believe that they need to be at least #5 in order to provide instantaneous anchorage of shear stirrups.

 

[Lomarandil]

KootK, if you remember where that #5 limit comes from, I'd love to read up on that. I'd guess that I'm alright since my stirrups are more often #2 than #3-4, but would like to understand the thinking.

----
just call me Lo.

 

[Agent666]

That #5(16mm) limit was in older ACI318 versions, cannot seem to find it's equivalent in 2019 version,but it's probably in there somewhere. But I suspect that in addition to the reason kootk gave that it is simply a bar size that works ok from a detailing perspective and also provides a decent capacity to tie members together without really having to think about things. Smaller sized bars might tend to buckle for example using typical wider stirrup spacings where the top bars have not been specifically designed as compression reinforcement. More empirical based than theoretical I suspect regarding the size.

Sanira, refer to 9.7.3.8 in ACI318-19 for the development limits regarding 1/3 & 1/4 strength limits kootk noted for beams (similar limits exist for slabs as well). Theses rules of thumb are actually codified.

 

[Sanira]

Thanks everyone for your reply.Agent666- Isnt the section 9.7.3.8 in ACI318-19 about the termination of bottom reinforcement? I think KootK is saying saying about the fraction of positive moment we need to use as a negative reinforcement in supports.

 

[KootK]

I think KootK is saying saying about the fraction of positive moment we need to use as a negative reinforcement in supports.

That is definitely what I was getting at.

KootK, if you remember where that #5 limit comes from, I'd love to read up on that.

I may have just made it up somehow. The relevant 318-19 provision is below. The mechanisms in play, as I understand it, are:

1) Sort of a mechanical grabbing of the longitudinal bar by the stirrup hook to improve anchorage. I find this a bit dubious frankly.

2) Limiting transverse beam crack widths in the vicinity of the anchorage and, thus, improving its resistance to splitting.

 

[r13]

KootK and I must have took the same class on this. A slight difference, I usually provide the top face with 1/3, or 1/2 of positive reinforcement with hook at ends. The top hooks will be overlapped with hooks of the bottom steel to protect the open ends in sitting beam situation. #5 will be the minimum size, #6 and larger are preferred for ease of hanging stirrups.

 

[Agent666]

Isnt the section 9.7.3.8 in ACI318-19 about the termination of bottom reinforcement?

It is, but most structures have some continuity and are indeterminate. So along a practical span you'll provide some reinforcement for the max positive moment for example, and as per 9.7.3.8.2 a minimum 1/4 proportion of the positive reinforcement is required to be continuous and becomes essentially the compression face bars in the negative flexural regions. Similarly 9.7.3.8.4 might govern the bars required in the compression face in the positive moment regions.

That was what I was getting at, sorry for not being clearer or explaining.