ACI 318 - Two-way Flat Slab Detailing

[RandyM81]

Hello,

I've run into a debate on permitted splice locations per ACI 318 figure 13.3.8. For a slab without drop panels the figures show that splices are permitted in the column strips between an interior support out to the 0.30ln span location. Does this region apply to both top reinforcing and the bottom reinforcing, or does it apply only to the continuous bottom reinforcing?

Your opinions and any references would be much appreciated.

Thanks,

Randy

 

[hokie66]

Bottom continuous bars only. The diagram seems self explanatory to me.

 

[RandyM81]

Maybe I'm reading to much into it, but it seems like you can interpret the region as all of the reinforcing between the two dashed lines, top and bottom. No?

As a follow up, if the diagram permits splicing of the bottom reinforcing only at the support, is splicing of the top steel therefore expressly prohibited?

What about splicing of wall dowels to slab reinforcing at the exterior supports, both top and bottom? Is that prohibited because the figure doesn't identify these areas as regions where splices are permitted?

Unfortunately I have an elevated deck that got poured with top steel spliced over columns and I am tasked with determining whether the slab needs to be demolished or repaired in some way or if it is acceptable as is.

Again, thanks all for any advice and references.

 

[hokie66]

The leaders from the top bars are just to define where the bottom bars can be lapped. Refer Paragraph 13.3.8.5 and the Commentary for the requirements and the reasons for continuous bottom bars.

I don't think there is express prohibition of top bar splicing, because everybody (almost everybody, apparently) knows you don't splice top bars over supports. Good luck with your determination. How did this happen? Was there no inspection?

I don't see how the wall to slab junction relates to this.

 

[RandyM81]

It was missed in the shop drawing review and the inspector missed it in the field. One floor went in this way, caught it on the next floor.

A typical slab detail similar to Fig 13.3.8 was included in the design drawings, but we're not going to just throw it back at the contractor.

The slab-wall junction relates in that you may have similar negative moments to an interior support, but it's quite common to splice the top slab reinforcing with the wall dowels at this high tensile stress location with no apparent issue.

Top bar splicing over the interior support may not be best practice, but has the Code been violated? Is repair necessary? Is anyone aware of any reference material that can be cited to assess whether Class A splices at this location would fail to develop the yield strength of the bar?

Thanks.

 

[rscassar]

The statement "Splices shall be permitted in these regions only" is written for the bottom reinforcement.

I would not be splicing top reinforcement over supports. This is the most highly stressed region of the slab. It wouldn't be uncommon for the stress in the reinforcement to be close to the 60ksi even at service loads. The thread attached has comments on splicing reinforcing in high tensile regions.

thread167-258462

Was the elevated deck inspected prior to placing concrete? The contractor should not have placed any concrete before the reinforcement was inspected by the certifying engineer. If the engineer did specify splicing on their drawings then these should be inspected to ensure that lapped splices are placed correctly. There is a strong case for the deck to be rejected.

Most engineers will specify top reinforcement to be spliced at midspan and bottom reinforcement to be spliced over supports.

 

[hokie66]

I don't think a perimeter slab-wall junction should ever be designed for "similar negative moments to an interior support".

As to whether Class A splices are adequate, read Clause 12.15.2.

 

[RandyM81]

I meant Class B Splices... certainly a Class A splice would be considered insufficient.

Reading 12.15.2 with 12.14.1 suggests that while the splice locations are not good practice, they are not prohibited and a Class B splice does meet the code requirement. Do you guys agree?

If the code is satisfied the question becomes whether sound judgment can allow the condition to remain. About 60% of the top bars in the column strip were spliced, with the remainder continuous over the support. Is there an objective way to assess whether the splices are safe other than to say, "it's not recommended".

Ordering demolition would be a bad result for everyone involved. If you think the splices are unacceptable as installed can you think of any remediation options?

 

[hokie66]

My approach would be to reanalyze the slab, with the information you know about how it was built. It is possible that you have excess capacity in the bottom and can redistribute some moment. Perhaps in that way you could justify the spliced bars, discounted a bit, but that would be a judgment call. When analyzing the slab, use the d based on the chairs actually used.

 

[rscassar]

I wouldn't say that the perimeter walls experience similar design moments as internal supports.

If the wall was CMU, I would not rely on any moment transfer into the wall. A concrete wall I would allow for some moment transfer but certainly not to the extent of that for an internal support.

What I read from ACI-318 is that it is encouraged where possible to locate lapped splices away from areas of high tensile regions. I don't see anywhere where it is explicitly prohibited.