Concrete Pedestal Reinforcement Detailing

[CollinW]

I feel like I am going crazy trying to reinvent the wheel so I wanted some additional advice. My office has a detail that they have used a couple of times. It is a pedestal that sees large shears mostly through pipe friction. The pedestal and footing are joined at a construction joint and designed per ACI. I have a few questions, but any insight helps:

1. Is there special detail required at the top of the longitudinal pedestal rebar. I have never seen any special detail on pedestals like this, but the bars technically have no development at the top. In corbel design there are often special detailing as this tension bar would be welded or specially attached. Why is that not required here?

2. ACI 25.4.2.1 states that the minimum development length is 12", so does that mean that the minimum pedestal height is 12". At the top of the pedestal the actual development length is 0 and the As,req is also 0 (no moment). So per ACI 25.4.10.2 you can reduce the development length, but to a min of 12". This whole thing feels weird because making the pedestal longer also increases the moment. Do we even need to develop these bars at all? I want to say we do if we have the construction joint, but we can potentially remove this if needed. ACI 22.5.1 for shear friction requires full development on either side.

3. Is it adequate to just develop pedestal rebar vertically in the footing? There was a long thread here discussing T-joints and I understand the idea of bending the bars and lapping them horizontally. But when the footing is thick enough, can't we just develop them vertically? Maybe we should check the rebar for a breakout cone/anchorage, but other than that I don't see any issues.

 

[HTURKAK]

In order to make clear , will you please provide more information regarding the anchorage requirements, spring support , and loads .?..
Apparently , there is no anchors which may be subject to tension and the pedestal / sleeper subject only to shear due to pipe friction and vertical loading.
If this assumption is correct , my comments will be ;

1- In this case , there is no need for special detail . If there were tension anchor rods , the development length of the vertical bars would be required but measured from the bottom of anchors. In your case , the uppermost two tie layers will help but if you want feel comfortable , you may consider the use of 90 degr. hook end anchorage.
2-It is true that , as per ACI 25.4.2.1 the minimum development length is 12". However that does not mean that the minimum pedestal height is 12". You may develop STM and provide min. end anchorage.

3-In your case if the footing is thick enough, you can just develop them vertically. ( in case of retaining wall found. the situation may differ ).

If you draw the concrete break out cone due to friction shear ( one of the probable failure mode ) the uppermost two layers would be effective to resist shear failure.

 

[CollinW]

Thanks for the response. Your assumptions were correct, but I will add some more detail for context. There are rebar anchors attached to a wear plate in the center of the section, but that is about it. We are assuming no uplift on these bars and all horizontal loads are transferred through friction. Additionally, the springs are just there to provide a bit of extra clamping for vibrations, but no major loads.

The two things that I didn't follow are below, could you please expand a bit:

You may develop STM and provide min. end anchorage.

If you draw the concrete break out cone due to friction shear ( probable failure mode ) the uppermost two layers would be effective.

 

[HTURKAK]

You may develop STM and provide min. end anchorage.

If you draw the concrete break out cone due to friction shear ( probable failure mode ) the uppermost two layers would be effective.

- If pedestal height less than 12 in., you may use the analogy of shear key or corbel. Develop STM and provide U shape reinf . or min. 90 degr hook.

- Regarding concrete break out cone, pls look picture below. ( copy and paste from Anchorage Design for Petrochemical Facilities=





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