Column joint at pile cap. 1

[NewbieInSE]

Dear Engineers,

Please see the figure below and provide your respected comment to clear my confusion.

 

[MIStructE_IRE]

That’s not a moment resisting connection for me. The anchorage is surely insufficient?

I would normally bob the bars anyway, even if pinned. How else do you tie the rebar? Looks like a weak detail.

 

[NewbieInSE]

Looks weak to me too. Looks like dowel detail for the pinned base.

I look forward to a more detailed response as to why it's insufficient.

 

[hokie66]

I would always cog the starter bars for constructability. But I can never remember a time when I considered, for analysis purposes, the column connection to a pile cap to be a rigid connection.

 

[NewbieInSE]

hokie66, in that case, the starter bar needs to be developed only for compression, right?

 

[NewbieInSE]

If I develop the starter bars for tension, will it work well to transfer the moment from the main bars to the Footing?

 

[MIStructE_IRE]

If the moment is to be transferred into the pile cap then shouldn’t the pile cap have top reinforcement also?

 

[KootK]

One great way to do this kind of thing is using strut and tie methodology (STM). That said, I recognize that is time consuming and often pile caps are designed using simpler methods. That said, I still recommend generating a strut and tie model conceptually and letting that guide your decisions about rebar disposition and anchorage. Ask yourself these questions:

1) Do I have rebar everywhere that I would need a tie for the STM model?

2) If I were to check the nodes of this STM model, would they have any chance of working given how I've chosen to anchor, or not anchor, the reinforcement?

In this case, based on the STM below this would leave me to the following conclusions:

3) There should be top steel in the cap.

4) The pile rebar should extend as high as possible and terminate in hooks. Even this wouldn't usually satisfy a rigorous STM check but it's a heck of a lot better than nothing.

6) The column rebar should extend as near as possible to the bottom of the cap and terminate in hooks.

It's also important to note that simply developing a bar into concrete does not always guarantee that it cannot be ripped from that concrete, taking a frustum of concrete with it.

All of the above assumes that your moment results in a net uplift at the piles.

 

[NewbieInSE]

KootK, Thanks.
At first, I have to confess I haven’t learned the STM analysis yet. That said, I just want to know the following from your perspective.


3) There should be top steel in the cap. >> Okay.

4) The pile rebar should extend as high as possible and terminate in hooks. Even this wouldn't usually satisfy a rigorous STM check but it's a heck of a lot better than nothing. >> Okay, it’s a good practice, I think. However, is this required because you have responded using STM? In normal practice, I think piles are considered pinned at top, in that case, maybe a minimum embedment of pile reinforcement to satisfy compression dev. length would satisfy. Is my thinking ok? Or is it so that, when column base considered rigid and if we utilize STM model, then pile reinforcement should extend till the top and end in hook to satisfy specific demand arising from STM?

6) The column rebar should extend as near as possible to the bottom of the cap and terminate in hooks. >> Well, it is a good practice, I think. Actually what I emphasized in my post was: is developing the starter bar for Class B Lap splice above and below the pile cap top sufficient to transfer moment from column to pile cap. I don’t know why, it just seems to me inappropriate since column main reinforcement has not been taken into the piloe cap.

It's also important to note that simply developing a bar into concrete does not always guarantee that it cannot be ripped from that concrete, taking a frustum of concrete with it. >> yeah, like an anchor does.

All of the above assumes that your moment results in a net uplift at the piles. >> it happens.

 

[KootK]

However, is this required because you have responded using STM?

No, not in my opinion. STM only draws attention to reinforcement requirements that exist no matter what analysis method is employed. That said, STM is a reinforced concrete method of design. If you're willing to rely on unreinforced concrete design methods (anchor theory) other things may be possible. I don't support such approaches in situations like this, however, so I'll not speak to them in detail.

In normal practice, I think piles are considered pinned at top, in that case, maybe a minimum embedment of pile reinforcement to satisfy compression dev. length would satisfy.

I disagree where the piles would be in tension. The detailing that I've suggested is not meant to create fixity at the pile head but, rather, to allow the pile to engage the full flexural depth of the pile cap. A surprisingly useful technique in these situations is to imagine them upside down. Imagine the situation shown in the sketch below where I've just put all of the piles in tension for simplicity. How do you feel about your detailing now?

Actually what I emphasized in my post was: is developing the starter bar for Class B Lap splice above and below the pile cap top sufficient to transfer moment from column to pile cap.

It is not if you mean to follow strict reinforced concrete design principles. Like I said earlier, if you're willing to consider anchor theory instead, the connection can certainly be shown to have some moment capacity.

I don’t know why, it just seems to me inappropriate since column main reinforcement has not been taken into the piloe cap.

That doesn't bother me. The starter bars should be lapped to the column main reinforcement such that, for all intents and purposes, the column cage does extend into the pile cap.

 

[KootK]

A fundamental tenet of reinforced concrete design is that, as much as possible, one ought not rely on unreinforced concrete to carry tension loads. See below.

 

[NewbieInSE]

Thanks a lot, KootK. for making things clear with your elaborate answers.

 

[KootK]

Happy to help NewbieInSE.