Concrete strut connection at top of wall

[ChiEngr]

Hello,

I have posted about this before, but had to make some design changes. I have a concrete strut (primarily an axial member and taking small flexural demand) which needs to be shifted up towards the top the wall it connects to. I have the following questions:

1. For this strut to behave as an idealized fixed-fixed member, don't I need the vertical bar in the wall to be developed above and below the strut? Below is not a problem, but I don't have a large distance of wall continuing above the strut.

2. If I end up just designing this strut as a pinned member, does that mean that I am essentially counting on the strut to crack at its ends? Intuitively, I don't see how this strut will be subjected to net tension due to the large compressive force it is taking.

3. Based on #2 above, if the section is in net compression, can the strut still behave as a pinned member even though the moment transfer into the wall is questionable?

Thanks for the help.

 

[KootK]

If you need the end of the strut fixed for some reason then, yeah, you need proper detailing to the wall (and wall stiffness) to justify that. I won't spend too much time on this aspect though just yet since I feel that a pin ended strut is your path here.

It would be helpful here to know where you are delivering the compression in your strut to. Are you sending it into the surrounding diaphragm?

 

[ChiEngr]

Thanks for the response Koot. The strut is between two walls receiving roughly equal lateral earth pressure. That is the source of the compression in the member. Do you have any thoughts on #3 above? Does my question make sense?

 

[KootK]

The strut is between two walls receiving roughly equal lateral earth pressure.

Is the strut integral with a surrounding, elevated concrete slab?

3. Based on #2 above, if the section is in net compression, can the strut still behave as a pinned member even though the moment transfer into the wall is questionable?

I would say that the moment transfer into the wall makes it more plausible that the member might behave as pinned.

There's always continuity in this kind of construction. Nothing is ever truly fixed or truly pinned. The best you can do is to make intelligent, conservative assumptions where that is possible and detail for crack control based on early load history stress patterns.

 

[ChiEngr]

The structure is a bio-retention structure consisting of perimeter walls and supporting footings. I implemented concrete struts along the wall length in order to mitigate the size of the footings. The size of the footings would be prohibited by large existing utilities, so this was the best option I had to go with.

 

[KootK]

Ah... I get it now.

How are things looking at this joint with respect to punching shear? That would be my first stop with something like this. That punching shear will be exacerbated by:

a) Any moment in the joint and;

b) Any contractor desire to build the beam into a wall bearing pocket.

Conceptually, I'd rotate this thing 90deg and imagine it as a joint between a floor slab and and exterior column. Let that inform your detailing.

In a similar vein, it could also be a creative application for stud rails.

 

[ChiEngr]

I am way good in punching shear. Part of the reason is because the walls are 12" thick and the axial demands are around 30 kips. If this strut was fixed-fixed, the moment would be less than 10 kip-ft. I am not too concerned about punching.

I was just conceptually trying to figure out the best way to detail this, pinned or not, given that I have a good amount of axial compression in this strut. Also, looking at the situation rotated as you show, you can see that the wall bars are not developed to the left due to the wall not extending far beyond the face of strut. Therefore, can the moment not be resisted by the wall? That is what I am trying to conceptualize, but I imagine the problem is a little more complex and nuanced than that.

 

[KootK]

I was just conceptually trying to figure out the best way to detail this, pinned or not, given that I have a good amount of axial compression in this strut.

Detail it like it's fixed. This is almost always the right approach when there will be some fixity and where rampant cracking may compromise serviceability, shear transfer etc.

That said, I'd be designing the strut for axial load as though it were pinned unless I was seriously desperate (it wouldn't be the first time).

Also, looking at the situation rotated as you show, you can see that the wall bars are not developed to the left due to the wall not extending far beyond the face of strut.

Many slab to edge column connections share this feature. I agree, development past the strut would be ideal. That said, if not having the development is good enough for slab/column connections, then I'd argue that it should also be good enough for your weird strut thing.

Therefore, can the moment not be resisted by the wall?

Probably not. But, then, the same can be said -- and often is said -- of pretty much every slab to edge column roof connection out there. And we let those ride.

I think that many an engineer would be happy with the first detail below, often without the green hook. It's pretty similar to what you've already proposed.

If you feel like being a high minded hard ass, then something like the second detail would probably be the order of the day.