Offset Transfer Columns in Thick PT Slabs 1

[jkeng]

Hi Team,

Long time lurker, first time poster.

I know this is a pretty hot topic on these forums - though being relatively green and new to strut and tie modelling, I would appreciate some advice on developing a strut and tie model for a column transfer in line with the below preliminary sketches.

My questions are in line with the below:

1. How do you define the height of the bottom node? Tough to find guidance on this in the standards I have looked at. The example above is just using the area below mid-depth of the slab as it is likely already in compression.

2. Will both nodes 1 and 2 act as CCT nodes in line with the below? I am pondering whether at node 2 in particular, the continuous slab surrounding the node can act as a restraint (surrounding concrete being in compression), and in turn make node 2 a CCC node.

3. If the cardinal directions are checked and justified with this same model, is placing the reinforcement in these cardinal directions (rather than connecting the plan centroids of each column) justified?

Any help/thoughts would be greatly appreciated!

Thanks very much.

 

[NAFTALI-HAKOHEN]

I think youve seriously overcomplicated your calculations!

And my calculation gives a larger value for the horizontal tension and compression in the slab (836kn) see attached calculation

The horizontal slab forces need to be resolved with a view to the larger structure ( next bays/frames etc)

Normally the compresion is okay, the tension in the slab needs to be resolved, or the PT compression should neutralize this tension IMO

NAftali

 

[jkeng]

Hi Naftali,

Appreciate the response.

Determining the tie force as you have done above is not so much the issue in my case. In terms of resolving this, my intention is to provide tie reinforcement fully anchored at the top and bottom nodes to have a local rigid transfer system rather than relying on the broader structure.

The trigonometry part in my calculations was carried out as a means of checking the strut and nodes as I believe to make this a safe design you would need to ensure:
1. The strut does not fail
2. The nodes do not fail
3. Horizontal reinforcement can deal with the tie forces

I suppose my main concerns come down to defining the CCT/CCC node at the bottom.

Thanks very much,

JK

 

[KootK]

1. How do you define the height of the bottom node? Tough to find guidance on this in the standards I have looked at. The example above is just using the area below mid-depth of the slab as it is likely already in compression.

You need to decide on the story that you mean to tell about where those lateral forces will go once they leave the vertical face of the nodes. If you're pushing against a compression block of concrete then one option for your node height is the depth of that compression block.

2. Will both nodes 1 and 2 act as CCT nodes in line with the below? I am pondering whether at node 2 in particular, the continuous slab surrounding the node can act as a restraint (surrounding concrete being in compression), and in turn make node 2 a CCC node.

This, again, depends on your story telling for the load path beyond the node. Most often, where you have top and bottom reinforcement developed well past the nodes, the nodes become CCC nodes, top and bottom. At the node proportions that you're looking at, it would probably be pretty tough to anchor rebar within the nodes for a CCT setup anyhow.

 

[jkeng]

You need to decide on the story that you mean to tell about where those lateral forces will go once they leave the vertical face of the nodes. If you're pushing against a compression block of concrete then one option for your node height is the depth of that compression block.

Understood. Thanks mate.

This, again, depends on your story telling for the load path beyond the node. Most often, where you have top and bottom reinforcement developed well past the nodes, the nodes become CCC nodes, top and bottom. At the node proportions that you're looking at, it would probably be pretty tough to anchor rebar within the nodes for a CCT setup anyhow.

If the slab is continuous in each direction at the nodes, is it not the case that installing unique/specified "tie" reinforcement such that it is located within the node and fully developed past the nodal face in tension renders it a CCT node? And if this reinforcement exceeds the tie force we can be done with it?

I suppose a sound approach in my mind for dealing with the tie force would be:
1. Assume a CCT node at the top and a CCC node at the bottom.
2. As the concrete at the top is likely in tension, design tie reinforcement to sit within the node and be fully developed at the tension face of the node.
3. Check the compression block at the bottom can resist the tie force in bearing.
4. If the compression block at the bottom fails in bearing, check a CCT node at the bottom and reinforce accordingly.

Keep in mind the load in the example is relatively small compared to some real-world examples.

Thanks again,

JK

 

[KootK]

If the slab is continuous in each direction at the nodes, is it not the case that installing unique/specified "tie" reinforcement such that it is located within the node and fully developed past the nodal face in tension renders it a CCT node?

Yes, I agree that is the case if you're using a designated tie local to the node as you suggest rather than the "surrounding concrete" argument. Analogous to bar anchorage with an anchor plate.

And if this reinforcement exceeds the tie force we can be done with it?

Yes, so long as the other node checks work out taking the hit for a CCT instead of a CCC.

2. As the concrete at the top is likely in tension...

Ok, now I think that I see what you're laying down. You mean to treat the top and bottom differently because of the presence of flexural tension & compression in conjunction with the axial transfer. I see the logic.

 

[rapt]

You missed the final check for the CCC node. Where does the compression force go to? Just getting it into the slab is the first stage. What is bracing the slab against horizontal sway forces induced.

 

[jkeng]

You missed the final check for the CCC node. Where does the compression force go to? Just getting it into the slab is the first stage. What is bracing the slab against horizontal sway forces induced.

Hi Rapt,

Thanks for that - As a specialist PT floor plate designer, I would think the best way to deal with this would be for the designer to raise the calculated tie force with the project structural engineer just to get confirmation that the load has been accounted for in the framing (though I would suspect it should be if the project structural engineer is allowing the offset in the first place!). Otherwise, I am not sure there is a way to deal with this within the floor plate in it's isolation.

 

[slickdeals]

IMO, there is no global horizontal force, only localized forces. We are dealing with two vertical elements and a slab that's supporting it.

 

[cliff234]

Jkeng – Depending on the size of the column load and magnitude of the offset (you have a significant offset here), I would seriously consider increasing the size of either the column above or column below for one floor to better overlap the columns and allow for a smoother load path. We see this all the time on our projects and (depending on the load) will almost always ask for one of the columns to be made bigger for one floor. Architects will usually go along with our request when we explain the reasons.

Every walking column is unique, and our goal is always to provide simple, safe, and economical designs that require as little effort on our part as possible. (It was challenging enough making a profit on projects before walking columns existed. Now that architects discovered that engineers will allow us to walk their columns, it’s getting more difficult!) The mantra in our office is, “Don’t do a thesis”.

And the addressing the column design is only half the challenge. There are always horizontal forces the floor diaphragms that must be addressed. I’ve often seen other engineers’ drawings with significant column walks and little evidence that they addressed those forces. We’ve had column walks that induced horizontal kick forces perpendicular to slab edges in excess of 1,000k – and these are real forces that occur 100% of the time! I’m guessing those engineers just assume the computer is taking care of the slab design. WRONG!

 

[cliff234]

slickdeals - Walking columns do induce global horizontal forces. Sometimes those forces are significant. Engineers need to compute those forces and determine their load path - usually to a shear wall.

 

[slickdeals]

Cliff - I am not disagreeing with you that a walking column will induce global horizontal forces on the overall system due to the slabs over multiple levels coupling and resisting the moment. But in this case with the slab thickness that's provided, I see this more of a transfer slab rather than a walking column situation. OP can correct me if I am off. I fully agree with your assessment of "Don't do a thesis".

 

[cliff234]

slickdeals - You're correct. No global horizontal forces for the transfer slab example shown! Sorry about that!

 

[rapt]

There will be a global sway effect!

Plus

The Compression strut only goes into compression if there are bracing elements to resist the compression force. It cannot just be assumed to be absorbed into the slab if there are no resisting forces to cause compression. The slab itself does not provide the compression resistance. The slab is acting as a compression diaphragm to transfer the compression force to some form of resisting element.

The tension ties only go into tension if the tension ties actually tie to bracing elements to resits the tension force.

Neither of these have been mentioned.