Is it possible to direct the load from the beam to column (offcenter)

[BauTomTom]

Hi

Does somebody know if it is possible to transfer the load from a offcenter beam into "center load" for the column?

Surely the arrangement of the reinforcement in the head of the column can have a positive influence but how to do it and how much does it actually help?

Please see the attached sketch, there you can see the problem.

Now am really curious which answers will come

BauTomTom

 

[ishvaaag]

You can through an orthogonal centering beam, like in foundation. The orthogonal centering beam has to take the load of your beam and be anchored to fixed point beyond. Then you can center the support of your centering beam on your column.

 

[BauTomTom]

TO ISHVAAAG:

oouu sorry but somehow I really don't know how you mean it with the orthogonal centering beam. In foundations? where? I wish I could see a sketch

TomTom

 

[normm]

bau

I do not agree with where you are showing the cranked bar. The reinforcement should be where tension is likely to develop.
I have attached a sketch of suggested detail.

 

[Lion06]

Well, when you get any distance from the top the axial load will be in the center of the section (remember from strength of materials that the axial load always acts at the centroid of the section). The only problem is there is moment as well.

If you have a pinned top and base then the moment will go from Pe at the top to 0 at the bottom, there's really no way around that.

 

[csd72]

can also be taken out by bending in the slab over (if you had one) but as you have drawn it no chance. The eccentricity will cause bending in the column.

 

[ishvaaag]

Like in this sketch, your beam to be unloaded centered on the co lumn at top right. It is just an archimedian lever.

 

[BauTomTom]

TO NORMM:


Thanks a lot for your sketch. To be honest you actually really pointed something very important for me.
This one shear link on top. I really need to make sure that I place more links on top. Thanks for this reminder.
You just say more links. Is the rule here as well 0.6x the normal spacing? The way you do it by overlapping?

My sketch is actually little bit wrong because it is a continues column. Must I still close up the column (right below the beam)
As you indicated? You say “TOP BAR CLOSER TO RESIST ECCENTRIC MOMENTS” even by continues column?

So my diagonal bar, wont transfer any axial load to the side?


TO LION06:

Maybe the load will go with the distance more to the center but really from the eccentric beam to the middle of the column?
Pinned top and bottom? Not in this case, I see it as fixed bottom and fixed top. So the moment go through the whole length.


TO CSD72:

Bending the slab over it? The slab is actually carried by the beam.

TO ISHVAAAG:

Thanks for your sketch. Archimedian level, I like this item.
I had the same idea to somehow center the load on the column with a perpendicular beam. I actually have even one. Let me rather upload a new sketch with the real
Situation. The first sketch was simplified.

But how can I create a kind of a “bearing” which would concentrate the load of the “beam cross” in the center of the column?
Surely the column will be cast up to the beam without any gab, and the column corner reinforcement must also continue up to the column above.
And the Column actually must be connected (not pinned) with the beam to create a stiff frame.

Any idea how to solve this?

BauTomTom

 

[Lion06]

I don't think you have a fixed condition at the top. Unless the bending stiffness of the slab and torsional stiffness of the beam exceed the bending stiffness of column (which looks pretty deep in the direction under consideration), then you have, at best, a partially fixed condition.

Think of a W8 beam framing into the strong axis of a W24 column (just for demonstration purposes) with a full moment connection. The beam feels a lot more like a fixed connection than the column does.

I see a similar condition here.

 

[BauTomTom]

I think I should make better sketches, which are more closer to the real dimensions. Ok you catched me the sketch is out of scale.

The column is 800x300mmm and 3000mm high. The beams are 220x300(plus 250 slab) so with the slab as a T beam 220x550mm

BauTomTom

 

[ishvaaag]

In any case whatever the degree of fixity you attain there you can use the same principle.

However if yo make the connection continuous, you can only provide the centering as a secondary mechanism of resistence -except introducing actively a centerning moment- for out of rigidity and direct connection the load will enter through inclined struts to the column below at some more outwardly part of the section.

To make the load enter vertically you must make the centering beam strong enough to pass what at the crossing to the central support point, and use some unloading (pot or whatever) device; and even then it may not be entirely avoidable that out of friction some inclination at the introduction appear.

 

[ishvaaag]

Introducing a centering moment would achieve the wanted continuity and centering the load; it would mean to force downwards the left end of the centering beam in my sketch and anchor it there with something able to keep the centering moment at work. It may be made with jacks or some bolts or cables forcing the beam to the convenient deformation against some anchoring device on some competent part of the structure.

 

[BauTomTom]

TO ISHVAAG

Ja this is an idea to force the beam in a specific deformation.

But as I showed the reinforcement with a diagonal bar from the beam to the left side-column-reinforcement or something simmilar, you wouldn't recommend?

BauTomTOm

 

[csd72]

BauTomTom,

We finally get a clear sketch of the full situation!

If your reinforcing bars are continuous past the beam depth then you really need to treat this as a continuous frame and analyse it accordingly to get the frame moments at the joint.

If the column is meant to act as a pinned member then you need to remove the continuous bars. Then using a bridge type elastomeric bearing could achieve the (nearly) concentric loads that you are asking for.

Since the column is only 2 floors high and has a much greater potential moment capacity than the beam then I fail to see what the problem is with the eccentricity. I was taught in my day that it was not worth skimping on column reinforcement for the following reasons:

1. the column reinforcement is normally less than 1% of the total reinforcement by weight.
2. Strong column weak beam is a much more robust configuration than weak column strong beam.

 

[ishvaaag]

And the inclined rebar would mean scarce or no benefit -if any- to center the load because it follows the path of something that we may ordinarily think is competent enough, the concrete struts forming from load to support.

 

[BAretired]

The diagonal bar marked 'C' in the original sketch does not transfer any load from the edge of column toward the center.

Statics cannot be changed by adding reinforcement in a particular location. Reinforcement must be placed such that it resists tensile forces caused by statics. Bar 'C' does not do that.

BA

 

[csd72]

BA,

you took the words right out of my mouth.

Rebars do not change the nature of the imposed loads only the ability of the members to take those loads.