Another concrete corbel question 3

[Robbiee]

Hi everyone,
An Existing corbel is to support a new precast beam. The corbel doesn’t have the capacity to resist the reaction from the beam. Can we provide plates like the ones shown in the attached sketch welded to existing embedded plates in the column to reduce the load applied on the corbel? We thought we can do load distribution between the corbel and the new plates based on relative stiffness. How?
1- Apply a unit load on the corbel and the plate and calculate deflections due to bending, compression, and shear,
2- the load carried by the corbel/the load by plate = deflection in plate/deflection in corbel.
Is this correct?
If yes, what do you think can go wrong? BTW, we know that carrying all the load on the corbel is more certain approach, but the corbel, that is one of many, need be demolished and re-built, which we don't mind doing if other ways are not available.

 

[hokie66]

Compatibility of deflections and distribution of load is not the way to go with this problem. Either the web connection or the corbel will fail first, then all the load goes on to the other element.

 

[csd72]

I agree with hokie.

The corbel is infinitely stiffer than the angles you show an will fail before any significant load gets taken by the angles (it already is taking all the existing load).

How are the existing corbels failing?

 

[Robbiee]

Csd72,
The corbels are not failing. They are built now. The beams are not istalled yet but they have higher loads now than what the corbels were designed for.

 

[kslee1000]

Enlarge the cornel by make it deeper if you could.

 

[haynewp]

Watch for rotation restraint due to the plates and moment into the column.

 

[Robbiee]

kslee1000,
We were hoping the plates can do it because this is much faster way. In our relative stiffness analysis, we assumed a cracked concrete section and full fixity for the plates, which is why we have uncertainty in this solution. The result of this analysis showed about 65% load to be carried by the corbel and 35% by the two plates, which would be adequate.
haynewp,
We were to carry only vertical shear on the new plates. We're allowing for horizontal movements by providing slotted connections.
Any ideas are welcomed.

 

[kslee1000]

Your detail creates a end point with partial moment restrain, which is not easy to evaluate. If you have confidence in the proportion of load sharing, then, yes, it is a much friendly solution. However, be prudent, watch out on effects on the precast beam end, column, and the plates.

One positive thing is your detail tends to diminish the lateral load on the corbel. Good luck.

 

[JAE]

Ailmar, I'd still say hokie has a good point. There is, in reality, a high difference in stiffness between the side plates and the corbel. Since the corbel is so much stiffer than the plates, it will initially take most of the vertical load.

If "most of the vertical load" is more than what the corbel can safely support, it may then experience damage, distress or failure. This distress would mean the corbel, to some extent, moves or shifts such that the load is quickly transferred to the plates, which cannot take the vertical load all that well based on your sketch.

It all doesn't look like a good solution to me.

A better solution would be to strengthen the corbel directly. Possibly by constructing a support corbel beneath it.

 

[civilperson]

Plates on both sides of the column/corbel with through bolts can be designed for the entire reaction of the beam. Grout any voids to bring the sides into plane.

 

[kslee1000]

Add'l precaution:

The precast beam, when loaded, tends to rotate upward. The rotation is restrained by the plates with a couple in them. Watch out for the extra pulling/pushing forces - are both the column and the precast beam are capable to resist the forces without doubt.

 

[JAE]

kslee1000 - I think the OP mentioned that there were horiz. slots in those plates.

 

[csd72]

Ailmar,

I dont mean how is it literally failing, I mean, how are the numbers saying the corbel is going to fail - i.e. what is the critical design criteria. Is it reinforcement or is it concrete strength.

If it is failing due to insufficient reinforcemet then you can provide additional external reinforcement by way of 2 angles and a bolt each side wrapping around the column.

If it is failing due to concrete strength then you may be able to get insitu testing to justify a higher strength.

 

[kslee1000]

JAE:

If thus, I reserve my doubts on effectiveness of this connection scheme.

 

[JAE]

Me too.

 

[Robbiee]

Csd72,
both steel and concrete are failing.
ksledd1000 and JAE,
So you don't see this connection to work?

 

[JAE]

I think the two plates above that you show don't provide the stiffness requried to adequately share the load....just from an initial view of the detail.

So...no, it doesn't look right to me.

 

[csd72]

then you would need to do both my suggestions.

 

[kslee1000]

A quick gut feeling call. If there are slots at beam end connection, because of the freedom in movements, a majority of the vertical load is likely to fall back on the corbel rather get into the plates. I wouldn't trust this scheme.
Think twice.

 

[csd72]

Ailmar,

One possibe suggestion that no-one has thought of - remove the load from the corbel entirely by removing the masticord pad, and provide a bracket that can take the entire load.