Concrete Beam to Wall Connection

[gmf]

I am currently working on a parking slab replacement project in Canada. The parking slab is covered with asphalt and serves as surface parking for a hotel. Below the slab is an under ground parking garage. The slab is a one-way slab spanning approximately 20ft. between beams cast monolithicly with the slab forming T-beams. The slab is supported at each end by concrete foundation walls and there are 4 interior T-beams. The T-beams span approximately 26ft. between columns. The T-beams are supported at each end by concrete foundation walls and there are 4 interior columns. The slab, beams and columns are being replaced and walls are remaining.

My question involves the T-beam connection to the wall. Can an adequate connection be established between the new T-beam and existing concrete foundation wall to assume continuous construction and therefore analysis of the beam/column/wall as a concrete single-storey frame? I am finding that I need a negative moment to develop at the beam to wall (end connection) in order to reduce negative moments at the first interior column and as a result reduce the moments in the first interior column (Note, that there is not much room for negative moment development over a 12" wall).

New and existing members are as follows:
9" thick slab
19.5" deep T-beam with 24" wide web
12"x24" column (h = 7ft.)
12" thick existing foundation wall.

Any help would be greatly appreciated.

gmf

 

[BAretired]

You don't say why the floor is being replaced. Is it as a result of corrosion of steel, concrete failure or simply a change in geometry?

If the existing wall reinforcement is carefully preserved during demolition of the floor and the concrete in the wall is deemed sound, you should be able to develop some continuity between the new beams and the existing wall. It might be a good idea, however to limit the end restraint of the beam to wL^2/24.



BA

 

[JedClampett]

Why are you getting a negative moment in the interior columns? Besides the minimum moment required in 10.12.3.2, I'm not seeing it. Model the beams as continuous over the columns and the column as a pinned support. I suspect that is the way the original design was done.

 

[gmf]

The floor is being replaced due to severe corrosion of the reinforcing steel over a majority of the slab and beams.

gmf

 

[kslee1000]

Exam and try to figure out if the original wall and the T beam were casted by one pour, or there was a construction joint below the T beam.

For the latter case, demo the wall to the construction joint with a width = 2*horiz bar development length + bw, then rebuid with the preserved bars.

For the former, on top of the width mentioned above, I would demo the wall below further to at least one development length of the wall bar.

Watch out for bonding development along the new-existing concrete interfaces.

 

[gmf]

I have designed my beams as continuous over my interior columns and the column as a pinned support. I am now wondering how to adequately detail my column to beam connection to make sure I have a pinned support. Any additional help would be greatly appreciated.

gmf

 

[ali07]

you can't have an ideal pin like roller support, it's just a matter of design. If there is no difference in spans, you should not worry about. Otherwise you are attracting moment in columns to make it pin to idealize your assumption.

 

[jike]

I would normally make a rigid connection between the beam and the column and design the column to take whatever bending develops. It is difficult to detail a "pinned" connection at this point. If a pinn is desired, I would probably use separate straight dowels between the column and beam located at the CG of the column.

I would not count on the wall to restrain the beam for negative moment but would add top bars in the beam if any incidental negative moment did develop.

 

[kslee1000]

I have never done that before. However, here is an idea.

Cast the column with a steel plate at the beam-column interface. The plate shall equipped with anchor studs on both top & bottom to transfer the horizontal shear at the interface, yet allow the beam to under go small rotation. The important thing is some type of flexible sealant shall be used to conceal the plate, and protect the upper edges of the column from spalling.

 

[jike]

I have seen some highway bridge details where they used straight dowels and exp jt material around the perimeter of the column to create a semi-pin condition.

 

[hokie66]

Just because you model the beam as pinned at the columns, you don't have to build it that way.

If the negative moment at the first column is too high, just make the beam wider. You don't have to rebuild to the same profile as the original structure.

 

[BAretired]

hokie,

The problem was that the first interior column was so stiff that it 'hogged' too much moment from the beams. So after consulting with EngTips, gmf has analyzed it as a pin ended column, reducing the column moment to zero. Now he is wondering how to detail a pin ended column.

The column is 12 x 24 and 7' high, so its stiffness will be pretty high if the 24" dimension is parallel to the beams. This is a case of the column being too stiff for its own good.

One idea is to curtail the tension reinforcement at the underside of beam so the column cannot develop any serious moment.

Another is to reinforce the column in a reasonable way and let the tension steel yield. Some cracks will develop in the column. Is that a problem?

A third idea is to reduce the depth of the column. A 12 x 12 column will not attract as much moment but the rotation of the members at that node will be dictated by the beam stiffness, so the column will rotate to accommodate them.

BA

 

[hokie66]

I don't see any problem in letting the top of the column crack. If you look at most carpark structures, the columns nearest the perimeter have some flexural cracks near the top due to shrinkage shortening of the slab. On the other hand, as the OP is building a new structure, he could change the column size, provided the carparking layout allows. The thing is that if the beam works with the assumption of pinned supports, then typical column to beam detailing just adds to the strength, doesn't detract.

I would still lean toward making the beam wider if the moments are so high as to present a problem at the first interior column.

 

[gmf]

Just some additional information associated with above comments. The 12" column dimension is parallel with the beam so any bending is around the weak axis. The parking layout limits the width of the column to 12".

I have contacted additional resources in my area and they to suggest providing a dowel at the centre of the column and tie to lower layer of upper steel in the beam provided shear can be resisted at the column to beam interface.

gmf

 

[hokie66]

You are worrying about nothing. With the column oriented that way, the column moment can be ignored. Just assume the beam is supported on knife edges, then detail the column with all bars bent into beam in the normal way. Trying to create a hinge where none is required will just sacrifice robustness of your structure.