Redistribution of negative moments
Redistribution of negative moments
(OP)
Instead of redistributing a maximum of 20% of the negative moment at the ends of a concrete beam, can I just design a beam as simple support and assume no end moment transfer to the column? Any justification in the ACI code?






RE: Redistribution of negative moments
That might be a conservative design for the beam, but not the column. I think this was talked about recently here about the effects of ingoring the fixed ends of the beam.
How is the beam connected to the column? If its monolithic you will have moment in your column and it will be undersdesigned.
RE: Redistribution of negative moments
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Redistribution of negative moments
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Redistribution of negative moments
RE: Redistribution of negative moments
A common practice is to design beam as simply supported and still put a certain amount of negative reinforcement to prevent cracking at service level.
With this 'rational' method - that is in reality a plastic design, no moment connection detailing with stirrup in the joint is required (shear transfer).
There is a lot of debate between engineers about if it is a good or bad practice. Any opinion ?
In my opinion it is good because, Imagine a transfer beam where the column cannot take the moment or the negative reinforcement is badly developed (anchored), the joint capacity is reduced and maximum negative moment cannot be achieved in the beam. Thus, the beam moment capacity can be dangerously reduced at mid span after redistribution of moment if there is not enough bottom reinforcement (designed with lower moment due to negative moment at support).
RE: Redistribution of negative moments
Yes, you will. I was not suggesting to remove it, only saying that the moment will be there if the steel is there. Kinda like the "if you build it, they will come analogy".
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Redistribution of negative moments
Can you detail the dowels to limit moment transfer? Maybe sleeve the dowels in the top half of the beam to take away the tension in the dowels?
RE: Redistribution of negative moments
That approach might work if this were a bearing connection, but the OP said he is using shear friction to connect to the face of an existing column, and the theory of shear friction is that the load is carried by friction, which requires tension development of reinforcement to provide the clamping force.
I don't believe in shear friction for that type connection, and would suggest to db7 that he needs to at least key into the column to provide a beam seat.
RE: Redistribution of negative moments
Depending on the loads you are dealing with, I agree with hokie66 that providing a "positive" connection by means of beam seat would be better.
It is a question of chicken or the egg. You are trying to create a simple shear connection, but doweling the bars provides fixity.
Can you provide some more information? Is this a "flying beam" or will it have a slab on top. If so, how is the new slab attached to the existing slab. You can add top bars in the flange of the beam, cut grooves in the existing slab and develop that reinforcing better than epoxy into the column.
RE: Redistribution of negative moments
ver gets dowelled in.
RE: Redistribution of negative moments
RE: Redistribution of negative moments
I have done this, and we "hung" the slab below using threaded rods and plate washers to connect the two. If you truly do not have access below, be cautious about your connections to the slab. Be sure mechanical anchors are pretensioned, or the adhesives are properly selected, mixed, and placed.
RE: Redistribution of negative moments
RE: Redistribution of negative moments