Existing Pier Tie In

[someengineer]

Hey,

I am wondering if anyone has any insight into a particular problem that I'm working on.

We are building a new structure that is connecting into an existing one. The area of intrest currently is the grade beam and column piers.

I have a lightly loaded column that is to sit on a new concrete pier. This pier needs to be tied into an existing pier which sits on top of a large caisson (therefore the caisson will be eccentrically loaded). I am trying to get an understanding of how much shear transfer you can get over a new-to-existing concrete interface.

I antcipate doweling into and roughening up the surface of the existing pier. I am referencing CSA A23.3-04 Cl 11.5 and it has some equations for the factored shear plane resistance. I have a fairly large area to work with (500mm wide by 2440mm deep) and based on the equation it appears that I can get around 600kN shear capacity just by roughening up the surface of at an amplitude of 5mm (from cohesion and friction).

To me this seems very high (if I were to roughen the whole surface), I was anticipating maybe having to put in shear keys but if I can get enough capacity by roughening up the surface then that would be ideal.

Is there a minimum number of dowels that are required in order to maintain this shear friction? The code speaks about shear friction reinforcement which is required. I am wondering if this is determined by the required cross sectional steel area needed to resist the shear force (with appropriate tension development lengths into each side of the shear plane).

Does anyone have any thoughts on this? I hope that I explained myself well enough.

Thanks!

 

[cessna98j]

While I'm not familiar with the Canadian code, the ACI shear friction capacity is directly related to the amount of reinforcing steel crossing the shear plane. The new concrete / existing concrete interface is used to factor up or down the calculated strength up or down, depending on the configuration.

ACI 11.7.4: Phi*Vn=Phi*Avf(fy)(mu); where (mu) is the coefficient of friction - which varies depending on whether you've got roughened concrete, smooth concrete, etc.

 

[BAretired]

You must have at least enough reinforcement to resist the bending moment between the new and existing pier, namely P*e where P is the new column load and e is the eccentricity from the face of the old pier.

Even when there is pure shear on the shear plane, you need some reinforcement to hold the new and the old concrete together, i.e. to prevent them from separating, a clamping force you might say.

Bars used across the shear plane must be developed each side of the shear plane.

BA

 

[someengineer]

Thank you for the responses. I will ensure that there is enough reinforcement to resist the eccentric moment and hold the concrete together (with the appropriate tensile development lengths).

I imagine that as the concrete shrinks there will be some seperation from the existing concrete, so I will ensure there is enough shear resistance in the dowels themselves to resist the shear load. Coupled with the cohesion and frictional resistance I should have a strong connection.

Thanks again. Please let me know if there is anything I am missing.

 

[BAretired]

You might want to review these threads on a similar topic:

http://www.eng-tips.com/viewthread.cfm?qid=240866&page=105

http://www.eng-tips.com/viewthread.cfm?qid=136748&page=445

http://www.eng-tips.com/viewthread.cfm?qid=267680&page=16

BA