Construction Joint design
Construction Joint design
(OP)
I am designing a construction joint on a base slab. I have found a detail that resembles the picture attached. I see the vertical bars will be taking part of the shear. The question is are the horizontal bar groups accounting for shear friction?
Can i completely rely on the vertical bars for shear or are these horizontal groups needed?
I will be finding the shear using the strip method. Do I distribute this shear load across the whole shear plane or do I apply this shear load per foot across the shear plane? I would think applying this load per foot across the shear plane would be overly conservative. But at the same time i do not know the exact location of this shear.
Can i completely rely on the vertical bars for shear or are these horizontal groups needed?
I will be finding the shear using the strip method. Do I distribute this shear load across the whole shear plane or do I apply this shear load per foot across the shear plane? I would think applying this load per foot across the shear plane would be overly conservative. But at the same time i do not know the exact location of this shear.






RE: Construction Joint design
RE: Construction Joint design
RE: Construction Joint design
May I enquire what this slab is for? It obviously cannot be for any typical building project !
Can the thickness be reduced by any means?
RE: Construction Joint design
RE: Construction Joint design
1. Construction Jt - a construction cold joint, continuity is to be maintained by cont. bars, or dowels.
2. Control Jt - a weakened surface, or through, joint that is mainly used for shrinkage/crack control.
3. Expansion Jt - as name suggests, it should be constructed to allow free movement of the concrete segaments. Depends on the application, the joint can be grouted at later stage, or filled by flexible joint material. The shear capacity could be achieved by concrete shear key(s), or sliding dowels (one end fixed, one end free with injection of suitable grout material). Continuous reinforcing should be avoided.
Your case, from the thickness of slab and the joint detail you have selected, suggests a expansion joint is required. From past experience, the shear key works with proper dimension and geometry to prevent vertical movement due to differential settlement of the adjacent segments and shear. If there is concern of relative rotation, add dowels as mentioned above. For your slab, I would recommend using more than one shear key (2 looks good). However, I don't like the reinforcing detail shown on the attached sketch (look sloppy), closely spaced small, trapezoidal, bars bend to shape with adequate development length and horiz. reinforcing is recommended (to protect key from undesirable cracks).
RE: Construction Joint design
"...If there is concern of relative rotation/longitudinal offset due to sliding, add dowels as mentioned above."
RE: Construction Joint design
Do you know where i can get any literature about the design of shear keys and their reinforcement?
RE: Construction Joint design
I don't think there is explicit discuss on design of shear keys, and it is unclear how the shear stress behaves accross the keyway. However, you may understand better by reading ACI requirements on design of corbel. Though the geometries would be different, the concept is the same, especially true for smaller keys. Also, you will need to familize with design of shear friction reinforcement.
I anticipate you will need deeper (vert. dim), accross length (horiz. dim) keys, for such case, the bearing force usually will be low enough for concrete to handle, however, I will simply let rebars to carry the entire load through concept of shear friction to simplify the design process. I might try, at a later time, to upload examples of shear key reinf. I used to use.
RE: Construction Joint design
RE: Construction Joint design
Here is a lift key detail similar to the one you had in mind. Please note only the additional bars around the keyway are shown for illustration purpose. Use the smallest bar size that would work, for ease of fabrication.
RE: Construction Joint design
There is still one thing that bothers me....I am still iffy on the shear calculation. Using the strip method you find a resultant shear force at a certain point on the cut. Do you design for that shear force per foot along the cut. Or do you distribute that shear force along the cut?
RE: Construction Joint design
RE: Construction Joint design
RE: Construction Joint design
Sorry to leave you in dark for so long. I too don't quite follow your question, but hope the explanation below helps.
The shear key in discussion is to provide lock mechanism in two mass bodies. The load on the key usually is a uniform line load in nature, so does the reaction. You can visualize it as precast pipes with tougue and groove linking the adjacent pieces, it prevent uneven vertical movement, but still allow small amount of lateral free movement (mainly shrinkage & expansion). This type of joint is more prevalent for at grade, below grade, or under water civil works, it is rarely seen in buildings (for which keys are usually having bars accross the joint to maintain continuity).
Back to your question on load distribution, as mentioned above, the load on this type of key is usually uniform in nature, however, you may have localized concentrate loads, if it is the case, yes, you can distribute over a finite width (a region bounded by 35-45 degrees to each side of the load). If two loads are closely placed and the concrete mass is quite thick (deep), there would have stress overlap, then just add the stresses up and check an unit strip for shear friction.
If the above does not cover your situation, you may need to upload a sketch, or describe your question further (what is the application, type od structure, geometry, how it is loaded...etc).
RE: Construction Joint design
RE: Construction Joint design
RE: Construction Joint design
This key is provided along the length of the concrete slab/mat to provide inter-lock between structure units, for which free lateral movement is essential, but vertical movement is not desirable. I am aware of "corbel", but don't have clue on the term "corbel joint", so I couldn't comment on its viability. But, note that a trapezoidal key is always preferred over the square/rectangle key for the reasons that the latter is more more difficult to vibrate around the right angle corners, which would result in poor quality; and the latter is prone to crack due to stress concentration around the sharp corners.
Yes, a shear key is intended to receive paired loads in a up-down manner (one unit up, one unit down).
RE: Construction Joint design
As far as I can see, the OP never said this was a movement joint. And a trapezoidal key is not always preferred, in fact they don't serve the purpose intended. When a joint opens horizontally, it stops transmitting vertical load unless the supported part slides down, thus creating a step.
RE: Construction Joint design
I would avoid using keyed/inclined joints for a high load/movement joints as described by hokie. It is also worth noting that the bearing stresses within the key increase as the joint opens (reduced bearing area).
Corbelled joints have the advantage of having a horizontal seat on which proprietary bearing strips (that permit movements and rotations) can be installed that ensure movements are not restrained and direct the load away from the edge of the concrete (prevents damage). In this instance only a very short corbel would be required (huge depth and shear capacity) so compaction issues should be able to be resolved (I imagine there would be a pour break somewhere anyway?).
Bengal – make sure that you design the joint as fully 'open' for whatever system you adopt.
RE: Construction Joint design
RE: Construction Joint design
I could only say the real/anticipated lateral movement (due to moderate temp. variation) is very minor compared to vertical movement. It may not be the best locking device, but been proven in many projects (heavy civil works) I have delt with. Safe to say, the key is only one part of a whole huge system, it needs many other features to make it works as intended. Without knowing the whole, I was only introducing the basic features of the key I felt it was the best match with the description and sketch he has provided -a combined shear key and expension joint for mass concrete structures. From his questions, I sense he is in the screening stage, the final decision will be made by a person your-like. agreed?
OzEng80:
No need to apology, you were expressing your view point. Also, I would agree with you on corbel if it is for isolted heavy concentrated load. Or it could be used for line load as in the case of a concrete ledge/seating. Maybe that's what you meant. Most importantly, we need to know more on the whole system that the OP is working on, otherwise, everything we provided are merely loose details, not the end closure.