huangyhg

construction joint design
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.
how thick is this slab? keys in slabs are usually poor practice, as the outside parts tend to break off, leaving no shear resistance. i don't see any bars or dowels crossing the construction joint plane, and thus don't understand how you consider that the bars shown would assist in developing shear across the joint.
that slab is going to be pretty thick, 8-10 feet.
slab 8 to 10 feet thick (or is that 8 to 10 inches thick ?). thats one mighty thick slab.
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?

if the slab is 8 to 10 ft thick, then i assume it is a mat foundation slab, either supported on the ground or on piles. either way, construction joints in such a slab should be located where shear is minimal, such as near centre span or centred over supports. that way, you just need to continue all flexural reinforcement through the joint, make sure the joint is rough and clean, and cast the second section. relying on keys, dowels, or shear friction in such a massive structural element is not wise in my opinion.
first to define the name of joints (per my personal dictionary
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).
correction:
"...if there is concern of relative rotation/longitudinal offset due to sliding, add dowels as mentioned above."
kslee1000
do you know where i can get any literature about the design of shear keys and their reinforcement?
bengalengineer:
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.
o, thank you for all your help.
bengalengineer:
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.

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