huangyhg

horizontal cold joint in one-way slab/joist construction?
i have a situation in one-way concrete joist construction where the final (5) trucks were held up and ended up being poured after the rest of the concrete had set. this has created a horizontal cold joint in at least one girder, and several joists as well as a vertical cold joint in several joists at the end of the span where the shear is highest.
i am trying to find information about how this is handled and to verify capacities for shear flow across this joint using in-place reinforcement for shear friction calculations.
any guidance or references that anyone could provide would be greatly appreciated!!
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usually this is a classic case of simply determining the section properties and q value at the joint to determine the horizontal shear at the joint.
shear = vq/i
v is the vertical shear at the point in question
q is the first moment of the area on one side of the horiz. joint
i is the moment of inertia of the total section.
once you have the shear (in kips/in or si equivalent) then you simply can use the aci shear friction values in chapter 11 of aci 318.
the trick would be using the appropriate surface roughness coefficient in the shear friction formula.
jae-
i considered this, but what steel would i use for shear friction? the stirrups are already being used as vertical shear steel, i can't double dip and use it for shear friction reinforcing also, can i? if i assume them to be transferring shear across that vertical joint, how will they also act to prevent the diagonal shear cracking?
i thought i re
see pca notes on aci 318-05 example 12.6
chapter 17 of aci addresses this. double-dipping is permitted. i've got a similar condition, except it hasn't been constructed yet. the contractor is proposing to pour the monolithic slab and beam in separate pours, so i have to investigate the horizontal shear.
if your stirrups are in tension (regardless of the reason why) this will cause compression on your shear interface.
in typical perpendicular shear the tension steel is used even though it is there for bending-same principal.
there is a clause in the australian code that covers this exact case, i will see if i can find it for you tonight.
jae
for calcultaing q we need the cross sectional area of either side of joint into the arm. what will be the arm? is it from center of area on either side of joint into it's centroid to neutral axis. also we assume neutral axis at geometric center of concrete beam with rebars?
i've always neglected the rebar. just used the gross cross sectional properties of the beam and the effective width portion of the slab.
q is: (area outside the horizontal plane you are seeking shear at) x (distance from na of that area to the na of the total section)
i agree with what folks have typed so far and can only add one thing that could be forgotten: development of the stirrups. this could be a challenge depending on the cold joint elevation.
i think you can also refer to section 17.5 in aci for horizontal shear strength. i believe they give provisions for this sort of thing.

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