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horizontal cold joint in one-way slab/joist construction?

horizontal cold joint in one-way slab/joist construction?

horizontal cold joint in one-way slab/joist construction?

(OP)
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!!

RE: horizontal cold joint in one-way slab/joist construction?

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.

RE: horizontal cold joint in one-way slab/joist construction?

(OP)
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?

RE: horizontal cold joint in one-way slab/joist construction?

I thought I remembered a provision of doing just what you describe but when I went searching today I couldn't find it. Could it have possibly been in a previous code and was edited out?

RE: horizontal cold joint in one-way slab/joist construction?

See PCA Notes on ACI 318-05 Example 12.6

RE: horizontal cold joint in one-way slab/joist construction?

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.

RE: horizontal cold joint in one-way slab/joist construction?

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.

RE: horizontal cold joint in one-way slab/joist construction?

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?

RE: horizontal cold joint in one-way slab/joist construction?

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)

RE: horizontal cold joint in one-way slab/joist construction?

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.

RE: horizontal cold joint in one-way slab/joist construction?

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.

RE: horizontal cold joint in one-way slab/joist construction?

What is the allowable horizontal shear strength of reinforced concrete beam with cold joint?

RE: horizontal cold joint in one-way slab/joist construction?

Section 17.5 provides a fairly straightforward analysis method, without having to calculate Q.  Specify that the cold joint is roughened to 1/4", and you can get pretty decent capacity using the stirrups you already have there.  There is a limit to the available capacity, beyond which you will have to design as JAE describes above.

RE: horizontal cold joint in one-way slab/joist construction?

i don't have the code handy, but wouldn't you have to compute the Q initially to come up with the demand stress at the critical section?  

i believe that that particular section (don't shoot me if i'm wrong) states provisions on strength vs. the actual analysis method.  i'll check it tommorow when i get to the office.

wouldn't it also be a good idea to provide a bonding agent such as a sika material to the existing concrete?  

one can also drill into the beam web vertical bars for shear friction i believe.

RE: horizontal cold joint in one-way slab/joist construction?

In ACI 318-05, Chapter 17, Vnh is the nominal horizontal shear strentgh. Vu is the vertical shear from analysis?
Or we need to convert back the shear stress obtained from formula VQ/bI multiply by bv x d .

RE: horizontal cold joint in one-way slab/joist construction?

"Vu is factored shear force at the section considered"

"Vnh shall not be taken greater than 80bvd"

if the Vu is larger than the phi x 500bvd, you place shear friction steel.

RE: horizontal cold joint in one-way slab/joist construction?

Swivel63,
Appreciate your input but please read question carefully.
You answered
"Vu is factored shear force at the section considered"
Let me repeat my question here
"Vu is the VERTICAL shear from analysis?
Or we need to convert back the HORIZONTAL shear stress obtained from formula VQ/bI multiply by bv x d ."

RE: horizontal cold joint in one-way slab/joist construction?

if you read the code, it says "Vu is factored shear force at the section considered" and the section where it says this is the horizontal shear force section.

that would lead me to believe that the Vu they're talking about is actually a Vuh (horizontal) that you derive from Vuv (vertical) x Q / I (in kips/in).

RE: horizontal cold joint in one-way slab/joist construction?

(OP)
I think they dumbed it down for us and we are supposed to use the vertical shear (Vu).  How would you go about calculating a horizontal shear?  You could get a shear flow in K/in or a shear stress in K/in^2, but unless you select an arbitrary length of beam to apply that to you can't get a horizontal shear force.  Since ACI gives no guidance on what length of beam to apply the horizontal shear flow or horizontal shear stress to (and doesn't even mention it), I would think that the Vu is the vertical shear force.

RE: horizontal cold joint in one-way slab/joist construction?

i think that's the same problem i had when i first came across this issue.  but depending on your stirrup spacing or your shear friction bar spacing, i guess you could convert it into a kip/in reading.  

bv - width of cross section at contact surface
d - (same as everywhere else in the code)

however in the commentary it says that "the definition of d used in chapter 11 for determination of vertical shear strength is also appropriate when determining horizontal shear strength."  

in the PCA notes it says that

"section 17.5.1 requires full tranfer of horizontal shear forces by friction at the contact surface, properly anchored ties, or both.  Unless calculated in accordance with 17.5.4, the factored applied HORIZONTAL shear force Vu < phi x Vnh, where phi x Vnh is the horizontal shear strength."  

it doesn't specify the length of which you would take to apply to your Vu, though.  



RE: horizontal cold joint in one-way slab/joist construction?

(OP)
17.5.4 says that as an ALTERNATIVE to 17.5.3, you can use any segment length you choose and then they specify Vu as horizontal shear.  
I think the fact that they are distinguishing this Vu in 17.5.4 from the Vu in 17.5.3 means that Vu in 17.5.3 MUST be the vertical shear, not horizontal.  If the Vu in 17.5.3 were a horizontal shear, there would be no need for 17.5.3 because it would be covered by 17.5.4.

RE: horizontal cold joint in one-way slab/joist construction?

which year are you looking at?  i'm looking at '02.  

17.5.3 states
(17.5.4 in ACI - 05)

"as an alternative to 17.5.2, horizontal shear shall be permitted to be determined by computer the actual change in compressive or tensile force in any segment, and provisions shall be made to transfer the force as horizontal shar to the supporting element."  

i took that section of the PCA Notes out of the '05 edition.  but nonetheless...the '02 edition is the same:

"section 17.5.1 requires full tranfer of horizontal shear forces by friction at the contact surface, properly anchored ties, or both.  Unless calculated in accordance with 17.5.3, the factored applied HORIZONTAL shear force Vu < phi x Vnh, where phi x Vnh is the horizontal shear strength."  

seems pretty cut and dry to me.  don't let the "alternative" fool you, just because it's referenced in two different locations as well as two different analysis it doesn't mean that they're not the same.  if it doesn't explicitly say that it's a horizontal (like the PCA notes state) or vertical shear, one shouldn't make assumptions of it being so.  

plus there's also basic mechanics theorems that state the relationship between horizontal and vertical shear stress.  that might help out in this discussion.

RE: horizontal cold joint in one-way slab/joist construction?

(OP)
I am looking at '05.

Is the following quote of yours from ACI or the PCA notes?
"section 17.5.1 requires full tranfer of horizontal shear forces by friction at the contact surface, properly anchored ties, or both.  Unless calculated in accordance with 17.5.3, the factored applied HORIZONTAL shear force Vu < phi x Vnh, where phi x Vnh is the horizontal shear strength."  

Either way, 17.5.3 just says, "Unless calculated in accordance with 17.5.4, design of cross sections subject t horizontal shear shall be based on Vu<phiVnh where Vnh is the nominal horizontal shear strength is accordance with 17.5.3.1 through 17.5.3.4"

All of that typed, they are clearly making a distinction between 17.5.3 and 17.5.4.  17.5.3 says as typed above.  17.5.4 is the section where it allows you to arbitrarily select a length of beam to calc out your shear flow and apply that horizontal Vu.  
They wouldn't make two sections for the exact same provision.  
Also, I understand the basics mechanics relating vertical to horizontal shear stresses, but unless you arbitrarily select a length of beam to apply it to you can not get a horizontal shear FORCE.
If, in 17.5.3, they meant for you to select a length of beam and calc the shear flow to get a horizontal shear that would be exactly the same as they are allowing in 17.5.4.  The fact that these two sections are seperated and reference each other as alternatives leaves no doubt in my mind that they are not implying the same method or procedure.

RE: horizontal cold joint in one-way slab/joist construction?

one of them is from the ACI code and the other is from the PCI notes.  the one you're referencing is the PCA notes page 12-10.

in both editions, the quote is verbatim to each other (previous editions).  the PCA notes clearly say that it's a horizontal shear force and not a vertical shear force.

i ran into this exact same question a year ago in "what's the length to use to calculate the Vu from qu calculation."  

17.5.4 states

"as an alternative to 17.5.3, horizontal shear shall be permitted to be determined by computing the actual change in compressive or tensile force in any segment, and provisions shall be made to transfer the force as horizontal shar to the supporting element."

but as i've said before, just because there are two different sections it doesn't mean one is for vertical shear and one is for horizontal shear.  in fact, PCA clearly states that it is a horizontal shear.  

RE: horizontal cold joint in one-way slab/joist construction?

(OP)
Then what was the outcome of the discussion?
What is the length to use with qu?

RE: horizontal cold joint in one-way slab/joist construction?

LOL, he looked up into the sky and said use "d" or something like that.  this coming from a man who was on the 318-99 and 02 voting commitees.

seriously, lol.

but i checked one of my concrete books.....horizontal shear stress and vertical shear stress are the same.

essentially, it said to use your vertical shear and resolve it into a unit stress.  that would be the same as your horizontal shear stress.  then compare that to the code strenght equations.  

RE: horizontal cold joint in one-way slab/joist construction?

(OP)
well, I guess that settles that...
Do you use a cracked I of 0.35 Ig when calculating qu?

RE: horizontal cold joint in one-way slab/joist construction?

i've always used full Ig.

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