Pretension Shear Equation 6

[Johns20188]

Hi everyone,

I am looking at using equation 22.5.8.3.1a from ACI 318-14 to calculate shear capacity of topped prestressed hollow core plank. What confuses me is why does the shear capacity depend on Vd, the shear from unfactored dead loads? Why does our capacity increase as Vd increases? It doesn't make sense to me that adding more load increases your shear capacity.


Here's the equation:
Vci=0.6*sqrt(fc')bw*d+Vd+Vi*Mcre/Mmax

Thank you.

 

[rapt]

No Vd is not "the decompression term". Decompression is a combination of Vd + Vi * Mcre / Mmax.

In composite sections, Vd is separated from the rest as the calculation for Mcre is only for loads applied to the composite section and using the composite section properties, and Vd is applied to the original precast shape only.

I am not sure why this is a puzzle at all. It is fully explained in ACI318 - 14 R22.5.8.3.1. Your musings are incomplete and complicating it.

 

[r13]

rapt,

Yes, you are correct. It was a puzzle to me as I don't have access to the code, and too far away from my class days. In my try, I wasn't sure I had a clear understanding of the full term of V[sub]i[/sub]*(M[sub]cre[/sub]/M[sub]max[/sub]), though I knew it was the prestress effect on the ultimate design shear less the dead load shear, which was added in front of this term. So in response, I call V[sub]d[/sub] the "decompression effect", not the whole term. My whole point was the second term and the third term are pre-stressing effect, not the physical concrete shear capacity as in conventional reinforced concrete.

I appreciate your explanation, so led me to find the correct word - "decompression. Thanks.

 

[Johns20188]

Rapt,

I guess the reason for this thread was that, initially, it was just a little confusing why Vi was multiplied by Mcre/Mmax while Vd was on its own. I could just read the commentary and accept that for some reason Vd alone accounts for the precomposite decompression part. But this thread is meant to discuss the equation more in depth. It is definitely overcomplicating it but I enjoyed delving deeper into the equation.