Load Rating K Calculation for Min. Reinf. (MBE 6A.5.6.)

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Nate
Load Rating K Calculation for Min. Reinf. (MBE 6A.5.6.)

I am load rating an existing bridge. My rating factors are low, and I've traced this back to the 'K' reduction factor (MBE 6A.5.6) that is applied to capacity in the Rating Factor Equation. K is meant to account for concrete members that do not satisfy the minimum flexural reinforcement provisions of AASHTO 5.7.3.3.2.

Calculating K appears fairly straightforward: K = Mr / Mmin <= 1.0; where Mr = phi Mn, Mmin = MIN(1.2Mcr, 1.33Mu).

However, when I try to replicate PG Super's K I cannot arrive at the same value that PGSuper calculates. In the attached file, I selected the 'Design - Inventory' truck at location 'Span 1 Girder C, (0.1Ls) 2.500' for my spot check. At this location PGSuper calculates K to be 0.606. If I try to replicate this I get the following:

Mr = 95.13 kip-ft
phi = 0.9
Mn = 105.70 kip-ft
Mmin = 209.30 kip-ft
1.2 Mcr = 209.30 kip-ft
Mcr = 174.42 kip-ft
(Girder C Det. Rep. > Moment Capacity Details > Min. Reinf....)
1.33 Mu = 258.71 kip-ft
Mu = 194.52 kip-ft
(Girder C Det. Rep. > Moment Capacity Details > Min. Reinf....)

K = 0.455

What am I overlooking? Thanks for any feedback!

Rick Brice
K calculation

Nate,

PHI = 1.0 for precast-prestressed girders so Mr = PHI*Mn = 105.70k-ft
In LRFD 6th Edition 2012, AASHTO changed MrMin from Min(1.2Mcr,1.33Mu) to Min(Mcr,1.33Mu) (See 5.7.3.3.2). We got the software doing the computation correctly, however, we didn't get the graphic in the report updated. The graphic has MrMin = Min(1.2Mcr,1.33Mu) and this is misleading you... sorry about that. I'll get it fixed ASAP.

At any rate, MrMin=Min(174.42,258.71)=174.42.

So K = 105.70/174.42 = 0.606 just as reported by PGSuper.

Nate
Thanks for the quick response

Thanks for the quick response Rick! This clarifies things.

-Nate

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