# NCEES Practice Exam - Lateral Bridge PM #903 - Strange use of 'R' Value



## clutchmoves2022 (Mar 20, 2022)

Bridge engineers out there - I'm probably looking at this wrong, but I'm a bit confused by how NCEES applied the R value in the solutions. Anyone else get caught up with this question? Sorry if this gets in the weeds...

NCEES's solution to problem 903 uses R=1 for shear and axial actions, allegedly due to Art. 3.10.9.4.3.d. They use R = 5 for moments, as expected. I'm not sure if it's correct to use R = 1 for shear, since Art. 3.10.9.4.3.d seems to only apply if the plastic hinge forces are used to obtain the shear demands. The method used in the problem is based on Art. 3.10.9.4.2 which seems to allow R = 5 for shear and axial actions. It's true that the solution they have is conservative, and avoiding shear failure before flexure would be guaranteed this way....

Anyone struggle with this?
Also noticed that the solutions use γ_DC = 1.25, while the AASHTO 8th Ed. uses γ_DC = 1.00 for Extreme I.


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## BridgeEngineerLBC (Mar 21, 2022)

I haven't solved the NCEES problems yet, but plan on doing so in 2 weeks or so. As far as your question, it all depends on the seismic zone classification of the bridge. Per AASHTO Article 3.10.9.3, all elastic seismic forces (P, V, M) are divided by R for bridges in Seismic Zone 2. Per Article 3.10.9.4.3d, if the bridge is in Seismic Zone 3 or 4, then only the elastic seismic moment is divided by R.

All load factors for EE-I should be taken as 1.0 per AASHTO 8th Edition, so it looks like NCEES didn't update this part of the solution when they went from the 7th to 8th Editions. The load factors for EE-I used to be 1.25 for DC, 1.50 for DW, 1.0 for EQ, etc. When I took the vertical practice exam last fall, I noticed a few errors in one of the solutions. I believe the PM solution to the bolted splice problem used single shear plane (Ns = 1.0) to solve the problem, but bolted splices are always in double shear, so Ns = 2.0 should've been used.


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## clutchmoves2022 (Mar 26, 2022)

Thanks! I was a bit confused about the applicability of 3.10.9.4.3d, but this makes sense!


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