Question on AASHTO LRFD 2014 sec 3.10.9.2 Seismic Zone I 1

[Stewie_]

Hi, Dear All,

AASHTO LRFD 2014 sec 3.10.9.2 tells that "For all other sites in Zone 1, the horizontal design connection force in the restrained directions shall not be less than 0.25 times the vertical reaction due to the tributaiy permanent load and the tributaiy live loads assumed to exist during an earthquake."
Is this tributary dead/live load should be factored?
If it is, then the factored EQ becomes
gamma_EQ*0.25*(gamma_DL*R_DL+gamma_LL*R_LL).
Or it should be
gamma_EQ*0.25*(R_DL+R_LL).
This is about fixed pier dowel design. I just don't feel good using the load factors twice.
Please help.

Thanks,
Stewie

 

[BridgeSmith]

The loads should be factored in accordance with the Load Combination Table in 3.4.1. See the commentary on the Extreme Event I load combination for discussion of gamma EQ values for Live Load.

You will also need to apply the correct resistance factor from 6.5.4.2 to the capacity calculated in 6.13.2.12.

 

[Stewie_]

Hi, Dear HotRod10,

Thank you so much for the help.
My question is about how to calculate H_EQ according to AASHTO LRFD 2014 3.10.9.2.
I found some examples from online, but the procedures are different from each other.
1. BDM Example 9 from CODOT:
Unfactored reactions are used.
H_EQ = 0.25*(R_DL + R_LL) in the example.
2. FHWA LRFD example:
Service dead load is used. The gamma_EQ is assumed as 0. My understanding is FHWA example uses unfactored dead load and factored live load to calculate the earthquake effect.
H_EQ = 0.15*(R_DL + gamme_EQ*R_LL) in the example, where gamme_EQ = 0.
3. Section 12 2008 NYDOT bridge manual:
Factored reactions are used, and they are Strength I load factors.
H_EQ = 0.1*(1.25*R_DC + 1.5*R_DW + 1.75*R_LL) in the example. (This is an example following AASHTO LRFD 2007.)
4. MNDOT Transmittal Notice (2016-03):
AASHTO LRFD 2014 sec 3.10.9.2 is modified to "Design for a minimum horizontal connection force equal to 15% of the Strength I limit state vertical reaction."
H_EQ = 0.15*(1.25*R_DC + 1.5*R_DW + 1.75*R_LL) in this case.

I guess the answer is to use factored/unfactored reactions to calculate H_EQ depends on which state the project is in.

Thanks,
Stewie

 

[BridgeSmith]

Various states have either adapted the AASHTO provisions, while others have simply codified the AASHTO provisions for the seismic hazard (or lack thereof) in their state. CDOT has followed the AASHTO provisions, except that they used a gamma_EQ = 1.0. The FHWA example could be out of date by a few editions; they do that fairly often. However, service DL = 1.0 DL = unfactored DL, so it's consistent with the spec. in that regard. The spec. allows gamma_EQ values from 0.0 to 1.0. The commentary sort of half-heartedly recommends a value of 0.5 for "a wide range of" traffic levels.

Anyway, I guess you could look at what your state DOT uses and consider using that, or just use what's in the current spec. As I said, the load combination table is fairly clear, I think. The only part that's left open is the value of gamma_EQ, which you would base on the typical traffic level for the bridge. If it's a heavily-used roadway, maybe use a value closer to 1.0. If OTOH, it's on a rarely used county road, the chances of a heavy truck being on the bridge during an earthquake may be so remote that you don't even want to consider it, and you would use 0.0.