bugbus
Structural
- Aug 14, 2018
- 533
Clause 15.15 states that: "Unless subject to special study, the abutment forces determined from the static or dynamic analysis shall be multiplied by the design ductility factor (mu) to obtain the design abutment forces."
This requirement seems to come from Austroads Technical Report (AP-T200-12, Clause C4.7.7). The gist of that report is that abutments are more prone to damage in an earthquake and should be assessed elastically.
That clause states the following:
"The static analysis procedure is a single mode approach, using design horizontal forces reduced for ductility. Although this is suitable, and conservative for determining the design moments in plastic hinges, it is non-conservative when abutment reactions are considered, since the analysis approach implies that these will also be reduced by the ductility factor. In fact, higher mode effects, which have only minor significance to plastic hinge moments, can significantly increase the abutment reactions above the value predicted from the lateral analysis using ductility-reduced horizontal forces. This is the reason that failures of abutments and shear keys are the most common form of damage to bridges in earthquakes.
Unless special studies are carried out to determine the magnitude of abutment forces, these should conservatively be assumed to be equal to the unreduced elastic spectrum forces. This implies multiplying the reactions determined from the analysis defined in Cl. 4.7.6 by the design system ductility factor."
In other words, the abutments need to be assessed elastically with mu = 1.0 (effectively).
Is there any situation where an abutment could be considered "pier-like" in terms of its flexibility and ability to form plastic hinges, thus allowing the use of a ductility factor > 1.0? I suppose this is what is meant by "special study", but just wondering if anyone has gone through this process before?
My (limited) understanding is that typical abutments are basically locked into the ground and thus very stiff compared to piers, and also less able to reliably/predictably undergo plastic deformation for the same reason.
This requirement seems to come from Austroads Technical Report (AP-T200-12, Clause C4.7.7). The gist of that report is that abutments are more prone to damage in an earthquake and should be assessed elastically.
That clause states the following:
"The static analysis procedure is a single mode approach, using design horizontal forces reduced for ductility. Although this is suitable, and conservative for determining the design moments in plastic hinges, it is non-conservative when abutment reactions are considered, since the analysis approach implies that these will also be reduced by the ductility factor. In fact, higher mode effects, which have only minor significance to plastic hinge moments, can significantly increase the abutment reactions above the value predicted from the lateral analysis using ductility-reduced horizontal forces. This is the reason that failures of abutments and shear keys are the most common form of damage to bridges in earthquakes.
Unless special studies are carried out to determine the magnitude of abutment forces, these should conservatively be assumed to be equal to the unreduced elastic spectrum forces. This implies multiplying the reactions determined from the analysis defined in Cl. 4.7.6 by the design system ductility factor."
In other words, the abutments need to be assessed elastically with mu = 1.0 (effectively).
Is there any situation where an abutment could be considered "pier-like" in terms of its flexibility and ability to form plastic hinges, thus allowing the use of a ductility factor > 1.0? I suppose this is what is meant by "special study", but just wondering if anyone has gone through this process before?
My (limited) understanding is that typical abutments are basically locked into the ground and thus very stiff compared to piers, and also less able to reliably/predictably undergo plastic deformation for the same reason.
