Live Load Capacity Factor and Dynamic Load Allowance
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Lomarandil
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I'm no AASHTO expert, but off the top of my head, do you mean the Impact and Live Load Distribution Factors?
I do a little light bridge work, but nothing extraordinary.
Per the AASHTO LRFD Article 3.2, Dynamic load allowance is defined as “an increase in the applied static force effects to account for the dynamic interaction between the bridge and moving vehicles.”
It used to be referred to as impact load, and the symbol IM is still used to defined dynamic effects. To determine the force effect, PLL+I due to live and dynamic loads; PLL+I = PLL*(1+IM). IM is dependent on the component analyzed and the limit state used. IM = 75% for deck joints at all limit states, 15% for all components at the fatigue and fracture limit states, and 33% for all components at other limit states.
This factor is not applied to design lane loads (though it is applied to truck loads), centrifugal and braking forces, and pedestrian loads. Also it is not applied to a variety of components including retaining walls, foundations below grade, wood components, or components listed by the governing body.
I do not have the 6th edition of the LRFD manual, but I do find the Structural Engineering Reference Manual offers good coverage on this for those taking the structural depth PE or even the SE exam.
Also, you can take a look at a document by the USDOT that covers the LRFD Bridge specs: USDOT Bridge nhi 08048
Per the AASHTO LRFD Article 3.2, Dynamic load allowance is defined as “an increase in the applied static force effects to account for the dynamic interaction between the bridge and moving vehicles.”
It used to be referred to as impact load, and the symbol IM is still used to defined dynamic effects. To determine the force effect, PLL+I due to live and dynamic loads; PLL+I = PLL*(1+IM). IM is dependent on the component analyzed and the limit state used. IM = 75% for deck joints at all limit states, 15% for all components at the fatigue and fracture limit states, and 33% for all components at other limit states.
This factor is not applied to design lane loads (though it is applied to truck loads), centrifugal and braking forces, and pedestrian loads. Also it is not applied to a variety of components including retaining walls, foundations below grade, wood components, or components listed by the governing body.
I do not have the 6th edition of the LRFD manual, but I do find the Structural Engineering Reference Manual offers good coverage on this for those taking the structural depth PE or even the SE exam.
Also, you can take a look at a document by the USDOT that covers the LRFD Bridge specs: USDOT Bridge nhi 08048
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