# Impact the same as dynamic allowance?



## McEngr (Dec 5, 2011)

Is the dynamic allowance the same as impact? The reason I ask is because it appears that the SE Practice problems refer to a problem that asks for impact negative moment of a slab at a support. It appears that this is an error.


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## McEngr (Dec 5, 2011)

Answering my own question here. The answer is that the DLA replaces the impact factor.


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## ipswitch (Dec 7, 2011)

McEngr said:


> Is the dynamic allowance the same as impact? The reason I ask is because it appears that the SE Practice problems refer to a problem that asks for impact negative moment of a slab at a support. It appears that this is an error.


I don't use dynamic allowance for anything, I use impact factor for LL. They are different animals. I think chap 3 of AASHTO talks about this.


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## ipswitch (Dec 8, 2011)

ipswitch said:


> McEngr said:
> 
> 
> > Is the dynamic allowance the same as impact? The reason I ask is because it appears that the SE Practice problems refer to a problem that asks for impact negative moment of a slab at a support. It appears that this is an error.
> ...


Depending on what design standard you use. ASSHTO LRFD calls the impact factor "dynamic allowance" Other bridge specs call it impact factor. I just call it impact factor.


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## McEngr (Dec 8, 2011)

ipswitch, you forced me to double check myself yet again. I'm glad you posted again as I was going to be scratching my head for a few days trying to figure out the difference again.

I think the word allowance is a big problem from an interpretation standpoint. The implied meaning is that it is optional, but it clearly is a requirement.


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## ADB (Dec 8, 2011)

AASHTO just changed terminology and the way it is computed. AASHTO Std. Specs used a different formula that was dependent upon the span length. LRFD uses 1.33.


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## McEngr (Dec 8, 2011)

Thank you ADB. It helps to have forums like these to get clarification. I hope to pay it forward during this study process...


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## ipswitch (Dec 8, 2011)

ipswitch said:


> ipswitch said:
> 
> 
> > McEngr said:
> ...


I was shooting at the hip the first post until I checked the AASHTO this morning when I arrived at the office.


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