RC 3-Span Slab Bridge Fatigue Analysis following AASHTO Standard Spec Design

[JHart_11]

I am looking for some direction on checking the fatigue for a 3-span slab bridge (40ft-50ft-40ft) that needs to be designed following the AASHTO Standard Spec since it is part of a rehab of an existing bridge. In Section 8.16.8.3 it gives the formula for the fatigue stress range that is not to be exceeded as Ff = 21-0.33*fmin+8*(r/h). It goes on to define fmin as the "algebraic minimum stress level, tension positive, compression negative in kips per square inch" but does not give any further guidance on how to determine fmin. Can anyone tell me how I actually calculate fmin? Any and all assistance would be greatly appreciated. Thanks!

 

[bridgebuster]

Look at the attached it shows how to calculate the tension and compression per the Standard Specs.

 

[JHart_11]

Thank you bridgebuster for the example. So my understanding is that fmin = fs - fc, is this correct?

 

[bridgebuster]

you're welcome.


f_min is the sum of fatigue truck moment at the point under consideration(could be + or -) plus dead load moment, divided by A_sjd. You compare that to Gamma times delta f, where delta f is your stress range divided by A_sjd.

If f_min >Gamma delta f you're OK

Check you're clients requirements if the loads are factored or unfactored. I've seen it both ways.

 

[JHart_11]

This is exactly what I needed. Thanks for the information.

 

[bridgebuster]

You're welcome