fatigue moment

[rnorth]

can someone walk me through the steps for calculating fatigue moment for a simple span bridge, span 80', rolled section, service load on one beam: 625 plf DL, 425 plf LL.
thanks
code is AASHTO 2002, using LFD

 

[Dinosaur]

I suppose the fatigue moment is equal to the fatigue stress range x the Section modulas for the beam, Sx. Because of the presence of diaphragm connector plates, I use a fatigue stress range of 13 ksi (Category C) to design rolled beams. Some engineers only check the fsr (fatigue stress range) at specific points where these connector plates exist, but I don't care for that myself. What if the fsr was exceeded only 2 inches from the connector plate? Would you still say it was a category B 2 inches from a stiffener and catagory C at the stiffener? I have worked with engineers that check the fsr at the top surface of the flange since that is where the weld is. Again, I don't care for this. The heat affected zone (HAZ) is well into the thickness of the flange. Good luck. 80ft spans with rolled beams is going to be tough. I'm sure you will be using at least 36 inch deep beam or maybe a meter beam for this.

 

[rnorth]

thanks. so far I have:
take unfactored moment and shear from the live load only, increase it by 1.15 for dynamic load allowance then multiply by 0.75 for the fatigue load factor.
also: above dead load should be 890 plf

 

[Dinosaur]

I am not sure if this is correct because I think there is a fatigue truck now. I think you are supposed to do a fatigue envelope using this truck.

In the old days, I was expected to use the values from my "normal" analysis to check fatigue. But because I think there is now a fatigue truck, I don't think you can just use that straight forward approach anymore.

On the other hand, I believe your approach would be conservative, but likely not economical.

I haven't designed a bridge to the new LRFD code so I may not have my facts straight on this.

Good luck.

 

[shin25]

AASHTO 2000 LFD is standard specification not LRFD. So, there is no different truck for fatigue.

 

[bridgeart]

I have the following "slightly related" question to the initial post: AASHTO LRFD 2004 defines fatigue truck essentially as the HS20 truck with spacing between the two rear axles fixed at 30ft and 15% impact (load factor for fatigue equal to 0.75).

Article 3.6.1.4.1 then states that "... the fatigue load shall be ONE design truck ...". Does this mean that for the design of a transverse member (e.g. cross girder), fatigue truck is applied only in one lane, although there may be multiple lanes available? (My interpretation is that it is not necessary to apply fatigue truck in multiple lanes.)

Thanks!

-ok

http://www.bridgeart.net

 

[bridgebuster]

rnorth - what are you trying to do? A rolled beam would be checked for fatigue unless you had Category C or higher detail(s).

Perhaps you should get a copy of "A Fatigue Primer for Strucutral Engieers" by the National Steel Bridge Alliance. It contains examples for various types of bridge fatigue calculations.

bridgeart - I've done it the same way for floorbeams; one truck positioned to give the maximum load.