×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

fatigue moment

fatigue moment

fatigue moment

(OP)
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

RE: fatigue moment

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.

RE: fatigue moment

(OP)
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

RE: fatigue moment

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.

RE: fatigue moment

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

RE: fatigue moment

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
www.bridgeart.net

RE: fatigue moment

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources