Bridge Wind Load Analysis - Calculate moment on steel beam flange

[CivilSigma]

Hi Folks,

I need help calculating moments from wind load on the top and bottom flanges of steel bridge beams. Attached is a problem statement to calculate the maximum bending moment in the top and bottom flanges of the beam.

My approach was:

1. Find wind load tributary to top and bottom flange
2. Calculate moment as wl^2/8

Some after thought:
3. I tried to divide the calculated moment by 4 since beams are connected?
4. Is there a load factor in ASSHTO that I'm missing?

How would you solve such a problem?

 

[Lomarandil]

I won't (and you shouldn't) speak to anything that was on the exam, assuming the standard confidentiality agreement was in place. In broad terms, here are a few things that often trip up engineers the first time they analyze wind on bridge girders.

• When deck is in place, we consider the leeward girders shielded, and only apply wind along the windward projected elevation.
• Note that the wind tributary to the top flange will include deck thickness, curb, barriers, etc. However, we typically consider the concrete deck diaphragm as the load path for that wind, and are allowed to neglect any lateral stresses in the top flange (e.g. from compatibility).
• Assuming the presence of diaphragms along the span length, we would usually consider the windward bottom flange to be supported by those diaphragms, and continuous across them. So not a simple span wl^2/8
  
  Other than that, the main thing that trips people up is mixing and matching AASHTO editions across the transition from service level wind to ultimate level winds

 

[CivilSigma]

I won't (and you shouldn't) speak to anything that was on the exam, assuming the standard confidentiality agreement was in place. In broad terms, here are a few things that often trip up engineers the first time they analyze wind on bridge girders.

Agreed... but who said anything about exams On a serious note, bridges are not my specialty, so thank you for your help.

• When deck is in place, we consider the leeward girders shielded, and only apply wind along the windward projected elevation.

Understood. So, there's no load sharing with the shielded beams' top flanges, even though they are all connected into the diaphragm?

• Note that the wind tributary to the top flange will include deck thickness, curb, barriers, etc. However, we typically consider the concrete deck diaphragm as the load path for that wind, and are allowed to neglect any lateral stresses in the top flange (e.g. from compatibility).

So, there will be no moment as a result of lateral wind load in the steel beam top flange which is braced by the deck?
Diaphragm action will result in some tension and compression forces into the flanges if we treat it as flexible correct? But I don't see how this will help me calculate flange moment.

• Assuming the presence of diaphragms along the span length, we would usually consider the windward bottom flange to be supported by those diaphragms, and continuous across them. So not a simple span wl^2/8

Do you mean the top flange? The bottom flange is un-restrained.

Other than that, the main thing that trips people up is mixing and matching AASHTO editions across the transition from service level wind to ultimate level winds

From ASSHTO Table 3.4.1-1, both the strength and serviceability load factors for wind are 1.0 (Seventh Edition). So, there would be no load factor increases correct?