Built up girders with uneven flange width and thicknesses

[YashKaran]

Hi all--

I was wondering why the top and bottom flange width and thickness vary for built-up shapes. I have my own reasoning which is that moments vary along the length of the girder since it is a continuous structure. Also, if the bottom flange is thicker at one location and the section modulus is more for Sx- versus Sx+ then which section modulus would you use to calculate the bending stress of the girder at the maximum moment location. Or would I use the formula Mc/I instead and choose the larger c (at the thicker flange) since I stays the same.

I have also attached a screenshot of the girder I am talking about. The girder in question has a much thicker flange at pier 3 which is because the negative moment at the pier is much higher than the positive moment at other locations. Any other thoughts or comments on why beam flange width and thickness vary along the length of the girder?

Thank you!

 

[BAretired]

I was wondering why the top and bottom flange width and thickness vary for built-up shapes. I have my own reasoning which is that moments vary along the length of the girder since it is a continuous structure. Also, if the bottom flange is thicker at one location and the section modulus is more for Sx- versus Sx+ then which section modulus would you use to calculate the bending stress of the girder at the maximum moment location. Or would I use the formula Mc/I instead and choose the larger c (at the thicker flange) since I stays the same.

I have also attached a screenshot of the girder I am talking about. The girder in question has a much thicker flange at pier 3 which is because the negative moment at the pier is much higher than the positive moment at other locations. Any other thoughts or comments on why beam flange width and thickness vary along the length of the girder?

It looks like a composite girder. For positive moment, the top flange includes a concrete slab. That is why the bottom flange is larger than the top flange.

At Pier 3, a negative moment region, the only composite action possible is the reinforcement in the slab, but it looks like the designer ignored that and considered just the steel section for negative moment. So for your analysis, Sx is probably valid for negative moment, but for positive moment, you would have to consider the composite section properties.