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CMAA 74 Welded Box Girder Proportions

CMAA 74 Welded Box Girder Proportions

CMAA 74 Welded Box Girder Proportions

(OP)
In CMAA 74 Paragraph 3.5.1 Proportions for Welded Box Girders, one of the equations is:

h/t ≤ 1000/sqrt(σy)

where,

h = height of web (in)
t = thickness of web (in)
σy = min. yield strength (ksi)

This formula does not exactly work the way one would expect. Consider this example:

Let's say I have decided I want a web height of 50 and I am using A36 Steel.

50/t ≤ 1000/sqrt(36)
50/t ≤ 166.7
t ≥ .300

Now I decide I want to use A50 Steel instead:

50/t ≤ 1000/sqrt(50)
50/t ≤ 141.4
t ≥ .354

How does it follow that I require a thicker web with a STRONGER steel? Checking the equation's end points confirms something weird is happening. As the strength of steel approaches zero, my required web thickness would approach infinity and vice versa.

Am I missing something or is this a typo in the print? I am using the 2010 edition. I'd appreciate it anyone can shed some light. Thanks!

RE: CMAA 74 Welded Box Girder Proportions

(OP)
Edit: I wrote the endpoint statement wrong. As strength of steel approaches infinity, my required web thickness also approaches infinity.

RE: CMAA 74 Welded Box Girder Proportions

Everyhandleistaken:
The higher you make the working stress, the more likely you are to buckle the web, unless you make it thicker. That’s the short answer. We would have to look a bit closer to see exactly where that equation came from.

RE: CMAA 74 Welded Box Girder Proportions

(OP)
Dhengr:
That's a very reasonable explanation. I hadn't taken into account that the formula compensates for the expected forces on the box girder.

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