CMAA 74 Welded Box Girder Proportions 1

[everyhandleistaken]

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

h/t ≤ 1000/sqrt(σ_y)

where,

h = height of web (in)
t = thickness of web (in)
σ_y = 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!

 

[everyhandleistaken]

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

 

[dhengr]

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.

 

[everyhandleistaken]

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.