Steel structures question 1

[jamsam75]

Bases on the AISC provisions, What is the major difference between the steel rolled W-shape beams and the built-up pale girders. and what are the limit state considered for both of them respectively?

 

[msquared48]

The obvious main difference is that the W sections only go up to 36" in depth, whereas, with proper design, plate girders can be designed and fabricated of any reasonable depth to meet the requirements of the engineering problem.

Mike McCann
MMC Engineering
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Motivation: Don't ask

 

[kelowna]

Very broad question.
Apart of what Mike said, tolerances are tighter with built up sections, and they allow to customize the size to your needs.
In terms of design, I am not aware of any significant differences

 

[abusementpark]

W-shapes are proportioned such that a lot of the stability limit states ( flange local buckling, web local buckling, web shear buckling, etc) are not applicable by default as noted in the code.

Plate girders are generally proportioned with more slender elements and all the applicable limit states will need to be checked. Connecting welds of the plates in the built-up shape with also have to be designed for proper shear transfer. With plate girders, there also comes some other design considerations to attenuate these additional limit states, like the decision to use transverse or longitudinal stiffeners to mitigate local buckling. Introducing these additional elements will result in additional design considerations regarding the proportioning of the stiffeners.

Quite simply: Plate girders are a helluva lot more complicated to design and detail than rolled shapes. They are generally only used for long-span (and/or curved) steel bridges. In most building applications, a structural steel truss will be used for long spans.