Design of partial fixity connections

[onsiteeng]

Hi All,
We have been reviewing a steel building where the designer has used partial fixity to reduce the moments in the beam. Magically! the partial fixity negative moment at the ends of the beam exactly matches the positive moment at midspan and therefore the lightest possible steel section can be selected. What baffles us is they use the same connection for multiple beam type with different partial end moments. The beam is a W12 x 16 with 12mm end plate that extends 3" above the top flange, has a mid plater stiffer on each side of the web and top plate stiffener on the top flange and 12 high strength(grade 8.8 16mm bolts. Sure looks like a fixed connection to us.

For checking deflection they assume fully fixity of both ends so it Seems like they are cherry picking the best of all scenarios for strength and serviceability to end up with the lightest beam they can use. Has anyone else seen this type of design and how would you design a connection to rotate the exact amount to match what you have specified in the computer model.

Thanks

 

[Tomfh]

For plastic design it's not so much that you tailor the end fixity to balance the moments, it's that the structure starts to yield meaning the positive and negative moments balance out in the collapse state. At service loads it may be valid to say its fully rigid with unequal negative and positive moment.

 

[Lomarandil]

Not having done plastic design much myself, what's the catch of that methodology? Is it simply that as long as the serviceability works out, and your hinges are in suitable locations, you're OK?

Speaking of, how does serviceability look with a W12x16?

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just call me Lo.