AISC B3.3 3

[peteotoole]

AISC B3.3 doesn't allow you to reduce moments "produced by loading on cantilevers, for design using partially restrained (PR) moment connections, or for design by inelastic analysis...and then further on in the commentary

[/indent]"This adjustment is appropriate only for cases where the inelastic redistribution of moments is possible. For statically determinate spans (e.g., beams that are simply supported at both ends or for cantilevers), redistribution is not possible; therefore, the adjustment is not allowable in these cases. Members with fixed ends or beams continuous over a support can sustain redistribution."

For the picture attached then, would this beam be available for moment reduction because as I read it in the commentary the beam "Must be either fixed at both ends or continuous over a support".

 

[Agent666]

I'd say that situation is statically indeterminate, therefore you can use redistribution.

Think of it like this, if you increase your loading to the point where a hinge occurs (say considering UDL on member), a hinge will form at the left-hand support first (i.e. member plastic capacity will be reached). Assuming the member has full lateral restraint and doesn't buckle, this formation of the hinge does not form a collapse mechanism.

You can continue to increase the load, and the negative moment at the left-hand support cannot increase as it is capped effectively by the plastic hinge capacity, so the increase in loading will cause a redistribution of the negative moment at the support to positive bending near midspan. If you increase the load enough, you will form another hinge near midspan. At this point you have formed a 'mechanism' because the system has three pins/hinges and with the roller support on the right end the beam can collapse.

But you can utilise redistribution to increase the load carrying capacity in this propped cantilever scenario. This is illustrated via my sketch below if above wasn't clear:-

Forget about what the code says, and go back to statics and first principles and analyse what occurs if you progressively increase the load within a system to determine if you can redistribute actions in any given situation.

https://engineervsheep.com

 

[271828]

Agree with Agent666.

It might help to take a look at the Continuous Beams chapter in Salmon, Johnson, and Malhas. (In some other books also, especially older ones.) The topic goes under the name "Plastic Analysis" or "Plastic Design."

 

[LeonhardEuler]

Agree, this is a case where plastic redistribution is possible.

I’m curious if there are any serviceability concerns in your design

 

[dik]

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