what is the accepted percentage from capacity to design moment connection

There is no accepted percentage. It should be designed for the moment at the end of the beam. You can design it for the full moment capacity of the beam, but that's likely overkill and overly expensive.

If you don't know the moments, how can you select the members? If you're just the connection designer, ask the EOR for the connection design loads.

It's usually specified in the contract documents. Some engineers want the reaction equal to 50% of the max UDL the span will support, some want 67%, some want half the web shear capacity. It varies. On a recent job, the EOR wanted full capacity of the HSS section at all connections... this meant welding a 'block' of steel at the joint and doing CJP welds for all HSS connecting... This wasn't what the EOR wanted... so he spec'd the real loads. The first three are the most common ones I encounter... and my work for the last three years is designing connections.

For moment connections, usually the moment is spec'd. Alternatively, if often the full moment capacity of the section.

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