Is there any way a portal frame with pin joints at the top of the columns can be stable? Are portal frames ever done with pin joints at the bases? (Aside from those with raft foundations/combined footings)?
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Yes a portal frame with at least one fixed (moment bearing) joint on top can be stable.
Portal frames with pin joints at the base could theoretically bear moment, provided they have fixed joints on top of the columns, but they are not advisable.
In fixed base and fixed top, a frame shares the moment at all joints and vibrates under lateral loads more in uniformity with the rest of the building. Also it lends itself to smaller cross sections, hence more ductile behavior and less stiffness. Too much stiffness in a portal frame invites all the shear concentrated over its length and is detrimental to the overall lateral performance of the building.
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$\begingroup$ Good response, just one question, though. Under what circumstances would we consider the base fixed? What kind of foundation (other than a raft foundation) can we consider able to take the bending moments that assuming fixity would force it to take. $\endgroup$ Commented Nov 28, 2018 at 7:04
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$\begingroup$ set the baseplate deeply embedded in the foundation, up to the depth required for the column to develop a plastic hinge. Tie the foundation with others with a deep moment beam designed to carry the momentum and shear created by rotation of the columns. $\endgroup$– kamranCommented Nov 28, 2018 at 7:50
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$\begingroup$ Portal frames can have all joints pinned on top, so long as at least one base is fully-fixed (also for moments), and the other is at least XY-fixed. $\endgroup$– WasabiCommented Nov 28, 2018 at 19:06