Are monoplane struts under tension, doubling the stress limit of the spar?
The spar on a strut braced wing effectively moves the bending stress out from the wing root to the strut attachment.  In other words, a strut braced wing is a cantilever wing from the strut attachment out, and will have the normal bottom cap in tension and upper cap in compression at that point.

Between the strut and fuselage, it's more complicated.  The spar along that section is like an inverted hammock being effectively supported at both ends.  It wants to bow up. At the root fitting, there are shear forces which depend on how far out the strut attachment is.  The forces are inboard and up if the strut attachment is closer to the tip, and inboard and down if the strut attachment is closer to the root (a Twin Otter's wing's root fittings are under inboard and downward shear because most of the wing is outboard of the strut attachment).  There is a 'sweet spot' point where you can attach the strut to the wing spar and the loads on the root fitting are straight-in compression with no vertical shear.  

This is why you will see astonishingly small bolts attaching the wing root to the fuselage on some airplanes (as in, half inch, or 5/8 inch diameter bolts at the strut attachments, and quarter inch bolts at the wing roots); if the strut picks up the wing at the right location, the loads at the root are almost all very moderate inboard shear loads.

As far as spar bending goes, the most highly stressed location is the cap strip above and below the outer strut attachment and that is where the spar has to be strongest in bending, with tension loads on the bottom and compression loads on top, in normal upright flight.