AISC 360, Chapter K of the spec deals with this problem. You could also assume the max moment to be PL/4 (P=2500lbs/4, L=5") and assume the bending capacity of the tube wall to be Fy*Z, where Z=2"*.25^2/4 and Fy=yield strength of tube (usually 46ksi for HSS members). The latter method is a...
Scully,
For the SCBF case, the max load that can be delivered to the system is either the shear capacity of the diaphragm, or a load that creates overturning of the footing. Remember that this "out" only applies to the tension force in the brace and not compression forces. In SCBF, the...