If you are "simply" talking about skirt supports (hotbox) on vertical pressure vessels and not nuclear reactor hotboxes, the following books all have simple structural design procedures:
"Pressure Vessel Design Handbook" - by Bednar
"Process Equipment Design" - by Brownell & Young
"Pressure Vessel Design Manual" - by Moss
"Pressure Vessel Handbook" - by Megyesy
"Structural Analysis and Design of Process Equipment" by Jawad and Farr;
"Mechanical Design of Process Systems - Vol 1" by Escoe;
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I also have case of HOT BOX design for the skirt support of tall column operating at very high temperature. Finite element analysis method been adopted to design the Hot box following the sections provided in the ASME section Viii Div2, Part 5.