Is the bearing bottom plate a compressive member? Is post-weld heat treatment required for extra-thick sections?

The base plate is not a compressive member; the post-weld heat treatment covers the welds between compressive members as well as those connecting them to non-compressive members.

Does it mean that the fillet welds between the gusset plates and the bottom plate need to be heat-treated, while the welds on the bottom plate itself do not require heat treatment?

The base plate is merely a rigid component and does not require heat treatment. Like many connection plates in steel frames, elements such as the foundation slab are also thick and do not require heat treatment

Are the welds between the rib plates and the gusset plates of the saddle, as well as the joints between the rib plates themselves, subject to heat treatment?

The support floor is not a compressive member, and post-weld heat treatment is not required in accordance with GB150. However, attention should be paid to the material, thickness, and stiffness of its welded structure; appropriate preheating or heat treatment should be carried out based on experience to reduce welding stress cracks resulting from the structural stiffness.

1 The fillet welds between the stiffener plate and the bottom plate shall be heat-treated. The requirements for the heat treatment of such fillet welds are not specified in the GB150 standard; the designer may set such requirements according to the actual circumstances. Corner welds of carbon-manganese steel structural members generally do not require post-weld heat treatment, as the welding dimensions are usually small. 2 Does the bottom plate’s own seam not require heat treatment? The heat treatment requirements for the joints in the support base plate are also not covered by the GB150 standard. If the base plate is particularly thick (for example, greater than 38 mm), it is likely because the base plate is subjected to large loads. If full-thickness butt welding is used, it is still recommended to carry out localized post-weld heat treatment. Non-pressure components can also be under load, and cracking must also be avoided. The main purpose of post-weld heat treatment is to relieve welding residual stresses. Whether post-weld heat treatment is required depends mainly on the size of the weld, rather than the thickness of the material.

Take a look at defining the compressed and uncompressed elements of the 150 and the fixity gauge

Thank you; it seems that the tolerance specifications apply only to the definition of the main pressure-bearing components