JStephen
Mechanical
- Aug 25, 2004
- 8,724
A design issue that comes up on occasion is a vessel supported on fairly short legs, with the vessel operating above ambient temperature.
To design the legs, I would usually assume the connection to the shell is "rigid", assume the legs are restrained horizontally at the foundation, and go from there, and that seems similar to the approach in Bednar.
The problem is, that with short heavy legs, if you use these same assumptions, then calculate radial deflections in the vessel from temperature and pressure, it will then give you very large radial forces in the legs. Attempting to beef up the legs is then counterproductive as it only increases the loads.
Meanwhile, if that leg can shift maybe 1/8" at the bottom, that load disappears entirely.
So how is that normally handled?
Is it reasonable to assume a base plate with bolts and grout is "fixed" for wind/seismic shear, but can shift enough to relieve thermal stresses?
Or assume the foundation can shift enough to relieve those forces?
Or use slotted base plates and bearing pad rather than a grouted column detail?
Or is this effect just neglected in the design?
A second question relating to the above-
In Bednar, he assumes allowable concrete stress of 0.25fc' under a baseplate, and I see similar recommendations elsewhere. Per the concrete codes, factored bearing can range up to 1.105fc' (including the A2/A1 factor) or with a load factor of 1.6 maximum, allowable bearing is about 0.69fc'. Is there some reason to use lower allowable stresses than what the concrete codes allow here?
To design the legs, I would usually assume the connection to the shell is "rigid", assume the legs are restrained horizontally at the foundation, and go from there, and that seems similar to the approach in Bednar.
The problem is, that with short heavy legs, if you use these same assumptions, then calculate radial deflections in the vessel from temperature and pressure, it will then give you very large radial forces in the legs. Attempting to beef up the legs is then counterproductive as it only increases the loads.
Meanwhile, if that leg can shift maybe 1/8" at the bottom, that load disappears entirely.
So how is that normally handled?
Is it reasonable to assume a base plate with bolts and grout is "fixed" for wind/seismic shear, but can shift enough to relieve thermal stresses?
Or assume the foundation can shift enough to relieve those forces?
Or use slotted base plates and bearing pad rather than a grouted column detail?
Or is this effect just neglected in the design?
A second question relating to the above-
In Bednar, he assumes allowable concrete stress of 0.25fc' under a baseplate, and I see similar recommendations elsewhere. Per the concrete codes, factored bearing can range up to 1.105fc' (including the A2/A1 factor) or with a load factor of 1.6 maximum, allowable bearing is about 0.69fc'. Is there some reason to use lower allowable stresses than what the concrete codes allow here?
