|
Answer:
|
COMPRESS allows the designer to specify a factor, typically
taken from the AISC, that is used to increase the allowable
stress for vessel supports such as skirts and legs. The latest
AISC Steel Design Standard has discontinued the 1/3 increase
in allowable stress that had been used for many years. The
reason is that recent building codes (ASCE 7, IBC, etc.)
consider loads in combination by reducing ("factoring")
the loads due to wind and seismic. But reducing the load while still
using the 1/3 increase in allowable stress amounts to a degree
of un conservative "double dipping". Thus the
AISC discontinued the 1/3 allowable stress increase for building
codes that use a factored load design method. Further
discussion on this topic was published by the AISC in Modern Steel
Construction (October, 2003) a copy of which may be found here:
The One-Third Stress Increase: Where is it now?
Keith Mueller, Ph.D. and Charles J. Carter, P.E., S.E.
The ASME Code continues to specify a "shape factor" of 1.2
for the case of pressure combined with external loads such as weight,
wind or seismic. One explanation for this is that "ASME
structures", namely cylinders, have paths of structural redundancy
because of the "shell" nature of construction, whereas the
general type of steel structure designed using the AISC Standard consist
of beams, columns and struts. All of these "AISC type"
structural elements may provide redundant paths, especially when
assembled into a statically indeterminate structure, but in the end
the nature of this type of construction may not provide as much
redundancy, or predictability, as a shell type of structure.
Note that the 1.2 factor is still applicable for cylinders subject
to pressure in combination with external loads per ASME Code rules
regardless which building code is used to calculate the external
load.
Regarding the "Skirt/Legs/Saddles Stress Increase" input in COMPRESS:
COMPRESS determines the skirt and vessel thicknesses
based upon the combined effects of wind or seismic
plus gravity loads. The combinations considered generally
correspond to specified load combinations in the selected
building code.
COMPRESS provides an input for "Skirt/Legs/Saddles
Stress Increase" to account for the increase in
allowable stress that may be permitted by a building
code when gravity loads are combined with wind or
seismic. For supports, the design of which is
not covered by the ASME Code, the stress increase is
usually taken from the AISC. Historically,
a factor of 1.33 has been used. However, the AISC recently
rescinded the 1/3 increase in the Allowable Stress Design
standard. A copy of the revision to the 9th Edition
ASD manual is available on the AISC web site:
Supplement No. 1 to the Specification for Structural Steel Buildings
Allowable Stress Design and Plastic Design
(See page 24 of the supplement, paragraph A5)
COMPRESS provides an input for "Skirt/Legs/Saddles
Stress Increase" on the Set Mode Options dialog to
account for the increase in allowable stress that may be
permitted when gravity loads are combined with wind or seismic.
Depending on the building code selected, COMPRESS will recommend
that either 1.0 or 1.333 (1-1/3) be used. It is recommended that
1.0 be used for newer building codes while 1.333 may be
appropriate for older building codes.
Depending on the combination of building code selected and
factor input, COMPRESS may provide a warning message indicating
that the support allowable stress increase factor may need to be
changed to be consistent with the building code used.
|
