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 unconservative "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).

Download The One-Third Stress Increase: Where is it now? by 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 as well as below:

Download Supplement No. 1 to the Specification for Structural Steel Buildings Allowable Stress Design & 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.