In December 2021 Meca has published a new version of MecaStack which made some revisions to the baseplate entry form as well as the calculations. We took a thorough look at the calculations which had been used for many years, and felt that there were some areas where we could be more thorough in our calculations.
In general our calculations follow the book "Process Equipment Design" by Brownell and Young (B&Y); however, this book doesn't cover every scenario that our users encounter, and so we have inserted other criteria where necessary. In addition, we have found that the book "Pressure Vessel Design Manual" by Dennis Moss (Moss) closely follows some of the same methodology as Brownell and Young, and so we have used it in some instances where it offered more clarity.
This article is to summarize the major changes:
Bolt Load:
We have offered for some time the ability for the user to select that the user can calculate the bolt tension using either the iterative method of B&Y, or to use the more straight forward equation provided by ASME STS-1.
STS: Pbolt = 4*M / (N*BCD) - V / N
M = Moment
N = Number of Bolts
BCD = Bolt Circle Diameter
V = Vertical Load
When using the STS equation, we then tried to use B&Y to determine the foundation bearing stress; however, this didn't seem quite right since we were making the decision to use the simple ASME STS-1 equation (STS doesn't tell you how to calculate bearing pressure). We revised this use a slightly different form of the STS equation to calculate the bearing pressure:
Fcmax = 4*M / [PI*Ds^2 * (OD-ID)*0.5] + V / [PI * Ds * (OD-ID)*0.5]
Fcmax = Maximum Bearing Pressure
Ds = Stack Outer Diameter
OD = Outer Diameter of Baseplate
ID = Inner Diameter of Baseplate
This also is applicable when the stack is supported on a steel base. When a steel base (rather than concrete) is selected, the the software automatically calculates the bolt load using STS and also uses the above equation for the bearing stress calculation.
Gussets for Chairs:
Brownell and Young uses the following equation for calculating the required gusset thickness for a baseplate chair:
tg = Pbolt / (18000*l)
tg = required gusset thickness
Pbolt = Maximum Bolt Tensile Load
l = width of gusset
After some investigation we determine that this was a simplification of the buckling from old versions of the AISC Steel Design Manual for column buckling, with the assumption that the steel had a 30 ksi yield strength and a modulus of elasticity of 29000 ksi. Users of our software use many different types of materials and so we wanted to make it more general to be applicable for any material specified by the user. Therefore we removed the above equation and replaced it using the current buckling criteria found in AISC 360-15.
Single Baseplate:
A single baseplate with or without gussets was revised to consider tensile loading in addition to compressive stress. The B&Y procedure only mentions the compressive stress, and most likely that one usually governs; however, we added a check to look at tension also.
Chair Top Plate:
The top (Compression) plate of a chair is calculated in B&Y using constants obtained from Table 10.6
At the bottom there is a confusing note about what to do when b/l is less than 1. After reviewing the interpretation that we had made and then looking at Moss to get more insight on the authors intent, we realized that we were not applying this note correctly in MecaStack. The approach we now take is that when b/l < 1, we do the following:
1) Invert b/l and use l/b instead.
2) The equations for Mx and My are flipped, so that they become My and Mx.
3) In the moment equation we b in place of l
This won't affect every calculation, but only those where b/l < 1.
Weld Stress:
The weld stress calculation was updated to use the latest procedure in AISC 360-15.