Allowable stress for structural members - t/R ratio
Allowable stress for structural members - t/R ratio
(OP)
Hi Guys,,
Just wanted to confirm with other engineers here is the R in t/R, the outer diameter of the structure member in question?
I copied the notes at the bottom of table 5.3, its says the following;
"The variables in the compressive stress equations are defined as follows:
l= unbraced length of the column, in in.; r= corresponding least radius of gyration of the column, in in.; t= thickness of the tubular
column, in in.; Y= unity (1.0) for values of t/R equal to or greater than 0.015; Y= (2/3)[100(t/r)] {2–(2/3)[100)t/r)]} for values of t/R less than 0.15."
Just wanted to confirm with other engineers here is the R in t/R, the outer diameter of the structure member in question?
I copied the notes at the bottom of table 5.3, its says the following;
"The variables in the compressive stress equations are defined as follows:
l= unbraced length of the column, in in.; r= corresponding least radius of gyration of the column, in in.; t= thickness of the tubular
column, in in.; Y= unity (1.0) for values of t/R equal to or greater than 0.015; Y= (2/3)[100(t/r)] {2–(2/3)[100)t/r)]} for values of t/R less than 0.15."





RE: Allowable stress for structural members - t/R ratio
RE: Allowable stress for structural members - t/R ratio
Im using API 620 - latest one with 2012 changes.
Ive looked at older versions and they have the same table and definitions.
I also curious is there a more dated version of this standard because I have noticed some typos here and there and even sometimes missing math operators like division and multiplication in formulas. For example I strongly believe the tubular axial loading allowable compressive stress whould be l^2/18000*r^2, not l^2*18000*r^2. You get a very small stress regardless of l and r values otherwise.
cheers,
pato
RE: Allowable stress for structural members - t/R ratio
On the compressive stress calculation, compare the line for tubes to the line above it for structural members- there is in fact a divide sign that is missing. Also, it's not obvious, but the r is part of the denominator- it is usually written (L/r)^2/18000, rather than split up as shown.
RE: Allowable stress for structural members - t/R ratio
When finding Y for the compression equations for structural members, the t/R ratio that governs Y is,
t(thickness of structural member) / R (distance from compression member's centreline to the sidewall)
is that right?
and second,
is the equation of the compression member the following,
18000(Y + ( 1 + (l^2/(18000r^2)) ) )
and finally,
the equation of Y for t/R less than 0.015(which by the way says 0.15, clear typo there) is the following,
Y = (2/3)[100(t/r)]*[2 –((2/3)(100(t/r))], where
t=member thickness
r=radius of gyration
Are my formulas and what each variable definition right?
RE: Allowable stress for structural members - t/R ratio
18000Y/(1+(L/r)^2(18000))
And,
You're right, there's a typo but, I think the correct number is 0.015.
Using 0.15 would be very conservative for a structural column, which I think API 620, Table 5.3 is intended for. Considering an 18 inch diameter column, the thickness would be 1.35 inch before the user would get away from the reducing coefficient Y.
Can anyone source this portion of 620? I'd also like to know the source of the formulas in API 620, Section 5.5.4, and API 650 E.6.2.2.3? These allowable compression stress equations seem related.
RE: Allowable stress for structural members - t/R ratio