On 04/09/2017 12:42 AM, James Waldby wrote:
On Sat, 08 Apr 2017 11:02:20 -0700, christine wrote:

....

For example, putting length 30, diameter 1.05, wall thickness 0.113,
and force 75# (assuming 4 beams each carrying 1/4 of 300#) into the
"Calculate Deflection for Round Tube Beams" calculator at the bottom
ofhttp://www.engineering.com/calculators/beams.htm shows deflection
of about 0.61" which might be unsafe as unequal loading could lead to
a cascade of failures.

That calculator probably assumes a point load at the center of the beam.
But supporting a sink, it seems like much of the load would be out near the
ends of the beam, putting much of the load into shear instead of bending.
....


That seems awfully big; I couldn't get the link at that site to work
with Firefox so I went to my old standby at www.engineeringtoolbox.com

I presume same 3/4" Sch 40 pipe which from the venerable Crane handbook
shows I=0.037 in^4 and I'd assume there's only a place for the support
on either edge so the load per each is 300/2--150 lb -- 150/30 = 5
lb/in. uniform load.

The beam calculator with a 27E6 modulus gives

Total Load : 150 (lb)
Length of Beam - L : 30 (in)
Moment of Inertia - I : 0.037 (in4)
Modulus of Elasticity - E : 27000000 (psi)
Distance of extreme point off neutral axis - y : 0.53 (in)
Support Force - R1 : 75 (lb)
Support Force - R2 : 75 (lb)
Maximum Stress - : 15963 (psi)
Maximum Deflection - : 0.0845 (in)

which is much more like I'd expect, altho a tenth of an inch is more
than I'd want to design for.

I agree with the other respondent who recommended just using wood; a
tubafor of nominal SYP with an equivalent load is 0.002"/ft owing to the
fact I== 0.67 instead of 0.037 or 20X as stiff.

OTOH, if the OP were adamant about the pipe idea, for 1" I=0.087 and
1-1/4" = 0.195 or ~2X and 5X the 3/4", respectively. Either of the
latter would be adequate although still a lot more of a hassle to deal
with probably than just framing in the opening with construction lumber
and appropriate supports for it.

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