API-650 11th Ed, Addendum 1 Questions

[richay]

I have a couple of questions regarding changes made in Addendum 1.

1) Paragraph 5.10.4.4 has changed the rafter spacing, providing a new (single) equation. The previous edition gave a spacing for both the inside and outside ends of the rafter. How is this new equation intended to be applied - is this a limit applied to the earlier inside/outside equations? If this is a replacement, how can it apply to both the inside and outside ends of the rafter?

2) Paragraph 5.11.2 has added a limit to "wl" (previously "wa"). This limit (0.90DH) has units of length^2 ??? The "wl" term has units of "force/length". Something seems amiss here ...

Any help or comments will be appreciated.


Richard Ay
COADE, Inc.

 

[JoeTank]

Regarding rafter spacing, the new equation applies to all situations. Unfortunately the equation only addresses plate spans between rafters. It does not consider the 3-sided support of the roof plate when near the tank shell. I'm sure this will generate several inquiries to the API code committee. I do not believe that the 0.90HD equation is correct, but I am still sorting through the derivation to figure it out.

Joe Tank

 

[Daianto]

There is a good series of blog posts on the API 650 addendum -see

http://itsdesign.blogspot.com/2008/07/api-650-11th-edition-addendum-1_17.html

and this post mentions these 2 sections--

10) 5.10.4.4 has been revised to provide calculations for the maximum spacing of the roof rafters. It also set the maximum spacing at 84". These limits apply to both inner and outer rafters.
11) 5.11.2 has been revised to add a limit on the value of wL for the wind condition. It also now specifies that tb is in the corroded condition. A calculation of the minimum annular ring width for wind has been added.