INQ-650-D29 – Rafter Spacing
Spring 2013
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Dear API Std 650 technical inquiry committee, good afternoon and thanks in advance.
This mail is a technical inquiry for API Std. 650, formatted as recommended in API Std
650 Appendix D.
Background:
When designing the roof of a supported cone roof per API 650 per item 5.10.4, a usual
subject about item 5.10.4.4 interpretation may occur.
API defines b as:
b = maximum allowable roof plate span, measured circumferentially from center-tocenter of rafters.
I understand, from definition, that b shall be measured in the circumferential direction,
but always in the outer ring formed by two rafters (see figure Rafters_spacing.jpg).
This way, the terms "center-to-center" refers about the center of the rafter in the
circumferential direction. And so, the arc measured is the biggest arc formed by the two
rafters (maximum roof plate span).
(See attached file: Rafters_spacing.jpg)
However, another possible interpretation is possible from the definition. Some designers
understand the term "center-to-center" in the radial direction of the rafter. And so, they
measure b in the circumferential direction (an arc). But the measure is made from the
center of one rafter in the radial direction (middle of the rafter) to the center of the other
rafter in the radial direction (middle of the rafter). This way, b is an imaginary arc placed
between the middle (radial direction) of the rafters.
Question:
Is the first interpretation right? When we consider the biggest arc formed by two rafters.
Balloted Question:
Q1) Does the maximum allowable roof plate span “b” given in 5.10.4.4 refer to the
largest circumferential span between any two adjacent rafters (typically at the outer limit
of the rafters if radial oriented)? Q2) Is it therefore true that the circumferential span from
center of rafter to center of rafter shall not exceed “b” in 5.10.4.4 at any roof location for
the applicable roof thickness used to calculate b?
Ballot Response:
A1) Yes
A2) Yes