Getting to know your AST’s fixed roof structure

Derek Slovenec from The Equity Engineering Group explains how cone roof support structures function and how to prioritize inspections Many aboveground storage tanks (ASTs) in service today were designed with a fixed cone roof, supported by a system of rafters and columns. Understanding the intended function of and interactions between components in the roof structure is critical to identifying potential signs of impending failure and employing effective mitigation. Derek Slovenec, senior engineer at The Equity Engineering Group, highlights key aspects of tank roof support structures and identify design requirements, potential failure modes, and areas of emphasis for inspection to empower informed decisions regarding their fixed roof ASTs. Roof rafters Roof rafters carry loads primarily in flexure and are designed for strength in accordance with Chapter F of AISC 360. A limit on spacing between rafters is set by API 650 (Section 5.10.4.4 in the 13th edition) to protect the roof plate from damage. There is also a slenderness limit of 300 set by API 650; this is intended to protect against lateral torsional buckling (LTB), which is the primary failure mode of rafters. This...