Chevron Floating Roof Support
Providing Safe Working Conditions
Under Floating Roofs
Types of Roof Collapse
• Roof Spiral
– this will result in the entire roof coming down
– the strength and condition of the legs have no effect on
this type of collapse
– this is the most dangerous collapse mode
• Vertical Collapse
– will not result in complete roof collapse unless this
initiates spiral mode above
– typically will result in excessive distortion in a local
area not complete collapse to the tank bottom
Sources Of Complete Roof
• Roof Spiral
– Roof anti-rotation device may not exist or may fail.
– Lateral support provided by a modern rim seal is
adequate to resist lateral movement. If the seal is
removed without adding temporary lateral restraint, the
roof can move laterally. The lateral movement alone is
not usually enough to cause problems beyond local leg
failures. However, it may initial rotational movement
which is almost always the source of complete roof
Sources of Local Leg Failures
• Vertical Collapse
– The flexibility of most floating roofs allow them to
withstand local leg failures without total collapse of the
– Landing floating roofs which are holding water or
product on the deck is the most common source of leg
– A properly landed roof with corroded or damaged legs
is also susceptible to collapse.
Prevention of Roof Spiraling
• Prevent Rotational Movement
– The Euler buckling mode of a floating roof leg determines its
strength. Without restraint to movement at the deck, the allowable
load of the leg is greatly decreased regardless of its condition or
– The forces required to prevent rotation are not excessive. Once the
roof begins to rotate the loading on back up systems are much
harder to quantify and design for. Properly engineered systems can
insure the spiral mode is avoided.
• Prevent Lateral Movement
– Lateral movement can be easily controlled by blocking the rim.
This limits potential movement of the roof to maintain stability.
Prevention of Vertical Roof
• Be sure roof is not landed with water or
product on the deck.
• Open the deck emergency drain to insure
the landed roof does not accumulate water.
• Inspect corrosion of legs and leg sleeves.
• Inspect plumness and alignment of legs.
Industry Standard Roof Support
• Insure rotational and lateral roof support.
– Unloaded roofs will not completely collapse when properly
restrained against rotation or lateral movement.
– The restraint must be in good working order. If this cannot be
verified, backup anti-rotation shall be used.
– Rim seals which are removed shall be replaced by rim blocks or
other lateral restraints.
• Inspect the condition of each leg.
– Test leg thickness. UT or use a hammer test.
– Verify plumbness and proper landing of legs. Insure legs are not
– Prevent loads from developing on the roof deck.
– Provide temporary support to questionable legs.
• Floating roof supports are designed very similar to fixed roof columns.
Fixed roof collapse is virtually non-existent because the fixed roof’s
attachment to the shell limits any movement at the top of the columns.
If floating roofs are similarly restrained against this movement they
will be highly reliable.
• Local buckling or collapse of legs can be sustained by most floating
roofs if anti-rotations and lateral support is present. Legs which are
susceptible to local collapse can easily be identified for corrective
action. Landed roofs should never be holding water or product on the
• Back up systems should directly address the sources of collapse.
Systems which are designed to support the roof after a failure mode is
initiated are difficult to design as the loading during this failure cannot
be easily predicted.