Fire Damage to Open Web Steel Joists

Posted by in Fire Damage, Steel | February 04, 2013

The determination of fire damage to open web steel joists is not always as obvious as the damage in the photo above, but the application of a few rules can make a preliminary evaluation of fire damage to open web steel joists somewhat simple.

Here are some tips for preliminary evaluation:

1.   Steel expands at a rate of .0000065 inches per inch per degree Fahrenheit.  This means that a 30 foot long open web joist exposed to temperatures of 400 degrees increases approximately 3/4 inches in length.  If this hypothetical joist is restrained from expansion at both ends it will either buckle like a yard stick (see photo above) or push through the structure at one or both ends.  Damage from heat induced expansion is the first evidence of damage to appear.  Look for a sweep (side-to-side bow) along the length of the joist.  The sweep may be from support to support or oscillate between the lines of bridging.

2.   The heat induced expansion of the steel may also effect the members within the joists.  Look for buckled members within the joists, especially nearer the area of the fire origin.  Diagonals in compression, top  chord members, and bridging (lateral bracing) are especially susceptible to buckling from exposure to excessive amounts of heat.  However, we have also observed diagonals in tension and bottom chord members buckled from exposure to excessive amounts of heat.  So, all members should be checked.

3.   Excessive heat (above 1200 degrees Fahrenheit for an extended period of time) may affect metallurgical changes in the steel.   It is a common contention that the steel material was damaged by exposure to fire.  However, it is extremely rare that the steel was raised to temperatures greater than the 1200 degrees Fahrenheit required to cause material damage to the steel without exhibiting other damages that lead to replacement.

The temperature of the fire or the steel may be estimated by the heat damage to the paint on the steel or other nearby building components.  Look for peeling, blistering, or checking in the paint coating.  A lack of damage to the paint indicates that temperatures of the fire and the steel were probably less than 600 degrees Fahrenheit, and thus, not high enough to cause damage to the steel material.  Most plastic materials melt or burn at temperatures between 180 to 300 degrees Fahrenheit, wood at around 450, and concrete begins to change color at around 550: thus, a lack of damage to these also indicates that the temperature of the steel did not exceed 1200 degrees Fahrenheit.

Thus, if a preliminary examination of the open web steel joists do not disclose any of these damages, the joists were probably not damaged by exposure to heat.  If there is a question, we recommend the procuring the services of a structural engineer or metallurgist experienced in the evaluation of heat damaged steel framing.  We would be please to assist you with such an evaluation.