Evaluation of Fire Damage to Structural Steel (Primer)

Posted by in Fire Damage, Steel | February 09, 2015

We often get called to evaluate fire damage to structural steel framing.  It may be an evaluation even as simple as one steel beam in a residence.  Some of the evaluations are relatively easy, others more difficult.  But here is a short list of conditions that might indicate that the steel member was effected by the heat of the fire or the subsequent extinguishing operations.

Has the paint peeled from the surface of the steel?  If the paint hasn’t peeled, the steel probably didn’t get hot enough to have a change in its chemical make-up.  If the paint has peeled from the heat of the fire, then the surface needs to be looked at more carefully for indications of material change.  This is a more unusual condition, but can result from localized exposure of the steel to intense heat.

Has the steel member deformed?  The most common evidence of fire damage to steel framing is excessive deformation, excessive twisting, excessive expansion, excessive sagging of a beam, excessive bowing of a column, etc.  See the Figures in this article.  The thinner the steel material is, the easier it is to be deformed.  Thicker steel has more mass to heat up and is able to conduct more heat away, and thus, takes longer to heat excessively.

 

FD-1 Top chord of steel joist and the bridging exhibit deformation from exposure to excessive heat.

 

Excessive expansion can damage welded or bolted connections, bearing attachments, or can damage concrete or masonry walls that restrain the ends of the beams.

Excessive sagging in a beam or bowing in a column can occur when the steel is heated enough to reduce the modulus of elasticity or the yield strength of the steel so that the weight on the beam bends it or the column buckles when the steel has been “softened” by the heat.  The modulus of elasticity is a measurement of the stiffness of the material: at higher temperatures the steel material softens and becomes less stiff.  The yield strength is a measurement of the material strength at excessive plastic deformation: at higher temperatures the yield strength is lower and the steel material is more easily deformed.

Examination of the heat damage to the various materials around the steel can give an indication of the temperatures that the steel was exposed.  Comparison of these with temperatures known to affect steel materials can give added insight into the cause of the damage and/or scope of the fire damage to the structural steel.

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