RESTRAINED VS. UNRESTRAINED
To most
structural engineers, code officials and architects, the terms "Restrained" and
"Unrestrained" are typically interpreted as referring to the connection of
structural elements at ambient temperatures. However, in the fire protection industry,
restrained and unrestrained are addressed at elevated temperatures, introducing the
concept of "thermal restraint".
The issue of
thermal restraint causes some controversy in determining whether an assembly should be
considered restrained or unrestrained. The classification of an assembly in one of these
categories has a bearing on the thickness of spray-applied fire resistive material (SFRM)
needed to satisfy code requirements. Higher SFRM thicknesses are typically required for
unrestrained ratings.
In order to
clarify whether an assembly should be considered restrained or unrestrained, one may refer
to the actual "Fire Test Standards of Building Construction and Materials".
According to Appendix X3 of ASTM Standard E119 and Appendix C of UL (Underwriters
Laboratories) Standard 263: "Floor and roof assemblies and individual beams in
buildings shall be considered restrained when the surrounding or supporting structure is
capable of resisting substantial thermal expansion throughout the range of anticipated
elevated temperatures. Construction not complying with this definition is assumed to be
free to rotate and expand and shall therefore be considered as unrestrained."
To assist in
determining this condition, ASTM Standard E119 and UL 263 also list general construction
classifications and whether they denote a restrained or unrestrained condition. This table
of classifications, which at one time appeared in the UL Fire Resistance Directory.
According to UL, this information is intended as a guide for the determination of
restrained conditions and is not meant as a specification. Engineering judgment must
therefore be exercised to determine what constitutes restraint to "substantial
thermal expansion". Furthermore, page 6 of the 1999 UL Directory states the
following: "Restrained conditions for the fire test assemblies are provided by
constructing floor, beam and roof test assemblies within nominal 14 ft x 17 ft frames of
composite steel/concrete cross sections having an approximate stiffness (EI/L) of 850,000
kip-in and 700,000 kip-in along the 14 ft and 17 ft sides, respectively." This
description provides structural engineers with the stiffness of UL's test frame so that
they have a basis of comparison when determining conditions of restraint for beams on a
project.
Due to the
level of analysis and interpretation required, there is often confusion as to whether a
building's construction shall be specified as restrained or unrestrained. Ultimately, the
determination of the conditions of restraint remain in the hands of the structural
engineer and the authority having jurisdiction. The Uniform Building Code (ICBO) criteria
requires that all construction design be considered unrestrained unless proven otherwise.
After
determining restrained vs. unrestrained conditions, the appropriate restrained or
unrestrained fire resistance rating must be utilized. Restrained and unrestrained fire
ratings specified for both beams/joists and assemblies are used to determine the required
fire protection material thicknesses which are listed in the UL Directory. Due to the
difference in thickness requirements between restrained and unrestrained hourly ratings,
this determination can often have a significant effect on both fireproofing requirements
and life safety integrity of the building.
Gary C.
Poindexter CSI, CDT
Isolatek International
CAFCO Fireprotecton Products