Difference Between Restrained and Unrestrained Beams

Promat 18/09/2017

structural steel protection

Restrained and Unrestrained, what does it mean?

• Restrained

The beam is mechanically fixed to the rest of the structure. During fire, there is normally elongation of a steel beam. In a restrained situation, the beam is blocked instead of elongation, the beam will be compressed. The compression results to a better performance during fire and the beam needs less thickness of SFRM.

• Unrestrained

 An unrestrained beam is without the full mechanical fixing between the beam and the slab, etc. For example, the slab is laying on the beam, both construction elements acting separately. During a fire, the construction elements are moving separately, there is more movement in the beam. To compensate this during a fire, there is a need for a thicker layer of SFRM material.

Below is a decision matrix about the compatibility between the UL design thickness and the on-site situation.

Table 1: Design Matrix

APPLIED THICKNESS ACCORDING TO UL DESIGN

ON-SITE SITUATION: RESTRAINED BEAMS

ON-SITE SITUATION UNRESTRAINED BEAMS

Restrained beams

Good design, fully compatible with UL approval

Failed design, wrong use of UL design. UL approval not valid for the project

Unrestrained beams

Good design, the thickness of SFRM materials is over engineered. Fully compatible with the UL approval

Good design, fully compatible with UL approval


On a project, only the structural designer can advise if a construction is designed as restrained or unrestrained - this is not an opinion, it's clearly stated in the UL and International Building Code (IBC)


Text from UL document (See references)
Determining restrained conditions
One source of information that can be used to help determine if a rated assembly is being installed in a restrained application (as reference in IBC section 703.2.3) is included in Appendix C of UL 263. The other is information included in the UL Guide Information for Fire Resistance Ratings - UL 263 (BXUV). Once the restrained versus unrestrained determination has been made and approved by the building official, the designer and involved contractors follow the published certification to determine the required fire protection to achieve the required rating.

Text from IBC code (See references)
703.2.3 Restrained classification.
Fire-resistance-rated assembles tested under ASTM E119 or UL 263 shall not be considered to be restrained unless evidence satisfactory to the building official is furnished by the registered design showing that the construction qualities  for a restrained classification in accordance with ASTM E119 or UL 263. Restrained construction shall be identified on the construction documents.

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