Difference Between Restrained and Unrestrained Beams

Promat 18/09/2017

structural steel protection
fire on structural steel
fire on structural steel
The Promat Middle East Technical Department experiences many questions and recently there has been an ever increasing number about the difference between restrained and unrestrained beams during the fire protection of structural steel - something that could have an awful consequences if not understood correctly. 

Using the wrong designs could lead to an unnecessary risk during potential fire and ultimately will not provide the fire-rating according to the Building Codes.

Under the UL designs (ANSI/UL 263 (ASTM E119) you can find two thickness of spray applied fire rated material (SFRM) for one fire rating - this is applicable to a lot of manufacturers, including Promat. One thickness is applicable to restrained beams (lower thickness) and another thickness for unrestrained beams (higher thickness).

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.

STRUCTURAL STEEL - WHAT DO YOU NEED TO CONSIDER FOR YOUR DESIGN?

Our experience on projects shows that the offered thickness of the SFRM, according to UL, is typically done based on the Restrained Beams designs - often due to the fact it is commercially more attractive, because of the reduced thickness of the SFRM. With this comes a significant risk for all involved including those that will eventually occupy the building. 

The majority of steel structures (80% or more) are built in an unrestrained situation, again the commercial aspects have a role to play here as the unrestrained steel structure is easier and more cost effective to build. 

This means that numerous projects are engineered: 

• Steel structure unrestrained
• SFRM thickness restrained

As we see in Table 1, this is not compatible with each other and the UL approval is not applicable to the project. 

OUR RECOMMENDATION FOR CALCULATING FOR STRUCTURAL STEEL PROTECTION

Before making a calculation, always check with the 'approved building official' if the steel structure is designed in a restrained or an unrestrained situation. Depending on the confirmation use the UL design thickness to below matrix. 

Table 2

CONFIRMATION FROM THE APPROVED BUILDING OFFICIAL

USE UL DESIGN THICKNESS FOR

Unrestrained beams

Unrestrained beams

Restrained beams

Restrained beams

If the design is unknown

Unrestrained beams (thickness covers both situations)


In our opinion, these points can be come up not only by error but also to enable commercial advantage, during the benchmark of price offers from different applicators/manufacturers it is essential to check what has been quoted - the restrained UL designs. Often the cheapest solution offers non-conformity to codes through non-compatible design for the project, this results in the inadequate protection in the project and the life of the building or the occupiers during possible fires. 

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