Report ID: 921

Published: Newsletter 59 - July 2020

Report Overview

A high rise building had to be remediated after it was found that there was insufficient encasement of structural elements.

Report Content

Building form
A reporter describes how a development was designed and constructed with both a traditional Steel Frame and a lightweight Steel Framing System (SFS). A mix of the two approaches was used with commercial units on the lower floors constructed using a steel structure due to the large spans along with acoustic detailing to separate the commercial from the residential above. The upper residential floors were of SFS construction.

This is a ‘high rise’ development as defined in the technical standards at the time of application. The period of structural fire protection was revised during the early design process from the required 2 hours (to meet the prescriptive Technical Standards requirement for ‘long’ duration of fire resistance) to 1.5 hours, achieved through a performance-based fire engineering approach. The agreed method for fire protection of the steel frame structure on the lower storeys was intumescent paint directly applied to the steel. Over this were several layers of plasterboard which were to form an acoustic separation from the commercial building to the residential buildings above.
 
The upper residential floors were SFS and were required to meet with tested forms of fire protection and acoustic separation. These were, as is generally the case, determined by the individual SFS supplier.

The SFS was a combination of cold rolled steel ‘C’ section studs with flat plate cross bracing brought together as panels which work in combination with hot rolled steel section columns and beams. In areas these resemble steel frames where for example the ground floor layouts differ from the upper floor layouts and walls do not stack. The designers’ details were developed on the basis of this information to account for fire separation and acoustic performance of the structure. The floors were profiled galvanised steel sheets with concrete infill to minimum 160mm depth. Bars as necessary to allow the concrete floor to meet the required and tested fire, structural and acoustic separation. Floors were supported onto a ’top hat’ detail or flange of steel beams. 

Fire protection of the structural walls was by over-cladding with plasterboard and the specification of this was determined by the certification provided by the SFS supplier and designer. In all but extreme load cases, the steel posts were sized to be within the depth of the wall panels. Wall structures were fire protected by the plasterboard sheets full height. The 'top hat' section and flanges supporting the floors were also fire protected by plasterboard sheeting.

There were some areas where hot rolled steel was required either to support larger floor spans, usually associated with open plan living areas, or where supporting walls from above do not align with those below. The location and size of these hot rolled steel beams was determined by the SFS supplier. These beams were predominantly encased in the concrete floor and depending on depth, were occasionally fire protected by the wall cladding. They sometimes presented themselves as a base flange or more obvious steel beam depending on depth.

In this case, the understanding of fire protection of these floor steel elements was incomplete, and they were not identified as requiring fire protection because they formed part of the floor structure. Specific scenarios of these steel beam encasements were not provided on the architectural details and the building was constructed without necessary fire protection encasement.

Identification & remediation
The issue was identified when a construction-phase inspection by the main contractor of the detailing on another phase of the development queried elements of encasement. Investigation led to an understanding that the encasement was insufficient, and the in-construction phase was remedied. A subsequent intrusive survey of the completed and occupied first phase of the development identified the same issue.

A combination of intrusive survey and re-visiting construction information allowed the designers and contractor to understand the extent of the issue and design a remedial encasement detail which provided for the 1.5 hour rating for the completed block. Tenant communication and additional fire alarm and management presence were put in place whilst a phased programme of remedial encasement actions was completed.

Considerations
Where a system build product is proposed which is reliant on specific components and detailing being in place, the system supplier and designer should also be party to the sign off process for construction detailing. The supplier should also have been requested to provide technical support to the design team and the opportunity for review of their information. A greater extent of typical details covering fire and acoustic separation should be provided on the Architectural details and verified by the system provider. Contractually there will be no detailing design relationship with the supplier or system designer, but as in this case, their system is reliant on the performance of adjacent components, so there should be a 'duty of care' to review the later production stages of the product are in accordance with defined specifications. 

In this case, the supplier and the system designer are separate entities. Where this relationship exists, the supplier and designer must both have a close working relationship with the design team and contracting organisation from the earliest opportunity.

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