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CROSS Safety Report

Fire protection of mixed hot/cold rolled steel structure

Report ID: 921 Published: 1 July 2020 Region: CROSS-UK


Overview

This report describes how a high-rise building had to be remediated after it was found there was insufficient fire protection encasement of structural elements.

Key Learning Outcomes

For clients, the construction and the design team:

  • The project management process should recognise the need for collaboration over boundaries where multiple parties are involved

  • It is beneficial if the engineering team, including those who create the fire engineering strategy, are involved throughout the project. This will ensure that the detailed design and construction meet the design intent.

  • Attention to detail is required at the interface of different construction systems and components to ensure fire safety systems are not compromised

  • It is good practice to document and circulate any alterations to the fire resistance to enable them to be properly understood and considered by the building control body and the relevant fire and rescue service

For civil, structural and fire engineers:

  • Fire engineered approaches resulting in reduced structural fire protection should only be carried out by people who are competent to do so

  • Where specialist systems are used, it is beneficial to have a close working relationship with the supplier from the earliest opportunity

  • The complexity of the interaction between structural integrity and fire must not be underestimated. Collaboration between both disciplines can help ensure all potential consequences are considered.

Full Report

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A reporter describes how a development was designed and constructed using both a traditional steel frame and a lightweight steel framing system (SFS). Commercial units on the lower floors required large spans. They were constructed using a steel structure. 

The upper floors were residential units and were of SFS construction. An acoustic ceiling separated the commercial units from the residential units above.

Fire resistance periods and fire protection methods used

This is a high-rise development as defined in the technical standards at the time of application. Early in the design process the period of structural fire protection was revised. The required 2 hours (to meet the prescriptive technical standards requirement for 'long' duration of fire resistance) was reduced to 1.5 hours. This was achieved through a performance-based fire engineering approach.

The agreed method for fire protection of the steel frame structure on the lower storey was intumescent paint. This was applied directly to the steel. Several layers of plasterboard were placed over this to form an acoustic separation between the commercial and residential units. 

The steel framing system (SFS) of the upper floors was 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. These worked in combination with hot rolled steel section beams and columns. In some locations, these resembled steel frames. For example, where the ground floor layouts differed from the upper floor layouts and where walls did not stack.

The designer’s details were developed on the basis of this information and were to account for fire separation and acoustic performance of the structure. The floors were profiled galvanised steel sheets with concrete infill to a minimum depth of 160mm.

Reinforcement bars were provided 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 directly on the flange of steel beams. 

Fire protection of the structural walls was provided by over-cladding with plasterboard. 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 full height plasterboard sheets. The top hat sections and the steel beam flanges supporting the floors were also fire protected by plasterboard sheeting.

How a lack of knowledge and details can compromise fire safety

There were some areas where hot rolled steel was required to support larger floor spans. These were usually associated with open plan living areas, or to support upper load bearing walls that did 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. Depending on their depth, they were occasionally fire protected by the wall cladding. In certain scenarios they sometimes presented themselves as a base flange or a more obvious steel beam depending on depth.

For the latter scenarios, the understanding of the fire protection for these steel elements was incomplete. The exposed beams were not identified as requiring any fire protection, because they formed part of the floor structure.

Specific scenarios for these steel beam encasements were not provided on the architectural details and the building was constructed without the necessary fire protection encasement.

Specific scenarios for these steel beam encasements were not provided on the architectural details and the building was constructed without the necessary fire protection encasement

Late identification of fire safety issues led to costly remedial works

The issue was identified during a construction-phase inspection by the main contractor. The contractor queried the encasement detailing of elements on another phase of the development. Investigations 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.

The designers and contractor carried out intrusive surveys and re-visited construction information. This allowed them to understand the extent of the issue and design a remedial encasement detail. The remedial detail 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.

Improving communication improves fire safety

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. However, when their system is reliant on the performance of adjacent components 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.

Expert Panel Comments

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Structural fire resistance is an area of significant concern and considerable complexity. Structural engineers are rarely familiar with details of fire engineering concepts, and fire engineers are not always familiar with structural response to fire.

Structural fire resistance is an area of significant concern and considerable complexity. Structural engineers are rarely familiar with details of fire engineering concepts, and fire engineers are not always familiar with structural response to fire

Involvement of the engineering team, including those who create the fire engineering strategy, should be retained throughout the project. This will ensure that the detailed design and construction meet the design intent.

Reducing the fire resistance period of a structure should be done with great care

Using fire engineering to reduce the period of fire resistance from the standard values (e.g. in the technical handbooks or Approved Documents) should be done with great care. Too often this is seen as an area to ‘value engineer’. It is done without properly taking account issues such as firefighter safety and the importance to society and the environment of preventing a building from collapsing locally or globally.

Occupant evacuation time is just one consideration when determining the degree of fire resistance to be provided by a structure. For higher consequence structures, the durations stipulated in the technical handbooks or Approved Documents should provide opportunity for the fire to burn out or be extinguished by the fire and rescue service, without the building reaching a state of instability.

Reducing the fire protection to the structure could lead to increased risk for firefighters and to others around the building should a collapse occur. It should be recognised that performance-based fire engineering can also be used to increase fire resistance if particular resilience objectives have been identified.

Understanding the fire strategy

Comprehensive guidance on a performance-based fire engineering approach to structural fire protection is available in the relevant British Standards. BS 7974 and PD 7974-3 (clauses 5.2 and 5.3) are particularly helpful.

To help others understand the fire strategy, the following should be considered:

  • The justification for any alteration in the fire resistance should be developed in association with a wide range of stakeholders

  • A qualitative design review (QDR) can be used to do this

  • Alterations should be documented and circulated to enable them to be properly understood and considered by the building control body and the relevant fire and rescue service

How collaboration over boundaries can reduce confusion

Safety issues such as those raised in this report stem from potential confusion over both technical and contractual boundaries. The overarching lesson is that the project management process must recognise the need for collaboration over boundaries. It may be useful to appoint one-party to oversee the achievement of the safety standards. They can also verify that the as-built structure complies.

Fire resistance is part of the structural design but given that the structural performance of a proprietary SFS system rests with the supplier, it may be difficult for the structural engineer to verify its performance from first principles. It is hoped that an experienced engineer would recognise this detail as requiring further investigation with the specialist subcontractor.

It is further hoped that future regulatory change will require that there will be a duty holder responsible for ensuring safety throughout projects such as this. Even better would be if the industry recognised the potential benefits and adopted such an approach prior to regulatory instruments being put in place.

Building control bodies have a duty in law to consult with the fire brigade for all building work. The consultation should include information about the design, such as fire engineering.

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