Development of a plug-and-play fire protection system for steel columns

Highlights

• Design of a fire protection made of only two components composed of rock wool boards and steel sheets.

• Conception of a plug-and-play connection system allowing for short installation times reducing risk exposure for technicians.

• 2D thermal models were created with FE software SAFIR and calibrated based on experimental tests.

• System certified against European norms(EN) to protect steel columns presenting box section factors going from 42 to 103 m⁻¹.

• System certified against EN to protect steel columns by maintaining their temperature below 550 °C when exposed to ISO834 fire for 120 min.

Abstract

The paper presents the development of an innovative cost-effective fire protection system for steel columns, which is quick and easy to install and to dismantle. In terms of assembly, this protection is designed to be a plug-and-play system, which is made of two U-shaped half-protections that encapsulate a column. They are composed of high-density rock wool boards arranged in U-shape steel sheets presenting a system of claws to ensure their connection. Small-scale experimental tests were performed to evaluate the insulating efficiency of the system and the thermal behaviour of the connection claws. Numerical models were then developed with the finite elements software ABAQUS and SAFIR and were successively calibrated based on experimental results. Subsequently, full-scale experimental tests were performed according to the European standard of regulation EN13381-4 and the results certified that the fire protection is effective for steel profiles with section factor ranging from 42 to 103 m⁻¹ to maintain the steel temperature below 550 °C when exposed to a standard fire for 120 min. Finally, a cost analysis was performed to attest the competitivity of the plug-and-play fire protection system by considering direct and indirect costs.

Introduction

Passive fire protection systems for steel structures have been investigated for decades. Their role is to delay the heating of structural components by isolating them from fire. In this respect, Islam et al. [1] studied the vulnerability of steel columns under the standard ISO 834 heating curve [2] through numerical analysis. Typically, existing fire protections are made of insulating materials which may be divided into three categories: intumescent paints, sprays and boards. Petukhovskaia [3] investigated and summarized passive fire protection methods for load-bearing structures in case of hydrocarbon fire. Leborgne and Thomas [4] illustrated three fire protection systems, i.e. intumescent paints, sprayed-based protections and board systems, and described their application with their advantages and limits. The National Institute of Standards and Technology (NIST) [5] reported an overview of existing fire protections for structural steel members with their advantages and disadvantages. Currently, almost all protections require important installation times on construction site, which represent costs and exposure to risks for technicians. A brief description of these fire protections is presented herein with their main characteristics.

An intumescent paint fire protection guarantees insulation efficiency for steel members by producing chemical reaction in event of fire. When heated to around 200 °C, an intumescent paint layer expands to form a layer up to 50 times thicker, which ensures the insulation of the member. The layer thickness to be applied depends on the targeted fire resistance. As depicted in Fig. 1a, paints offer a clean visual aspect. They can be applied by an off-site or in-site treatment depending on the project. In both cases, their proper application requires time and qualified labour which make these paints an expensive protection. Intumescent paints are suitable to protect complex structural connections and can be reused in case of building dismantlement. Lots of research is going on about intumescent coatings and its use in a performance-based fire engineering context when natural fire curves are used to investigate the structural fire behaviour. Lucherini and Maluk [6] reviewed intumescent coatings for the fire-safe design of steel structures. De Silva et al. [7] experimentally investigated steel elements protected with intumescent coatings. De Silva et al. [8] subsequently developed a procedure for modelling the intumescent coating in both simplified and advanced calculation methods based on experimental results. Griffin [9] modelled the heat transfer across intumescent polymer paints. Gardelle et al. [10] studied a silicon-based coating. Luangtriratana et al. [11] quantified the thermal barrier efficiency of intumescent coatings. Chen et al. [12] studied the performance of ultrathin intumescent paint for steel plates at elevated temperature. Mariappan [13] reviewed the recently developed intumescent coatings for structural steel. These numerous studies were encouraged by the increase of the market demand for intumescent paints in the past decade.

Sprayed-based fire protections can have different chemical compositions; as for example, they can be cementitious, gypsum, and vermiculite based. These products must be directly sprayed on structural members on site. The thickness to be sprayed also depends on the targeted fire resistance. Although the spray application is rapid, it is wet and requires a drying period which can delay the construction site progress. In fact, an area where a structure is being sprayed cannot be occupied by other workers. As depicted in Fig. 1b, the spray visual aspect suits only very low frequented areas, for instance basements and car parks. Sprays offer the advantages to cover complex structural geometries and to be a low-cost solution, but they cannot be easily reused in case of building dismantlement. Fulmer [14] developed and patented a cementitious spray without using any adhesive. Zhang and Li [15] recently developed a spray adopting engineered cementitious composite technology to address durability issues.

Fire protections made of boards can be made of gypsum, wood, or any other rigid insulating material such as rock wool. Protecting structural members with board requires a tailor-made installation on site. Boards need to be cut, attached to members with screws or glue and eventually finished to offer a good visual aspect if required. This is a time-consuming process. Even though boards can be made of cheap materials, they are usually an expensive solution due to the labour cost. As depicted in Fig. 1c, when made of gypsum, boards solutions present a clean visual aspect which can suits any sort of building occupancy. However, that is not the case for all boards, as for example the ones made of rock wool that must be subsequently covered to prevent contacts with users as they can be irritating for the skin. Globally, board systems used as fire protections for steel columns also benefit from research conducted in the development of fire resistance wall panels usually made of rock wool, mineral wool or gypsum. Zhang et al. [16] studied the fire resistance of tubular steel columns protected with gypsum boards, rock wool boards and blanket made of alumina-silica materials. Keerthan and Mahendran [17] numerically studied gypsum plasterboard panels under standard fire. Gottfried [18] developed and patented a board system to wrap structural steel members and made of several layers of insulating material. Zago and Keiser [19] patented a rigid protection system composed of different components, made of gypsum boards surrounded by a steel sheet, to be screwed together. Different metallic connectors were conceived to equip steel profiles flanges extremities and provide attaching surface to screw fire resistant gypsum boards. Parker [20] developed and patented a first version of such connectors and Ramos [21] developed and patented an alternative and continuous version of these connectors.

Considering existing fire protection systems, Akaa et al. [22,23] proposed an analysis technique to select the optimal system based on structural performance and cost criteria. Lim et al. [24] used numerical analysis to evaluate the performance of passive fire protections in processing facilities. Existing fire protection systems differ in their global cost, installation process and visual aspect, although they all delay the heating of structural steel members. Obviously, fire protection visual aspect is correlated with costs and depends on the construction type and the architectural aesthetics that is being sought. Good visual aspects usually require higher costs. However, the key disadvantage shared by these systems remains the time required for their installation. The application of these fire protections must be all undertaken on construction sites, except for the intumescent paints when applied off-site. This observation raises two questions: cost and safety. The direct costs of a fire protection system are the ones related to materials and labour. The indirect costs are related to maintenance and are generated within construction projects due to the installation time of the fire protection. The latter, in particular, can be significant in terms of delay, damage, or accessibility limitations for other workers. Regarding the safety, it is well known that construction sites are areas more exposed to risks and hazards than familiar environments such as manufacturing plants. It means that with a fire protection system that requires long installation delay, the exposure to risk for the installation technicians increases because workers must spend more time on site. Considering these two aspects, it seems relevant to develop an innovative fire protection system that addresses installation time issue, which intertwines costs and safety. Indeed, especially with current construction sites where the numbers of subcontractors and their interactions can be very important, it is in the community interest to design a safer and cost-efficient fire protection system. Furthermore, in order to maximise its cost efficiency, it is also important to consider maintenance requirements in terms of time and costs.

The scope of this work is to develop an innovative fire protection system for steel columns exposed to the standard fire by maintaining their temperature below 550 °C for 120 min. At 550 °C steel retains 62.5% of its strength and it can be deemed as a limiting temperature in a simplified approach for which the loads in the fire situation can be taken as 0.65 of the loads at the Ultimate Limit States as for EN 1993-1-2 [25].

The system is designed to protect steel columns with H, I and hollow square sections. It aims at simultaneously offering the advantages of the existing protection and to address their issues related to cost and safety. Therefore, the design of such a system started with the definition of the specifications to be fulfilled by the fire protection. Among these specifications listed in Table 1, the most innovative one is the plug-and-play connection system which facilitates short installation time and dismantlability. Indeed, the fire protection system must innovate in terms of installation ease and rapidity. The dismantlability aspect aims at meeting the sustainability criteria that current projects have sometimes to fulfil when submitted for tender process. It has to be noted that the fire protection system was designed to protect steel columns exposed to fire on four sides. Therefore, the system has not been yet tested to protect steel columns exposed to fire on less than four sides, steel beams and structural connections. Since, in principle it can be used in combination with other fire protections systems the appropriateness of boundary conditions at the top and at the bottom of the protection system were also investigated in this work.

The work investigated all these aspects: fire performance and cost effectiveness.

The paper is organised as follows: Section 2 describes the design of the fire protection system based on the specifications and on small-scale experimental tests. Section 3 reports the outcomes of small-scale experimental fire tests conducted on seven columns equipped with different fire protections. Section 4 presents the numerical models that were developed to analyse the behaviour of the protection when exposed to the standard fire curve. Section 5 reports the implementation of large-scale experimental tests and the methodology prescribed by the standard of regulation EN13381-4 [26]. For that purpose, five specimens, presenting different section factors, were protected by the same fire protection system. Section 6 analyses the thermal data obtained from the large-scale experimental tests and assesses the thermal efficiency of the protection. This assessment follows a method defined by the standard of regulation leading to the certification of the fire protection. Design recommendations are also provided. Section 7 details a cost analysis of the developed fire protection and compares it with other protection systems based on direct and indirect costs. Finally, Section 8 draws conclusive remarks along with future perspectives.