Evaluation and Repair of Steel Structure Damaged by Fire

Abstract

In recent years, steel structures have been widely used in buildings because of their advantages in self-weight and seismic performance. However, due to the great influence of temperature on steel structure, once a fire occurs, it may lead to the destruction of steel structure and even the collapse of the building. In this paper, through the assessment of a steel structure factory after fire, the damage degree of the components and structural integrity of the building are determined, which provides a reliable technical support for the subsequent repair and retrofit work. Based on the extent of damage identified, repairs are performed on the affected members. The analysis results of the rehabilitated structure show that this method is safe and effective.

1 Introduction

Fire statistics show that in all kinds of fire accidents, construction fire is the biggest threat to human life and property safety. Among them, the possibility for factory fires is relatively high due to the special production process, lack of safety awareness and other reasons [1].

Structural steel is a kind of material widely used in factory buildings with high strength, light dead weight and superior seismic performance. However, the fire resistance is very poor, usually in the building fire within 15 min, the steel structure of the building will lose its bearing capacity, the building will soon collapse, causing huge casualties and property losses [2]. Therefore, it is necessary to evaluate the fire damage of the building to provide reliable evidence for the subsequent repair and retrofit of the building [3]. In this study, the evaluation of fire damaged structural members is carried out, so as to determine the damage class of fire and put forward the corresponding retrofit methods.

2 Application Example

2.1 Description of the Building

The factory building is a single-storey light steel structure with portal frame. The eaves height is 8.5 m. The building is constructed in two phases. The first phase (1–21 axes) was built in 2004, with a second phase (21–24 axes) following in 2006. The building is basically rectangular in plan, as shown in Fig. 1. The building dimensions are 96 m in width × 180 m in length × 10 m in height. The top elevation of the steel beam at the eave is 8.5 m, forming a north–south 3% drainage slope. The column spacing is mostly 8 m (except for the two spaces on the west side which are 6 m).

Fig. 1

Plan view of the factory building

The primary structural frame includes columns, purlin, and roof steel H-beams. The portal frame columns are all welded H-bar with constant section. The side columns are H550 × 150 × 6 × 10, H550 × 225 × 6 × 10, H550 × 250 × 6 × 10, and the middle columns are H450 × 350 × 6 × 12, H350 × 250 × 6 × 10, H350 × 250 × 5 × 10. The wind columns are H320 × 250 × 6 × 10 and H320 × 175 × 6 × 10. The beams are welded H-bar with variable section, H(750–850) × 200 × 6 × 12, H(600–850) × 200 × 6 × 12, and H(600–700) × 200 × 6 × 12. Roof purlin is cold-formed thin wall Z-shaped steel (Z200 × 70 × 20 × 2.5). Steel beams and columns are generally connected with 6M20 or 8M20 high-strength bolts. The columns are connected to the foundations with a combination of base plates and anchor bolts.

2.2 Information About the Fire

On May 7, 2019, an electrical fire broke out between 22 and 24/(1/D)-E axes in the northeast corner of the second phase of the factory building (see Fig. 1). Before the fire, plastic products (motor fans) stacked in the northeast corner of the building belong to flammable substances. Because it is mainly used for storage and occupies a limited area, there is no active sprinkler system in the building. The plastic was flammable, causing the fire to spread quickly. Luckily, the fire was quickly brought under control. The fire was observed to last approximately 10 min. The fire covered about 240 m². Combustibles stored in the area were completely destroyed.

3 Damage of Steel Structural Members After Fire

A systematic investigation of the structure was carried out after the fire. It was found that the direct fire zone was limited to the 23–24/(1/D)-E axes. The reason was that the surrounding walls of the fire area were block or brick masonry firewalls or explosion-proof walls. All these walls with a 2-h fire rating effectively prevented the fire from spreading. The main damage was in the northeast corner. No significant fire-induced damage was observed in other areas. Plaster falling from the enclosure walls was observed in areas outside the fire due to high temperature.

3.1 Assessment of Damage to Columns

Though the overall structure did not collapse, many steel structural components were severely damaged in the fire region (Fig. 2). Some steel columns exposed to fire suffered reduction in performance and instability that result in localized buckling of the flanges or overall buckling. Some of the columns experienced minor damage, such as falling of fireproof coating due to elevated temperature.

Fig. 2

Damage views of building following the fire accident: a overall view of fire damaged area; b buckling and large deflection of roof steel beams and purlins; c flexural deformation of roof panels; and d falling of fireproof coating on steel columns (Images by Huabo Liu.)

3.2 Assessment of Damage to Horizontal Framing Members

The heat from the fire visibly damaged steel members located directly above. The color-coated steel sheet roofing was affected and it exhibited visually apparent deformation, spalling, and discoloration. Although there was no complete collapse, several large depressed areas were observed on the roof of the building. Inspection of the interior of the building showed that purlins were exposed to extreme temperatures and failed in a buckling mode as the steel lost its strength. The horizontal structural components supporting the roof were badly damaged. The damage to roof steel beams and purlins included (a) large in-plane deflection in the steel beams and purlins; (b) buckling of the roof beams and (c) out of plane deflection due to buckling, as shown in Fig. 2.

4 Evaluation of Steel Components

According to Standard for Appraisal of Engineering Structures After Fire (T/CECS252-2019), the steel components after fire are evaluated and rated. Fire damaged steel is typically classified in four classes:

• Class I—Members with no damage or slight deformations that are not easily detected by visual observation. Performance and safety of the component have not been affected by fire.

• Class II_a, II_b, or III—Members with visible deformation. According to the component fireproof coating, local deformation, overall deformation, connection damage, the most serious damage class is taken as the component damage class.

• Class IV—Severely damaged components, such as excessive overall deformation, severe residual deformation, cracking or fracture, local buckling, bolt breakage, etc.

According to the standard, the evaluation and rating results of steel components after fire are shown in Table 1.

Table 1 Evaluation and rating results of steel components after fire

5 Retrofit Methodology

5.1 Repairing Decisions

Based on the results of the steel component assessment, taking into account the method to be used, the extend of intervention, etc., decisions are made concerning repair of the structure after fire.

For Damage class I, only redecoration is required.

For Damage class II, superficial repair of slight damage will suffice, such as non-structural or minor structural treatment (restoring fireproof coating on steel columns, etc.).

Damage class III, generally damaged steel, will be retrofit to satisfy increased strength demands. For these columns or beams, retrofit is more economical than replacing fire-damaged components.

Damage class IV, severely deformed steel, will be removed and replaced.

5.2 Structural Shortcomings in Details

The factory building was built in 2004, when the “Technical specification for steed structure of light-weight Buildings with gabled frames” CECS102:2002 was just issued [4]. The current national standard, “Technical code for steel structure of light-weight building with gabled frames” GB51022-2015 [5], has improved in all aspects compared with the original regulations. Therefore, it does not meet the requirements of current design specifications. The main deficiencies of the detail measures are:

• Smaller section size of components compared with current standard.

• Inadequate support system to form the spatial structure system effectively.

• Lack of adequate lateral support out of plane.

To sum up, the safety of the factory building does not meet the requirements of the current relevant specifications, and retrofit measures should be taken. Treatment measures include: (1) supplement the support system, (2) adding necessary roof knee braces, (3) adding rigid tie rod between side columns to reduce the calculated length out of plane, and (4) strengthening the section of the main components, etc.

5.3 Structural Reanalysis

According to the final retrofit methodology, a structural analysis was performed to assess performance of the building and identify deficiencies (see Fig. 3), using the new components and structural information. The transverse plane model is calculated as a portal frame. The column foots are taken as rigid connection, and the beam-column joints are partially rigid and partially hinged according to the actual connection mode. The length of the roof oblique beam out of plane is determined by the spacing between the knee braces.

Fig. 3

Model of structure

The results of structural calculation show that the structure has good performance and meets the requirements of current specifications. Results of the analysis are subsequently used to repair and strengthening the damaged structure.

6 Conclusion

In this study, the building was not designed in accordance with current building regulations. Structural damage due to fire further complicates retrofitting. Based on the proper assessment, an appropriate and cost-effective retrofit methodology is performed.

For the steel structure, once the fire occurs, if no fire prevention measures are taken or the fire is not put out in time, the steel structure will collapse quickly. If the fire damage is very severe, demolition is recommended on the basis of safety and life-cycle economic analysis. If the fire is not serious, the structure can be retrofitted according to the usability and durability of the structure to prevent the occurrence of secondary disasters. Post-fire evaluation is critical to the safety of a building in the future. How to make the repair and strengthening of steel structure after fire reasonable, economical and effective is a problem worth further study.