*2.1. Overview of the Structure and the Case*

The building consists of six partitions with different sizes and an administration office. The design and all the relevant details of the structure were provided by the company that undertook the design and the construction of the building. The steel truss roof structure, which is supported on steel-reinforced concrete columns, covers a total area of 30,180 m<sup>2</sup> .

The slope of the roof is 1% in each direction. The parapets of 15 and 25 cm heights surround the roof, which is equipped with a siphonic drainage system. The layout of the structure is presented schematically in Figure 1; the damaged parts are marked in white. The entire structure is made of seven partitions, including an administrative office and six halls where the industrial manufacturing took place.

**Figure 1.** Layout and damaged regions of the roof structure.

The main dimensions of the unit truss system are presented in Figure 2. The steel carrier system consists of a double-layer truss with a depth of 2 m. The bars of this system have circular tube cross-sections with different wall thicknesses and diameters depending on their predicted loads. At both ends of the bars, screw threads are located to enable connecting those members to the mero system. The mero system includes steel spherical parts that have screw holes. It is noted that no

eccentricity at the nodes is observed after the assembly of the individual members. The blocks are divided from each other with expansion joints.

**Figure 2.** Unit truss structure.

The steel space truss roof structures are mounted on reinforced concrete columns that are 11 m tall with an 80 cm by 80 cm cross-section. The number of columns and their configuration can be seen in the plan view of the building, which is given in Figure 1. No failure, no deformation, and no cracks were observed on these columns during the investigations. Only some broken wings were observed on their top section, which was assumed to have happen during the collapse of the roof. Thus, it was concluded that the columns did not play a significant role in the collapse mechanism of this building. On top of the columns, the spherical joints are welded to the bearing plates. Figure 3 represents an overview of the real building structure, and Figure 4 shows the damaged roof of the space frame system on the night of the incident.

**Figure 3.** Overview of the roof structure.

**Figure 4.** Damage to the roof's space truss system.

On the day of the incident, more than 500 lightning events were detected in the region by the British Meteorological Office's (The Met Office) ATDnet (Arrival Time Difference Network) system and the Vaisala Global Lightning Dataset GLD360. It was determined that lightning struck the building and the lightning rods worked. Meteorological data indicate that the thunderstorms were rainy and the wind blew from 40 to 60 km/h from the north and northeast directions and up to 70 to 80 km/h locally. The rainfall was 22.6 mm during the collapse, although equally heavy local rainfalls had occurred several times in recent years. Around the roof, there are several siphonics at 5 cm height. The heights of the parapets on the roof are 15 cm and 25 cm. Although no scuppers are required to be used on such a shallow parapet, there are several scuppers of 15 cm height on the roof structure.
