# Numerical Investigation of the Collapse of a Steel Truss Roof and a Probable Reason of Failure
## 4 Results and Discussion
### Global Analysis with Mechanical Loads and Thermal Effects  
With these results obtained from the global analysis it was found that increasing horizontal forces may cause damage, various damage scenarios were investigated by changing the loads applied to the space frame system. As the first case, the vertical load was increased considerably and no damage was predicted in the real damaged zones A and F, as given in Figure 1. As the next case, the structure was  
Figure 16: Resultant displacement distribution of the column support connection. Figure 15: Damage on the support to which the space truss roof system was attached. tested by increasing the horizontal load. Although the horizontal load value was increased to a level that cannot be reached by wind and similar factors, the damage that occurred cannot be explained with these combinations of loads. However, the structure was found to be more sensitive and vulnerable to the horizontal loads. Finally, it was understood that the space frame system with the columns is safe against vertical loads (snow, water, etc.) and horizontal loads (wind, etc.). It is obvious that different factors must have been present in order to explain the real damage in the structure. Therefore, considering the natural events occurring on the night of the event, their effects on the structure were considered. It was assumed that reasons such as heavy rain that leads to ponding on the roof with a slope of 1% and a plausibly clogged water discharging drainage system would not cause damage similar to that which occurred. Besides, around the rooftop surface there are several scuppers with 15 cm height. Even with the assumption of full ponding of the roof with 15 cm height, it can be confidently claimed that the chosen loads for the analysis were on the cautious side and led to conservative results, which do not point out any damage predictions similar to the real damage. On the other hand, a large number of lightning strikes were detected on the night of the incident. The effects of lightning depend on the energy of the lightning itself. The energy of lightning, which is impossible to fully determine, depends on many parameters, such as the altitude of the cloud where the lightning originates. It is well known that this discharged energy heats up a considerable amount of air surrounding the core of the lightning. The temperature rise is so rapid and significant that it can start fires near the location where lightning strikes [37]. Furthermore, when lightning strikes a building, the electric current passes through the structure, which leads to a sudden heat generation and a rapid increase of temperature [29; 37; 38]. Besides, quite large shock forces occur due to the interaction of large masses of hot and cold air around it, and also due to the humidity of the air and the ground, which is called the blast effect. In addition, it is known that the presence of water amplifies the blast effect and interacts with the structure under electric current [34]. Therefore, it is thought that these great shock forces can cause instant damage. Even though it is judged that the structure is heated, it is impossible to predict or evaluate the exact or even approximate rise of temperature. For the purpose of understanding the underlying factors that led to the damage shown in Figure 1, the effects of temperature change on the structure were investigated using a simplified approach. The space frame system was given a temperature increase of 50 \({}^{\circ}\)C. Lightning is most likely to cause a local thermal shock and not heat up the entire structure. However, it is impossible to know the number of lightning strikes, the generated heat, the temperature rise, generated blast, etc. Based on these assumptions, a 50 \({}^{\circ}\)C temperature increase was applied to the global model alongside the external loads as described in the previous global analysis. A new static model was built and the results are given in Figures 17-21.