*Article* **Numerical Investigation of the Collapse of a Steel Truss Roof and a Probable Reason of Failure**

**Mertol Tüfekci 1, \* , Ekrem Tüfekci <sup>2</sup> and Adnan Dikicio ˘glu 2**


Received: 6 October 2020; Accepted: 1 November 2020; Published: 3 November 2020

**Abstract:** This study investigated the failure of the roof, with steel truss construction, of a factory building in Tekirdag in the northwestern part of Turkey. The failure occurred under hefty weather conditions including lightning strikes, heavy rain, and fierce winds. In order to interpret the reason for the failure, the effects of different combinations of factors on the design and dimensioning of the roof were studied. Finite element analysis, using the commercial software Abaqus (Dassault Systèmes, Vélizy-Villacoublay, France), was performed several times under different assumptions and considering different factors with the aim of determining the dominant factors that were responsible for the failure. Each loading condition gives out a characteristic form of failure. The scenario with the most similar form of failure to the real collapse is considered as the most likely scenario of failure. In addition, the factors included in this scenario are expected to be the responsible factors for the partial collapse of the steel truss structure.

**Keywords:** steel truss; roof structure; partial collapse; finite element analysis; lightning strike

## **1. Introduction**

Engineers aim to make human life easier and to enhance life quality. Using mathematical calculations and experimentation, engineers try to predict the behaviour of a system and design it accordingly [1]. However, there have been cases that ended in structural failures, and some of these failures caused financial loss or even cost lives. Hadipriono studied nearly 150 major failures of structures around the world and determined that the major failures were due to lateral impact forces [2]. Moreover, Klasson published a survey covering failures of slender roofs [3]. Even the simplest structures, which have the most predictable behaviours, fail under unexpected conditions that exceed the designated safety margins [4,5].

Trusses are one of the most widely used and easy-to-design light structures [6]. They are able to carry very large loads relative to their own weights over very large spans. This is one of the main reasons that truss-type structures are preferred for building roofs and bridges. The possible loads are standardised to help engineers design structures in a very simple and straightforward way [7]. The standardised loads can be multiple times greater than the structures' own weight. In the case of unexpectedly excessive loads of accumulated snow or rainwater, failure of the designed structure may be unavoidable [8,9]. Geis et al. studied more than 1000 snow-induced building failure incidents all over the world [10]. An accumulated mass may cause failure in various ways. Excessive weight loads acting on the structures lead to a different load distribution than that of the designed distribution due to some members entering the plastic region and/or buckling [11–13].

In addition, the loads caused by dynamic effects such as earthquakes could lead to failure [14–16]. Structures are more vulnerable to dynamic loads than they are to static loads [17–19]. A way to

improve the performance of structures against dynamic loads is to add damping to the structure [20]. Earthquakes are not the only sources that cause dynamic loads on structures. One of the main dynamic effects that may lead to sudden or progressive collapses is wind [21–24]. The wind is a very significant factor that leads to lateral loads to which building structures are relatively more sensitive than they are to vertical loads [25].

A crucial dynamic effect that is a very complicated phenomenon is lightning strike [26]. A lightning strike can cause partial damage or complete failure of structures [27,28]. Specifically, there are cases where the main reason reported for structural failure is lightning strike [29–32]. In Australia, it is reported that 21% of insurance claims for damage to buildings are caused by lightning strikes [33]. A lightning strike can affect a structure in many different ways, with the two most dangerous processes being blast (local and rapid pressure oscillations) and heating [34]. Previous research proved the heating effects of the lightning strike phenomenon [35,36]. It is known that a lightning strike increases the temperature rapidly in a very short period of time [29,37]. The electric current that is caused by the lighting strike heats up the structure so that it can start fires [38]. Besides, fire alone can also be the main factor that leads to structural failure [39–42].

Lightning is one of the key points of this study. Of course, structures are not affected just by one of the consequences of the lightning strike phenomenon. Instead, the combination of these enumerated parameters creates a synergy effect, which increases the resultant influence of the factors so that the combination becomes greater than a superposition of these contributing load components [43]. The effects of a lightning strike can be even more dramatic on the structures when there is accumulated water on top of the structure, since the local blasts are amplified by the presence of the water and thermal changes influence the structure [34,44].

There are different types of failure mechanisms as well as different types of reasons for the collapse. Thus, it is possible to find possible reasons for the failure just from the mode/form of the failure. One of the most important types of failure mechanisms is fatigue [45]. Fatigue may lead to the progressive collapse of structures [46,47]. Progressive collapse may happen instantly following the changed load distribution of members due to buckling and/or failure of some members [48,49]. However, it does not have to happen at the time of the initial failure of individual members but can happen after a certain number of members have failed successively [50,51].

There has been experimental research focusing on the mechanical behaviour of some specific components and/or structures that are used in trusses [15,52–55]. In past studies by various researchers, real roof structures were put through experimental testing to analyse their mechanical behaviour [15,56,57]. However, experimentation is not always possible and/or feasible. Therefore, numerical studies are conducted to predict the mechanical behaviour as well as to predict plausible failures. For truss-type structures, the finite element method is usually employed since it is one of the most powerful tools in engineering [14,22,57,58]. Pieraccini et al. studied the collapse of a spatial truss roof of a gym building in North Italy during a moderate snowfall by using the finite element method [59]. They reported that the elements and connections made by ductile or brittle materials influenced the bearing capacity of the roof structures and created geometric imperfections.

On the other hand, not all the failures occur slowly. As mentioned earlier, some collapses occur suddenly under excessive, impulsive loadings. This study aimed to study a case in which a sudden partial failure of a steel truss roof of a factory built in July 2011 happened during heavy weather conditions on 22 October 2012 in Cerkezkoy Tekirdag in the northwestern part of Turkey [8]. The main focus was on developing a plausible theory for the failure by employing numerical analyses performed using the finite element method, which covers various loading conditions and their combinations. Not only the truss roof but also the combination of the truss roof with the column supports were analysed. Thus, the results of numerical analyses were used to compare the obtained failure modes to the real failure, considering different criteria for the failure. The extreme loads due to the ponding of rainfall or snow accumulation and the temperature shock due to the thunderstorm were considered in the analyses. Even under these potential mechanical overloads, the observed damage was different

from the actual damage: The damage caused by these excessive loads occurred in different regions than those in which the actual damage occurred. While the studies in the literature mostly describe failures due to vertical loads caused by water/snow accumulation or horizontal loads caused by wind, this study showed that the critical regions created by temperature change are similar to the actual damaged zones.
