Ponti ad arco in acciaio: analisi del comportamento al fuoco secondo diversi scenari

Analisi sulla vulnerabilità al fuoco dei ponti ad arco in acciaio

I ponti sono opere strategiche e devono essere progettati per resistere a condizioni di carico di esercizio ed eccezionali.

Tuttavia, la progettazione strutturale attuale dei ponti non tiene esplicitamente in conto l’effetto di un incendio. Infatti, al contrario della maggior parte delle strutture ed infrastrutture (edifici e gallerie), non esiste uno specifico obbligo normativo che impone al progettista di verificare un ponte secondo criteri di resistenza al fuoco, benché il rischio di incendio non sia trascurabile, come si evince dalla letteratura scientifica.

Di fatto questo aspetto può comportare un’alta vulnerabilità al fuoco dei ponti e in caso di un incendio ci si può attendere quindi un significativo impatto sulla funzionalità della rete infrastrutturale. Il presente lavoro si inserisce in questo contesto analizzando la vulnerabilità al fuoco di un sovrappasso ad arco in acciaio con impalcato a lastra ortotropa.

Differenti plausibili scenari di incendio sono stati considerati, quali ad esempio camion che trasportano merci, al di sotto del ponte e sono stati modellati secondo modelli di incendio che si basano su curve d’incendio nominali e naturali, quest’ultime sviluppate secondo modelli di fluidodinamica numerica (CFD).

Sono state quindi sviluppate una serie di analisi termomeccaniche in modo da individuare i modi e i tempi di collasso, nonché il comportamento deformativo che può causare la perdita di funzionalità dell’opera.

Leggi di seguito il testo originale della memoria CTA.

Abstract

Bridges are strategic infrastructures and must be designed to withstand operating and exceptional load conditions.

However, the current structural standards of bridges does not explicitly consider fire actions. In fact, unlike most structures and infrastructures (buildings and tunnels), there is no specific regulatory obligation that requires the designer to verify a bridge according to fire resistance criteria. However, the fire risk is not negligible, as highlighted by the scientific literature.

This aspect can lead to a high vulnerability to the fire of bridges and in the event of a fire, a significant impact on the functionality of the infrastructural network can therefore be expected. The present work fits into this context by analyzing the fire vulnerability of an arched steel overpass with an orthotropic slab deck. Different plausible fire scenarios, such as heavy goods truck, were considered below the bridge and were modelled according to nominal curves and natural fire curves such as computational fluid dynamics (CFD).

A series of thermomechanical analyses were then developed to identify the failure modes and times of collapse, as well as the deformation behaviour that can cause the loss of functionality.

>> Potrebbe interessarti anche: Collasso di strutture in acciaio esposte al fuoco: la corretta analisi con la modellazione numerica
Per gli edifici in acciaio, soprattutto quelli non protetti uno degli strumenti utilizzati per verificare la cinematica del crollo a causa di incendio è il metodo degli elementi finiti. Vediamo in cosa consiste.

Introduction

Fire is an action that can severely damage bridge structures, which are not generally designed with fire resistance criteria. In addition, to assess the possibility of structural collapse, which can occur despite the beneficial effect of the ventilation that cools the hot gas that spread during the fire, it is often essential to check the extent of the deformations in the structure. In fact, too high deformations not only may cause the loss of functionality of the bridge, with severe repercussions for vehicular traffic, but can also cause damage to systems and underground services, both urban and extra-urban, often incorporated into the structure, with even more extensive consequences.

In 2002, average annual losses of $ 1.28 billion from fire-damaged bridges were estimated in the United States alone. In particular, the fires that are triggered on the infrastructural network are mainly caused by collision of vehicles with combustible materials and gas explosion from a leaking pipeline attached to the bridge structure [1,2]. Therefore, their intensity can be exceptionally high, also due to the fact that collisions produce the rapid ignitions of highly flammable materials.

Bridges and overpasses with girders and with cables / stays made of steel and steel-concrete composite are therefore particularly vulnerable, since: i) being in most cases not designed with fire resistance criteria, the load-bearing capacity decreases rapidly due to the rapid heating of the steel structural elements and ii) are often made with statically determined schemes with low robustness in case of fire. Furthermore, the closure of a bridge can have significant repercussions on the infrastructural network that are also economic.

Case history

The bridge geometry, material models and other assumptions are provided in this section. The bridge analyzed in this paper is an an unprotected single span steel arch overpass, as shown in Fig.1. The bridge is 65 m long and 14.7 m wide and it consists of an orthotropic slab deck with 2 traffic lanes.

The transverse beams are placed every 3 m. Fig. 2 shows its cross section. The bridge has one arch along the span that is centered in the middle of the bridge deck. The steel bridge deck is fully fixed at the junction to the arch and provides lateral stability to the arch. Two different steel grades were used. In detail high strength steel grade (fy≥750 N/mm2) was adopted for the hangers, while steel grade S355 (EN 10025-2, 2019) was adopted for all other elements, i.e. steel deck and arch.

CONTINUA LA LETTURA NEL PDF.

L'articolo è tratto dalle memorie del XXVIII CONGRESSO C.T.A. che si è svolto a Francavilla al Mare (Chieti) dal 29 settembre al 1 ottobre 2022.