Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ResFrameFireSeismic (Resilient steel frame against fire and seismic hazards)

Teaser

Conventional seismic-resistant structures are designed to experience significant damage under moderate-to strong earthquakes and this results in socio-economic losses such as injuries, high repair costs and disruption of the building use or occupation. To address this issue...

Summary

Conventional seismic-resistant structures are designed to experience significant damage under moderate-to strong earthquakes and this results in socio-economic losses such as injuries, high repair costs and disruption of the building use or occupation. To address this issue, researchers have developed modern seismic-resilient frames that can avoid inelastic deformations (i.e. damage) in structural members. Fire is another type of loading, which can cause significant damage and collapse. Moreover, fire after strong earthquakes is also a highly probable catastrophic event as it has been seen after recent earthquakes (e.g. Indonesia 2009, Chile 2010). Despite the fact that the risk of fires is high after strong earthquakes, seismic resilient structural systems have not yet been studied against fire and fire after earthquake loading. This project studies the behaviour of modern seismic-resilient self-centering post-tensioned steel frames against fire loading and proposes modifications of their structural details so that, apart from seismic resilience, fire robustness can be also achieved. The project aims to develop, for the first time, an innovative steel frame characterized by the unique combination of minimal damage seismic behaviour and robustness against fire and post-earthquake fire loading.
The objectives of the project are:
1. To experimentally assess the fire-resistance and the failure modes of undamaged and pre-damaged PT connections.
2. To develop high-fidelity numerical models for PT connections that can be used for non-linear
thermal/structural analysis.
3. To simulate the thermal/structural response of PT connections at elevated temperatures and to define the appropriate structural detailing for fire resistant connections.
4. To propose fire design criteria and simplified models for PT connections with the goal of achieving fire
resistant SC-MRFs.
5. To develop numerical models for SC-MRFs, which are appropriate for thermal/structural analysis, seismic simulations, and FAE simulations.
6. To propose practical fire and FAE design procedures for SC-MRFs in the context of Eurocodes 3 and 8.

Work performed

The behaviour of SC-MRFs using PT connections with WHPs under fire was studied in detail. Sophisticated 3D finite element models were developed in MSC Marc and non-linear transient thermal/structural numerical analyses were carried out. A closed cavity was adopted to overcome the difficulties associated with the calculation of the view factors due to the complex geometry of the connections. The numerical techniques used for the solution of the thermal problem were validated through comparisons with experimental result and results obtained according to the recommendations of EN 1993-1-2 (CEN 2005). Apart from the SC-MRF using PT connections, the corresponding MRF with the same cross-sections and rigid full-strength welded beam-column connections was also studied for comparison purposes. Other parameters that were studied are the use of longitudinal beam web stiffeners in the PT connections, the fire protection of the PT bars, and the fire protection of the WHPs. Based on the numerical results, the following conclusions are drawn:
• The numerical techniques used for the solution of the thermal problem can predict correctly the temperature field of the heated frame during fire exposure.
• During the early stages of fire exposure, the behaviour of the heated beam of the SC-MRF is governed by the rotational and longitudinal restraints induced by the surrounding structure. Local buckling takes place at the web of the beam near its supports. At the 45th min of fire exposure, the WHPs are activated and very quickly reach their plastic resistance due to the high temperature. After this point, the beam deflection increases rapidly, which indicates collapse.
• Decompression of the PT connection and subsequent activation of WHPs depend on the magnitude of the axial force of the beam. This axial force is not constant and depends on the restraint provided by the columns but does not depend on the force of the PT bar.
• The behaviour of the PT bar is controlled by the restrained part of its thermal expansion, which induces compressive stress, and thus results in prestress force loss.
• During the early stages of the fire exposure, the mid-span deflections of the MRF increase rapidly and they are higher than those of the SC-MRF. However, the critical time for the SC-MRF is close to that of the MRF due to the beneficial effect of the catenary action that takes place in the case of the latter.
• Fire protection of the PT bar does not affect the behaviour of the frame.
• Using beam web stiffeners in the PT connections slightly improves the fire resistance of the SC-MRF.
• Fire protection of the WHPs increases the survival time of the SC-MRF considerably.
The results of the action were presented in the 12th HSTAM 2019 International Congress on Mechanics. Moreover, the results of the project are summarized in four papers which are the following:
1) Numerical assessment of the fire behaviour of steel post-tensioned moment-resisting frames, under revision in Journal of structural engineering
2) Fire and fire after earthquake design procedures for self-centering moment resisting frames, is currently prepared and will be submitted in Journal of constructional steel research
3) Experimental and numerical investigation of fracture of steel at elevated temperatures, submitted in Journal of structural engineering
4) Experimental and numerical investigation of fracture of steel at elevated temperatures after cyclic loading, currently prepared and will be submitted in Journal of structural engineering

Final results

The project goes beyond the state-of-the-art by developing, for the first time, an innovative steel frame characterized by a unique combination of minimal-damage seismic behaviour and enhanced fire resistance and has the potential to achieve high seismic resilience along with fire and FAE robustness. To achieve this goal, the project will develop and calibrate models in a unique way, i.e. models will be calibrated against experimental results from fire and from mechanical-fire tests. These high-fidelity models will be then used to conduct numerical simulations to enable practical design rules within the context of Eurocodes to be developed. Therefore, the project provides an original as well as a comprehensive and solid contribution to earthquake, fire and multi-hazard engineering.

Website & more info

More info: http://www.steel.civil.upatras.gr/index.php/contact/.