Extreme loads can arise from accidents such as vehicle collisions or airplane crashes, as well as deliberate acts of terrorism or military attacks involving blasts and fragmentation. Blast overpressure can also occur accidentally, for example, from explosions of hazardous materials such as gas. Distinguishing between accidental and deliberate loads is crucial for designing appropriate protection measures. The repercussions of extreme loading events can be devastating, leading to injuries, loss of life, economic setbacks, and significant social disruption. These consequences result not only from the direct effects of impacts or explosions, but also from secondary factors such as structural collapse, which is particularly concerning due to its potential for widespread devastation and substantial losses. Efforts to enhance the protection of concrete structures have focused on understanding the properties of construction materials and how structures respond to impact and blast loads. This document presents a comprehensive overview of RILEM TC 288-IEC, aiming to provide essential guidance for designing concrete structures to withstand extreme dynamic loads. This emphasizes the importance of a thorough understanding and accurate modelling of loading scenarios and material behaviour. By implementing the strategies outlined in this document, engineers can enhance the safety and resilience of structures facing such challenges.

This study investigated the effect of fire on the ultimate load-bearing capacity of reinforced concrete (RC) slabs. The structural response of RC circular specimens subjected to static load conditions after exposure to a hydrocarbon fire on one side of the specimen was examined. Two fire exposure times were considered (60 and 120 min) in addition to reference non-exposed specimens. The static response was evaluated in the damaged specimens in residual conditions after natural cooling from the elevated temperatures. The temperature distribution across the thickness of the slabs and their load–displacement response was measured. The decrease in the stiffness of the slabs due to the thermal exposure was studied by means of direct ultrasonic pulse velocity (UPV) measurements made before and after the fire tests. The decrease in global stiffness was partially accounted for by UPV measurements. The experimental results showed two peaks in the load-deflection response of the slabs. The first peak was related to an arching mechanism introduced by the specific set-up used. The second peak, corresponding to the ultimate load, occurred due to tensile membrane action at large deflections. While the former was strongly affected by the fire exposure, with the load being halved after the 120-min exposure, the latter was not greatly affected by either the presence of fire or the exposure time. Simplified mechanical models were used to explain the behavior of the RC slabs during the tests.

Structural modelling of wind turbine shallow foundations on granular soil strata deals with combined complex phenomena, such as soil-structure interaction, material nonlinearities, local stress concentration in both reinforced concrete foundation and soil. Numerical finite element analyses are performed employing three-dimensional elements for both concrete foundation and soil and one-dimensional elements for reinforcement. The results of the nonlinear static simulations are validated with experimental results of recent tests carried out on 1:15 scaled models having reinforcement ratios and stress conditions similar to the prototype foundations designed according to current codes. The validated modelling strategies are also applied to investigate the role of nonlinearities in both soil and concrete foundation, emphasizing the parameters and the nonlinear constitutive laws that significantly affect the foundation response. Three different reinforcement layouts are considered in both testing and simulations of the prototype: (a) complete prototype reinforcement, (b) prototype without shear reinforcement and (c) prototype without shear reinforcement and with 50% of remaining reinforcement replaced by steel fibres. A critical comparison is also made with models employing elastic two-dimensional shell finite elements lying over elastic supports, typically adopted for design purposes.

The current effort towards the progressive switch from carbon-based to renewable energy production is leading to a relevant spreading of both on- and off-shore wind turbine towers. Regarding reinforced concrete shallow foundations of onshore wind turbine steel towers, possible reductions of reinforcement may increase their sustainability, speed of erection, and competitiveness. The article presents the results of an experimental program carried out at Politecnico di Milano concerning both cyclic and monotonic loading, simulating extreme wind conditions on 1:15 scaled models of wind turbine steel towers connected by stud bolt adapters to reinforced concrete shallow foundations embedded in a sandy soil. Three couples of foundation specimens were tested with different reinforcement layouts: (a) similar to current praxis, (b) without shear reinforcement, and (c) without shear reinforcement and with 50% of ordinary steel rebars replaced by steel fibers. Additional vertical loads were added to the small-scale models in order to ensure similarity in terms of stresses. The test results allowed to (i) characterize the mechanical behavior of the foundation element considering soil-structure interaction under both service and ultimate load conditions, (ii) assess the foundation failure mode, (iii) highlight the role of each typology of reinforcing bars forming the cage, and (iv) provide hints for the optimization of these latter.

The design of massive structures, like cast-on-site foundations for wind towers, is not an easy task for design. In fact, this kind of structure is not specifically taken into account by Eurocode 2 standard. The huge concrete volume requires a huge amount of reinforcement, even if only the minimum amount is selected, and it is not clear if the lateral confinement exerted by the truncated cone geometry, mainly loaded along only one radial direction, corresponding to that characterized by the fastest wind direction, really requires the huge transversal reinforcement conventionally introduced in current wind towers. Within the research programme developed in 2019 between Enel Green Power, the Enel Group company dedicated to the development, construction and operation of renewables across the world, and Politecnico di Milano, an experimental investigation, carried out on prototypes characterized by reduced scales (1:15 for the whole structure and 1:4 for the investigation of the only core foundation, where the cylindrical stem is anchored), was conceived to calibrate a reliable modelling with the aim of being extended to full-scale structure, optimizing the required reinforcement. In this perspective, the use of fibre reinforced concrete can significantly improve the construction time by giving a specific toughness to the whole foundation, limiting also the crack opening and the correlated durability problems in the serviceability limit states.

Advanced numerical modeling of high-strength concrete (fc>60 MPa) structures designed to withstand severe thermal conditions requires detailed and reliable information on the mechanical properties of the material exposed to elevated temperatures. The only uniaxial compressive strength variation with temperature is not enough to satisfy the large number of parameters often required by advanced nonlinear constitutive models. For this reason, a complete experimental investigation is required. The paper takes a commonly used high-strength concrete (fc=73 MPa) as an example to describe a comprehensive experimental approach instrumental to the parameter definition and calibration of common constitutive models for concrete. The present study not only studied the overall compressive and tensile behavior of the case study material, but also investigated the effect of elevated temperatures on the specific fracture energy and the evolution of internal damage, in residual conditions after a single thermal cycle at 200°C, 400°C, and 600°C.

Reinforced concrete (RC) submerged floating tunnels (SFTs) represent a possible solution for crossing wide, deep fjords, and is considered for the E39 highway route along the Norwegian west coast. With regard to SFTs, the specific accidental scenario that is under investigation is the combined action of fire and subsequent internal explosion, as this is a crucial safety design condition for this type of structure. To assess the structural performance of reinforced concrete structures under combined fire and blast actions, gas burner equipment and a shock tube device were used to generate high temperature and blast loading, respectively, on RC circular slabs. A proper set of instruments consisting of thermocouples embedded in the specimens, accelerometers and ultrasonic pulse velocity (UPV) equipment made it possible to capture the behaviour of the slabs under the combined fire and blast actions and to distinguish the specific role of fire and blast. Simplified numerical tools such as an equivalent elastic single degree of freedom (SDOF) model and a linear elastic finite element (FE) model were used to interpret the experimental results. By considering all combinations of three fire exposure times and two shock waves, the effect of damage accumulation from a combined action of fire and subsequent internal explosion was mapped. A reliable benchmark for numerical models was obtained.

Ferry-free coastal route E39 is a project that aims to design a coastal highway without ferry connections. Wide and deep fjords along the Norwegian coast make submerged floating tunnels (SFT) an alternative to conventional structures. In the unfortunate situation of accidental events, the SFT reinforced concrete (RC) structure may become damaged, affecting its load carrying capacity. RC shells can be a representative component of the SFT concrete structure, and it is relevant to study if RC shells can carry the combination of fire and subsequent blast load. An experimental program has been performed in conjunction with numerical simulations in order to enable the risk analysis and the feasibility study of SFT in the E39 project. A shock tube equipment has been used to evaluate the influence of high temperature on the blast load carrying resistance. In addition, static and material tests have been performed for further understanding of the slab behaviour. Nonlinear analyses for static loading and preliminary dynamic analyses have been performed, in combination with standard fire curve load effects. Temperature-dependent material properties have been used based on the results from the material tests. This paper gives an exemplary overview of (sometimes preliminary) results. Both the experimental and numerical results have revealed that high temperature leads to a significant internal damage of the material, affecting the response of RC slabs. In addition, statics tests results for slabs subjected to high temperatures show a significant reduction of the load carrying capacity.