Добірка наукової літератури з теми "Impact Dynamic Loads"

Fibre-reinforced composite materials are becoming important in many areas of technological application. In addition to the static load, such structures may be stressed with short-term dynamic loads or even dynamic impact loads during their lifespan. Impact loading of structural components produces a complex process, where both the characteristics of the design itself and the material parameters influence the resultant behavior. It is clear that fibre reinforced concrete has a positive impact on increasing of the resistance to impact loads. Results of two different impact load tests carried out on drop-weight test machine are presented in this report.

The practices of hazardous and unique facilities’ construction imply that specific attention is paid to the issues of safety. Threats associated with crash impacts caused by moving cars or planes are considered. To ensure safety of these construction sites it is required to know the potential dynamic loads and their destructive capacity. This article considers the methodology of reducing dynamic loads associated with impacts caused by moving collapsing solids and blast loads to equivalent static loads. It is demonstrated that practically used methods of reduction of dynamic loads to static loads are based in schematization only of the positive phase of a dynamic load in a triangle forms are not always correct and true. The historical roots of this approach which is not correct nowadays are shown; such approach considered a detonation explosion as a source of dynamic load, including TNT and even a nuclear weapon. Application of the existing practices of reduction of dynamic load to static load for accidental explosions in the atmosphere that occur in deflagration mode with a significant vacuumization phase may cause crucial distortion of predicted loads for the construction sites. This circumstance may become a matter of specific importance at calculations of potential hazard of impacts and explosions in unique units — for instance, in the nuclear plants. The article considers a situation with a plane crash, the building structure load parameters generated at the impact caused by a plane impact and the following deflagration explosion of fuel vapors are determined.

Fibre-reinforced composite materials are becoming important in many areas of technological application. In addition to the static load, such structures may be stressed with short-term dynamic loads or even dynamic impact loads during their lifespan. Impact loading of construction components produces a complex process, where both the characteristics of the design itself and the material parameters influence the resultant behavior. It is clear that reinforced concrete with fibers has a positive impact on increasing of the resistance to impact loads. Results of two different impact load tests carried out on drop-weight test machine are presented in this report.

Fibre-reinforced composite materials are becoming important in many areas of technological application. In addition to the static load, such structures may be stressed with short-term dynamic loads or even dynamic impact loads during their lifespan. Dynamic effects can be significant especially for thin-walled shell structures and barrier constructions. Impact loading of construction components produces a complex process, where both the characteristics of the design itself and the material parameters influence the resultant behavior. It is clear that reinforced concrete with fibers has a positive impact on increasing the resistance to impact loads. Results of impact load tests carried out on drop-weight test machine are presented in this paper. The results are supplemented by compression strength test.

Pavement distresses are induced by mechanistic responses in pavement structure subjected to dynamic loads of moving vehicles. Pavement surface evenness deteriorates as pavement distresses propagate, which results in dynamic axle loads and faster pavement deterioration. It is vital to consider the dynamic axle load spectra to predict pavement deterioration using traffic-monitoring data. This study aimed to evaluate the effect of dynamic loads and overweight traffic on asphalt pavement overlay performance using mechanistic–empirical (M–E) pavement analysis. The relationship between dynamic load coefficients (DLCs), axle loads, and international roughness index (IRI) was obtained for accurate quantification of dynamic axle loads. Then the dynamic axle load spectra were derived by shifting the static axle load spectra in weigh-in-motion (WIM) data, given the DLC value. AASHTOWare Pavement ME software was used to analyze pavement performance with static and dynamic axle load spectra, and the impact of overweight traffic on asphalt pavement overlay performance. The impact of dynamic loads on reflective fatigue cracking was distinguished at an early stage of the service period and eliminated after the 10-year analysis period, when the propagation of reflective cracking reached a specific level. On the other hand, the consideration of dynamic axle loads increased the impact of overweight truck traffic on pavement distresses, and pavement structures of major highways tend to be more sensitive to overweight traffic because of greater DLC excitement at higher operational speeds.

In construction engineering, rock is an important building material. During the construction process, layered rock masses are typically subjected to varying dynamic load disturbances under triaxial loads. It is thus essential to investigate the mechanical response of layered rocks under various disturbances of the triaxial loads. By using a three-dimensional SHPB, triaxial dynamic compression tests with various impact dynamic load disturbances and identical triaxial static loads were carried out on sandstones with differing bedding angles. The impact pressures were 0.8, 1.2, and 1.6 MPa, and the bedding angles were 0°, 30°, 45°, 60°, and 90°. The results showed that the ductility of the sandstone considerably increased under triaxial static loading. With the increasing bedding angle, the sandstone’s dynamic strength and coupling strength first declined and subsequently rose. As the impact pressure increased, the reflective energy ratio, peak strain, and dynamic growth factor of the sandstone essentially rose progressively. The bedding angles and dynamic loads had a major impact on the damage pattern of the layered sandstones. Additionally, a constitutive model considering bedding angle, dynamic load, and static load was established and verified. The constitutive model was able to accurately characterize the dynamic behavior of the rock under load disturbances.

A ship impact is a dynamic phenomenon and the dynamic global load effects can be significant, especially for small platforms where dynamic loads from the ship impact can be larger than the extreme environmental loads and the ship impact can govern the design of the platform. This paper describes a detailed procedure for dynamic analysis of fixed offshore platforms exposed to ship impacts. The procedure includes: • a consistent description of the motion of the vessel and dynamic interaction with the platform during the impact; • a realistic description of the global dynamic behavior of the platform during the impact; • detailed calculation of the transient hydrodynamic pressure forces acting on the vessel during the impact; and • a realistic description of the local deformation zone at the point of impact. The equations of motion for the vessel and the platform are solved simultaneously in the time domain, and the overall dynamic loads acting on the platform during the impact are determined by means of the modal superposition principle. The procedure has been applied for the design and subsequent risk analysis of three small tripod tower-type platforms to impacts from drifting supply vessels. The effect of the number of mode shapes used for representation of the dynamic behavior of the platforms, and the influence of the transient hydrodynamic pressure forces have been investigated. Critical velocity tables for different impact situations have been developed. For nearly all the situations investigated, the critical collapse criterion was overturning of the platform due to pull-out of piles in tension.

Fibre-reinforced composite materials are becoming important in many areas of technological application. In addition to the static load, such structures may be stressed with short-term dynamic loads or even dynamic impact loads during their lifespan. Dynamic effects can be significant especially for thin-walled shell structures and barrier constructions. Impact loading of construction components produces a complex process, where both the characteristics of the design itself and the material parameters influence the resultant behavior. It is clear that reinforced concrete with fibers has a positive impact on increasing the resistance to impact loads. Results of impact load tests carried out on drop-weight test machine are presented in this paper. The results are supplemented by static modulus of elasticity.

The mechanical response characteristics of mudstone from the ingate roadway of the west ventilation shaft in Yuandian No. 2 coal mine, Huaibei City, Anhui Province, China to dynamic loads were quantified in single- and cyclic-impact compression tests, using the split-Hopkinson pressure bar test device. The dynamic stress–strain relationships and the failure characteristics of mudstone samples under different impact loads were analyzed systematically. Considering the “rate effect” of the mudstone dynamic strength, the dynamic strength criterion of mudstone was proposed, and the dynamic damage constitutive model of mudstone was established, based on the statistical damage theory. In response to single-impact loads, with increasing impact pressure, the mudstone peak stress and strain gradually increased, and the peak stress and average strain rate increased nonlinearly. In response to cyclic-impact loads, with an increasing number of impacts, the mudstone peak stress first increased and then decreased, and the peak strain increased gradually. With increasing impact pressure, the number of impacts to the samples’ failure decreased gradually. By parameter identification and comparative analysis of the test results, the proposed dynamic damage constitutive model of mudstone was validated. The model can be used for stability analysis of roadway-surrounding rock under dynamic loads.

In this study the dynamic response of a ship structure to impact loads is investigated. The ship motion is fully three-dimensional and the ship structure is modeled as a three-dimensional elastic beam. Finite element methods are used to digitize the equations of motion of the system. The forces on the ship are interactive with the ship motion and position so that a full dynamic analysis is essential. Two main problems are considered: i) Estimation of hull damage when a ship collides with another ship, floating structure or fixed installation. A particular aspect of this analysis which has not previously been examined analytically involves estimating damage to the bottom of ship when it runs aground. Depending on the nature of the ground the ship may be pierced and significant amounts of steel may be torn, or the ship may ride over a sand bar without tearing but with noticeable denting and bending. In such grounding studies it has been necessary to introduce certain strength coefficients, realistic values of which have not been determined, but for which sensible estimates have been made. The results of a numerical study into grounding and collision damage illustrate clearly that ship speed is the major variable in the damage process. In particular the effect of subsequent angular motions incurred during a high speed collision can cause secondary but also significant collisions further aft. It is believed that these aspects of collision and grounding, and the related problems associated with collision whilst maneuvering, have not been investigated previously. ii) Bending stresses induced in ice-breaking ships during operation in ice. In this second class of problems two modes of operations are considered; continuous operation in level ice without loss of speed, and high speed ramming of ice ridges in which the ship is brought to rest. In the continuous ice breaking mode, the impulse loads are relatively low but periodic. The period of the impulse loads varies linearly with ship speed and also depends on the hardness and thickness of the ice. Since the ship is an elastic system with natural frequencies of the same order as impact frequency, some interesting response conditions have been identified leading to large flexural bending stresses in the ship. In the ramming mode,' two response states are of importance., The initial impulse at the bow of the ship, when contact is first made, causes the ship to respond primarily in its first flexural mode with possibly large bending stresses developing during the first second after impact. The ship then rides onto the ice in a "beaching mode" causing large quasi-static bending stresses in the hull which reach a peak after five seconds or so. Both of these peak bending situations have been investigated and their dependence on speed, hull" stiffness, bow angle, and ship speed has been established. In the past few years some data obtained from ships operating in the Beaufort sea has been released, both for continuous ice-breaking and for ramming. Whenever possible those data have been compared with the results predicted by the numerical method developed here. The agreement is shown to be very good.
Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate

This thesis performs analytical, numerical and experimental studies to investigate the influences of various parameters that affect the drop-weight impact tests, and the impact responses of monolithic and precast reinforced concrete beams, and beam-to-column joints. Influences of the drop weight test setup configurations and measurement methods on impact force and impact response of beams are examined. Performances of precast concrete beams and beam-to-column joints subjected to impact loads and their impact resistance capacities are investigated.

The dynamic responses of monolithic and segmental concrete bridge columns under impact loads were numerically and analytically investigated in this study. Theoretical models to predict the vehicle impact loading profile on reinforced concrete columns and to determine the dynamic capacity of the columns were developed. A practical procedure for engineers to design reinforced concrete columns against vehicle collisions, as well as an effective strengthening method to increase the dynamic capacity of the columns was proposed.

The Rollover of heavy vehicles operating in the construction, mining and agricultural sectors is a common occurrence that may result in death or severe injury for the vehicle occupants. Safety frames called ROPS (Rollover Protective Structures) that enclose the vehicle cabin, have been used by heavy vehicle manufacturers to provide protection to vehicle occupants during rollover accidents. The design of a ROPS requires that a dual criteria be fulfilled that ensures that the ROPS has sufficient stiffness to offer protection, whilst possessing an appropriate level of flexibility to absorb some or most of the impact energy during a roll. Over the last four decades significant research has been performed on these types of safety devices which has resulted in the generation of performance standards that may be used to assess the adequacy of a ROPS design for a particular vehicle type. At present these performance standards require that destructive full scale testing methods be used to assess the adequacy of a ROPS. This method of ROPS certification can be extremely expensive given the size and weight of many vehicles that operate in these sectors. The use of analytical methods to assess the performance of a ROPS is currently prohibited by these standards. Reasons for this are attributed to a lack of available fundamental research information on the nonlinear inelastic response of safety frame structures such as this. The main aim of this project was to therefore generate fundamental research information on the nonlinear response behaviour of ROPS subjected to both static and dynamic loading conditions that could be used to contribute towards the development of an efficient analytical design procedure that may lessen the need for destructive full scale testing. In addition to this, the project also aspired to develop methods for promoting increased levels of operator safety during vehicle rollover through enhancing the level of energy absorbed by the ROPS. The methods used to fulfil these aims involved the implementation of an extensive analytical modelling program using Finite Element Analysis (FEA) in association with a detailed experimental testing program. From these studies comprehensive research information was developed on both the dynamic impact response and energy absorption capabilities of these types of structures. The established finite element models were then used to extend the investigation further and to carry out parametric studies. Important parameters such as ROPS post stiffness, rollslope inclination and impact duration were identified and their effects quantified. The final stage of the project examined the enhancement of the energy absorption capabilities of a ROPS through the incorporation of a supplementary energy absorbing device within the frame work of the ROPS. The device that was chosen for numerical evaluation was a thin walled tapered tube known as frusta that was designed to crush under a sidewards rollover and hence lessen the energy absorption demand placed upon the ROPS. The inclusion of this device was found to be beneficial in absorbing energy and enhancing the level of safety afforded to the vehicle occupants.

The Rollover of heavy vehicles operating in the construction, mining and agricultural sectors is a common occurrence that may result in death or severe injury for the vehicle occupants. Safety frames called ROPS (Rollover Protective Structures) that enclose the vehicle cabin, have been used by heavy vehicle manufacturers to provide protection to vehicle occupants during rollover accidents. The design of a ROPS requires that a dual criteria be fulfilled that ensures that the ROPS has sufficient stiffness to offer protection, whilst possessing an appropriate level of flexibility to absorb some or most of the impact energy during a roll. Over the last four decades significant research has been performed on these types of safety devices which has resulted in the generation of performance standards that may be used to assess the adequacy of a ROPS design for a particular vehicle type. At present these performance standards require that destructive full scale testing methods be used to assess the adequacy of a ROPS. This method of ROPS certification can be extremely expensive given the size and weight of many vehicles that operate in these sectors. The use of analytical methods to assess the performance of a ROPS is currently prohibited by these standards. Reasons for this are attributed to a lack of available fundamental research information on the nonlinear inelastic response of safety frame structures such as this. The main aim of this project was to therefore generate fundamental research information on the nonlinear response behaviour of ROPS subjected to both static and dynamic loading conditions that could be used to contribute towards the development of an efficient analytical design procedure that may lessen the need for destructive full scale testing. In addition to this, the project also aspired to develop methods for promoting increased levels of operator safety during vehicle rollover through enhancing the level of energy absorbed by the ROPS. The methods used to fulfil these aims involved the implementation of an extensive analytical modelling program using Finite Element Analysis (FEA) in association with a detailed experimental testing program. From these studies comprehensive research information was developed on both the dynamic impact response and energy absorption capabilities of these types of structures. The established finite element models were then used to extend the investigation further and to carry out parametric studies. Important parameters such as ROPS post stiffness, rollslope inclination and impact duration were identified and their effects quantified. The final stage of the project examined the enhancement of the energy absorption capabilities of a ROPS through the incorporation of a supplementary energy absorbing device within the frame work of the ROPS. The device that was chosen for numerical evaluation was a thin walled tapered tube known as frusta that was designed to crush under a sidewards rollover and hence lessen the energy absorption demand placed upon the ROPS. The inclusion of this device was found to be beneficial in absorbing energy and enhancing the level of safety afforded to the vehicle occupants.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.02.09 – динаміка та міцність машин. – Національний технічний університет "Харківський політехнічний інститут" Міністерства освіти і науки України, Харків, 2017 В дисертації вирішена актуальна науково-технічна задача визначення динамічного напружено-деформованого стану захисних елементів машинобудівних конструкцій при імпульсному та ударному навантаженнях для забезпечення їх міцності та ефективного використання при експлуатації. На основі тривимірної моделі швидкісного деформування елементів конструкцій, з урахуванням пружно-пластичних скінченних деформацій і динамічних властивостей матеріалів отримані залежності розподілу напружень від швидкості удару за просторовими та часовими координатами в елементах конструкцій. Виявлені нові особливості процесу швидкісного деформування елементів при локальних навантаженнях, що відрізняються визначенням розмірів обмеженої зони напружень з великими градієнтами, утворенням кратерів тощо. Отримані залежності між напруженнями та швидкостями удару в тришаровому елементі для окремих шарів та деформаціями в шарах в залежності від швидкості ударника.
The thesis for a candidate of technical science degree in speciality 05.02.09 – Dynamics and Strength of Machines (engineering sciences) – Kharkov National University "Kharkov Polytechnic Institute", Kharkiv, 2017. In the thesis, the actual scientific and technical problem of determining the dynamic stress-strain state of the protective elements of machine-building structures under impulse and shock loads solved to ensure their strength and effective use during operation. The thesis proposes an improved three-dimensional model of high-rate deformation of structural elements, which is different by taking into account elastic-plastic finite deformations and dynamic properties of materials. Based on the proposed model, the dependences of the distribution of stresses on the speed of impact on spatial and temporal coordinates in structural elements made of various materials obtained. New features of the process of high-rate deformation of elements under local loads detected, differing in the definition of the size of a restricted stress zone with large gradients, the formation of craters and the process of unloading with the appearance of residual stresses and damages. Dependencies between stresses and impact speeds in a three-layer element for individual layers and deformations in layers depending on the speed of the impactor obtained. The dynamic stress-strain state changes significantly both in space coordinates and in time. Therefore, even for thin-walled constructions, the use of the theory of plates and shells is undesirable, since in this case the law of stress distribution over the thickness is preliminarily assumed, and part of the stresses perpendicular to the middle surface are not taken into account at all. The processes of high-speed deformation occur both in the elastic and in the plastic stage and partially accompanied by rather large deformations. Therefore, the work uses three-dimensional models, even for thin-walled structures. From a mathematical point of view, such problems are essentially non-linear and require analysis of a three-dimensional dynamic stress-strain state. The problems of high-rate elastic-plastic deformation of elements of cylindrical structures are considered. It is shown, that the largest displacements and stresses develop in local zones and in the case when the speed is increase up to V ≥ 150 m/s, the area of intense displacements and stresses is R ≤ (10-12) r, where r is the radius of the zone load. These features of the dynamic stress-strain state make it possible to isolate the corresponding region of the element and to make refined calculations for it using a denser grid. A number of practical problems of analyzing the stress-strain state of the elements of the gas turbine engine corps under shock loading considered which differ in the purpose, geometric characteristics and properties of the materials. It is shown, that the largest displacements and stresses develop in bounded zones and rapidly decrease in spatial coordinates both in time and in unloading. It is shown, that when the blade fragment is detached, as well as the foreign particles fall into the flow at the working speeds of the gas turbine engine rotation, the stress intensities do not exceed the prescribed boundaries. In some cases, preference is given to two-layer structures, since they resist shock loads better, than single-layer ones with a larger thickness of the same material.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.02.09 – динаміка та міцність машин. – Національний технічний університет "Харківський політехнічний інститут" Міністерства освіти і науки України, Харків, 2017 В дисертації вирішена актуальна науково-технічна задача визначення динамічного напружено-деформованого стану захисних елементів машинобудівних конструкцій при імпульсному та ударному навантаженнях для забезпечення їх міцності та ефективного використання при експлуатації. На основі тривимірної моделі швидкісного деформування елементів конструкцій, з урахуванням пружно-пластичних скінченних деформацій і динамічних властивостей матеріалів отримані залежності розподілу напружень від швидкості удару за просторовими та часовими координатами в елементах конструкцій. Виявлені нові особливості процесу швидкісного деформування елементів при локальних навантаженнях, що відрізняються визначенням розмірів обмеженої зони напружень з великими градієнтами, утворенням кратерів тощо. Отримані залежності між напруженнями та швидкостями удару в тришаровому елементі для окремих шарів та деформаціями в шарах в залежності від швидкості ударника.
The thesis for a candidate of technical science degree in speciality 05.02.09 – Dynamics and Strength of Machines (engineering sciences) – Kharkov National University "Kharkov Polytechnic Institute", Kharkiv, 2017. In the thesis, the actual scientific and technical problem of determining the dynamic stress-strain state of the protective elements of machine-building structures under impulse and shock loads solved to ensure their strength and effective use during operation. The thesis proposes an improved three-dimensional model of high-rate deformation of structural elements, which is different by taking into account elastic-plastic finite deformations and dynamic properties of materials. Based on the proposed model, the dependences of the distribution of stresses on the speed of impact on spatial and temporal coordinates in structural elements made of various materials obtained. New features of the process of high-rate deformation of elements under local loads detected, differing in the definition of the size of a restricted stress zone with large gradients, the formation of craters and the process of unloading with the appearance of residual stresses and damages. Dependencies between stresses and impact speeds in a three-layer element for individual layers and deformations in layers depending on the speed of the impactor obtained. The dynamic stress-strain state changes significantly both in space coordinates and in time. Therefore, even for thin-walled constructions, the use of the theory of plates and shells is undesirable, since in this case the law of stress distribution over the thickness is preliminarily assumed, and part of the stresses perpendicular to the middle surface are not taken into account at all. The processes of high-speed deformation occur both in the elastic and in the plastic stage and partially accompanied by rather large deformations. Therefore, the work uses three-dimensional models, even for thin-walled structures. From a mathematical point of view, such problems are essentially non-linear and require analysis of a three-dimensional dynamic stress-strain state. The problems of high-rate elastic-plastic deformation of elements of cylindrical structures are considered. It is shown, that the largest displacements and stresses develop in local zones and in the case when the speed is increase up to V ≥ 150 m/s, the area of intense displacements and stresses is R ≤ (10-12) r, where r is the radius of the zone load. These features of the dynamic stress-strain state make it possible to isolate the corresponding region of the element and to make refined calculations for it using a denser grid. A number of practical problems of analyzing the stress-strain state of the elements of the gas turbine engine corps under shock loading considered which differ in the purpose, geometric characteristics and properties of the materials. It is shown, that the largest displacements and stresses develop in bounded zones and rapidly decrease in spatial coordinates both in time and in unloading. It is shown, that when the blade fragment is detached, as well as the foreign particles fall into the flow at the working speeds of the gas turbine engine rotation, the stress intensities do not exceed the prescribed boundaries. In some cases, preference is given to two-layer structures, since they resist shock loads better, than single-layer ones with a larger thickness of the same material.

AbstractIn the harbour of Hamburg a new bridge crossing the Suederelbe at Elbe km 620 has been planned. The bridge is part of the BAB26 motorway. Contracted by DEGES, a German company for highway planning and design, BAW determined the ship impact loads on the basis and methodology of EN 1991-1-7 (2006) on a site - specific basis, BAW (2016). The Suederelbe Bridge crossing will have a length of 695,60 m with main span bridging the fairway over 350 m. The pylon height of the main piers is planned to be 150 m. The clearance height of the Suederelbe crossing should be 53 m above sea level. The two pylons and main piers of the Suederelbe bridge crossing, which are to be positioned close to the bank, are at risk of ship impact. About 6,000 seagoing vessels up to 33,000 dwt are passing the future bridge. Fleet structure, ship passages, speeds, accident rates and nautical conditions were analysed and ship impact loads for the piers and protective structures were determined using a load model on one hand and a collision model on the other hand. Load and collision model are probabilistic and based on corresponding distributions of the decisive influencing parameters, whereby this approach represents state-of-the-art technology. Based on the collision model, the average time that the eastern pylon is being impacted is expected to be 470 years, while the average time for the western pylon is expected to be 1,240 years.The impact loads for the east pylon are dynamic loads with FFdyn = 17.5 MN for frontal impact and FLdyn = 4.0 MN for lateral impact. Out of the models the impact load can be determined as dependent on the distance of the piers (pylons) to the fairway center line. The variation of the span distance of the pylons show the plausibility of the developed and used modelling.

The present paper studied the dynamic response of an underwater system with its navigation plate rotated relative to the main body until it was blocked by an energy absorber. In this process, the relation between fluid-driving moment and speed of main body, as well as the relation between rotation angle of the plate and design parameters of absorber, was investigated through combined finite element method and finite volume method. Before the plate contacted with the energy absorber, it was modeled by linear elastic material, the movement process was solved by finite volume method with dynamic boundary. When the plate started to contact and crash with the absorber, it was modeled by elastic-plastic material, and the interaction of fluid-structure coupling was simulated by explicit finite element method in LSDYNA and finite volume method in FLUENT. The two-way data exchange on the interface between fluid and structure was carried out through equivalent force and moment on each patch of the interface. In addition, the simulation accuracy on large plastic deformation of absorber was verified through a group of drop hammer experiments. After the energy absorber was crushed to ultimate shape, the open angle of plate reached the maximum value and the plate kept relative static to the rigid body. The maximum structural stress and deformation, the opening time and angle of the plate were evaluated by numerical method. It is demonstrated that the proposed method can effectively predict the dynamic response of underwater system under impact loads, and both the absorption capability of the block and the speed of moving body affect the dynamic response history and structural safety.

Abstract This work presents the results of a study on the amplification of live load in the vertical direction due to derailment of a train, i.e. derailment impact. The study was performed using numerical models based on a single span precast prestressed concrete girder bridge, with a range of typical stiffness values. Two representative transit vehicles were considered. The transit vehicles were modeled in LS-DYNA, accounting for all geometry, stiffness and damping of the car body and suspension system. The bridge was idealized as a spring-supported rigid surface, with the spring stiffness related to the computed stiffness of the full composite bridge structure. The model simulated the condition of the vehicle derailing and falling vertically through the height of rail and supporting plinth to impact the deck of the bridge. Two different drops were considered, representing the height of rail only and height of rail plus plinth structure. Derailment impact was estimated as the difference between effects of the static load applied to the rails and the dynamic force due to the derailed vehicle impacting the deck. The study also examined the effects of structure stiffness by considering two additional values of stiffness values, representing the range of stiffness expected in typical transit structures. The results showed that the peak amplification of live load due to derailment varied from 300% to 625% over the range of stiffness and vertical train drop considered. The results are based on simplified conditions and compare only peak amplification but are high enough to warrant a more detailed examination of and approach to the derailment forces currently used in North American codes to determine more accurate loads.

This paper is aimed at providing a better understanding of the potential energy absorption benefits of components fabricated using fused deposition modeling (FDM) additive manufacturing. Using FDM, it is possible to print three-dimensional (3-D) objects created through the use of computer-aided design and computer-aided manufacturing software coupled with computer codes that enable the layer-by-layer deposition of material to form the 3-D component. Also known as direct digital manufacturing or 3-D printing, AM offers the benefit of being able to rotate printing orientation during processing to manipulate the design build and ultimately control mechanical and structural properties when subjected to dynamic loads. In this work, tensile test specimens were first fabricated to characterize the general mechanical behavior of the of 3D-printed Acrylonitrile Butadiene Styrene (ABS) material to assess its potential strain rate dependency. The mechanical evaluation under the quasi-static load was also necessary to determine the properties necessary to characterize the dynamic evolution of ABS in compression at various strain rates. ABS specimens were subsequently subjected to high strain rate deformation through the use of the Split Hopkinson Pressure Bar. During compression a new phenomenon described as a multistage collapse in which the samples undergo multiple stages of contraction and expansion was observed as the impact load was applied.

Ships and offshore structures are often exposed to various types of repeated impact loads, such as wave slamming, floating ice impacts and ship collisions which will cause large deformation or even fracture. With imperfections due to the process of construction or damage caused by accidents, the load carrying capacity of structures will decrease. This paper investigates the load carrying capacity of aluminum alloy plate with an initial crack under repeated impact loads by means of experiments and numerical simulations. In the experiments, the prepared specimens with crack and without crack are impacted repeatedly up to plate perforation by releasing a hemispherical-headed cylindrical hammer. Numerical simulations are carried out with ABAQUS/Explicit software. The numerical models are built according to the actual experimental conditions. Comparison of the numerical predictions with the experimental results shows reasonable agreement. It is found that aluminum alloy plates under repeated impacts are sensitive to initial cracks. The fracture mode and plastic deformation of aluminum alloy plates can also be affected.

Abstract A rigid subsea spool is used to connect the riser of a jacket platform to oil export pipeline in Johan Sverdrup oil field. The location is within the lifting zones of the platform. Consequently, the dropped object hazard has potential high risk and needs to be checked. This paper presents a numerical model on accessing the structural dynamics of subsea spool under the dropped container impact loads by using de-coupled local and global model. The impact impulse was obtained from local impact analysis by Abaqus Explicit solver, in which deformations from container and pipeline are both captured. The global model was built by using inhouse program utilizing ANSYS APDL macros. A simple input file is only needed for end users. The nonlinear pipe and soil interaction is included in a simplified manner. The model comprises of static and dynamic analysis parts. The static analysis captures the in-place configuration and the functional loads. The dynamic analysis is a restart with inherited stress state from static analysis. The impact impulse was applied by point loads in a certain time range. The nonlinear soil stiffness was approached by spring elements (compression only). The dynamic analysis was done in a longer time, ensuring to capture any dynamic effects. The interface loads at the riser stick-out and riser anchor are both extracted and discussed. Concluding remarks have been made accordingly.

This paper presents an improved methodology for evaluating the dynamic responses of shipping casks subjected to the sequential HAC impact loads. The methodology utilizes the import technique of the finite-element mesh and the analytical results form one dynamic analysis using explicit numerical integration scheme into another dynamic analysis also using explicit numerical integration scheme. The new methodology presented herein has several advantages over conventional methods. An example problem is analyzed to illustrate the application of the present methodology in evaluating the structural responses of a shipping cask to the sequential HAC loading.

Abstract Dynamic fragmentation through high-rate impact generates large numbers of fragments with various shapes and sizes. The fragmentation failure mode is an important part of the protection capacity of advanced ceramics which typically feature high strength and low density but fail in brittle modes. The penetration resistance of these brittle materials has been linked to the fragment size and shape created through impact in the literature [1]. Such studies have shown that particular fragment size and shape combinations can more effectively erode incoming projectiles, presenting a possible route to improve penetration resistance. These results stand in contrast to other studies that examine links between penetration resistance and material properties (e.g. fracture toughness or stiffness) which have sometimes resulted in contradictory correlations. Boron carbide has received a strong focus in the literature in recent years as an advanced ceramic with one of the highest specific strengths and lowest densities [2]. Yet boron carbide exhibits poor penetration resistance at higher loads, a phenomenon that some researchers attribute to a phase transformation termed “amorphization” [2]. To better understand the protection capacity of boron carbide under high rate loading, we use a laser-driven micro-flyer apparatus to impact boron carbide specimens.

Describes the computer program SPAN which computes the nonlinear transient response of a submarine pipeline, in contact with the ocean floor, to wave and current excitation. The dynamic response of a pipeline to impact loads, such as loads from trawl gear of fishing vessels, may also be computed. In addition, thermal expansion problems for submarine pipelines may be solved using SPAN. Beam finite element theory is used for spatial discretization of the partial differential equations governing the motion of a submarine pipeline. Large-deflection, small-strain theory is employed. The formulation involves a consistent basis and added mass matrix. Quadratic drag is computed using a nonconventional approach that involves the beam shape functions. Soil-resistance loads are computed using unique pipeline-soil interaction models which take into account coupling of axial and lateral soil forces. The nonlinear governing equations are solved numerically using the Newmark Method. This manual presents the discretized equations of motion, the methods used in determining hydrodynamic and soil-resistance forces, and the solution method.

Housing issues usually play a major role in urban studies, but are often overlooked as a factor in rural development. This policy brief explores aspects of the dynamics of the ‘frozen’ rural housing market in the Nordic Region, with a specific focus on the role of financing, the part played by municipalities and the potential benefits of a larger rental market.Housing is generally seen as a human right, a consumable that serves as the framework for our lives. However, at the same time, real estate is a financial commodity on the market. In many rural areas, the market value of houses is low – often considerably below the cost of construction. In consequence, it is very difficult to obtain loans to build or buy. This ‘freezes’ the market and has a strong impact on rural development overall, in effect acting as a boost to the trend towards urbanisation and the depopulation of rural areas. We will explore ways to counteract this dynamic.

Previous FSRI led research projects have focused on examining the fire environment with regards to current building construction methods, synthetic fuel loading, and best-practices in firefighting strategies and tactics. More than 50 experiments have been previously conducted utilizing furniture to produce vent-limited fire conditions, replicating the residential fire environment, and studying the methods of horizontal ventilation, vertical ventilation, and positive pressure attack. Tactical considerations generated from the research are intended to provide fire departments with information to evaluate their standard operating procedures and make improvements, if necessary, to increase the safety and effectiveness of firefighting crews. Unfortunately, there still exists a long standing disconnect between live-fire training and the fireground as evident by continued line of duty injury and death investigations that point directly to a lack of realistic yet safe training, which highlights a continued misunderstanding of fire dynamics within structures. The main objective of the Study of the Fire Service Training Environment: Safety, Fidelity, and Exposure is to evaluate training methods and fuel packages in several different structures commonly used across the fire service to provide and highlight considerations to increase both safety and fidelity. This report is focused on the evaluation of live-fire training in acquired structures. A full scale structure was constructed using a similar floor plan as in the research projects for horizontal ventilation, vertical ventilation, and positive pressure attack to provide a comparison between the modern fire environment and the training ground. The structure was instrumented which allowed for the quantification of fire behavior, the impact of various ventilation tactics, and provided the ability to directly compare these experiments with the previous research. Twelve full scale fire experiments were conducted within the test structure using two common training fuel packages: 1) pallets, and 2) pallets and oriented strand board (OSB). To compare the training fuels to modern furnishings, the experiments conducted were designed to replicate both fire and ventilation location as well as event timing to the previous research. Horizontal ventilation, vertical ventilation, and positive pressure attack methods were tested, examining the proximity of the vent location to the fire (near vs. far). Each ventilation configuration in this series was tested twice with one of the two training fuel loads. The quantification of the differences between modern furnishings and wood-based training fuel loads and the impact of different ventilation tactics is documented through a detailed comparison to the tactical fireground considerations from the previous research studies. The experiments were compared to identify how the type of fuel used in acquired structures impacts the safety and fidelity of live-fire training. The comparisons in this report characterized initial fire growth, the propensity for the fire to become ventilation limited, the fires response to ventilation, and peak thermal exposure to students and instructors. Comparisons examined components of both functional and physical fidelity. Video footage was used to assess the visual cues, a component of the fire environment that is often difficult to replicate in training due to fuel load restrictions. The thermal environment within the structure was compared between fuel packages with regards to the potential tenability for both students and instructors.

Under the United States Department of Homeland Security (DHS) Assistance to Firefighter Grant Program, Underwriters Laboratories examined fire service ventilation and suppression practices as well as the impact of changes in modern house geometries. There has been a steady change in the residential fire environment over the past several decades. These changes include larger homes, more open floor plans and volumes, and increased synthetic fuel loads. This investigation examined the influence of these changes to the fire behavior and subsequent impact on firefighter tactics relative to horizontal and vertical ventilation and suppression. It is anticipated that the results of this investigation will be incorporated into improved firefighting tactics and decision making to reduce firefighter injuries and fatalities. Vertical ventilation has been used successfully but also resulted in firefighter fatalities in the past, as it is not easily coordinated with suppression and other fire ground tasks such as horizontal ventilation. It is not straightforward for firefighters to train on the effects of vertical ventilation since fire service training structures and props do not allow for ventilation-limited fire conditions with representative fuel loads and floor plans that will be encountered on the fire ground. Thus, guidance on the effectiveness of vertical ventilation comes from experience gained during real incidents, but under many different fire ground conditions. This has made it difficult to develop comprehensive guidance on the coordination of vertical ventilation with other firefighter tactics, and how these tactics may influence the fire dynamics in the burning home. The purpose of this study was to improve the understanding of the fire dynamics associated with the use of vertical ventilation so that it may be more effectively deployed on the fire ground. Two houses were constructed in the large fire facility of Underwriters Laboratories in Northbrook, IL. The first house was a one-story house (1200 ft, three bedrooms, one bathroom) with a total of 8 rooms. The second house was a two-story house (3200 ft, four bedrooms, two and a half bathrooms) with a total of 12 rooms. The second house featured a modern open floor plan, two-story great room and open foyer. A total of seventeen experiments were conducted varying the ventilation locations and the number of ventilation openings. Ventilation scenarios included ventilating the front door and a window near the seat of the fire (with modern and legacy furnishings) to link to the previous research on horizontal ventilation, opening the front door and ventilating over the fire and remote from the fire. Additional experiments examined controlling the front door, making different sized ventilation holes in the roof and the impact of exterior hose streams. The results from the experiments led to identification of tactical considerations for the fire service to integrate into their education and fire ground strategies and tactics where applicable.

Stormwater management is an ongoing challenge in the United States and the world at-large. As state and municipal agencies grapple with conflicting interests like encouraging land development, complying with permits to control stormwater discharges, “urban stream syndrome” effects, and charges to steward natural resources for the long-term, some agencies may turn to constructed wetlands (CWs) as aesthetically pleasing and functional natural analogs for attenuating pollution delivered by stormwater runoff to rivers and streams. Constructed wetlands retain pollutants via common physical, physicochemical, and biological principles such as settling, adsorption, or plant and algae uptake. The efficacy of constructed wetlands for pollutant attenuation varies depending on many factors such as flow rate, pollutant loading, maintenance practices, and design features. In 2018, the culmination of efforts by Clackamas Water Environment Services and others led to the opening of the Carli Creek Water Quality Project, a 15-acre constructed wetland adjacent to Carli Creek, a small, 3500-ft tributary of the Clackamas River in Clackamas County, OR. The combined creek and constructed wetland drain an industrialized, 438-acre, impervious catchment. The wetland consists of a linear series of a detention pond and three bioretention treatment cells, contributing a combined 1.8 acres of treatment area (a 1:243 ratio with the catchment) and 3.3 acre-feet of total runoff storage. In this study, raw pollutant concentrations in runoff were evaluated against International Stormwater BMP database benchmarks and Oregon Water Quality Criteria. Concentration and mass-based reductions were calculated for 10 specific pollutants and compared to daily precipitation totals from a nearby precipitation station. Mass-based reductions were generally higher for all pollutants, largely due to runoff volume reduction on the treatment terrace. Concentration-based reductions were highly variable, and suggested export of certain pollutants (e.g., ammonia), even when reporting on a mass-basis. Mass load reductions on the terrace for total dissolved solids, nitrate+nitrite, dissolved lead, and dissolved copper were 43.3 ± 10%, 41.9 ± 10%, 36.6 ± 13%, and 43.2 ± 16%, respectively. E. coli saw log-reductions ranging from -1.3 — 3.0 on the terrace, and -1.0 — 1.8 in the creek. Oregon Water Quality Criteria were consistently met at the two in-stream sites on Carli Creek for E. coli with one exception, and for dissolved cadmium, lead, zinc, and copper (with one exception for copper). However, dissolved total solids at the downstream Carli Creek site was above the Willamette River guidance value 100 mg/L roughly 71% of the time. The precipitation record during the study was useful for explaining certain pollutant reductions, as several mechanisms are driven by physical processes, however it was not definitive. The historic rain/snow/ice event in mid-February 2021 appeared to impact mass-based reductions for all metals. Qualitatively, precipitation seemed to have the largest effect on nutrient dynamics, specifically ammonia-nitrogen. Determining exact mechanisms of pollutant removals was outside the scope of this study. An improved flow record, more targeted storm sampling, or more comprehensive nutrient profiles could aid in answering important questions on dominant mechanisms of this new constructed wetland. This study is useful in establishing a framework and baseline for understanding this one-of-a-kind regional stormwater treatment project and pursuing further questions in the future.

1.1 Macroeconomic summary Economic recovery has consistently outperformed the technical staff’s expectations following a steep decline in activity in the second quarter of 2020. At the same time, total and core inflation rates have fallen and remain at low levels, suggesting that a significant element of the reactivation of Colombia’s economy has been related to recovery in potential GDP. This would support the technical staff’s diagnosis of weak aggregate demand and ample excess capacity. The most recently available data on 2020 growth suggests a contraction in economic activity of 6.8%, lower than estimates from January’s Monetary Policy Report (-7.2%). High-frequency indicators suggest that economic performance was significantly more dynamic than expected in January, despite mobility restrictions and quarantine measures. This has also come amid declines in total and core inflation, the latter of which was below January projections if controlling for certain relative price changes. This suggests that the unexpected strength of recent growth contains elements of demand, and that excess capacity, while significant, could be lower than previously estimated. Nevertheless, uncertainty over the measurement of excess capacity continues to be unusually high and marked both by variations in the way different economic sectors and spending components have been affected by the pandemic, and by uneven price behavior. The size of excess capacity, and in particular the evolution of the pandemic in forthcoming quarters, constitute substantial risks to the macroeconomic forecast presented in this report. Despite the unexpected strength of the recovery, the technical staff continues to project ample excess capacity that is expected to remain on the forecast horizon, alongside core inflation that will likely remain below the target. Domestic demand remains below 2019 levels amid unusually significant uncertainty over the size of excess capacity in the economy. High national unemployment (14.6% for February 2021) reflects a loose labor market, while observed total and core inflation continue to be below 2%. Inflationary pressures from the exchange rate are expected to continue to be low, with relatively little pass-through on inflation. This would be compatible with a negative output gap. Excess productive capacity and the expectation of core inflation below the 3% target on the forecast horizon provide a basis for an expansive monetary policy posture. The technical staff’s assessment of certain shocks and their expected effects on the economy, as well as the presence of several sources of uncertainty and related assumptions about their potential macroeconomic impacts, remain a feature of this report. The coronavirus pandemic, in particular, continues to affect the public health environment, and the reopening of Colombia’s economy remains incomplete. The technical staff’s assessment is that the COVID-19 shock has affected both aggregate demand and supply, but that the impact on demand has been deeper and more persistent. Given this persistence, the central forecast accounts for a gradual tightening of the output gap in the absence of new waves of contagion, and as vaccination campaigns progress. The central forecast continues to include an expected increase of total and core inflation rates in the second quarter of 2021, alongside the lapse of the temporary price relief measures put in place in 2020. Additional COVID-19 outbreaks (of uncertain duration and intensity) represent a significant risk factor that could affect these projections. Additionally, the forecast continues to include an upward trend in sovereign risk premiums, reflected by higher levels of public debt that in the wake of the pandemic are likely to persist on the forecast horizon, even in the context of a fiscal adjustment. At the same time, the projection accounts for the shortterm effects on private domestic demand from a fiscal adjustment along the lines of the one currently being proposed by the national government. This would be compatible with a gradual recovery of private domestic demand in 2022. The size and characteristics of the fiscal adjustment that is ultimately implemented, as well as the corresponding market response, represent another source of forecast uncertainty. Newly available information offers evidence of the potential for significant changes to the macroeconomic scenario, though without altering the general diagnosis described above. The most recent data on inflation, growth, fiscal policy, and international financial conditions suggests a more dynamic economy than previously expected. However, a third wave of the pandemic has delayed the re-opening of Colombia’s economy and brought with it a deceleration in economic activity. Detailed descriptions of these considerations and subsequent changes to the macroeconomic forecast are presented below. The expected annual decline in GDP (-0.3%) in the first quarter of 2021 appears to have been less pronounced than projected in January (-4.8%). Partial closures in January to address a second wave of COVID-19 appear to have had a less significant negative impact on the economy than previously estimated. This is reflected in figures related to mobility, energy demand, industry and retail sales, foreign trade, commercial transactions from selected banks, and the national statistics agency’s (DANE) economic tracking indicator (ISE). Output is now expected to have declined annually in the first quarter by 0.3%. Private consumption likely continued to recover, registering levels somewhat above those from the previous year, while public consumption likely increased significantly. While a recovery in investment in both housing and in other buildings and structures is expected, overall investment levels in this case likely continued to be low, and gross fixed capital formation is expected to continue to show significant annual declines. Imports likely recovered to again outpace exports, though both are expected to register significant annual declines. Economic activity that outpaced projections, an increase in oil prices and other export products, and an expected increase in public spending this year account for the upward revision to the 2021 growth forecast (from 4.6% with a range between 2% and 6% in January, to 6.0% with a range between 3% and 7% in April). As a result, the output gap is expected to be smaller and to tighten more rapidly than projected in the previous report, though it is still expected to remain in negative territory on the forecast horizon. Wide forecast intervals reflect the fact that the future evolution of the COVID-19 pandemic remains a significant source of uncertainty on these projections. The delay in the recovery of economic activity as a result of the resurgence of COVID-19 in the first quarter appears to have been less significant than projected in the January report. The central forecast scenario expects this improved performance to continue in 2021 alongside increased consumer and business confidence. Low real interest rates and an active credit supply would also support this dynamic, and the overall conditions would be expected to spur a recovery in consumption and investment. Increased growth in public spending and public works based on the national government’s spending plan (Plan Financiero del Gobierno) are other factors to consider. Additionally, an expected recovery in global demand and higher projected prices for oil and coffee would further contribute to improved external revenues and would favor investment, in particular in the oil sector. Given the above, the technical staff’s 2021 growth forecast has been revised upward from 4.6% in January (range from 2% to 6%) to 6.0% in April (range from 3% to 7%). These projections account for the potential for the third wave of COVID-19 to have a larger and more persistent effect on the economy than the previous wave, while also supposing that there will not be any additional significant waves of the pandemic and that mobility restrictions will be relaxed as a result. Economic growth in 2022 is expected to be 3%, with a range between 1% and 5%. This figure would be lower than projected in the January report (3.6% with a range between 2% and 6%), due to a higher base of comparison given the upward revision to expected GDP in 2021. This forecast also takes into account the likely effects on private demand of a fiscal adjustment of the size currently being proposed by the national government, and which would come into effect in 2022. Excess in productive capacity is now expected to be lower than estimated in January but continues to be significant and affected by high levels of uncertainty, as reflected in the wide forecast intervals. The possibility of new waves of the virus (of uncertain intensity and duration) represents a significant downward risk to projected GDP growth, and is signaled by the lower limits of the ranges provided in this report. Inflation (1.51%) and inflation excluding food and regulated items (0.94%) declined in March compared to December, continuing below the 3% target. The decline in inflation in this period was below projections, explained in large part by unanticipated increases in the costs of certain foods (3.92%) and regulated items (1.52%). An increase in international food and shipping prices, increased foreign demand for beef, and specific upward pressures on perishable food supplies appear to explain a lower-than-expected deceleration in the consumer price index (CPI) for foods. An unexpected increase in regulated items prices came amid unanticipated increases in international fuel prices, on some utilities rates, and for regulated education prices. The decline in annual inflation excluding food and regulated items between December and March was in line with projections from January, though this included downward pressure from a significant reduction in telecommunications rates due to the imminent entry of a new operator. When controlling for the effects of this relative price change, inflation excluding food and regulated items exceeds levels forecast in the previous report. Within this indicator of core inflation, the CPI for goods (1.05%) accelerated due to a reversion of the effects of the VAT-free day in November, which was largely accounted for in February, and possibly by the transmission of a recent depreciation of the peso on domestic prices for certain items (electric and household appliances). For their part, services prices decelerated and showed the lowest rate of annual growth (0.89%) among the large consumer baskets in the CPI. Within the services basket, the annual change in rental prices continued to decline, while those services that continue to experience the most significant restrictions on returning to normal operations (tourism, cinemas, nightlife, etc.) continued to register significant price declines. As previously mentioned, telephone rates also fell significantly due to increased competition in the market. Total inflation is expected to continue to be affected by ample excesses in productive capacity for the remainder of 2021 and 2022, though less so than projected in January. As a result, convergence to the inflation target is now expected to be somewhat faster than estimated in the previous report, assuming the absence of significant additional outbreaks of COVID-19. The technical staff’s year-end inflation projections for 2021 and 2022 have increased, suggesting figures around 3% due largely to variation in food and regulated items prices. The projection for inflation excluding food and regulated items also increased, but remains below 3%. Price relief measures on indirect taxes implemented in 2020 are expected to lapse in the second quarter of 2021, generating a one-off effect on prices and temporarily affecting inflation excluding food and regulated items. However, indexation to low levels of past inflation, weak demand, and ample excess productive capacity are expected to keep core inflation below the target, near 2.3% at the end of 2021 (previously 2.1%). The reversion in 2021 of the effects of some price relief measures on utility rates from 2020 should lead to an increase in the CPI for regulated items in the second half of this year. Annual price changes are now expected to be higher than estimated in the January report due to an increased expected path for fuel prices and unanticipated increases in regulated education prices. The projection for the CPI for foods has increased compared to the previous report, taking into account certain factors that were not anticipated in January (a less favorable agricultural cycle, increased pressure from international prices, and transport costs). Given the above, year-end annual inflation for 2021 and 2022 is now expected to be 3% and 2.8%, respectively, which would be above projections from January (2.3% and 2,7%). For its part, expected inflation based on analyst surveys suggests year-end inflation in 2021 and 2022 of 2.8% and 3.1%, respectively. There remains significant uncertainty surrounding the inflation forecasts included in this report due to several factors: 1) the evolution of the pandemic; 2) the difficulty in evaluating the size and persistence of excess productive capacity; 3) the timing and manner in which price relief measures will lapse; and 4) the future behavior of food prices. Projected 2021 growth in foreign demand (4.4% to 5.2%) and the supposed average oil price (USD 53 to USD 61 per Brent benchmark barrel) were both revised upward. An increase in long-term international interest rates has been reflected in a depreciation of the peso and could result in relatively tighter external financial conditions for emerging market economies, including Colombia. Average growth among Colombia’s trade partners was greater than expected in the fourth quarter of 2020. This, together with a sizable fiscal stimulus approved in the United States and the onset of a massive global vaccination campaign, largely explains the projected increase in foreign demand growth in 2021. The resilience of the goods market in the face of global crisis and an expected normalization in international trade are additional factors. These considerations and the expected continuation of a gradual reduction of mobility restrictions abroad suggest that Colombia’s trade partners could grow on average by 5.2% in 2021 and around 3.4% in 2022. The improved prospects for global economic growth have led to an increase in current and expected oil prices. Production interruptions due to a heavy winter, reduced inventories, and increased supply restrictions instituted by producing countries have also contributed to the increase. Meanwhile, market forecasts and recent Federal Reserve pronouncements suggest that the benchmark interest rate in the U.S. will remain stable for the next two years. Nevertheless, a significant increase in public spending in the country has fostered expectations for greater growth and inflation, as well as increased uncertainty over the moment in which a normalization of monetary policy might begin. This has been reflected in an increase in long-term interest rates. In this context, emerging market economies in the region, including Colombia, have registered increases in sovereign risk premiums and long-term domestic interest rates, and a depreciation of local currencies against the dollar. Recent outbreaks of COVID-19 in several of these economies; limits on vaccine supply and the slow pace of immunization campaigns in some countries; a significant increase in public debt; and tensions between the United States and China, among other factors, all add to a high level of uncertainty surrounding interest rate spreads, external financing conditions, and the future performance of risk premiums. The impact that this environment could have on the exchange rate and on domestic financing conditions represent risks to the macroeconomic and monetary policy forecasts. Domestic financial conditions continue to favor recovery in economic activity. The transmission of reductions to the policy interest rate on credit rates has been significant. The banking portfolio continues to recover amid circumstances that have affected both the supply and demand for loans, and in which some credit risks have materialized. Preferential and ordinary commercial interest rates have fallen to a similar degree as the benchmark interest rate. As is generally the case, this transmission has come at a slower pace for consumer credit rates, and has been further delayed in the case of mortgage rates. Commercial credit levels stabilized above pre-pandemic levels in March, following an increase resulting from significant liquidity requirements for businesses in the second quarter of 2020. The consumer credit portfolio continued to recover and has now surpassed February 2020 levels, though overall growth in the portfolio remains low. At the same time, portfolio projections and default indicators have increased, and credit establishment earnings have come down. Despite this, credit disbursements continue to recover and solvency indicators remain well above regulatory minimums. 1.2 Monetary policy decision In its meetings in March and April the BDBR left the benchmark interest rate unchanged at 1.75%.

This paper presents experimental and theoretical investigations on progressive collapse behavior of steel framed structures subjected to an extreme load such as fire, blast and impact. A new capacity-based index is proposed to quantify robustness of structures. An energy-based theoretical model is also proposed to quantify the effect of concrete slabs on collapse resistance of structures. The experimental results show that the dynamic amplification factors of frames subject to impact or blast are much less than the conventional value of 2.0. The collapse process of frames in fire can be either static or dynamic depending on the restraint conditions and load levels. It is necessary to account for the failure time and residual strength of blast-exposed columns for assessing the collapse resistance of structures subject to explosion. Two collapse modes of steel frames under blast or impact are found: connection-induced collapse mode and column-induced collapse mode. In case of fire, a frame may collapse due to either column buckling or pulling-in effect of beams. The energy dissipation from elongation of slab reinforcement and additional resultant moment greatly contribute to the collapse resistance of structures.