Progressive Collapse Resisting Capacity of RC Flat Slab Buildings with Varying Spans and Storey Heights

With no beams, reinforced concrete flat slab buildings are typically built to advance urban growth and to meet the architectural needs of large spans and low storey heights. Its behaviour to avoid a progressive collapse must therefore be investigated. In this research, the progressive collapse resistance of six-storey RC flat slab buildings with varying span lengths and floor heights is assessed by subjecting the building to three different instances of instantaneous removal of columns in the first storey, performing dynamic progressive collapse analysis as per GSA guidelines, and comparing the evaluated joint displacements and chord rotations at column removal locations with the permissible chord rotation for flat slab buildings as per DoD guidelines. The results have shown that the studied flat slab building with all different span lengths and floor heights is prone to progressive collapse. It is also observed that the vertical displacements and chord rotations at column removal positions increase as the span lengths and storey heights are increased alternately.

Experimental and Numerical Investigation of 3D Dam-Break Wave Propagation in an Enclosed Domain with Dry and Wet Bottom

Dam-break flood waves represent a severe threat to people and properties located in downstream regions. Although dam failure has been among the main subjects investigated in academia, little effort has been made toward investigating wave propagation under the influence of tailwater depth. This work presents three-dimensional (3D) numerical simulations of laboratory experiments of dam-breaks with tailwater performed at the Laboratory of Hydraulics of Iskenderun Technical University, Turkey. The dam-break wave was generated by the instantaneous removal of a sluice gate positioned at the center of a transversal wall forming the reservoir. Specifically, in order to understand the influence of tailwater level on wave propagation, three tests were conducted under the conditions of dry and wet downstream bottom with two different tailwater depths, respectively. The present research analyzes the propagation of the positive and negative wave originated by the dam-break, as well as the wave reflection against the channel’s downstream closed boundary. Digital image processing was used to track water surface patterns, and ultrasonic sensors were positioned at five different locations along the channel in order to obtain water stage hydrographs. Laboratory measurements were compared against the numerical results obtained through FLOW-3D commercial software, solving the 3D Reynolds-Averaged Navier–Stokes (RANS) with the k-ε turbulence model for closure, and Shallow Water Equations (SWEs). The comparison achieved a reasonable agreement with both numerical models, although the RANS showed in general, as expected, a better performance.

Experimental and analytical evaluation of disproportionate collapse flat-plate buildings

Reinforced concrete flat plate buildings without continuous integrity reinforcement may be vulnerable to disproportionate collapse if a supporting structural member was lost in an abnormal event. This research forces on the evaluation of potential of disproportionate collapse in older flat-plate structures subjected to the loss of a supporting column in extreme loading events. If a supporting column fails, then the load was carried by that column must be redistributed to the surrounding slab-column connections, which in turn may results in a disproportionate collapse over an entire building or a large portion of it. This progression can occur if the punching shear strength of the surrounding connections is not sufficient. In order to make the most accurate determination of the potential for disproportionate collapse of flat plate structures, this research seeks to accurately evaluation the punching shear capacity of slab-column connections using the conditions present in a potential collapses event. The in-plane lateral restraint provided by the floor slab can enhance the punching shear strength of surrounding slab-column connections and may be significant. In addition, the post-punching capacity of the original failed slab-column connection may reduce the amount of load to be redistributed to the surrounding connections. In order to investigate the effects of lateral restraint and post punching capacity, six restrained and unrestrained static tests was conducted at 1% and 0.64% reinforcement ratios. The static tests showed that the punching shear capacity can be increased 2-8% as lateral restraint stiffness varies from 17 to 75.6kN/mm but the increase is highly related to the in-plane lateral restraint stiffness. The tests also indicated that the slab without integrity reinforcement can develop 54% of maximum post-punching strength after punching. However, this capacity decreases dramatically as the deflection increases to a large amount after punching failure. Since isolated slab-column testing cannot fully represent behaviors of an actual building, multi-panel testing was done at a sub-structure system level. The specimens consisted of two 9 column portion of a flat plate building, one tested with an exterior column instantaneous removal and another tested with an interior column instantaneous removal. The tests further investigated the dynamic load redistribution, punching, and post-punching responses in a flat-plate structure. The multi-panel tests (with interior and exterior column removal) showed that flat-plate slabs are vulnerable to disproportionate collapse at load levels of approximately 50% of their design capacity. The recorded lateral movements on columns in the tests verified the existence of compression membrane forces in continuous slab panel. Compressive membrane forces form after a column removal and gradually transition to tension membrane forces at deflections approaching the slab depth. Punching failure did not happen in compressive membrane phase, but in the tension membrane phase and tests showed that pre-existing damage in flat-plate structures (from prior overloading or shrinkage cracking) may impede the formation of compressive membrane forces in the slab. Dynamic removal of a supporting column resulted in a dynamic load amplification factor (DLAF) of approximately 1.3. Therefore, surrounding connections need to be able to carry at least 30% more than the predicted redistributed static load in a collapse analysis."

Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions

A high-resolution, two-dimensional numerical model is used to study the moisture redistribution following homogeneous ice nucleation induced by Kelvin waves in the tropical tropopause layer (TTL). We compare results for dry/moist initial conditions and three levels of complexity for the representation of cloud processes: complete microphysics and cloud radiative effects, likewise but without radiative effects, and instantaneous removal of moisture in excess of saturation upon nucleation. Cloud evolution and moisture redistribution are found to be sensitive to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water, whereas the cloud radiative heating induces upward advection of the cloudy air. The latter results in an upward (downward) flux of water vapour if the cloudy air is moister (drier) than the environment, which is typically when the environment is subsaturated (supersaturated). Only a fraction (~25% or less) of the cloud experiences nucleation. Post-nucleation processes (ice depositional growth, sedimentation, and sublimation) are important to cloud morphology, and both dehydrated and hydrated layers may be indicators of TTL cirrus occurrence. The calculation with instantaneous removal of moisture not only misses the hydration but also underestimates dehydration due to (i) nucleation before reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal from sedimenting ice particles below the nucleation level. The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative estimates of cloud-induced moisture redistribution on the basis of case-by-case calculations.

A modelling study of moisture redistribution by thin cirrus clouds

A high resolution 2-dimensional numerical model is used to study the moisture redistribution following homogeneous ice nucleation induced by Kelvin waves in the tropical tropopause layer (TTL). We compare results for dry/moist initial conditions, and three levels of complexity for the representation of cloud processes: full bin microphysics and radiative effects of the ice, ditto but without radiative effects, and instantaneous removal of moisture in excess of saturation upon nucleation. Cloud evolution and the profiles of moisture redistribution are found to be sensitive to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water. On the other hand, the cloud radiative heating induces upward advection of the cloudy air. This results in an upward flux of water vapour if the cloudy air is moister (or drier) than the environment, which is typically when the environment is subsaturated (or supersaturated). The numerical results show that only a small fraction (less than 25%) of the cloud experiences nucleation. Sedimentation and reevaporation are important, and hydrated layers in observation may be as good an indicator as dehydrated layers for the occurrence of thin cirrus clouds. The calculation with instantaneous removal of condensates misses the hydration by construction, but also underestimates dehydration due to lack of moisture removal from sedimenting particles below the nucleation level, and due to nucleation before reaching the minimum saturation mixing ratio. The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative conclusions regarding the role of thin cirrus clouds for the moisture distribution in the TTL and stratosphere.

Dynamic Analysis of Systems With Varying Constraints

This paper deals with constrained dynamical systems which are subject, during motion, to the replacement of one set of constraints with another. The theory of imposition and removal of constraints is used to formulate equations governing motions of such systems. To this end, the terms minimally constrained state (MCS), phase of motion, transition, and transition conditions are introduced. These terms are used to denote, respectively, state of a system subject only to constraints which are not removed throughout the motion, period of time during which an MCS system is subject to one set of constraints, event characterized by the instantaneous removal of one set of constraints and the imposition of another, and conditions, satisfaction of which initiate a transition. The indicated formulation enables the simulation of motions of the systems in question, including the evaluation of changes in the motion variables associated with the transitions. The formulation is particularly efficient in that the impulses arising during the transitions are automatically eliminated. The formulation is used to simulate motions of a number of systems, including a legged machine.