Hamed Salem

Computer-Aided Assessment of Progressive Collapse of Reinforced Concrete Structures according to GSA Code

A building is subjected to progressive collapse when a primary vertical structural element fails, resulting in the failure of adjoining structural elements, which cause further structural failure, leading eventually to partial or total collapse of the structure. The failure of a primary vertical support might occur because of extreme loadings such as a bomb explosion in a terrorist attack, a car colliding with supports in a parking garage, an accidental explosion of explosive materials, or a severe earthquake. Different design codes address the progressive collapse of structures attributable to the sudden loss of a main vertical support such as the General Services Administration (GSA) code and the Unified Facilities Criteria (UFC). The alternative path method (APM) is the main analysis method for evaluating the hazard of progressive collapse in the two codes. The APM requires that the structure be capable of bridging over a missing structural element, with the resulting extent of damage being localized. In the current study, a progressive collapse assessment according to the GSA code is carried out for a typical ten-story RC-framed structure. The structure is designed according to the building code requirements for structural concrete. Fully nonlinear dynamic analysis for the structure is carried out using the applied-element method (AEM). According to the GSA code, a primary vertical structural element is removed, and the collapse area is investigated. The investigated cases include the removal of a corner column, an edge column, an edge shear wall, internal columns, an internal shear wall, and a corner shear wall. The numerical analysis showed that, for an economic design, the analysis should consider slabs and cannot be simplified into a two- or three-dimensional frame analysis. Neglecting the slabs in the progressive collapse analysis is a very conservative approach that may lead to uneconomic design. The RC structures designed according to American Concrete Institute guidelines met the GSA limits and were found to have a low potential for progressive collapse.

Applied Element Method Analysis of Porous GFRP Barrier Subjected to Blast

Numerical analysis of highly dynamic phenomena represents a critical field of study and application for structural engineering as it addresses extreme loading conditions on buildings and the civil infrastructure. In fact, large deformations and material characteristics of elements and structures different from those exhibited under static loading conditions are important phenomena to be accounted for in numerical analysis. The present paper describes the results of detailed numerical analyses simulating blast tests conducted on a porous (i.e. discontinuous) glass fiber reinforced polymer (GFRP) barrier aimed at the conception, validation and deployment of a protection system for airport infrastructures against malicious disruptions. The numerical analyses herein presented were conducted employing the applied element method (AEM). This method adopts a discrete crack approach that allows auto cracking, separation and collision of different elements in a dynamic scheme, where fully nonlinear path-dependant constitutive material models are adopted. A comparison with experimental results is presented and the prediction capabilities of the software are demonstrated.

Punching shear strength of reinforced concrete flat slabs subjected to fire on their tension sides

Abstract The effect of fire on punching strength of flat slabs is experimentally investigated. An experimental program, consisting of fourteen one-third scale specimens pre-exposed to fire on their tension side and tested under concentric punching, is carried out. The main investigated parameters are the duration of exposure to fire, the concrete cover and the cooling method. Specimens are subjected to direct flame for 1.0, 2.0 and 3.0 h, respectively. Concrete covers of 25 mm and 10 mm are used for test specimens. Two cooling methods are employed; gradual cooling in air and sudden cooling with water applied directly to the heated surface of the slabs. It was found that exposure of slabs to fire resulted in a reduction of up to 18.3% and 43% in cracking loads and ultimate punching loads, respectively. Concrete cover was proven to have a significant effect on level of temperature in tension reinforcement. A reduction in punching strength of up to 14% was observed for specimens with 3 h exposure to fire compared to those with one hour exposure. Sudden cooling was found to reduce punching strength by 25% compared to specimens gradually cooled. A simplified mechanical model for calculating fire effect on punching capacity is proposed and found to be in good agreement with the experimental results.

Numerical Collapse Analysis of Tsuyagawa Bridge Damaged by Tohoku Tsunami

AbstractIn March 2011, the Tohoku tsunami swept over the east coast of Japan killing more than 15,000 people and missing more than 2,500 people. The tsunami resulted in collapsing of more than 400,000 buildings and washing away of more than 250 coastal bridges. In this study, the collapse of Tsuyagawa bridge damaged by the Tohoku tsunami is numerically investigated using the applied element method (AEM) because of its advantages for simulating structural progressive collapse. The AEM is a discrete crack approach that allows the elements separation, fall, and collision with each other in a fully nonlinear dynamic scheme of computations. The analysis was demonstrated to efficiently simulate the bridge collapse, showing that the bridge collapsed at a water speed of 6.6  m/s initiated by flexural failure of its piers. The study of bridge strengthening showed that the collapse water speed could be increased by 22 and 29% compared to Tohoku tsunami if piers are strengthened by a 100-mm RC jacket and 20-mm thick...

Prediction of Bridge Behavior Through Failure: A Case Study Of The Minnesota I-35W Bridge Collapse

The I-35 Bridge over the Mississippi River in Minneapolis, Minnesota catastrophically failed during the evening rush hour on August 1, 2007, collapsing into the river. In the years prior to the collapse, several reports cited problems with the bridge structure. This research analytically investigated the cause of the collapse using the Applied Element Method. The bridge was modeled using construction drawings, with relevant structural details and loadings. Structural details included the steel truss, gusset plates, concrete slabs, concrete piers, while structural loading included traffic and construction. AEM provided the cause of collapse of the I35-W Bridge. The cause of collapse was found to be the failure of the gusset plates at connections L11 and U10, which well agreed with the field investigations of the collapsed bridge. The under-designed thickness of the plates, their corrosion, and over loading due to traffic and construction loads at time of collapse were the reasons for the bridge collapse.

Predictive Seismic Load History Comparison of AEM Model Prediction vs. Empirical Results

This paper reviews the analysis and associated results of seismic acceleration in due to ground impact from a demolition. Several large structures were planned to be demolished and the concern arose that the impact from these structures would damage sensitive instruments nearby. Applied Element Method (AEM) was used to predict the force-time history from the demolition. As the structures were demolished, there were several different impacts that were used as multiple force-time histories. This data in turn was applied to the ground to predict the force-time history in the soil. Extreme Loading for Structures (ELS) which utilizes AEM was used to model a 240 meter diameter area of the soil. This included seven different soil layers based on provided soil data. An additional layer of 80 meters in depth was modeled below these to reduce the effect of boundary conditions. Results from the blind analysis are compared to a simplified lumped parameter model and actual data gathered in the field during the demolition. The results from this analysis are compared to empirical results and are found to correlate closely.