Microstructure Evolution, Mechanical Properties and Strain Hardening Instability of Low and Medium Carbon Quenching & Partitioning Steels

Ramadan N. Elshaer,Mohamed K. El‑Fawakhry,Ahmed I. Z. Farahat 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.6

The effect of quenching after martensitic finish (QAMf) or quenching & partitioning (Q&P) on microstructure evolution,mechanical properties, and strain hardening instability of low and medium carbon hot rolled steels were investigated. Twoheats of low and medium carbon steels were cast in an induction open furnace. The chemical composition of low carbonsteel is 0.16C–0.27Si–1.47Mn–0.02Al while medium carbon steel is 0.49C–0.30Si–0.91Mn–0.03Al. They were hot-rolledat 1200 °C for 30 min followed by air cooling. The microstructure after hot-rolled gives bands of ferrite and pearlite for0.16 wt% low carbon steel. On the other hand, 0.49 wt% medium carbon steel produces coarse pearlite islands surrounded byferrite phase. To enhance mechanical properties, it was necessary to modify the microstructure of low and medium carbonsteels using QAMfor Q&P processes. The resultant matrix of microstructure after QAMfand Q&P processes containedferrite, bainite, lath martensite, and retained austenite for 0.16 wt% low carbon steel, and polygonal ferrite, lath martensite,and retained austenite for 0.49 wt% medium carbon steel, respectively. In low carbon steel, QAMfprocess increased uniformelongation from 6.6 to 13.5% (105% increase) while ultimate tensile strength (UTS) improved slightly from 645 to 692 MPa(7% increase). However, in medium carbon steel, Q&P reduced uniform elongation from 12.4 to 4.8% (61% decrease) whileincreased UTS from 769 to 1242 MPa (61.5% increase). It is worthy to mention that QAMfprocess exhibited strain hardeninginstability zone (7.8% strain before necking) compared to hot-rolled process (0% strain before necking). On the otherhand, Q&P process highly decreased strain hardening instability zone (0.77% strain before necking) compared to hot-rolledprocess (3.4% strain before necking).

Progressive Collapse Assessment of Multistory Reinforced Concrete Structures Subjected To Seismic Actions

Ahmed Elshaer,Hatem Mostafa,Hamed Salem 대한토목학회 2017 KSCE JOURNAL OF CIVIL ENGINEERING Vol.21 No.1

Progressive collapse is a catastrophic partial or total failure of a structure that mostly occurs when a structure loses a primary component like a column. Some international standards have started to consider progressive collapse resistance in various approaches. In this study, the ‘Unified Facilities Criterion’ guidelines were used in assessing the structure; these guidelines represent one of the codes that discuss progressive collapse using sophisticated approaches. Three-dimensional nonlinear dynamic analyses using the ‘Applied Element Method’ were performed for a structure that lost a column during a seismic action. A parametric study was made to investigate the effect of different parameters on progressive collapse. In this study, a primary structural component was assumed lost during an earthquake. The studied parameters were the location of the removed column in plan, the level of the removed column, the case of loading, and the consideration of the slabs. For the study cases, it was concluded that the buildings designed according to the Egyptian code satisfies the progressive collapse requirements stated by ‘Unified Facilities Criteria’ (UFC) guidelines requirements with a safety factor of 1.97. Also, it was found that losing a column during a seismic action is more critical for progressive collapse than under gravity load. Finally, this study elaborated the importance of considering the slab in progressive collapse analysis of multistory buildings in order to include the significant catenary action developed by the slabs.

Variation in wind load and flow of a low-rise building during progressive damage scenario

Ahmed Elshaer,Girma Bitsuamlak,Hadil Abdallah 한국풍공학회 2019 Wind and Structures, An International Journal (WAS Vol.28 No.6

In coastal regions, it is common to witness significant damages on low-rise buildings caused by hurricanes and other extreme wind events. These damages start at high pressure zones or weak building components, and then cascade to other building parts. The state-of-the-art in experimental and numerical aerodynamic load evaluation is to assume buildings with intact envelopes where wind acts only on the external walls and correct for internal pressure through separate aerodynamic studies. This approach fails to explain the effect of openings on (i) the external pressure, (ii) internal partition walls; and (iii) the load sharing between internal and external walls. During extreme events, non-structural components (e.g., windows, doors or roof tiles) could fail allowing the wind flow to enter the building, which can subject the internal walls to lateral loads that potentially can exceed their load capacities. Internal walls are typically designed for lower capacities compared to external walls. In the present work, an anticipated damage development scenario is modelled for a four-story building with a stepped gable roof. LES is used to examine the change in the internal and external wind flows for different level of assumed damages (starting from an intact building up to a case with failure in most windows and doors are observed). This study demonstrates that damages in non-structural components can increase the wind risk on the structural elements due to changes in the loading patterns. It also highlights the load sharing mechanisms in low rise buildings.

A low-cost expandable multi-channel pressure system for wind tunnels

Moustafa Aboutabikh,Ahmed Elshaer,Haitham Aboshosha 한국풍공학회 2022 Wind and Structures, An International Journal (WAS Vol.35 No.5

Over the past few decades, the use of wind tunnels has been increasing as a result of the rapid growth of cities and the urge to build taller and non-typical structures. While the accuracy of a wind tunnel study on a tall building requires several aspects, the precise extraction of wind pressure plays a significant role in a successful pressure test. In this research study, a lowcost expandable synchronous multi-pressure sensing system (SMPSS) was developed and validated at Ryerson University’s wind tunnel (RU-WT) using electronically scanning pressure sensors for wind tunnel tests. The pressure system consists of an expandable 128 pressure sensors connected to a compact data acquisition and a host workstation. The developed system was examined and validated to be used for tall buildings by comparing mean, root mean square (RMS), and power spectral density (PSD) for the base moments coefficients with the available data from the literature. In addition, the system was examined for evaluating the mean and RMS pressure distribution on a standard low-rise building and were found to be in good agreement with the validation data.

Structural performance of an electricity tower under extreme loading using the applied element method- A case study

Jason Ah Chin,Mauricio Garcia,Jeffrey Cote,Ellen Mulcahy,Jonathan Clarke,Ahmed Elshaer 한국풍공학회 2022 Wind and Structures, An International Journal (WAS Vol.34 No.3

The resiliency of electricity transmission and distribution lines towards natural and man-made hazards is critical to the operation of cities and businesses. The extension of these lines throughout the country increases their risk of extreme loading conditions. This paper investigates a unique extreme loading condition of a 100-year old distribution line segment that passes across a river and got entangled with a boom of a ship. The study adopts the Applied Elements Method (AEM) for simulating 54 cases of the highly deformable structural behaviour of the tower. The most significant effects on the tower’s structural integrity were found to occur when applying the load with components in all three of the cartesian directions (i.e., X, Y and Z) with the full capacities of the four cables. The studied extreme loading condition was determined to be within the tower’s structural capacity, attributed to the shear failure of the anchor bolts, which acted as a sacrificing element that fails to protect the transfer of tensioning load to the supporting tower.